in backscattering as if it were a n element of hypothetical atomic number Z. K h e n Z is substituted in the equation for the appropriate period, the per cent backscattering is predicted with accuracy. The alternative method of predicting backscattering from the intrinsic scattering of the elements and their knoivn weight fraction gives an identical answer only if all atoms are in the same period. The Z concept applies also to hydrogen-bearing compounds, and the correction for the hydrogen deficit is straightforward (4). This means that if Z is calculated for a hydrogen-bearing compound and then substituted in the appropriate equation, the per cent backscattering so computed will be higher than that observed, but in dircct proportion to the hydrogen content. DISCUSSION
The technique described here is susceptible to extensive improvement, particularly from the experimental and instrumental point of viem. Until recently such obvious improvements have been held in abeyance until the fundamental facts were established. Most readers will share the author’s dissatisfaction with the empirical nature of these relationships. However, in view of the inherent complexity of the backscattering phenomenon, the author is compelled to regard these
regularities as little short of astonishing, particularly because they are so susceptible t o precise measurement and have exhibited no single exception to these rules. If one cannot ( d ) , a t this moment. deduce these regularities from a simple theory based upon fundamental assumptions, one is a t least in possession of the facts from which a coherent theory may be deduced. Backscattering is not the sole puzzle concerning beta particle phenomena. The very manner in which beta particles are emitted in the process of radioactive decay is not completely understood. These investigations have raised more questions than they have answered. That the results are highly geometry-dependent has been emphasized. It mould be of great interest to know how the scattering is distributed in angle and how the backscattered betas are degraded in energy. The emphasis in this work has been to gain a broad and comprehensive picture of the relative behavior of most elements and compounds under a restricted but reproducible set of conditions. The results have been interesting, a t least, and perhaps not without profit. ACKNOWLEDGMENT
Grateful
acknodedgment
is
ex-
pressed to the following members of this laboratory: G. P. Apprill, K.H. Ashley, A. S. Coffinberry, D. T . Cromer, H. Eberline, Earl Fullmari (who grew the reference crystals), E. R. Jette, L. E. Lanham, J. R. Lilienthal, A. F. Nalmberg, C. F. Mete, TV. S. Miner, J. R. Mosley, D. 11. Olson, J. G. P o d i t e s , V. G. Rexroth, Jr., A. R. Ronzio, E. Staritzky, R. G. Sturgess, LL H. Tattan, and J. T. TTaber. For extensive tabulations and assistance with the manuscript the author is also indebted to Mrs. H. K. Schmitt and Prestn Ritter. LITERATURE CITED
(1) Allen, S. J., Phys. Rev. 29, S o . 3 , 177 (1909). (2) Bothe, W.,Ann. Physik [6] 6 , 44 (1949). ( 3 ) McClelland. J. A.. Proc. Rou. SOC. London 80, 501 (1908). (4) hluller, D. c., ASAL. CHEM. 29, 978 (1957). (5) Muller, R. H., P h y s . Ret). 93, 891 (1954). ( 6 ) Muller, R. H., Lonadier, F., unpublished work, Los Alamos Scientific Laboratory, Los Alamos, N. XI. (7) hliiller, R. H., White, R. R.,unpublished work, Los Alamos Scientific Laboratory, Los Alamos, X. hl. \
,
RECEIVEDfor review October 1, 1956. Accepted February 11, 1957. XVth International Congress of Pure and Applied Chemistry, Lisbon, Portugal, September 1956. Work performed under the auspices of the U. S. Atomic Energy Commission.
Interaction of Beta Particles with Organic Compounds DORIS CLEGG MULLER University of California, Los Alamos Scientific Laborafory, Los Alamos,
b The general principles which govern the backscattering of beta particles from atoms and molecules are applied to some representative organic compounds. The anomaly of hydrogen is resolved and a reliable value is fixed for it. The behavior of isomers is described and proof is given that the backscattering of compounds is accurately predictable from the intrinsic scattering of the constituent atoms.
