(81) Shirane, G., Jona, F., Pepinsky, R., Proc. I.R.E. 43, 1738 (1955). (82) Simpson, W. S., Woods, H. J., Nature 185, 157 (1960). (83) Sociktk franqaise de Min6ralogie et Cristallographie, 1 Rue Victor Cousin, France, “Index Of Crysta11o-
graphic Supplies”; also available from Wm. Parrish, I. U. Cr. Commission on Crystallographic Apparatus, Philips
Laboratories, Irvington-on-Hudson, N. Y. (84) Soviet Physics, Crystallography, translation of KristallograJiya by American InstituteOf xew (85) raphy, TayFrjRiley, New‘‘X-Ray York, 1961. (86) Taylor, H. F. J . A p p l . Chem. London 10, 317 (1960). (87) Trueblood, K. M., Phys. T o d a y 14, 45 (1961).
*.,
w.,
(88) Wade, F. A,, Matton, R. B., “Elements of Crystallography and Miner-
New
‘’O’
(89) Wheatley, P. J., “Determination of Molecular Structure,” Oxford Univ. Press, Oxford, 1959. Woolfson, p\I. hI., Methods in X-Ray Crystallography,” Oxford Univ. Press, Oxford, 1961.
Magnetic Susceptibility Quite often the editors receive suggestions concerning subjects which have not been covered or not covered regularly in our annual reviews. In some cases there are subjects in which work being done i s too limited to warrant regular reviews; in other cases the field or technique may be one which has not generally been considered to be analytical in nature but which has a potential for analytical applications. Magnetic susceptibility, or magnetochemistry as it is also called, i s one example. The editors are pleased, therefore, to present a review on this topic prepared b y L. N. Mulay, who has done much work in the analytical applications of magnetic susceptibility.
Review of Fundamental Developments in Analysis
Instrumentation and Some Analytical Applications of Magnetic Susceptibility I , N. Mulay, Deparfmenf o f Chemistry, University of Cincinnafi, Cincinnati 2 I , Ohio
A
of magnetism to a study of chemical problems are numerous and have been developed since the days of Faraday. However, the term magnetochemistry appears to have been introduced for the first time in 1935 by Bhatnagar and Mathur in India, who wrote the rather extensive “Physical Principles and Applications of Magnetochemistry.” I n spite of the age and usefulness of magnetism, a study of its applications to chemical problems seems to have been restricted to certain specific schools. This may be attributed in part to a lack of commercial availability of instruments for measuring magnetic susceptibility of chemical compounds under a variety of experimental conditions. Fortunately, with the advent of the techniques of nuclear magnetic resonance, the interest in magnetochemistry within and outside these schools has been revived. Considering this renewed interest, a presentation of a review in this area, written from the standpoint of analytical chemists, appears to be in order. This being the first review of its type, an attempt will be made to point out especially the salient features and trends PPLICATIOSS
in instrumentation and to indicate some analytical applications of magnetic susceptibility. References are included which go back earlier than our usual reviews; this should not, hoR-ever, be regarded as a comprehensive review on magnetochemistry itself. NOMENCLATURE AND THEORY
It is not possible to summarize within the available space the meaning of the vast number of terms used in magnetochemistry and to outline its theories. The reader is, therefore, referred to many excellent texts listed in the next section. The most important magnetic parameters on which a discussion of this review rests are the volume and specific magnetic susceptibilities and the magnetic moments. The first may be defined as the ratio of the intensity of magnetic field induced inside a unit volume of a substance to t h a t of the applied field. The specific or mass susceptibility x is obtained by dividing the volume susceptibility K b y the density. The magnetic moment, from a physical point of view, may be defined
as the turning effect which a magnetic dipole, arising from the “spin” and “orbital” motions of electron(s), experiences when placed in a unit magnetic field. Considering the recent advances in our knowledge of the magnetic susceptibilities and moments of nuclei of elements, i t has become necessary to distinguish between these and the properties ascribed to electrons. Thus the term “electronic susceptibility” often refers to the magnetic susceptibility of electrons; i t corresponds to the static or bulk magnetic susceptibility x and is related to the magnetic moment g. This term should not be confused with electrical susceptibility which is related to the electric dipole moment. It may also be pointed out that a new term “ferrimagnetism” to indicate ferromagnetism arising from atoms in two kinds of sites has been introduced by Ye61 (120). This has no relationship to antiferromagnetism or to the valence state nomenclature such as ferro(cyanide) and ferri(cyanide). It may be surmised that as yet no satisfactory theory has been developed for ferromagnetism and antiferromagnetism. Indeed the work of N e d (120) VOL. 34, NO. 5, APRIL 1962
343 R
Table 1.
Type Diamagnetism
Effect of External Field on Substance Feeble repulsion I < H
Paramagnetism
Attraction I>H
Ferromagnetism
Intense Metals like iron, cobalt, nickel, and Attraction I >> H their alloys r-FenOa
Examples dost inorganic conipounds, except those containing ions of transition elements; organic compounds except free radicals. Certain compositions like stainless steel, special Cu-Si alloys (eg., 5-cent coin) Salts and certain complexes of transition elements, “odd” electron molecules like NO, Oxygen. Free radicals such as triphenyl methyl
Types of Magnetic Behavior (cf.
724)
Magnitude of Specific Comments Susceptibility Dependence of Susceptibility on on Origin x a t 20’ C. Temperature Field Caused by orbital Negative and very None theoreti- Kone motion of elecsmall cally. Small detron(s). Hence, (“1 x 10-6) pendence, atit is a universal tributable to propertv. Most rhange in state perceptible n-hen of aggregation all electrons are of system with “paired,” that temp. is when they have no permanent “spin” moment Caused by spin Positive and small oo 1 Xone and (usually) (“100 x 10-6) orbital momen- It is sufficientlv (Curie lam’, tum of (unlarge to mask ‘ or paired) electhe underlying 1 trons. The sysdiamagnetism X CQ tem contains permanent magnetic dipoles (moments) with no interaction Caused by “do- Positive and very Dependence is Dependence demains” or latcomplex. Bescribed by hyslarge tice of particles (“1 x 102) yond a certain teresis curves containing electemp. (Curie trons with parpoint) magnetallel spins ization drops Positive interacand shows paration among dimagnetic bepoles havior Atom with upper Positive and very Kone state separated small from ground (”1 x 10-6) state by energy interval large compared to KT. System has no permanent magnetic moment Paramagnetism of an “electron gas”
Temperature in- Feeble dependent or attraction Van Vleck paramagnetism
KMn04 Co(111)ammines
Pauli or free Feeble electron paraattraction magnetism
Metallic K and Na (vapors)
Antiferromagnetism
Feeble attraction
Ferrimagnetism
Feeble attraction
KNiFs, MnSe, TisOI Two lattices of Positive and very Complex dependence. Up to a critical particles having ferrites small temp. (antiferromagnetic Curie point electron spins in 1 x 10-7 to or See1 temp.) magnetization inone lattice anti2 x 10-6 creases with temp., then it decreases parallel to those in another lattice. Negative interaction among magnetic dipoles FeCr104 Interpenetrating Positive and small Positive dependence on field lattices with un1 x 10-5 equal numbers of electrons with antiparallel spins. Simultaneous unequal interaction among dipoles NiCl, or CoC1, a t Parallel or anti- Positive and small Positive dependence on field 1 x 10-6 liquid HZtemp. parallel alignment 0: moments in domains
-1
Metamagnetism Feeble (may be reattraction garded as a special case of antiferromagnetism with low Nee1 temp.). It shows field strength dependence
344 R
ANALYTICAL CHEMISTRY
x
10-6
has succeeded in clarifying many aspects of the phenomena of ferro- and antiferromagnetism, which are complex as compared with those of dia- and paramagnetism. A classification (124) of various types of magnetism and their finer subdivisions in relation to many complex chemical compounds is given in Table I. GENERAL LITERATURE
A number of books on magnetism under the general title of Electricity and hfagnetism have been published. However, only a few describe magnetism in relation to matter and have been written by a numbpr of physicists (10, 26, 72, 16.4, 1 7 7 ) . Information on magnetic properties of materials such as semiconductors and alloys is available in books ($7) specially written in these areas and in solid state physics (92). These generally discuss the concepts of lattice and charge carrier susceptibilities which contribute to the over-all susceptibility of a semiconductor (49, 98). I n the area of magnetochemistry itself, a treatise by Bhatnagar and Mathur (14) referred to earlier appeared almost four decades ago; yet even today, i t
Table
hlethod Gouy
II.
stands out as a good reference for spectroscopic nomenclature and certain calculations. It is particularly helpful in calculations of diamagnetism of organic molecules using Pascal’s constants for the atomic susceptibilities and the constitutive correction constants for different types of bonding. This was followed by Klemm’s “Magnetochemie” (94) and two editions of Selwood’s “Magnetochemistry” (152); the 1956 edition is by far the most comprehensive contribution in this area. Significant review-type articles by a number of authors (61, i24, 126, 128, 145) appeared during the last decade. Some sections on magnetic susceptibility have appeared in “Techniques of Organic Chemistry” (108) and other similar compilations (83, 90). An extensive discussion on analytical applications of magnetic susceptibility by the author (114) is scheduled t o appear in the “Treatise of Analytical Chemistry.” Some of the useful tables on magnetic susceptibility constants are to be found in another compilation (113). Most extensive tables of susceptibility constants are given by Foex, Gorter, and Smits (63). Recent trends in magnetic measurements are discussed by Palmer (130).
INSTRUMENTATION
Magnetic susceptibilities are generally measured by the socalled (A) uniform field and ( B ) the nonuniform methods; (C) some derived and related methods have also been developed in recent years. Table I1 summarizes practical aspects, such as applicability and limitations of these methods. The Gouy and Faraday methods belong to categories ( A ) and ( B ) , respectively, and are regarded as standard classical methods. These along with their innumerable modifications have been described extensively in the literature (10, 14, 101, 1 i 4 , 152) and the references listed therein. Information on microtechniques is given by Cunningham (45). Hence, no special effort will be made here to outline their physical principles and operational procedures; only a few of the more recent modifications will be reviewed. I n addition, a discussion will be presented of some practical aspects and new components on the market such as magnets, power supplies, balances, etc., required for the assembly of the Gouy and Faraday type magnetic balances. A review of the derived and related methods is given a t the end of this section.
