MAGNETIC SUSCEPTIBILITY BALANCE JUDSON L. IHRIG and ROBERT G. CALDWELL University of Hawaii, Honolulu, Hawaii
The use of a permanent magnet instead may frequently make the construction of an instrument prartiral for such laboratories whose budgets are limited. This paper desrrihes the ronstruction and operation of a magnetic susceptibilit,y balance xhich may be huilt easily and economically. Its simplicity commends it as an instrument for lahoratory instruction; a t the same time it has proved sensitive enough for magnetochemical research. THE G O W METHOD The most usual method for t,he determination of magnetic susceptibility is t,hat of Gouy. The principles of t,he method have been giveo in detail by S e l ~ o o d . ? , ~ Briefly, the procedure consists of measurement of the force exerted upon a cylindrical tube containi~rgthe sample in the presence of a magnetic field. This forre is related to the magnet,ir susrept,ihility of the material hy the expression:
where K 1 and K z are the volume susceptihilities of the substance and the surrounding medium, respectively. HI and Hpare the magnetic field strengt,hsa t the hottom and top of the cylindrical tube, respectively, and -4 is the cross-sectional area of the tube. In practice, the tube is plared in the field so that its lower end is in the region of greatest intensity and its upper portion is in a region of negligible field; this has the effect of eliminat,ing H2 from the calculations. K2 is the volume susreptibility of air; it too map be disregarded if t,he air surrounding the sample is replaced with a gas, surh as hyRECENT years magnetic susceptibility measurements drogen or nitrogen, vhich bas a very small volume have been applied to diverse fields of chemistry. Magsusceptibility. netic susceptibility now appears to be taking its place I n the Gouy method this force along the axis of the with dipole moment, molecular spectroscopy, and X-ray tube may be equated to QAW,where g is the gravitational diffraction as a commoll tool in the study of molecular constant and Aw is the apparent change in weight of structure, B~~~~~~of this trend, has the sample due to the influence of the magnetir field. been give11 to the inclusion of magnetic susceptibility Suhstanres whose electrons are all aired are repelled experiments in the laboratory courses in physical chemby the field; they have negative susceptihilities and are istry or instrumentation. Schulerl has described a very Other substances (free radsaid to be diamagnetic. simple illstrument for instructional purposes, l-his lcals and certain salts of transition metals, for example) apparatus, though practical for laboratory exercises, which possess one or more unpaired electrons are atdoes not possess the sensitivity or precision needed for Their tracted to the field and are ralled paramagnetic. research work. susceptibilities are positive in sign, as are their Aw in one instruDespite the advantages of ment both teaching and investigational functions, some lahoratories have been reluctant to set up equipment for magnetochemical work because of the expense in2 S ~ ~ w o o P. n , w.,"Magnetochemistrv," Interscience ~ u h volved. The costly item of equipment is the large elec- lishers, Ino., New York, 1943. SELWOOD, P. W.,in A. WBISSRERGER,Editor, "Phwieal tromagnet usually employed and its power supply. Methods of Organic Chemistry," 2nd. ed., Intersrienro Puhlishers, Inc., New York, 1954, Part 3, pp. 2451-90. SCHULER, R. H., J. CHEM.EDUC., 27.591-4 (1950).
IN
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The experimental procedure consists of weighing the sample in and out of the applied magnetic field. Recause the area A and the field HI are difficult to measure accurately, the apparatus is usually calibrated with two substances of known susceptibility. Air, water, benzene, and nickel chloride solutions are among the common calibrating agents. It is best to choose calibration substances which give rise to forces of approximately the same magnitude expected for the unknown substauces under investigation. THE APPARATUS
Figure 1 shows the complete apparatus. The sample tube is suspended by a fine copper wire from the arm of an old Kuhlmann microhalance. An outer glass jacket encloses the sample tuhe to protect it from drafts which would otherwise cause deflections of the tube several times as great as those produced by the magnetic field. The details of the suspension are shown in Figure 2. The sample tuhe itself is made of 13-mm. Pyrex tubing and is about 20 cm. in over-all length. The tube is made sufficiently long so that the upper end will be in a region of negligible field strength when the lower end is positioned between the pole pieces of the magnet as shown in the drawing. The cross-sectional area is made as large as possible so as to have the maximum amount of sample acted upon by the field. Care must be taken, however, to prevent actual contact of tube with jacket, and between jacket and pole pieces. Closure of the tube is a 7/25 ground-glass joint. The glass housing is made in two sections, the lower end of 32-mm. Pyrex tubing narrowing t o 18 mm. so as to fit between the magnet pole pieces. This lower section of about 14 inches is connected to the fixed upper portion, of 18-mm. tubing and 25 inches long, by means of a Z8/42 joint. This allows access to the sample tube. The upper portion is fixed to the balance support with a rubber gasket in such a manner as to prevent drafts. I t is also supported by a sponge-rubber ring clamped in a brace of brass strip attached to the main apparatus frames. This may be seen in Figures 1and 3. The latter figure is a close-up of the magnet and its mounting. The magnet support is a heavy-duty automobile sdssors jack firmly bolted to the base of the apparatus frame. This jack was chosen because of its high lift and also because the lifting mechanism has both a coarse and a fine adjustment which permits an accurate and reproducible positioning of the magnet poles about the sample tube. Moreover, its cost (about $12) is less than half that of a similar laboratory jack. When raised into position around the tube, the magnet is still some three feet from the microbalance and has no detectable effect upon the weighings. During measurement of the weight of the sample and tube, the magnet is lowered t o its extreme position which results in a negligible field about the sample. The magnet itself is an Alnico magnetron magnet originally used in one of the larger portable radar trailers. These mav still be found in the sumlus market or in salvage yards where they are used to pick up nails