S
STUDIES on the backscattering of beta particles by matter h a r e been made in this laboratory for the past 3 years. The general lams and conclusions have been described ( 1 , 2). It is now known that the relative backscattering is a discontinuous function of atomic numYSTEMATIC
N. M.
ber, but strictly linear with 2 in each period of the periodic system. The scattering by compounds is accurately predictable from the intrinsic scattering of the constituent atoms. This Ivork deals with the behavior of a few representative organic compounds. Khile these are in no way different in behavior from other substances, they do afford a simple means of handling the anomaly of hydrogen, which exhibits negative scattering-that is, less than zero scattering, presumably due to absorption effects. It has been possible to establish the exact value and, therefore, to apply the small but definite correction to the observed scattering of hydrogen-bearing compounds. The experimental details and instrumental techniques have been described by Muller (1). As before,
these results are concerned with the relative backscattering of 2.18-m.e.v. beta particles from yttrium-90. The relative backscattering for elements in periods I1 to VI, inclusive, is known (1); for the present purpose, only those which are more commonly encountered in organic compounds are listed in Table I. I n this table the fourth digit in the values for elements 6 to 9, inclusive, is for computational use only. The values of all elements in Table I are believed to be accurate to somewhat better than 0.1%. No assumptions are to be made about the state of aggregation of each element. The values for nitrogen, oxygen, and fluorine obviously do not refer to the gaseous state, for which the backscattering would be far less. These values are simply those which apply ‘401. 29, NO. 6, JUNE 1957
975
Table 1. Backscattering of Elements Common in Organic Compounds Z Element Backscattering
6
49 16
17 35 53
Carbon Yitrogen Oxygen Fluorine
5.230 6.461 7.692 8.923 15.953 16.920 29.560 36.208
Sulfur
Chlorine Bromine Iodine
BS = 1.23112
rigorously as the intrinsic scattering for that element, as it may occur, in definite weight fraction in any compound. From the values in Table I, the backscattering of any compound may be computed by multiplying the intrinsic backscattering of each element by the weight fraction of that element in the compound and totaling for all other atoms in the same way. As a n illustration of the general method for calculating the backscattering of compounds, the case of Teflon is chosen. From Table I the intrinsic backscattering of carbon is 5.230% and that of Auorine is 8.923%. For Teflon, the weight fraction of carbon is 0.24017 and of fluorine, 0.75983. The backscattering is:
+
BS = (5.230 X 0.24017) (8.923 X 0.75983) = 8.037%
Direct observation yields a value of S.045%, a difference of 0.008% *or a relative error of 8 parts in 8037 (0.099%). I n many cases the error is greater than this, but in the case of perfect crystals of pure substances, the error is one half to one third of this magnitude. However, this mode of computation is restricted to compounds in which all the atoms are in the same period. -4general approach is afforded by the Z concept proposed by hIuller (2). According to this principle, the compound, B,C,, has a value of
where the A’s are atomic weights and the Z’s, the respective atomic numbers. The denominator is actually the minimal formula weight and the equation could be written: Z
=
+
Z B ~ B Zcfc
ANALYTICAL CHEMISTRY
- 2 157
(2)
If Z for Teflon is substituted in this equation, the predicted per cent backscattering is 8.037, a value identical with the one computed previously. The agreement arises only because carbon and fluorine are in the same period. Further examples of the general validity of the Z principle are given by hiuller (1). This principle is now applied to the evaluation of the hydrogen anomaly. Accurate measurements on the backscattering of several solid hydrocarbons have supplied the information needed for this purpose. The intrinsic value for hydrogen, so obtained, is satisfactory for the prediction of other hydrogen-bearing compounds. hfeasurements on paraffin, naphthalene, trans-stilbene. and polvstyrenc are summarized in Table 11. If each Z value is substituted in Equation 2 for period I1 elements, the predicted backscattering is uniformly too high. The anomaIous behavior of hydrogen is obtained directly, however, because the deviations are proportional to the hydrogen content. If we add the necessary corrections t o the equation for period I1 in the form: BS = 1.23112
- 2.157 - BE f a
(3)
where f~ is the weight fraction of hydrogen and BE is the desired intrinsic scattering of hydrogen, then the values of BE so obtained are shown in column 6. The value of BH is constant to slightly better than 1%. The values in column 7 are expressed in per cent backscattering per per cent hydrogen a form which is useful in making corrections to observations on substances for which the hydrogen content is expressed in per cent. The value of -10.3801, for the intrinsic backscattering of hydrogen is satisfactory for the correction of all hydrogen-bearing compounds By comparison with the values of other elements, this “apparent” negative backscattering of hydrogen seems to be rather large; in some compounds of low hydrogen content and for many