Summary of Important Aspects of Methods of Measuring Magnetic Susceptibilities
Physical Nature General Field of Sample Requirements Applicable to Uniform field. Dia- and Para- Powdered solids, pure liquids and Recommended magnetic only solutions. and easily available range with (Adaptable for measuring x of a electromagnets gas surrounding is 3000 to 15,000 a known samoe. Permanent magnets up to ple)
Approximate Minimum and Temperature Convenient Size Control Accuracy of Sample 0.5-g. solids 5-ml. Generally =t1%, Is possible over a wide range. liquids (Macro may be improved From liq. HP or to I t O . l Y0 , ( sepscale), Few mg. liq. Np temp. t o arate density or micrograms Eeveral hundred measurement in solution can degrees may be required; accube handled in obtained. racy depends on special apparatus
packing
5000 oe.
Quincke
Same
Generally &O.l% Pure liquids and -5ml. solutions. (Adaptable for measuring T of a gap above the meniscus of a known liquid) Pure liquids and -2 ml. Same Same solutions (143) [Adaptable to flow systems (69) and gases] Few mg. (micro- i o . 1 7 0 Dia-, para-, and Generally useful techniques are ferromagnetic for powdered materials solids (liquids also available) may be handled in special containers)
Same
Limited range depending on f.p. and b.p. of syetem
Same
above
Rankine
Low fields, 15 to 100 oe.
Faraday
Temperature conField strength trol is possible range same as over a wide for Gouy balrange (same as ance, but giving etated for Gouy nonuniform field technique) with a conEtant field gradient Temperature conExternal fields not Generally to dia- Solids and liquids 0.5-g. solids, 5 ml. Accuracy genertrol over a wide ally better than required except and paramagliquids range is rather & O . l % but d e in a study of netics. Ferrodifficult with pends on dielecferromagnetics magnetics may the r.f. method, tric characterbe studied in but is adaptable special appaietics. in other inductratus ance methods.
Induction methods (a.c., including r.f. and d.c.)
as
VOL. 34, NO. 5, APRIL 1962
345 R
., .i ,I.
Figure 1. Recording Gouy magnetic balance in outhor's laboratory
. .
__
.
-.
. .
.
..
(a.) Iflagnets. ne introuuct1on of nuclear magnetic resonance techniques during the last decade has greatly improved t h e availability of permanent and electromagnets with interchangeable pole faces and variable pole gaps. Some of the prominent manufacturers of electromagnets (a t o f) and of permanent magnets (e and f) are listed below: (a,) Harvey Wells, Inc., Framingham, Mass. (b.) Varian Associates, Palo Alto, Calif. (c.) Pacific Electric Motor Co., Oakland, Calif. (d.) Newport Instruments Ltd., Newport - Pagnell, Bnckinghamshire, England. (e.) Indiana Steel Co., Valparaiso, Ind. if.) .. , Arnold Eneineerine Co.. Marengo, Ill. Where cost is not a Drohlem. it is indeed advantageous to duy a commercial magnet and a matching d.c. power supply. However, it is possible to build low cost electromagnets to give moderate fields of about 5000 oe. (Gauss). A number of publications (28) describe the construction of such magnets. It is even possible to convert a large transformer into a workable electromagnet merely by cutting a pole gap in the core of the transformer a t a suitable place. One such design is described by Broersma ( S I ) . (b.) Direct Current Source and its Control. Direct current supply obtained from a generator may be used for energizing electromagnets. However, the fluctuations in the avI
346 R
~
ANALYTICAL CHEMISTRY
..
.. .
erage supply are usually so large t h a t they ruin the precision of measurements. Devices for compensating such variations are described, but are cumbersome t o build. A battery supply is always advantageous, b u t this requires a constant maintenance and limits a prolonged use of the batteries. Several circuits for stabilized power supplies have been described in literature (9, 11, 19, $6). A circuit especially adaptable for energizing an electromagnet is described by Figgis and Nyholm (61). Here again, if the cost of such equipment is not of consequence, it is preferable t o buy regulated power supplies to match requirements of currents from commercial sources such as the Varian Associates, the Harvey Wells Co., etc. These units provide regula tion of current to within 1 part in lo4 or better, which is more than adequate for the magnetic susceptibility measurements. It should be noted that regnlatiou of current may not necessarily regulate the field in the pole gap. Devices for regulation of the magnetic field itself are described in literature (22) and should be adapted particularly with homemade electromagnets. Fortunately, the special construction designs used by manufacturers of electromagnets usually provide a regulation of magnetic field which is adequate for magnetic susceptibility measurements. A setup shown in Figure 1 has been in satisfactory operation in the author's This laboratory for some time. arrangement provides the use of one electromagnet mounted on a movable platform and equipped with inter-
changeable pole gaps for the Gouy and Faraday techniques. It has also proved useful for other magnetic work such as the magnetic anisoti'opy and magnetic resonance studies. Permanent magnets constructed from alloys such as Alnico, Permendur, etc., which have a high remanence and high coercive power provide fields as high as 10,000 oe. in a pole gap of 2.5 em. with tapered poles about 5 em. in diameter. Small permanent magnets giving fields of ahont 2000 oe. may be recovered from war surplus Magnetron magnets. The cost of a permanent magnet, not involving an exceptional degree of field homogeneity (required for nuclear magnetic resonance work) is usually less than the combined cost of a comparable electromagnet and a stabilized source for direct current required for energizing it. This and the availability of a steady field over long periods constitute major advantages of a permanent magnet. On the other hand. ~,some mechanical device for moving the permanent magnet over a sufficient distance is required to permit a measurement of force on the sample with and without the field. Perma. 1 ~~~,~~~ I nenr magnew nave m e aisauvamvagt: 111 that they cannot be used for studying the susceptibility m a function of field strength which is useful in detecting the presence of ferromagnetic impurities in the sample. Some variation in the field may be obtained through the use of magnetic shunts. The usefulness of permanent magnets lies especially in measuring changes in magnetic susceptibility occurring as a function of time in certain photochemical and polymerization reactions, and those involving free radicals. The choice of field strength depends on the size of the sample, the dimensions of the sample tube, and on the sensitivity of the technique for measuring force. Larger forces, obtained a t higher fields, can be measured more accurately than small forces; as such, no upper limit may be placed on the magnetic field. For normal working, fields up t o 15,000 oe. are adequate. Taking into consideration even .the most seusitive technique of measuring force, a lower limit a t about 2000 oe. is desirable for routine work. (c.) Balances, Automatic Recording Devices, and Helical Springs. A . number of analytical, semimicro-, and microbalances in the single and double-pan styles are manufactured b y Ainsworth and Sons, Cahn Instrument Co., Fisher Scientific Co., and Testing Equipment Co. in the U. S. A,, b y E. Mettler in Switzerland, Sartorius Werke A. G. in Germany, and by Stanton Instruments in England. A balance using B double cantilevered beam is also made by the Testing Equipment Co. These manufacturers, .1
1.
~
1
FIXED TUBE
---d7l
MOVABLE TUBE -
,
,
SILVER CHAIN
1/ LUCITE BOX L E A D S OF NON-INDUCTIVE HEATING
HOOK
COILS
Figure 2. Temperature control for varying temperatures over a narrow range (Used with author's Gouy balance)
and several others not listed here, also make equipment for the automatic recording of weight, and many provide a suspension for attaching to the bottom of the pan, which is useful for the Gouy method. For studying a continuous change in magnetic susceptibility as a function of temperature or changes which may occur as a function of time in chemical and physical processes, it is convenient to have a n automatic recording device. Selwood (161, 152) describes equipment for thermomagnetic analysis, especially of ferromagnetics. It is generally possible to convert any balance, such as the spring, and double-pan, or the singlepan to automatic recording without too much difficulty. Gordon and Campbell (70) present an excellent review of a number of ingeneous methods adapted by investigators and manufacturers for recording changes in weight. Microbalance techniques are also available (89). Helical springs made of silica are manufactured by Worden Laboratories, Houston, Tex., and may be used conveniently for measuring the magnetic force on the sample in the Faraday- type magnetic balance. (d.) Requirements of Field Strengths and Sensitivity of Balance. Paramagnetic susceptibilities which are very large in comparison with the diamagnetic produce appreciable changes in the weight of a sample on application of a magnetic field. Changes produced by paramagnetic
samples of a few tenths of a gram placed in a field as low as 3000 oe. may be measured with a n analytical balance with a sensitivity of 0.2 mg. However, accurate measurement of diamagnetic susceptibilities of samples of the same size call for a better accuracy (&0.01 mg.) in weighing and the use of higher fields. A happy compromise for measurements on both para- and diamagnetics in fields ranging from 3000 to 7000 oe. is to be found in the use of a semimicro balance with a loading capacity around 50 grams or better. The changes in weight may be observed conveniently merely by adjusting the rider or the chain, on the conventional double-pan balance or on the optical (deflection) scale of the single-pan balance. I n studying a variation in the magnetic susceptibility of a system it is advantageous to restrict the changes in weight to the range provided by the rider, chain, or the optical scale. This is readily done by adjusting the field strength of the electromagnet. The single-pan balance has a number of advantages over the conventional double-pan type and was first adapted for the Gouy technique by the author (115) in 1950. Many single-pan models are equipped with a good damping system (usually air), which facilitates a quick weighing within a few seconds. The enclosed gram and centigram weights are handled by a remote control and thus remain protected. An optical deflection system is employed to read
the milligrams; the fraction up to a hundredth or better of a milligram may be read off from an external vernier arrangement in the semimicro and micro versions of the single-pan balance. The built-in optical deflection scale incorporates the advantages of the external microscope method of observing deflections used in the sensitive Theorell (172) and Michaelis (108) modifications of the Gouy technique. Apart from these mechanical merits, some single-pan balances-e.g., the Mettler-facilitate weighing under conditions of constant load and hence of constant sensitivity. This feature appears desirable if i t is necessary to study samples varying in sizes from a few tenths of a gram to several grams. (e.) Temperature Control. Production of temperatures above t h a t of room is easily obtained with a n electric cylindrical furnace, the coils of which are wound noninductively t o prevent stray magnetic fields. T h e temperature regulation is accomplished with adequate relay mechanisms, or by motor-driven rheostats to within 0.1' C. of the desired temperature. I n the low region, temperatures provided by freezing mixtures, low boiling liquids, and liquified gases are also easily obtained (50, 149) using a Dewar vessel and a jacket surrounding the sample tube. The temperature of the sample remains constant so long as the freezing agent is maintained a t a n adequate level above the sample tube. It is easier to measure the temperature of the atmosphere immediately surrounding the sample tube with a thermocouple than of the sample itself. However, if care is taken to let the sample stand a t a fixed temperature for about 10 to 15 minutes, it quickly reaches a thermal equilibrium with its surroundings; this renders a direct measurement of the temperature of the sample unnecessary. Nevertheless, for work in a narrow range such as -10" to 60" C. not involving very accurate measurement of temperature, a small thermometer dipped right into the sample has been found to be useful in the author's laboratory. The details of this arrangement are shown in Figure 2 . This eliminates the somewhat cumbersome use of a thermocouple and potentiometer, normally required for large temperature ranges and more accurate measurement of temperature. This setup was used by the author with a Gouy balance in studying the changes in the magnetic susceptibility of aqueous solutions containing paramagnetic ions. The tube containing the sample was weighed a t room temperature and the weights used were left unchanged. I t was removed and cooled to a desired temperature using a n adequate freezing agent. Any condensed moisture was VOL.