and bolts. Another source is old conveyer machinery where many were installed to remove steel filings and chips from the material being processed. The magnet shown was purchased in poor condition for $5. I t is important to obtain the largest size possible in order that the poles have a reasonable diameter. The larger the diameter of the poles, the larger the volume of homogeneous field H,, and the greater the reproducibility of the measurements. The magnet used in the present apparatus is fitted with truncated conical pole pieces of soft iron with faces 1.9 cm. in diameter. The pole gap is 2 cm. This arrangement gives a sufficient volume of homogeneous field to permit the vertical positioning of the tube to be done simply by eye. Most surplus mag-
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nets need recharging, -. which is readily done. In this case the magnet was wrapped with 318 turns of No. 10 insulated copper wire and a momentary current of 400 amperes a t 75 volts was passed through. This was supplied by a standard d:c. welding outfit. A field strength of about 5550 gauss was obtained. he whole apparatus-is supported by a sturdy frame made of 4-X4-inch timbers-fastened -with 9-inch machine bolts. Cross bracing and platforms for the mag-
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net assembly and the microbalance are of 2-X4-inch lumber. The balance is supported by a thick glass plate insulated from the frame hv four larne ., rubber' stoppers. This constrnction is essentially free of troublesome vibrations. The balance itself is a rostly item, although with the now prevalent practice of having organic analyses performed by outside professionals, many laboratories have sensitive bdances which are no longer in constrant use. The Knhlmann model used with this apparatus was particularly easy t o modify. I t was necessary only t o remove the pan-arrest mechanism. On balances with pan arrests operating from the inside of the case, it is also necessary to drill a small hole directly under the left-hand pan stirrnp t o permit attachment of the suspension wire. With the fairly low field strengths oh-
tween tube and jacket, the apparatus is %llowedt o come t o thermal and mechanical equilibrium for 30 minutes. Then the actual weighings are made. It was found that reproducible results were obtained with a series of six weiahinw - - made alternatelv with. and then ~ i t h o n t the presence of the field. Since microweighings are always subject t o thermal deflections caused by the presence of the operator, it is necessary t o devise a routine procedure in which the numher of operations and their time of duration are held fixed. Only in this way mill runs on different substances be comparable with each other and with the calibration values. The exact details of a standardized procedure, which depend upon the balance used and the environment of the entire apparatus, will almost automatically suggest themselves t o the operator. I n any case, the rnut,ine is easy t o devise and use, requiring no more practice than is necessary for any precision instrument. With the equipment descrihed, a precision in weighing of from *2 t o 3 micrograms was obtained. Calibration with water and air showed that this prerision corresponded t o a volume susceptibility increment of from 1.2 t o 1.8 X 10-lo c.g.s. units. Since all but a few literature values of snsceptibilit,y are given t o only 1X 10-%.g.s. units, the apparatus was considered sufficiently sensitive. Cross rhecks of the calihration were made using acetone and henzene. The value for acetone was in agreement mith the literature value to better than one part in 500, and that for benzene mas higher than the literature value by two parts in 600. These calibrations demonstrate the qualities of this simple apparatus. The equipment was used successfully in an investigation of semiquinone-type intermediates in the oxidation of benzoin t o benzil.' ~~
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ADVANTAGES AND LIMITATIONS
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tainable mith inexpensive magnetron magnets it is necessary t o use a rather sensitive balance. The large, old type of instrument with a sensitivity of or mg. is, however, a satisfactory alternative to a microbalance. In general, the gres.ter the field strength available, the more leeway one h2.s in the choice of balance, OPERATION AND PERFORMANCE
The experimental procedure consists of the weighing of sample and tube both in and out of the magnetic field. The difference in weight Aw may then be related t o the volume susceptibility of the material a s mentioned previously. First, the filled sample tube is attached t o the suspension wire and the lower jacket fitted. After it is certain that there is no contact he-
The principal advantages of this apparatus are its simple construction and its low cost which did not exceed 825, since a balance was available. The ease of operation, compared with more complex appa,ratus, makes the outfit suitable for use in physical chemistry and instrumentation laboratory courses. The major limitation is that imposed by the use of permanent magnets in general, i. e., a nonvariahle field strength. Although the extreme steadiness of permanent magnet fields is of advantage in work where each run is of many hours duration, the absence of field control makes it impossible to make a magnetic check for the presence of ferromagnetic impurities. Thus, the instrument is unsuited for catalytic and solid-state investigations as well as for absolute measurements on many "pure" inorganic salts which frequently have minute traces of ferromagnetic substances. It is true that fields of permanent magnets may sometimes he varied, but not in a simple, inexpensive fashion. The present design, then, is limited to studies of diamagnetic organic substances, free radicals, and certain selected inorganic compounds. Even if the presence of ferromagnetics is
' IHRIC,J. L., AND R. G. CALDWELL, to be published.
JUNE,1955 suspected, however, the apparatus may he used to measure susceptibility changes as might arise in the course of a reaction. A less severe limitation is that the apparatus as described is useful for measurements a t one temperature only. To make measurements a t temperatures different than ambient, it is necessary only to make the jacket surrounding the sample tube in the form of a Liebig condenser and circulate through it a liquid of the
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desired temperature. However, extremes of temperature, high or low, demand more elaborate accessories. These are described by Selwood. I n conclusion, it is hoped that the rather detailed exposition of a simple magnetic susceptibility balance will help to promote further investigation in a fascinating field. Those institutions who have not entered this work because of the expense may find this version of the apparatus suited to their needs.