hydrates, the correction becomes quite small, Having obtained the absolute value for BE,it is evident that Equation 3 can be rearranged to compute the backscattering from 2 for any hydrogenbearing compound, or if the backscattering has been measured, to compute Z. It may also be apparent that, when the hydrogen correction is made to the observed backscattering, the Z values will all lie on the linear plot relating backscattering to Z (or Z ) as defined by the equation for period
11. I n the future it may become profitable to extend this method to other compounds of reasonably high hydrogen content to obtain a more precise estimate for the intrinsic scattering of hydrogen. However, the value given here has been adequate for correcting the observations on the organic compounds. The question of how the value for hydrogen compares with that of deuterium was studied by comparing polyethylene and deuteropolyethylene and ordinary water and heavy water. There are marked differences in both cases, indicating that hydrogen and deuterium have markedly different backscattering (or absorbing) properties. This effect could probably be accounted for by the neutron which is present in the deuterium nucleus. This problem is the subject of further study. Since the nuclear forces acting upon the beta particles should be profoundly affected by the neutron-proton ratio, a t least for thr lighter elements, such inquiries would be enhanced by the examination of pure isotopes of some of the lighter elements. The average value for the intrinsic backscattering of hydrogen so obtained, if applied to those substances from which it was derived, yields values for the prediction of their backscattering as shown in Table 111; the average correction predicts the observations within an average error of 0.16% and a maximum error of 0.26%. Similar results with an average error of 0.2% have been predicted for the backscattering of methanol, ethanol, benzene, and acetone. Compounds involving heavier elements are of interest because these contain elements showing m-idely different
(1)
where the 2 ’ s are atomic numbers and thef’s are the corresponding weight fractions of the respective atoms in the compound. For Teflon, the Z value is 8.280, which indicates that it should backscatter as if it were an element of hypothetical atomic number 8.280. Its behavior in this respect should be 976
intermediate between oxygen ( Z = 8) and fluorine (2 = 9). The backscattering for all elements in period I1 (helium to neon) is expressed by:
Table II.
Compound trans-Stilbene Paraffin Polystyrene Naphthalene
Backscattering Measurements on Organic Compounds
Weight Fraction C 0.93289 0.85175 0.92257 0.’93709
Weight Fraction H
0.06711 0.14825 0,07743 0,06291
Obsd. BS, % 4.125 2.778 3.956
5,664 5.259 5,613
4.178
5.685
Z
hv.
BH - 10,297 -10.381 - 10.293 -10,551 - 10.382
BS of H per % H -0.1030 -0,1038 -0,1029 -0.1055 -0.1038
Table 111.
Backscattering Corrected for Hydrogen Defect
Cornpound trans-Stilbene Paraffin Polyst) relic
Naphthaitme
- c1Backscattering Calrd. Obsd. A 4 119 4 125 0 006 2 ii8 2 7 i 8 0 000 3 949 3 956 0 007 4 188 4 178 0 011
dv.
0 006
intrinsic scattering. d comparison of chloroform and carbon tetrachloride yielded the expected values. I n this case the relative scattering of the two, as calculated from the elements, was in the ratio of CC14/CHC13 = 1.0265, whereas the obscrved ratio was 1.0278, a relative error of 0.13Oj,. ISOMERS
Several cases of isomers have been studied. From all available evidence, isomers scatter identically. Fortunately, they differ in transmittance (or adsorption), and in predictable fashion. Two liquids, diethyl ether and l-butanol (both CaHloO),have been examined in great detail Both yield backscattering values agreeing with those computed from the elements, but in this connection, it may be more useful to compare the relative backscattering of the two. The ratio of the observed relative backscattering was found to be. ether/butanol = 1 00056
This means that if any difference exists between the two, it is less than 0.055%. The statistical error in counting (many millions of counts) was of the same order of magnitude, which indicates that there is no difference in these two substances within this limit of experimental error. Conversely, if it would serve any useful purpose, one could extend the counting times severalfold to search for a more precise indication of their identity. A detailed study in this laboratory (3) has established similar results for the isomers of CirHia-i.e., anthracene, phenanthrene, and tolane (diphenylacetylene). Their identical backscattering \vas established independently by measurements on powdered specimens compressed to practically theoretical density in a hydraulic press and also from solutions of these substances in a mixed solvent. That isomers can be differentiated from one another by beta particle techniques has been established in this laboratory, but not t o the high precision with which their identical backscattering can be demonstrated. Because backscattering was of primary interest, transmittance measurements were made with a simple experimental modification of the backscattering system described