34, NO. 5, APRIL 1962
3471
wiped off the tube which was then suspended inside the plastic box and weighed quickly with the magnetic field off and on. The preliminary adjustment of weight helped to make these weighings quickly within a few seconds, during which the temperature change was found to be less than 1' C. Similar procedure was followed at high temperatures which were obtained by heating the air inside the plastic box with electric coils which were wound noninductively on asbestos board and placed a t the bottom and back of the box as shown in Figure 2. The current was controlled by a Variac (variable transformer). An advantage of this method, which is restricted to the temperature range -10' to 60' C., is that i t did not become necessary to widen the pole gap to accommodate a heating furnace which would have weakened the magnetic field and affected the accuracy of susceptibility measurements. The production of any intermediate temperature not furnished by a freezing agent calls for construction of special cryostats. The following general principles have been used in their construction. A fen- references to their typical applications are cited as examples. (i) The sample is surrounded by a double-n-alled metal or glass jacket through which a liquid a t the desired temperature is circulated. This provides only a limited temperature range. Lorn temperature baths (-35' to about 65' C. with a regulation of 0.01' C.) are made by the WilkensAnderson Co., Chicago 51, Ill. (ii) A metal block (copper or lead) surrounding the sample is cooled by a cooling agent-e.g., liquid nitrogenand simultaneously heated by electricity. Regulation of heat gives a desired temperature between that of the coolant and room temperature. A Cryostat of this type was described by the author (116) for nuclear magnetic resonance studies. It can be used without the radio-frequency coil for the Faraday magnetic susceptibility technique. The temperature gradient over a sample height of about 2 cm. is negligible. (iii) A coolant such as liquid air is injected into a spiral opening in the metal block; the rate of flow controls the temperature. More sensitive control may be obtained by passing oxygen through a liquid nitrogen condenser and alloIying the liquified oxygen to drip into the metal block. The rate of oxygen gas passing through the condenser controls the rate of cooling [cf. Selwood (152)]. (iv) Liquid air or nitrogen is boiled under the sample so that cold vapors rise over a jacket surrounding the sample (85, 152). (v) The sample is surrounded by a double-walled jacket, which in turn 348 R
ANALYTICAL CHEMISTRY
is surrounded by a coolant (liquid nitrogen). The space inside the jacket is filled with helium gas, which acts as a heat transfer medium. Regulation of the pressure of helium gas by a vacuum pump (and some heating) controls the temperature (159). Figgjs and Kyholm (61) use air for heat transfer. Other descriptions of cryostats adaptable for susceptibility work may be found in reference (60). (vi) For work in the temperature region furnished by liquid helium, principles (i) and (iv) are difficult to adapt as liquid helium boils off very quickly (specific heat 1.25cal. per gram). Hence, a new conduction technique, originally developed for nuclear and electron magnetic resonance experiments, may be used for magnetic susceptibility measurements. (Such cryostats are presumably in operation in the Physics Department and Gordon McKay Laboratories of Harvard University. d n y description of this and similar cryostats has escaped this author's attention.) One end of a copper tube is immersed to variable extents in liquid helium and thus produces different temperatures a t the other end, inside nhich a small sample is placed. This limits the over-all volume over which a uniform temperature can be maintained; the technique appears promising for the Faraday-type magnetic balance, which uses samples of very small volume. In general, the production and control of temperatures between -190' C. to room temperature is easily accomplished with liquid nitrogen alone and several references to such cryostats other than the ones cited above are to be found in the literature (15). The use of liquid hydrogen for obtaining temperatures down to -163' C. is rather hazardous, and i t cannot be substituted in place of liquid nitrogen without introducing adequate safety precautions in the nitrogen cryostat. A design using liquid hydrogen (6) originally developed for NMR experiments appears to be adaptable for the Gouy and Faraday techniques. Scott (150) describes an elaborate cryostat which uses liquid helium, hydrogen, and nitrogen to give temperatures from 1.6" to 300" K. It is particularly useful for the Faraday technique. Another cryostat (62) originally developed for EPR work may also be used conveniently for temperatures between 4.2 and 77" K. The choice of a design for a cryostat depends on the specific nature of the magnetic study, the temperature range, the precision of temperature regulation, and finally on the particular technique used for the susceptibility measurement. One important criterion for techniques using a magnet is to make the stem of the cryostat as narrow as possible so that a small pole gap for obtaining high magnetic fields can still be used. A
Dewar flask with an oval cross section to fit into narrow pole gaps is described by Broersma (33). A helium Dewar with incorporated magnetic pole tips also has been described (55). Care must be taken to allow ample space for the free movement of the sample inside the cryostat. For work in the very high temperature region, precautions also must be taken to protect the sample and the pole faces from harmful effects of temperature. ( A ) . Gouy Magnetic Balance and Its Modifications. Several modifications of this balance have recently appeared in the literature: a review of important modifications prior t o 1956 is presented by Selwood (152). The novelty in these modifications lies partly in the use of a permanent magnet in place of an electromagnet and in the techniques used for suspending the sample and for measuring forces. Hila1 and Fredericks (81), for instance, use a method in which changes of the order of a fe1-i milligrams are measured with a magnet-solenoid arrangement, to a high degree of accuracy, not obtainable by the conventional method of swings. The magnet-solenoid method is to be found in some recording balances made by the A.R.A.LI. Co. in Lyons, France, by the Fisher Scientific Co., and the Niagara Electron Labs. in the U.S. A. There is indeed no limit to implementing ingeneous devices in an ordinary or a susceptibility balance to meet special requirements. d unique Gouy-type recording apparatus using an Ainsworth Model UMD chainomatic balance has been recently reported (41). An unusual feature is the addition of a quartz spring that is a l m y s kept under tension. The resulting change in transducer output is amplified with a Ganborn carrier preamplifier. X basic advantage of the Ainsworth balance is that i t permits a complete evacuation of the balance and the -n-eighing chamber and thus minimizes errors due to the buoyancy effects. I n the particular Gouy balance setup i t has a further advantage in that a correction is avoided for K ~ the , volume susceptibility of the medium surrounding the sample appearing in the following expression, force gAw = AH~(K - h o i nhere Aw is the change in a eight of sample on applying the magnetic field H , K is the volume susceptibility of the sample under investigation, and A is its cross-sectional area. h Gouy balance suitable for studies of adsorption of (paramagnetic) gases on adsorbents such as silica, alumina, etc., is described in the literature (152). The method may be regarded as a differential method in that the force of attraction on the sample (A) is counterpoised by a weight (B) similar in size to the sample. Any adsorption in cham-
ber A naturally changes the susceptibility of the sample and produces a change in weight in the magnetic field which may be applied temporarily. The change in weight itself may be followed by using the electromagnetic force device coupled to the (magnetic) counterpoise in chamber B. Another modification developed by Selwood (162) is found to be excellent for studying reactions involving changes from paramagnetism to diamagnetism or vice versa. These can be followed easily, as they produce significant changes in magnetic forces acting on the sample n hich is suspended horizontally from a bifilar suspension. However, with sufficient sensitivity changes in diamagnetism, for example, those resulting during polymerization of styrene, etc., could also be followed. A permanent magnet is employed for convenience and increased sensitivity. The horizontal sealed tube holds the reaction mixture. The changes in susceptibility are followed by observing the displacement of the tube with a micrometer microscope. The apparatus was calibrated by Selwood by filling the tube n i t h a dichromate solution which was gradually undergoing reduction with sucrose. This involved a change from the diamagnetic Cr20i- to the paramagnetic Cr3+ ion. Instrumentation of a Gouy balance n ith temperature control is reported by Earnshaw (61) and others (167). S e n omechanical networks have been also adapted by Hedgcock and hluir (77) for the Gouy method. They can be used in conjunction uith the Faraday method also. I n the instrument described, a sensitive electrodynamic balance is operated as a null instrument by allowing it to form part of a servomechanical netnork. The feedback system provides a stiffness of balance movement of 8 X 10' dynes per degree of deflection The sensitivity is such that changcs of e.m.u. per gram are detected in the susceptibility of large metallic samples having electrical resistances less than lo-$ ohm per cin. The effects of eddy currents in such samples induced by the magnetic field are eliminated. The balance appears t o be versatile for measurement on other samples and a t different temperatures. Henry and Rogers (79) describe a special balance for short length specimens and discuss the errors involved. -4simple Gouy balance for lecture demonstration and student use utilizes a Westphal balance (102). I n the author's laboratory a Gouy magnetic balance shown in Figure 1 rn as assembled from the following units in order to obtain a simplicity in its operation, and to permit a maximum flexibility in its application to a variety of research problems. Figure 1 shows
.