by Lluller ( 1 ) . I n this manner the basic backscattering system can be utilized to obtain relative transmittance measurements. Rather elaborate, but straightforward, corrections for backscattering are required in order to obtain the desired transmittancy. Improved cells are being constructed to provide simultaneous transmittancy and backscattering measurements, which are essential for studies on solutions and liquid mixtures. With this technique the relative transniittancy of diethyl ether and 1-butanol were compared. Contrary to the backscattering results, these substances exhibit different transmittancies (or absorbances). The average of a large number of observations yields the following ratio of absorbance: I-butanol/diethyl ether = 1.1288 The densities a t 23.3’ C. are 0.80734 for 1-butanol and 0.70966 for diethyl ether. The ratio of the densities a t 23.3’ C. is 1.1376. These results show t h a t density is the controlling factor in the relative absorbance, because the ratios agree within 0.86%, which is rrithin the limitations of the backscattering correction and temperature constancy. K i t h respect to the latter, in the Smith and Otvos (4) method for determining hydrogen in hydrocarbons a simultaneous measurement of density is made with high precision. In the above measurements temperatures were constant only to the degree afforded by the air-conditioning system. Lonadier and RIiiller (3) have confirmed the dependence of absorbance upon density for the isomers of CI4Hl0. Further work is being carried out with improved methods for absorbance measurements. Aside from the numerical values in each particular case, it is fortunate that there are measurable differences between isomers in a t least one aspect of beta-particle interaction. Otherwise, the identical behavior in the case of backscattering would leave much to be desired for analytical purposes, as these substances are easily differentiated by a dozen or more simple criteria Quite a few substances have been examined in the liquid and solid state by the simple expedient of providing a tall, thin-window cell with a cold finger insert. Backscattering can be measured and then, by filling the cold finger insert with a suitable refrigerant, the backscattering of the solid can be measured on the same sample. By removing the refrigerant and allowing the solid to melt, the liquid value can be checked. KOsignificant differences were observed in the backscattering from the liquid and solid phases. Unless far more precise measurements reveal very small differences, it is apparent that the density of the system has no effect on the backscattering. This conclusion is borne
out by the behavior of isomers, which show identical backscattering even though the densitips are different. It must mean that the beta particles traverse such relatively large distances within the substance that the same fraction of them return to be counted, regardless of thc density. These considerations do not apply to the case of tran3mittance or absorption, as has been shown above. The improved system, which is nearly completed, IT-ill permit the simultaneous measurcment of absorption and backscattering in thin layers of liquid. The backscattering values which the new technique affords are not those which have been discussed, because they do not correspond to a sample of “infinite” thickness, but their precise value is necessary for the calculation of true absorption. Despite the obvious and necessary improvements to be expected from the newer technique, the present method leaves no serious doubts about the identity of isomeric substances. The backscattering values as observed, combined with the known intrinsic scattering of the constituent atoms, establish the substance as one of several possible isomers. By then measuring the absorbance by use of the gold-faced reflector, one can decide which of the isomers is responsible for the observed value. This presupposes that the densities are knovn or have been measured. Small differences in densities of two or more isomers would make a decision difficult. Also, in the case of mixtures of isomers, extremely precise measurements would be required and it is possible that the method Jvould be inferior t o alternative methods such as infrared spectrophotometry. ANALYTICAL IMPLICATIONS
The author prefers to express the analytical aspects of this phenomenon as implications rather than applications. Thr emphasis in these and all related studies on backscattering has been to discover a general relationship between composition and relative backscattering. Because this has been established to the degree that one can predict the relative backscattering in a given experimental assembly, one can speak of applications, their advantages, and limitations for any element or any compound. The present equipment can be used to examine solids, liquids, and solutions, but it requires samples which are larger than might be desired. The sample stage requires a specimen which must cover a 3/*-inch circular aperture and must have a minimum thickness ( t ) Fhich can be expressed roughly as 0.422 inch/d, where d is the density. The latter value is approximately “infinite” thickness for the yttrium-90 betas. VOL. 29, NO. 6 , JUNE 1957
977