the arrangement used for enclosing the suspension system to prevent effects of air drafts. A Newport Type A electromagnet with adjustable pole gap, interchangeable straight and Faraday pole caps, giving a maximum field of about 13,000 oe. in a 1.5-cm. gap. (ilpproximate cost $1200.) A matched Newport Type-B d.c. power supply operating from a 110volt, 60-cycle mains giving a maximum current of 8 amp. a t 100 volts and a regulation of 1 part in 104. (Approximate cost $1700.) A Sartorius semimicro balance, with a sensitivity of 0.02 mg under normal working conditions. Range on optical scale = 100 mg. Loading capacity = 100 grams. (Approximate cost $1000.) A thin silver chain is attached to the bottom of this pan. A Sartorius photoelectric recording device (approsimate cost $600) and a Leeds &- Northrup recorder CSSOO). The recording device is best used with a Sartorius analytical balance A homr-made cryostat for regulating temperaturrs from -190' t o 200" C. The arrangemrnt facilitates displacement of air by nitrogen gas, whrnever required for arcurate work. Preparation of Sample for Gouy Method. lfET.4LLIC A S D CERTAIN NOSMETALLICSOLIDS. Samples of metals, alloys, glass, polymers, etc., may be obtained in narrow long uniform cylinders. The length should be such that the suspended end of the rod would lie in a field n hich is negligible in comparison nith the field applied a t the lou-er end. Usually a Imgth betu-een 10 to 15 em. is adequate. The diameter must be such that the rod will move freely inside the cryostat fitted between the smallest possible gap. Rods a few millimeters in diameter are suitable for work over a wide temperature range; hoLvever, rods 2 to 3 em. in diameter may be used with advantage for measurement a t room temperature by suspending them directly in the pole gap. For most purposes, a sample tube made of Pyrex glass is adequate. For very accurate measurements allowance must be made for the temperature dependence of its small paramagnetic susceptibility (76), which arises from certain impurities. The sample containers may be made from rather thinwalled tubing of uniform diameter (available from the l17ilmad Glass Co., Buena, N. J.) with hooks for suspension. A stirrup made of Pyres glass, copper, or silver wire may be used for supporting the tube. The general considerations for determining the size of the tube are the same as mentioned previously for rods of metals, alloys, etc. An etched mark may be made a t the top of the tube where the field is known to be negligible. This fixes the volume of the sample and
of the reference to be used. For measurements on powdered solids a tube about 5 mm. in diameter and 15 cm. long is convenient; for liquids and solutions a tube of the same length and about 1 cm. in diameter is suitable for work of moderate precision. For accurate work on solutions a semidifferential method is used (60, 108). This employs a tube partitioned a t the center where the field is applied. The pure solvent is sealed in the bottom part with a reservoir half filled. This allows for the expansion of the solvent a t higher temperatures and prevents the formation of a bubble of vapor a t the septum on cooling. The solution is placed in the upper compartment. It is easy to see that the magnetic pull on the two ends of this compensation tube Till be in opposite directions and will be almost cancelled if the susceptibility of the contents in the tn-o compartments is the same. Thus, by keeping the solvent unchanged in the lower compartment and varying the (concentration of the) solution in the upper compartment, changes in weight corresponding to a difference in the susceptibilities of the tTvo are obtained. This difference may be taken to correspond to the susceptibility of t8hesolute under conditions of no interaction bet.u-eenthe solute and the solvent. The limits of error in the differential methods of evaluating the susceptibility of a solute are considerably lower than in the method employing separate measurements on the solvent and solution. Finely ground powder should be used so that a maximum filling is obtained in a fised volume. Esperiments show that under normal \%-orking conditions, a loose packing or a tight ramming of the powder in the tube does not affect the susceptibility measurements appreciably, provided corrections, to be described below, are made for the presence of air pockets. Hon-ever, the ramming method may be preferred to the first as it facilitates a maximum filling and minimizes the chances of preferential orientation of the particles in the field. I n this method, small and nearly equal portions of the poxder are introduced in the tube and are packed by pounding after each addition n-ith the flat end of a ramrod which snugly fits the tube. By equalizing the portions and the number of strokes used in pounding, it is generally possible to obtain uniform packings and a reproducibility of n-eight to within 1%. Liquids and solutions do not pose the problems of packing and may be used with sample containers described previously. The sample tube may be sealed in the case of very volatile liquids. A dry bos with an atmosphere of nitrogen or special reservoirs and burets in ~ liquids is controlled hy which the f l o of the pressure of nitrogen gas may be used for filling easily oxidizable samples such VOL. 34,
N O 5 , APRIL 1962
349R
as the solutions of Cr(II), Fe(I1). Sealing the tube is desirable in this case and with reactive substances such as the free radicals; this requires calibration of each tube used for individual experiments. I n many cases a tube with a good ground glass stopper or with a highvacuum stopcock is adequate. CALIBRATION.I t is customary to use distilled water as a calibrating agent. However, distilled water contains appreciable amounts of dissolved air which is paramagnetic. The author prefers the use of "conductance water" which is boiled just prior to its use to drive off all air. The susceptibility of water is taken to be -0.720 X lou6 c.g.s. unit per gram. Selwood (162) has reviewed the use of other reagents as calibrants. Benzene is recommended as a reference, but its susceptibility depends on whether it is saturated with air (X = -0.7020 X 10-6 c.g.s. unit) or with nitrogen (x = -0.7081 X c.g.s. unit). Several workers have investigated the susceptibility of solutions of nickel chloride. The molar susceptibility of this salt is found t o be (4433 i: 12) X lod6 a t 20" C. The variation of the volume susceptibility of a 0 . l M nickel chloride solution with temperature as reported by Michaelis (108) is given in the following table. Magnetic Susceptibilities of 0.1 M Nickel Chloride Temperature, O
c.
K
0.0466 0.0443 0.0440
18
20 22 24 26 28
x
10-8
0.0437 .__ .
0.0434 0,0432
Although the susceptibility of a mixture is not a linear function of concentration in some cases, the susceptibility per gram of nickel chloride a t 20' C. is given by the following relation
x
=
[ 3 4 . 2 1 ~- 0.720 (1 - p ) ] X
where p is the weight fraction of NiCl, in the solution. The susceptibility of this solution is found to be independent of concentration near 30% Tu'iClt by weight. Hence, such a solution may be used conveniently for calibration. Figgis and Lewis (60) recommend mercury tetrathiocyanato cobalt, Hg [Co(CNS)14, as an all around calibrant for solids. It has a molar susceptibility of 16.44 x 10-6 It(o.570) a t 20' C. and is said to have exceptionally good packing properties. Recently, Curtis (46) suggested trisethylenediamine nickel(I1) thiosulfate as a calibrating agent. It has a molar susceptibility of 10.82 X 10-6 .t(O.4%) a t 20" C. French and Harrison (65) have discussed the accuracy of the determination
-
350 R
0
ANALYTICAL CHEMISTRY
of magnetic susceptibility of solids in relation to the presence of air-pockets in powdered solids packed up to a mark in a tube and the meniscus correction arising in the use of liquid calibrants filled up to the same mark. Assuming that air surrounds the sample, the following equation is derived:
x= where X xs
w
=
= =
W S =
d
di F Fs
= = =
Kair
= =
C
=
susceptibility per gram of solid susceptibility per gram of standard liquid (that is calibrant or reference) weight of solid packed up to a mark weight of standard liquid up to the same mark density of solid density of standard liquid Force (expressed as change in weight on applying magnetic field on solid only) Force on standard liquid volume susceptibility of air which may be taken as 0.029 X 10-6 under normal experimental conditions meniscus correction: for tubes of radius between 2 to 4 mm. and reference liquids such as water, benzene, and acetone, W c = 0.054 - 0.0037, h where W Sis weight of standard liquid and h is height up to mark on tube.
-'
( B ) Faraday Balance and Its Modifications. Critical reviews of several modifications of this method appeared in the literature (152). The novelty in most modifications lies in the force measuring techniques. The original Faraday method used a torsion head to measure the force on a sample placed in a nonuniform field. The sample is suspended from a torsion arm and is free to move horizontally. The torsion head is twisted to return the sample to the original place; the twist is a measure of the force required just to balance the magnetic force a t the zero position. Important modifications of this torsion method are known as Curie-ChBneveau, Curie-Wilson, and Oxley. In the Curie-ChBneveau method a small permanent magnet is moved toward and away from the sample to produce the gradient. An elaborate modification of special significance is described by Sacconi and Cini (147) and recently by other workers (48)* Sucksmith (166) used a sensitive method in which the sample is suspended from a phosphor bronze ring equipped with an optical system. The displacement of the sample is magnified several hundred times with this system. Cham drasekhar (40) has introduced special damping devices in the Sucksmith balance. Other workers (180) have replaced the optical system by a flat spiral spring which is linked to a dis-
placement transducer. The voltage from this is recorded or read on a n oscillograph. Improvements (161) for handling variable loads (1 to 80 mg.) have been incorporated in the Sucksmith balance. Milligan and Whitehurst (110) and Jacobsen and Selwood (86) have described a magnetic balance in which a quartz helical spring is used for measuring small changes in force. I n the writer's opinion, this is by far the simplest and most sensitive force measuring device which can be adapted readily for the Faraday technique, A helical spring, fibers, and buckets all made from quartz (obtainable from the Worden Laboratories, Houston, Tex.) have been used in the author's (lid) equipment. A right angle vacuum valve with enclosed metal bellows (made by Kepco Industries, New York) is useful for resetting the position of the pan a t the same point between the pole faces in the region of uniform field gradient, Most of the modifications mentioned so far use permanent or electromagnets with inclined poles or poles of special design (166) to produce a nonuniform field with a uniform field gradient, A new design for 4-inch pole faces is now available (80) to produce constant gradients over the 1- to 1.4-cm. region. Other sensitive methods have been developed by Smith (159), Cini (&), and Pacault, Duchene, and Baudet (1.27). I n a recent modification (164) of the Curie-ChBneveau method a small permanent magnet with straight pole faces is used. The required gradient is obtained by moving the permanent magnet vertically toward or away from the sample. A quartz helix spring and a measuring microscope are used for measuring the force on the sample. The magnetic susceptibility of a sample relative to that of a standard is simply calculated from the following:
where d is the deflection, i.e., one half the difference of maximum to minimum deflection observed during the movement of the magnet, m is the mass in grams. XS, is the susceptibility of the standard in e.m.u. per gram, and the subscripts 1, 2, and 3 refer, respectively, to the measurements of the sample plus pan, the standard plus pan, and the pan alone. The chief advantages of the method are that submilligram amounts of the sample are required for paramagnetic substances having a susceptibility of from 1 to 50 X lo6 e.m.u. per gram; small samples, about 10 mg. are required for weakly paramagnetic and diamagnetic substances to obtain a precision of better than 2y0; and an inexpensive
small permanent magnet and a simple experimental setup are needed. A continuation of work with this equipment has led to a method of measuring magnetic susceptibility which has been claimed to be a n absolute method not requiring a standard substance (17 4 ) . The susceptibility x is shown to be a function of the area, a, under the curve of sample displacement us. the distance of the magnet from the sample and of the maximum applied field €€,ax:
Here g is acceleration due to gravity and h is the measured static deflection of the helix. In the experimental procedure the area a of the curve (n-hich resembles the first differential of a Gaussian curve) is measured lyith a planimeter and H,,, is measured with a Gaussmeter. The usefulness of this method has been verified by other workers (37). With regard to the claim that it is an absolute method not requiring any standards (but requiring a precise measurement of H ) , it may be remembered, that if an exact measurement of the field H is made, for instance, in the Gouy technique, then it does not become necessary even here to use a standard. I n this case the volume susceptibility K (and hence the mass susceptibility x = Kjdensity) can simply be calculated from the basic equation gAW = l / 4 H 2 . ~where , the symbols have the same meaning as used previously. It is important to note that in the magnetic susceptibility methods the purpose of employing a standard with known susceptibility YS is to avoid a measurement of the magnetic field H and other parameters such as the area A of the cross section of the sample. This is possible because these factors cancel out in considering the ratio of changes in weight due to magnetic field of the sample to standard T.I’/TI’, which equals the ratio of the susceptibility of the sample to standard K / K , . Garber, Henry, and Hoeve (66) provide an elaborate description of a Faraday balance designed to give a reproducibility of better than 0.15y0; this was particularly used for measurements of magnetic susceptibility of copper, silver, and gold. A null technique especially useful for paramagnetics has been developed (158). It can be used over a wide range of magnetic field strengths without any readjustments and automatically detects the presence of ferromagnetic impurities. The balance is semiautomatic, uses small single crystals, and subtracts the contributions from (ferromagnetic) impurities and antiferromagnetic components. Another torque method (136) originally designed for ferromagnetic anisotropy measurements
on crystals is expected to be useful for susceptibility measurements. This employs a light beam-mirror-phototube network. An electromagnetic servobalance employing a differential transformer has been designed to handle large forces (5 grams) for ferromagnetic materials (36). It readily measures forces as small as 0.02 mg. This paper lists several references on servobalances. Special adaptations are described for measurements a t very high temperatures (175, 179). (C) Derived and Related Methods. The Quincke method (141). This method is related to the Gouy technique and is strictly applicable to liquids and solutions, and with some modifications to gases. I n this the magnetic force acting on the sample in the capillary is measured in terms of the hydrostatic pressure. I n the actual experiment the change, Ah, in the height of the meniscus with the field off and on is measured with a cathetometer. Paramagnetic liquids show an increase in height; diamagnetic liquids show a decrease. If p is the density of the liquid, then the hydrostatic pressure g. pAh is balanced by the magnetic force ‘/2 H 2 ( K - KJ] where H is the applied field, K is the volume susceptibility of the liquid, and K~ the volume susceptibility of the gas over the meniscus. With solutions exposed to air, the gas will consist of a mixture of air and the vapor of the liquid. Hence,
From this X , the susceptibility per gram is given by
where x, and p o are the gram susceptibility and the density of the gas over the mensicus. For practical purposes, the second term may be ignored when the reservoir is of a large diameter compared with the capillary and the susceptibility of the gas, X,, is small. This gives a simpler relation X
=
2gAh F
and is found advantageous in that an independent measurement of the density of the liquid is not required. It is not usually necessary to find the value of the applied field H , since the factor 2g/H2 can be eliminated by making measurements on a sample (subscript s) and on a reference (subscript T) under identical conditions :
Fields of about 26,000 oe. are recommended. Accuracy of about 1% in
the susceptibility measurement, which is comparable to that of the Gouy method is easily obtained, .particularly a t room temperature which can be regulated with a high degree of accuracy. Equation 1 shows that the susceptibility X , of a gas above the meniscus may be also determined by the Quincke method (cf. 14). I n practice reference liquid such as water in an atmosphere of hydrogen gas may be used In the initial experiment. Hydrogen gas has a susceptibility of only 0.02% of water ( K J . The experiment is repeated with a sample gas. If h, and h, are the changes in the height of the meniscus of the liquid in the two experiments produced by the same magnetic field, the susceptibility K of the gas is given by K
KO-
ho
- hg ho
Several modifications of the Quincke method applicable to liquids and gases are described in the literature (158). The Rankine Method ; Susceptibility of Gases. The measurement of susceptibility of a gas involves some difficulties. The volume susceptibilities of liquids and solids c.g.s. unit per range from 10-6 to cubic centimeter whereas the susceptibilities of gases and vapors are found to be much smaller (-1O-lO c.g.s. unit per cubic centimeter a t KTP). Measurements on a compressed gas which is expected to have a larger volume susceptibility are limited by the degree to which i t may be compressed and the size of the vessels that may be used for measurements. Further, most gases are diamagnetic and even a trace (1 part in a thousand) of oxygen (from air) which is markedly paramagnetic ( K = 0.162 X 10-6 c.g.s. unit) is enough to vitiate the measurements of diamagnetic susceptibilities by about 10%. Therefore, all gases and vapors must be purified and particularly freed from oxygen prior to measurement. Many methods for measuring the susceptibility of gases are related to the Gouy and the Faraday techniques. Son6 (168), for instance, used a partitioned glass tube with the Gouy technique; air under pressure of k n o m susceptibility was placed in one part and the other end was evacuated and sealed off. Air m-as then replaced by the gas and the forces on the sample were measured with a sensitive balance using an optical system. Stossel (16 e a r s entirely novel techniques and qome uqing :i combination of principles outlined in qections &4, B, and C have been deYeIoped t o meet special requirements. An ingeneous method is available (69) for studying biological and chemical processes which occur within small particles 1 to 100 microns in diameter. The force on a single diamagnetic 10micron particle can be made to be of the order of lop9 d y w in an inhomogenous field. Forces of this nature are measured in terms of the vclocity nhich the particle assumes in hydrodj naniic inotion depending on the viscority of the medium. A microscope is used for measuring the velocity (of the order of 1 micron per second) in an inhomogcneous field of a particle suspciitlcd in a medium. Susceptihility differenccs as sinal1 as 0.04 X have becn mensured for blood cells in salt solutions and also for polystyrene latex. In the writer's opinion this represents a significant advance in microtechniques. Another method (187) uses a torsion halance with very delicate quartz suspension fibers (0.5 micron diameter) for measuring susceptibilities of particles about 1 micron in diameter. d stereo microscope is used to observe the displacement of the sample in a pulscd field of 2000 oe. in l/60 second. Appreciable motion of suspension system is avoided during this small duration. Another unique method also uses high pulsed fields (163). The magnetic force on the sample is converted to a stress wave through the apparatus and excites a voltage on two-piezo electric crystals which is then measured. Measurements of carrier susceptibility of semiconductors by means of a torsion pendulum balance has been accomplished (67) with an accuracy of 0.037,. hlercier and Bovet (107) employ a torsion pendulum with sample in two symmetrical cavities. The period of the pendulum is determined very precisely. For small amplitudes of oscillation in the magnetic field, this period is found to be proportional to the magnetic susceptibility. In a bubble method (105), n-hich is somewhat related to the Quincke method, the force due to a field on an air bubble surrounded by a liquid is measured. This force is equal and opposite to that which would be experienced by the same volume of liquid when surrounded by air and kept in the same region of the field. The force is measured by tilting the spirit level type tube rnclosing the bubble; the tilt is controlled by a micrometer and
adjusted to bring the bubble in its original position in the absence of the field. The susceptibility of air and density of air have been assumed to be negligible in the calculations of susceptibilities of some liquids; this is not aln ays true. Several refinements in calculations and the technique ill have to he made to obtain better precision and accuracy than those reported. A ballistic circuit ( l a g ) for the measurement of magnetic susceptibili ties below 1' K. and a magnetic analog of a Wheatstone bridge (82) for susceptibility measurements have been reported. d transistorized device using two coils in a differential circuit (73) has been described for geological use. Neasurenientq on detachrd samples or outcrops of rocks are possible on the field. ; ispecial apparatus for measurements on glasscs u p to their annealing temperatures has lwcn constructetl (7). A magnetic torsion linlnncc is tlcscrilwl by Korovkin (95). Last of all, mention may he mntlc of magnetometers, which generally qemi to escape the attention of magnetocliemists. Some of them can be adapted readily for measurement of dia-, para-, and ferromagnetic susceptibilities, although some ii-riters design then] for specific uses such as measuring the magnetization of ferrornagnetics over a range of temperature. In some magnetometers ( 4 ) the torque is balanced by means of an electric current passing through the coil of a moving-coil meter movement \I-liich replaces the normal torsion head and mire; in others (2) a transducer is employed to convert the torque into a measurable electric current. A number of magnetometers employ some sort of a vibration of the sample in the magnetic field (64, 100). An elaborate setup is described by Foner (64), ivho lists references to several other vibration techniques. In his apparatus changes as small as to e.m.u. have been detected and a stability of 1 part in 10' is attained. It minimizes or eliminates errors in other methods and is useful for measuring magnetic susceptibility as a function of temperature, magnetic field, and crystallographic orientation. Most magnetometers are useful for thermomagnetic analysis which has been extensively studied by Selwood and his coworkers (152, 15s). The construction of an apparatus for studying magnetizations betmeen 2" and 1200" K. is given by Rimet (146). A special null method astatic magnetometer for geomagnetic measuremcnts has been also described (6). APPLICATIONS OF MAGNETIC SUSCEPTIBILITY
The applications of the magnetic technique are numerous as will be seen
GLDSS
MBCNET PSILE
TLCf
?lNG
FDCE
Figu winc
er bareo on
from the next few sections. Several applications arc related to a study 01 paramagnetic and ferromagnetic proper. ties which have many technological applications. Diamagnetism seems ta he somewhat limited in this respect; however, its applications to organic chemistry have been signscant. Although the techniques of nuclear and clectron magnetic resonance spectroscopy, developed during the last decade, are finding new applications every day, the relatively simple technique of magnetic susceptibility continucs to prove helpful as an exploratory tool, and in several cases the magnetic studies have preceded the more elaborate NMR and EPR investigations. As stated before, commercial equipment of general applicability for measuring magnetic susceptibility is not generally available. However, instrnments such as the oxygenmeters, permeameters, and coercimeters, designed for specific applications, are marketed by some manufacturers. Of these, the oxygenmeters will be described. Selwood (268)discusses many applications of permeameters and coercimeters. Recently permeameters have been used to test materials such as phosphor bronze, copper, filled rubbers, etc. (1). Analysis of Oxygen Content. Among the many physical methods that are adaptable for a continuous analysis and recording of oxygen content in a sample of gas, the methods widely used are based on a measurement of its magnetic susceptibility and on special effects produced in a magnetic field. Most methods take advantage of the fact that, among the few paramagnetic gases known, oxygen has a large magat netic susceptibility of 0.142 X
rnagneric
Internal view of Hays oxygen anal.yzer
20'
C. and standard pressure. All
other gases such as nitrogen, carbon dioxide, etc., which are constituents of air, are diamagnetic. This facilitates an analysis of oxygen in air and in other samples containing diamagnetic gases. Magnetic susceptihility data on oxygen have been used in conjunction with the electron magnetic resonance technique to study atomic recombination of oxygen (97). The surface recombination coefficient for oxygen atoms on quartz has been per colestablished to be 3.2 X lision and the second-order volume recombination process is about 5 X 10'6 mole-asec.-' The Hays oxygen analyzer is marketed by the Hays Corp., Michigan City, Ind. It is based on the inverse relation between the paramagnetism of oxygen and its temperature. It resembles an apparatus developed by Klauer, Turowski, and Wolff (93). Figure 3 shows two electrically selfheated identical nickel coils 3A and 3B on the outside of a glass tube 1. The winding 3A is placed between the poles of a small permanent magnet 4. The two windings form two legs of a Wheatstone bridge. The sample gas flows from the entrance 5 to the exit 6 and oxygen is attracted into tube 1 by the magnetic field. The gas is heated hy coil 3A, which decreases its susceptibility. Cooler gas entering a t 3A pushes the heated gas away from the field as shown by the arrow. This
--
.. . .