FF'ith the possible exception of t,, there are sample size limitations which can be decreased by improved instrument design. If the composition of a substance is known or suspected, its identity can be verified in a few minutes. This statement must be qualified by the obvious fact that there can be numerous combinations of atoms present in such relative proportions that their backscattering would be identical or very close, but the possibilities can be calculated. Similarly, if the kinds of atoms present in a compound have been established by elementary qualitative analysis, then the observed backscattering corresponds to a definite limited number of possible compounds. It is obviously impossible, at present, to measure the backscattering and then predict the composition uniquely. This is a limitation comparable to other methods such as electron or x-ray diffraction and infrared spectroscopy, which presuppose t h a t the characteristic pattern has been examined before and can be compared with the new specimen. As the backscattering of a compound is uniquely and precisely defined by the kind of atoms present and their relative
proportions, it is entirely possible that a computer system could be devised to calculate backscattering values for all possible permutations and combinations of atoms in compounds. If restricted to the relatively limited number of atoms commonly found in organic compounds, the requirements of this computer might not be too severe. In cases where only a limited number of compounds would come into consideration, the simple calculation can easily be performed in a few minutes. Table I s h o w that large changes in backscattering can be expected for the heavier elements. Halogenated compounds can be distinguished with high precision because of the high intrinsic scattering of the halogens and the large weight fraction which they possess. Since beta-particle techniques both in backscattering and absorption have been in use for several years in automatic gaging operations, it is obvious that these methods can be utilized in the continuous monitoring of process streams. I n such applications direct counting is not used; the counter is connected to a count-rate meter and recording potentiometer. The rate of response to composition changes involves several fac-
tors, all of ahich are well understood. It includes the counting rate and time constants associated with the recording system, and these must be such that the time lag in detecting a change in composition shall have a n acceptable value. These are engineering considerations which have been solved satisfactorily for the above-mentioned gaging application. The studies described here have established the physical and chemical factors from which the feasibility of a process-monitoring scheme could be established. LITERATURE CITED
(1) hIuller, R. H., ANAL.CHEM. 29, 969
(1957).
(2) hluller, R. H. Phys. Rev. 93, 891
(1954).
(3) Muller, R. H., Lonadier, F., unpublished work, Los Alamos Scientific Laboratory, Los Alamos, N. M. (4) Smith, V. N., Otvos, J. W., ANAL. CHEM.
26, 359 (1954).
RECEIVEDfor review October 1, 1956. Accepted February 11, 1957. XVth International Congress of Pure and Applied Chemistry, Lisbon, Portugal, September 1956. Work performed under the auspices of U S. Atomic Energy Commission.
Determination of Aluminum in Aluminum-Iron Alloys JOHN V. GlLFRlCH U. S. Naval Ordnance laboratory, Silver Spring, Md.
b The steadily increasing interest in the Alfenol type of alloy (aluminumiron containing 5 to 20% aIuminum) has necessitated development of an accurate method for determining the high aluminum content of these alloys. Application of these alloys in the field of magnetics requires a high degree of analytical accuracy because the composition of the alloy is critical for the desired magnetic properties. The present method is based on the adsorption of interfering elements by an ion exchange resin; the nonadsorbable aluminum is determined gravimetrically by precipitation with ammonium hydroxide.
T
determination of aluminum in the presence of iron has been recently studied by many investigators (2-4, 8, 9, I I ) , who, in most cases, were concerned with determining small amounts of aluminum in the presence of large amounts of iron or large amounts HE
978
ANALYTICAL CHEMISTRY
of iron plus other elements. K h e n the aluminum content exceeds 2 or 3%, these methods are either not applicable or inaccurate. The methods in the literature for the separation of large amounts of aluminum from iron yielded inconsistent or inaccurate results-for example, the electrolytic deposition of elements other than aluminum on a mercury cathode and the subsequent determination of aluminum in the solution are frequently erratic and unreproducible. The cyanide-oxyquinolate method has a major disadvantage in the difficulty of handling the aluminum precipitate obtained, due to the coprecipitation of the reagent. The separation of iron from aluminum by the use of sodium hydroxide is a disagreeable procedure and it is difficult to wash the precipitated ferric hydroxide free of aluminum. Ion exchange is becoming a n important tool of the analytical chemist. Although the principle has been known for a number of years, only fairly re-
cently have practicing analytical chemists investigated its use. The separation of metallic ions by an anion exchange resin has become a popular application ( 1 , 3, 6, I O ) . These separations depend on the fact that certain
BALL" JOINT\
(I\
y 2 m m . CAP TUBING
DOWEX-I RESIN GLASS
2 mm. CAP.STOPCOCK
Figure 1.
Ion exchange column