flow, often termed the magneac wmu, cools coil 3A and beats coil 3B. This difference in temperature changes the resistance of the two coils and produces an unbalance of the Wheatstone bridge. This is converted into a voltage unbalance, amplified, and transmitted to an indicator or a recorder. The unbalance is found to be proportional to the oxygen content of the gas. The instrnment is thus adaptable to an enclosed or a flowing sample of gas containing oxygen. An internal view of theinstrumentisshowninFigure4. Another oxygen analyzer (cf. f34) is marketed by the Beckman Co., Fullerton, Calif. (Figure 5). As shown in Figure 6 it measures directly the volume susceptibility of a gas in an inhomogeneous field, similar to the one employed in the Faraday technique. One end of a tesehody in the form of a glass dumbbell is placed between the poles of an Alnico permanent magnet. The test-hody is supported on a silica fiber, which acts aa a tension suspension. This measures the torque of the system ahout the axis of the suspension in terms of the deflection of a small mirror attached to the fiber. The torque is proportional to the magnetic field strength, its gradient, and the difference in the volume susceptibilities of the test-body and the gas surrounding it. A change in the susceptibility of this gas, resulting from a change in its oxygen content, produces a nearly proportional deflection. VOL. 34, NO. 5, APRIL 1962
355R
Analysis of Ions i n Solution, in Solid Mixtures, and i n Colloidal Systems Such a s Glass. T h e Wiedemann's additivitv law mav be used t o determine the concentration and oxidation state of naramaenetic ions ILL SUIIIIIIUII, LU WIIU IIIIXIIUIPJ,
aiiu
11,
glass. The method is capable of giving results to within 1% or better depending on the accuracy of the suseeptihility measurement. An obvious limitation is that there should not he any interaction between the solvent and the solute. In the case of paramagnetic
- fnrthar ......l_.".. .-....r"*"y,
inn. . "..l ,
II
I;rn;tnt+n I
;n ; r n n n n a A
;n
I
that the system must be magnetically dilute, that is, free from spin-spin interactions between adjacent ions. As a matter of fact, in recent years the technique has been profitably used for studying the nature of such interactions and to derive structural information. In the case of a diamagnetic salt in solution, its concentration (p%) may be obtained by measuring the specific susceptibility (X:) of the solution and using the following relation X: =
p Mol. wt. + x'nio' salt
+ (100 - p)~;,,
X,.,ion and XsnLon rep.resent the gram ionic susceptibilities. The specific susceptibility of water (Xb.0) is taken as -0.720 x 10-6. For a paramagnetic ion, x , , , ~will ~~ be dependent on the temperature and its effecitve magnetic moment. I n many cases X,,i,. may he calculated from Xcaebn =
N . mi' 3KT ~
The values of ionic susceptibilities and fieri are reported in the literature (63, 152). Solid mixtures, for instance, aluminum oxide, supporting paramagnetic ions are similarly analyzed (106, 152) and valuable information on the properties . of . the . ions _. or the . ^ reduced ..metal ... . is.
356 R
ANALYTICAL CHEMISTRY
the parti .eular snpport used is detnrminedseg)arately. I t is re:markable that in some cases the deter mination of the ratio of paramagnetic ions in two different oxidation states dis persed in colloidal systems such ay grass nas given results with as much precision as that of the spectroscopic methtods. According to de Jong (87), a glass containing 0.08% total iron in the Fe+3 and Fe+' states gave the following results for the percentage of Fe+? 44.5 to 47 from the ultraviolet spectrum, 44 f rom the infrared spectrum, and 45.5 from the magnetic susceptibility r l o t a which were treated according to the additivity law and assuming that the magnetic moment for Fef3 is 5.91 and for Fe+lis4.9OBohr magnetons. It must he pointed out that the limited studies in this area were undertaken not merely to find the concentration of the paramagnetic ions, but to unravel structural aspects of glasses containing these ions, Bishay (16) has recently studied the color and magnetic properties of iron in glasses of various types. This paper lists a number of references to magnetic studies on glass. Iron-containing glasses have been studied by Banerjee (8). Determination of Purity and Analysis of R a r e Earths. The shielding of 4 f electrons in rare earths makes t h e interactions between their adjaeent ions negligible. This corresponds t o the ideal situation of "magnetic dilution" and permits a magnetic analysis of rare earths. The purification of rare earths was formerly based on fractional crystallization, A plot of magnetic susceptihility vs. fraction number was useful in determining purity. A horizontal plateau in such plots indicated the isolation of a pure rare earth salt. Although elegant techniques of ion exchange are now used for the separation of rare earths, a determination of magnetic susceptibility is an excellent criteria for purity in addition to that of uyyy,
Magnetic Pole Pieces
Quartz Fiber
\ f Hallau
Figure 6. Schematic view of oxygen analyzer measuring system
fact, the two criteria are different. It is known, for instance, that a trace of a ferromagnetic impurity in copper can be detected only by the magnetic technique (152). The choice of the criterion for purity naturally depends on the final use of the material in question. The magnetic criterion has been applied successfully to the diamagnetic lanthanum and lutetium oxides. The analysis of rare earths is dependent on the application of the additivity law, discussed before. It has proved particularly useful for analysis of binary mixtures of lanthanum oxide (diamagnetic) and gadolinium oxide (paramagnetic). In general, a n analysis of a binary mixture is facilitated if the two components have widely different susceptibilities. Accuracy better than 1% may be achieved by a careful control of temperature during susceptibility measurements and by studying the solutions of their soluble salts instead of the solid oxides. Many texts (185) list the magnctic susceptibilities of rare earth oxides. Polymerization of Paramagnetic Ions. The magnetic susceptibility method has proved useful in studying the polymerization of paramagnetic ions in solution. A few typical examples are Fe(III), studied by the author (117, 119) and Mo(V), investigated by Sacconi and C h i (147). The technique is particularly useful for studying systems in which large changes in magnetic susceptibility of a solution take place due to conversion of a paramagnetic species to a diamagnetic one (or to one of a very low magnetic moment) or vice versa. Only relatively simple systems not containing too many species of varying magnetic properties can be studied effectively. The magnetic method has another limitation, in that its applicability is rather doubtful in studying polymerization of diamagnetic ions. The magnetic work on the dimerization of Fe(II1) in solution will be illustrated in some detail. The following
p H dependrmt equilibrium proposed by Hedstrom (78) was investigated:
possesses only one unpaired electron and, therefore, explains the small magnetic moment observed and also the significant absorbance due to iron (111). The author considered some structures for the dimer and concluded that the dimeric structure
,. 1 hc iiiagnetic susceptibilities were measurvd by the Gouy method for a solution caontaining 0.02, 0.04, and 0.0651 r'(,(C104)q,at a constant ionic strength (3.11 ?rTaC104)and a t different acidities. 'I lic simple temperature control shown 111 rigure 2 was used to study the susc( 1)tibility from - 10" to 50" C. Assuming that the dimeric species is tli:imagnetic, the concentrations of Fe+3 mid the dimer were calculated a t differvnt avidities and also over a range of ttmlmaturc. From this the dimerizatioii constant I\ as calculated a t differctnt temperatures. For freshly prcp a r ~ ld u t i o n s , t h r ICz2 T aliic a t room tcml)cmture wis about I . I 1 x 0-2 i n c\c~,llciit agrcenirnt I\ itli vnlur. tleI 1~ et1 from the spectroi)h~itoiiietric technique (117, 1 1 9 ) . lIr:~-urcnic.nts of the. S l I I 1 spin lattice rclaution timr TI for piotons in th(, samcL solutions (52) cwihrmcd in general tliv dianingnetic~ nnturc of the dimeric species and shor\ et1 satisfactory agreement TT ith the I d u e s (-8 x for K2?. It is non concluded that the dimerization is a fast reaction, and the apparent Increase in K Z 2may be attributed to polymerization processes other than that of dimerization. Recently the magnetic method was used by Niyake (111) to study the slow decoloration of thiocyanato iron (111) complexes in acidic solutions. This was also investigated spectrophotometrically. The magnetic data could be interpreted to support the idea that the decoloration n a s caused simply by a reduction of iron(II1) to iron(II), by S C S - ions, b u t such a simple model of the reduction could not explain the observcxd decoloration because the decrease in ahsorbance n as too small, being only about 207,. To eliminate this discrepancy, the formation of a dimeric complex of Fe(II1) and Fc(I1) during the decoloration reaction has been proposed. This dimer
is more likely than the c.orrcy)ontling structure with S C S bridges A significant contribution hv Eai nshaw and Lei& (52) dealing v i t h the theoretical and experimental aspects of polynuclear complexes m a y lie mentioned here. They studird t h r magnetic properties of a number of hinuclrar complexes of iron(II1) and chromium (111),with one and two bridging groups. I n certain caws involving oxo or 01 bridges magnetic interaction is observed through a reduction in magnetic moment of the metal ion. This has l m n attributed to the possibility of T bonding h c t w c n the metal ion and o-iygen of the bridge. Additional evidence for such bonding has been obtained from a study of infrared spectra. l h e author (118) also observed a small decrease in the magnetic moment of Cr(II1) during hydrolysis. This has been attributed to polymeric species of Cr (111). The solutions of sodium triphenyl boron, KaBPha in tetrahydrofuran are found to be diamagnetic (112). This surprising result may be explained by its apparent dimerization. The tri-lnaphthyl boron anion apparentl3- dimerizes in solution (75). Thermal Decomposition of Silver Oxide. Tobisan a (175) describes a magnetic technique for studying t h e susceptibilities at extremely high temperatures (-1200" C.), H e studied the rate of decomposition of silver oxide represented by
whcre the constants ai and bi depend on the temperature. The decomposition temperatures for silvcr oxide and silver oxide containing some carbonatr are 354' and 372' C. JIrtallic silver i ~ a s shown to be diamagnctic up to 1100" C.. the susceptibility of silver obtainod from decomposition n-as -0.48 X n.hile the susceptibility of the silvrr oxide was -0.21 x Thermomagnetic Analysis and Heterogeneous Catalysis. I t is difficult to summarize in a short space
the recent advances in this fascinating and rapidly growing branch of magnetochemistry. Selwood (153) gives a n extensive revim of work done up to 1956 in his and other laboratories. His recent work (151, 153) deals predominantly with studies of chemisorption of various gases on supported nickel catalysts. KO attempt will be macle here to summarize this vast area, which now stands out as a special topic by itself. The grneral effort is directed ton-ard elucidating the mechanism of hetrrogeneous catalysis; from this n-ork it is possible to extract specific malytical applications. For instance, a quantitat'ive study of particle size distribution in a catalyst is possible (152) since tlie (saturation) magnetization of a ferromagnet,ic has heen shonm to d(yend not only on tempcmt,urc, but also on particle sizc. Another olnious application is a qu:intitativc study of adsorption and chemisorption of gases Structural Aspects of Coordination Complexes. This is >-et another v m t area in which in:ignrtic susceptibilities and t'he mo nient s de rived t her c4r om have bern used. It is impossible t o indicate even briefly such appliwtions because of the variety of the complexes, and of t'lie bonding they involve. Referrnce must be, therefore, made to a recent article (60) and a number of books (SQ, ?2, 91, 104, 125, 13'2, 169) dealing with t'heir chemistry. h significant trend (60) has been to correlat'r the magnetic and spectral properties of complexes in relation to the crystal-field theory; this new and rapidly growing approach (.@?a,103,135,167') stems from the pioneering work of Van Vleck (17 7 ) andBethe ( I S ) . Free Radicals. Many organic and organometallic molecules a r r knon-n t o dissociate into free radicals under conditions of dissolution of so!ids, pyrolysis, and photolysis. From t h e magnetic point of view, free radicals are entities containing unpaircd electrons a n d as such behave as paramagnetics. The stable free radicals are easily studied by the magnetic susceptibility technique; absence of orbital contribution to magnetic moment somewhat simplifies the calculation of the magnetic moment' by the "spin only" formula. A study of free radicals n.ith extremely short lives of the order of microseconds is facilitated by the nen-er technique of clcctron paramagnetic resonance. Frer radicals are highly reactiw and particularly are autoxidizable in tlie dissolved stat'e, although t'hey might be somewhat stalde in the solid state. The solid state doc's not pose serious prohlems in the mcasurcment of their suscrptibilities. Thcse can be meuurrd by the Gouy method. However, sincc VOL. 34, NO. 5, APRIL 1962
357 R
the magnetic state of a free radical in solution is usually different from that in the solid, measurements for the two states provide a vast amount of analytical and structural information about the free radicals. Their study in solution naturally calls for a number of precautions; these include a prevention of autooxidation by carrying out the magnetic measurements in an inert atmosphere, prevention of interaction n i t h the solvent wherever possible, and prevention of the loss of solvent by evaporation, a normal precaution to be exercised in all work dealing with solutions. A vast number of magnetic studies have been reported in literature on free radicals in systems such as hexaaryl and hexaalkyl ethane, organometallics, semiquinones, porphyrins, highly conjugated systems, metal ketyls, and of metals in different solvents. Dainton, Wiles. and Wright (47) have shown that solutions of potassium in ethers are diamagnetic, unlike solutions of alkali metals in ammonia and amines (57,168). The literature has been reviewed by many authors, notably by Walling (182), Selwood (1529,Hutchison (833,Wheland (184),and Klemm (94). An account of the electron paramagnetic resonance studies is given by Ingram (84),Varian associate staff (278),and others (21,176). Study of Reactions. T h e oxidation of styrene ($3) and the reversible reduction of duroquinone (108) are typical examples of reactions studied in situ by the magnetic method. ,4 horizontal Gouy magnetic balance (172) is convenient for such studies. In studying the reduction of duroquinone i t was possible to avoid the correction for diamagnetism of the free radicals produced in the reaction, because the system was found to return to its original value of diamagnetic susceptibility. This behavior resulted from the reversible nature of the reduction of the diamagnetic duroquinone. Reductions with several other monosaccharides have bperi also investigated (170) in recent years. Studies of Polymerization of Diamagnetic Molecules. T h e Tvork of Farquaharson (58) on 2,a-dimethylbutadiene is representative in this area; rather estensive reviews (152) are also available. A significant neJv developniriit in organic polymers is reported by some Russian 11-orkers (1W). Polymers containing nitrogen and polar groups in a conjugated chain in a macromolecule show a rapid decrease in paramagnetic susceptibility which falls off rapidly with increasing field. This is said to he similar to a ferromagnetic behavior. The cloud of strongly interacting unpaired electrons is said to unite the whole structure into a single electronic unit. Preliminary experiments show 358 R
ANALYTICAL CHEMISTRY
that polyaiiiinoquinones can act as estremely effective catalysts for decomposition of hydrogen peroside and other oxidation-reduction reactions. Somen hat similar observations are reported by Krause (96). The polymers obtained from a reaction of benzidine and chloranil gave polymers with high susceptibility (-1.28 X lo6) showing a dependence on the applied field. The polyaminoquinones show strong internal hydrogen bonds and form compleses with metals. The susceptibility and further EPR data have been used to propose definite structures for the polyaminoquinones. .Additional comments on the ferromagnetism of (polymeric) nucleic acids are given a t the end of this review. Determination of Structures of Organic Molecules. An interesting application of diamagnetism, independent of a n y assumptions involving the use of Pascd's constants, is in the work of Pink and Ubbelhode (138). The magnetic susceptibility measurements and calculations have established that cyclo-octatetraene is a nonplanar, fourfold, conjugated aromatic system. The same conclusion is reached by using Pascal's constants, which have been used extensively in spite of their purely empirical nature. Study of Components of Blood a n d Related Compounds. T h e pioneering work in this area has been due to Pauling, Coryell, and their coworkers (44,133). The heme, which is an iron porphyrin complex, may be said to be the essential constituent of hemoglobin and related compounds. The terms ferri and ferroheme refer to the ferric [Fe(III)] and ferrous [Fe(II)] complexes of iron. The general procedure has been to study the magnetic susceptibility of such components or their derivatives and obtain the magnetic moment per gram atom of iron in these con~pounds. The magnetic criteria applicable to coordination compounds have been used to determine the valence state of iron and the bond type in these complexes. I n most casrs an octahedral d 2 s p 3 bonding is observed. Ferriheme chloride (hemin) has a magnetic moment of about 5.8 Bohr magnetons for iron, n hich corrtsponds to the theoretical \aluc 5.92 I3ohr magnetons for thc 5 tl electrons in Fc(II1). The iron i n ferro-hemoglobin, 15 hich is regarded as a conjugated protpin containing native globin and the fPrroheme is found to h a w a mommt of about 4.91 13ohr inagnetoni. 'I'his corresponds to the thcoretical x alue of 4.9 Bohr magnetons for the 4 d unlmired electrons in Fe(I1). T h r magnetic iiiommts observed for some derivatk es of iron protoporphyrin and porphyrin are shonn below using Rawlinson's scheme (144):
I
-s --s
1 s-
\,.,/ . Fe
.
//
s-
.
'\
1
1
Hemin (ferrihenie chloride) n
=
5, p = 5.91
Globin
Fe / \\
'
I ?-
-T n
=
R
Ferrihemoglobin cyanide, or ferrihemoglobin hydrosulfide 1, p = 2.50 (R = CN) p, = 2.26 (R = SH) I
--N
1
x'\ * /' * Fe .
Ferroheme n
=
4,
p =
4.00
Globin I
Fe /'
-N
1
'\
N-
' ItI '
Oxyhemoglobin ( R is -O=O c:trbonmonosyhemoglobin ( R = CO) n
= 0,p =
0
A
-
n
=
I
N-
Hematin (ferriheme hydroxide) p = 3 88
3,
h magnetic moment in the neighborhood of 2.83Bohr magnetons corresponding to two unpaired electrons has not been reported in any of the derivatives. The magnetic moment observed for iron in many derivatives depends on the esperimental conditions employed. It has heen now well established (117, 119) that the dimerization and polymerization of Fe(II1) ions in solution a t modcrate acidities are sufficient to depress the average magnetic moment per grani atom of iron. Hence, the magnetic moments observed in many cases, for instance, in hematin need a cautious interpretation. The effect of p H on the magnetic) moment of iron in ferrihemoglobin will be discussed under magnetic titrations. Magnetic moments varying between 2 to 5.89 Bohr magnetons have been
observed in ferrihemoglobin derivatives, containing imidazole, ammonia, azide, and ethanol groups. Some studies are reported on hemochromogens, which are compounds of ferroheme and denatured globin. RIyoglobin, catalase, and other iron-containing compounds have been investigated also. The n-ork on hemoglobin and related compounds has been discussed by Selwood (162) and by others (68, 74, 99, 121, 131). I n a recent study Brill and Killiams (30) have correlated the absorption spectra of ferric porphyrin complexes with the magnetic moments of iron. An analysis of the absorption spectra has been used to estimate the amounts of low spin and high spin complexes mhich exist in equilibrium mixtures. Magnetic, oxidation-reduction, and other chemical properties have been used to elucidate the nature of groups binding the iron in the hemoproteins, catalase, peroxidase, hemoglobin, and myoglobin. Their viork gives a n extensive bibliography in this area. Kinetic Studies. T h e work of Brill, Ehrenberg, and Den Hartog (28) on a magnetokinetic study of the reaction between ferrimyoglobin and methyl hydrogen peroxide represents a n unique application of their “magnetic susceptometer,” devised for studying kinetics of reactions and flow systems. I n earlier ivork, Theorell and Ehrenberg (173) used a specially modified Gouy method for their studies on the red compound formed by the reaction hetween horse ferrimyoglobin and methyl hydrogen peroxidase. Because of the instability of this compound at the fcrrimyoglobin concentration of 650 p M , some of the compound had reverted to fcrrimyoglobin during the relatively long time required for making measurements n i t h t h r Gouy balance. These authors made appropriate corrections for the ferrimyoglobin present using a spectrophotonietric technique and obtained a tentative value of 3000 X c.g.s. unit for the paramagnetic susceptibility of the red compound. According to Griffith (71 ) , who introduced other corrections, the revised value is 3300 X loe6 c.g.s. unit. The flon system type susceptonieter dcscribcd before has many a d i nntages over the Gaul method. It is capable of handling much smaller concentrations of the paramagnetic material and has a rcsponse of few tenths of a second. I n a kinetic study n i t h this apparatus, m c 4 y l hydrogen permidase n-as injected into a sample of ferrimyoglobin and the changes in the macnetic susc e p t i ~ i t ywere fol~onedon a recorder. The susceptibility of the reaction product IYRS calculated from X
where
compound
= X
unrencted
=
Ax
Ax =
los . AK final [ferrimyoglobin]
and represent’s the observed change in molar susceptibility. The molar susceptibility of the unreacted ferrimyoglobin existing in two forms n-as calculated on the basis of additivity relation, using at 20’ C. X,, = 13,980 X 10-6 for t.he brown form and XFeoa = 11,330 X lop6for the red alkaline form. I n the final analysis, Brill and coworkers (28) calculated the magnetic susceptibility of the reaction product to c.g.s. unit. The be (3300 =t500) rate constants for the reaction between ferrimyoglobin and methyl hydrogen peroxidase were obtained as a function of temperature, using the magnetic and spectrophotometric techniques. Significant differences were observed particularly a t lon- temperatures in the results obtained by the two techniques. The authors point out that the magnetically determined rate constants would not be expected to obey the Arrhenius relat,ion if two or more magnetic processes contribute to the observed kinetic curve. They considered the possibility of production and disappearance of free radicals, which was previously post’ulated by other workers in the reaction between ferrimyoglobin and hydrogen peroxide. Thus, the production and disappearance of free radicals affected the time course of magnetic susceptibility, whereas the spectrophotometric method generally followed the over-all conversion of ferrimyoglobin to the reaction product. The original paper contains several interesting aspects of this work and discussions on other reactions. Determination of Hemoglobin Concentration. T h e method was developed by Taylor and C’oryell (171). This is promising as i t gives rcsults more accurately than the conventional gasometric methods based on the dctermination of carbon monoxide capacity. The hemoglobin solution is reduced to ferrohemoglobin using sodium dithionite, T\’a2S204.Fcrrohenioglobin is paramagnetic. This is then saturated with carbon monoxide and converted to carbonmonoxy-ferrohenioglobin tvhich is diamagnetic. The difference between their susceptihilit’ies, therefore, represents the paramagnetic contribution of the ferrohcmoglobin. The observed differencc depends on the concentration of ferrohrmoglobin, which in turn corrcsponds to the original concentration of hemoglobin. A change in molar susceptibility of 12,290 X l o T 6 a t 24’ corrcsponds to one heme; the effective magnetic moment of ferrohemoglobin is taken to be 5.46 Bohr magnetons. I n these experiments the diamagnetism of water, dissolved salts, and prot,eins cancels out since the entire change in magnetic properties arises
from a change in the state of iron, escept for the negligible diamagnetism of added carbon monoxide. Magnetic Titrations. It was pointed out before t h a t i t is possible t o carry out a magnetic study of chemical processes, in which large changes in susceptibility arise due t o conversion of a paramagnetic species into a diamagnetic one (or t o a species with relatively small paramagnetism) and vice versa. Such study is particularly facilitated if the susceptibility changes in the reagent and its products are very small. A number of magnetic titrations of o.;yhemoglobin, ferrihemoglobin with diamagnetic reagents such as sodium dithionite, sodium hydroxide, and potassium cyanide have been described by Coryell and coworkers (44). The Gouy technique, which allows handling of large samples (20 to 30 ml.), is very convenient for such titrations. Other Biochemical and Biological Studies. During the past few years some workers (142) have studied magnetic susceptibilities of carcinogenic compounds in efforts to study the coexistence of antimitotic and carcinogenic action. They report that the susceptibility of chloramine, triethylene melamine, urethan, myleran, 6-mercaptopurine, and several others show very high susceptibility. The observed values are higher than the calculated ones and range from 7.5 X 10+ or 12 X This is ascribed to a spread of one or more 7 electrons in these molecules. Much Ivork in this area appears to be purely speculative in nature. I n 1959 Uliumenfeld (17, 18) reported work on the electron spin rcsonance in nucleic acids. Since then a number of workers have studied both the electron resonance and magnetic moments of a wide variety of preparations of nucleic acids. Shulnian and conorkers (156) sho\\ed that the electron spin resonance spectra reported by earlier workers exhibit a ferromagnetic rather than paramagnetic behavior. This has been attributed to trace quantities of iron n-hich were shown to be present in amounts sufficient to explain the observed magnetism, assuming that the magnetic ions exist as concentrated ferromagnetic aggregates rather than as dispersed paramagnetic centers. The entire field has been reviewed critically in a recent article (283). The authors suggest that the iron found in nucleoproteins is present in the living cells though not in the concentrated ferromagnetic form. The latter, an oxide-hydroxide type material approximating FesOc in coniposition, may be precipitated during the extraction of nucleic acids or may even form slowly in intact but dried orgaVOL 34, NO. 5, APRIL 1962
0
359 R
nisms. These views are supported by findings of other workers (181). Another interesting development is due to Senftle and Thorpe (166), who studied the magnetic susceptibility of normal liver and transplantable hepatoma tissue. It was shown that the magnetic properties of these tissues are different and that the difference is probably due in a large part to the amount of the water held by the cells in the two types of tissue. The water in the normal cells appears to be bound, whereas that in the tumor tissue appears to be partially free. ACKNOWLEDGMENT
The author thanks Indumati Mulay for her help in the compilation of references and for bringing to his attention the interesting magnetic lvork on nucleic acids and the P.R.I. for a research grant which also helped the writing of this review. LITERATURE CITED
( I ) Abbott, A., Kilgour, It., J . Sci. Znstr. 31, 155 (1954). (‘2) Bldenkamp, A. A., Marks, C. P., Zijlstra, H., Rev. Sci. Instr. 31, 544 ( 1960).
(3) Amey, W. G., Clark, W.R., Krantz, F. H., Williams, A. J., Jr., Communication and Electronics Paper 54-295, p. 1, November 1954. (4) Archenhold, W. F., Broxn, A. C., Thompson, J. E., J . Set. Instr. 36, 505 ( 1959). (5) Aston, J. G., Bolger, B., Trambarulo, R., Segall, H., J . Chem. Phys. 22, 460 (1954). (6) Atanasiu, G., Patrascu, S., Rev. Phys Bucharest 4, 273 (1959). (7) Bamford, C. R., Charnock, H., Phys. Chem. Glasses 1, 143 (1960). (8) Banerjee, B. K., Zndzan J . Phys. 33, 201 (1959). (9) Barnett, S. J., J . Appl. Phys. 23, 975 (1952). (10) Bates, L. F., “Modern Magnetism,” Cambridge University Press, 1961. 1) Bell, J. S., Wright, P. G., Electronzc Eng. 32, 394 (1960). 2) Berlin, A. 9., Blyumenfeld, L. A., Semenov, K. N., Izuest. Akad. Nauk S.S.S.R., Otdel. Khim. Sauk 1959, 1689
ijBkthe, H., Ann. Physik. 3, 133 ( 1929). 4) Bhatnagar, S. S., Mathur, K. X., “Physical Principles and Applications of Magnetochemistry,” MacMillan and Co., Ltd., London, 1935. 5 ) Birss, R. R., Lee, E. W.,J . Sci. Znstr. 37, 225 (1960). (16) Bishay, A., J . Am. Ceram. SOC.44, 16 (1961). (17) Bliumenfeld, L. A., Biofizilca 4, 3, 515 (1959). (18) Bliumenfeld, L. A., Kalmanson, A. E., Pei-Ken, Sheng, Doklady Akad. Nauk U.S.S.R. 124, 1144 (1959). (19) Block, R. B., Case Institute of Technology, Cleveland Tech. Rept. No. 6; U. S. Atomic Energy Commission Rept. U-4554,July 1959; Aruclear Sci. Abstr. 14, 678 (1960). (20) Bloembergen, N., Purcell, E. M., Pound, R. V., Phys. Rev. 73,679 (1948). (21) Blois, M. S.,Jr., Brown, H. M., Lemmon, R. M., Lindblom, R. O., Weissbluth, M.,eds., “Free Radicals in Biological Systems,” Academic Press, Yew York, 1961. 360 R
ANALYTICAL CHEMISTRY
(22) Blooni, .I\. L., Packard, 11.E., Sczence 122, 738 (1955). (23) Boardman, H., Pelwood, P. W.>J . Am. Chem. SOC.72, 137%(1950). (24) Bockris, J. O’M., Parsons, L). F., J . Scz. Znstr. 30. 362 (1953). (25) Bose, A . , Indian‘J. Phys. 21, 275 i194i). (26) Bozorth, R. ll., “Ferromagnetism,” Van Nostrand Sew York, 1951. (27) Bozorth, R. M.,Van Vleck, J. H., et al., “Magnetic Properties of Metals and Alloys,” American Society for Metals, 1959. (28) Brill, A. S., Ehrenberg, .1., Den Hartog, H., Biochzm. et Biophys. Acta. 40, 313 (1960). (29) Brill, A. S., Den Hartog, H., Legallais, Lr., Rev. Sci. Instr. 29, 383 (1958). (30) Brill, rl. S., Williams, R. J. P., Biochem. J . 78, 246 (1961). (31) Broersma, S., Am. J . Phys. 24, 500 (1956).(32) Broersnia, S., J . Chem. Phys. 26, 1405 ( 1957). (33) Broersma, S., Reo. Sci. Instr. 24, 993 (1953). (34) Ibid., 20, 660 (1949); “hlagnetic Measurements on Organic Compounds,” Martinus Kijhoff, The Hague, 1947; J . Chem. Phus. 17,873 (1949). (35) Brog, K. C., Case Institute of Technology, Cleveland, Tech Report No. 2, N. P.-8106, August 1959; ,Yuclear \ - - - - I
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