REPORT
FOR A N A L Y T I C A L
CHEMISTS
H o w Good Are t h e N e w Atomic Weights? b y E D W A R D W I C H E R S , National
Academy
of Sciences-National
Research
Council,
Washington,
D. C.
This is the second of two articles on atomic weights. In the February issue, page 23 A, A. E. Cameron discussed "The Determination of Atomic Weights by Mass Spectrometry" Γ
ΎΛ H E 1 9 6 1 REVISION OF THE INTERNATIONAL TABLE OF -*• A T O M I C W E I G H T S was based on t h e first comprehensive
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Dr. Edward Wichers was born in Michigan in 1892 and completed his undergraduate education with the A.B. de gree from Hope College in 1913. After receiving the Ph.D. degree at the University of Illinois, he became a member of the staff of the National Bureau of Standards. He served continuously at the Bureau until his retirement, as Associ ate Director, in 1962, except for the years 1 9 4 4 - 4 5 , which were devoted to uranium chemistry and special reagents at the Los Alamos laboratory. Dr. Wichers' early work on the preparation of very pure platinum metals and on analytical separations for these elements led to more general interests in the preparation and characterization of pure substances, especially those useful as primary chemical standards (see Anal. Chem. 3 3 , 23 A, 1961). Dr. Wichers has served continuously as a member of the A.C.S. Committee on Analytical Reagents from 1924 and as its Chairman f r o m 1943 t o 1 9 5 5 . He has also served as Chairman of the Society's Committee on Annual Reports on Atomic Weights from 1949 to date. In 1949 he was asked by the International Union of Pure and Ap plied Chemistry to reorganize the Union's Commission on Atomic Weights, which had lost nearly its entire member ship by death or retirement. He served as President of the Commission from 1949 to 1959 and since has con tinued as an Associate Member. In this capacity he col laborated with A. E. Cameron in preparing the text of the Commission's report for 1 9 6 1 . The present article is intended to extract from that report considerations of spe cial interest to analytical chemists.
review of experimental evidence since 1925 a n d t h e third in the history of t h e International Commission, t h e first having been made in 1909. ( B y action of t h e International Commis sion on Atomic Weights t h e title of t h e table was changed in 1961 t o t h e International Table of Relative Atomic Weights. Since t h e concept "relative" is implicit in t h e term "atomic weights" as used b y chemists, t h e word itself has not been added in this article.) T h e adoption of t h e new unified scale, based on 12 as t h e assigned atomic mass of carbon-12, caused a systematic change of four p a r t s in 100,000 in all values for atomic weights a n d thus m a d e a complete revision necessary. W i t h this revision in prospect it was a p p r o p r i a t e t o review consistently t h e large a m o u n t of experimental work t h a t h a d been reported in t h e interval 1925-61. This period marked the transition from t h e time when atomic weights were de termined exclusively b y chemical procedures, except for a few values based on measiirements of t h e relative densities of gases, t o t h e time when chemical work came t o a virtual stand still a n d was replaced b y mass spectrometry. D u r i n g this time new values for atomic weights were introduced into t h e I n ternational Table from time to time, b u t no a t t e m p t was m a d e a t a consistent re-evaluation of all p e r t i n e n t results. I n t h e 1961 revision no previously accepted values for atomic weights were changed, except for t h e systematic changes caused b y t h e adoption of t h e C 1 2 scale, without strong evidence t h a t t h e results of newer work were more re liable. This was in keeping with a long-standing policy of t h e I n t e r n a t i o n a l Commission on Atomic Weights. Because of the n e w scale a n d because of t h e critical re-examination of t h e atomic weight of every element, t h e 1961 Table of Atomic Weights is believed to be a more reliable compilation of these i m p o r t a n t constants t h a n a n y t h a t preceded it. T h e purpose of this article, written in conjunction with t h e article b y A. E . Cameron in t h e preceding issue of this Journal, is t o discuss the aspects of t h e 1961 Table t h a t should m a k e it more useful to chemists, to show what its limitations are, a n d t o point out possible means of further improvement. T h e discussion of some of t h e classical chemical work on atomic weights t h a t is involved in this t r e a t m e n t m a y have some incidental interest for analytical chemists. One of t h e factors contributing t o t h e reliability of the 1961 Table was t h e change from t h e element oxygen t o the nuclide C 1 2 as t h e reference species for t h e assignment of atomic weights. This change h a s removed the slight a m biguity of t h e former scale, which was caused b y n a t u r a l variations in t h e isotopic composition of oxygen. Because of VOL 35, NO. 3, MARCH 1963
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23 A
REPORT
FOR
ANALYTICAL
CHEMISTS
J. H. E. Mattauch ( 1 8 9 5 ) is Director of the Max Planck Institut fur Chemie at Mainz. After Aston he became the foremost practitioner of mass spectroscopy for the determination of atomic masses. He took the lead in persuading physicists to adopt the carbon-12 scale
this variation the atomic weight of oxygen varies within a range of about 1 p a r t in 100,000. So long as the atomic weight of another element was referred to t h a t of oxygen, it was obviously impossible to fix it with any higher accuracy, no m a t t e r how precise the measurement itself might be. This was not a m a t t e r of concern when atomic weights were derived either from chemical ratios or from gas density measurements. Neither of these procedures was capable of yielding values of higher accuracy. Neither did it m a t t e r for atomic weights derived from mass spectrometry if isotopic abundance measurements were involved, except in the few instances of elements for which calibrated abundance measurements were available. There are, however, 21 elements t h a t do not have natural isotopic forms. By reason of comparatively recent improvements in the physical determination of the atomic weights (more properly atomic masses) of single nuclides, the atomic weights of these 21 elements can be as24 A
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ANALYTICAL CHEMISTRY
F. W. Aston ( 1 8 7 7 - 1 9 4 5 ) was best known as the founder of mass spectroscopy and made many contributions to the early work on atomic mass measurements and isotopic abundance determinations. He introduced the scale of atomic masses based on oxygen-16
signed within limits of uncertainty much smaller t h a n 1 p a r t in 100,000. Highly consistent data on nuclidic masses have become available only recently. The two independent techniques, mass spectrometry and nuclear conversions, used for determining nuclidic masses have been used by groups of scientists of radically different interests. Neither group paid much attention to the results obtained by the other. Thus there existed two parallel sets of data on nuclidic masses which did not always agree within their supposed respective uncertainties. There was no agency comparable to the International Commission on Atomic Weights t h a t was responsible for resolving such differences or for choosing "best" values. This unfortunate situation was changed in 1960 by the a p pointment of a Commission on Nuclidic Masses, as an agency of the I n t e r n a tional Union of Pure and Applied Physics. Simultaneously there was a voluntary effort to reconcile the existing data which resulted in the first p u b -
A. O. Nier ( 1 9 1 1 ) is Head of the Department of Physics at the University of Minnesota. He has been one of the most productive contributors to mass spectroscopy, both in the determination of atomic masses and of isotopic abundances. He was the first to propose the carbon-12 scale
lication of a Table of Nuclidic Masses (2> in which all sources of information were critically evaluated and combined. A revision of this Table appeared in 1961. DEGREES OF ACCURACY
Thus there became available, just in time for the 1961 revision, highly reliable values for the atomic weights of 21 elements for which no naturally occurring isotopes have been discovered. I n the 1961 Tabic these atomic weights are given with a minimum of five digits, which means t h a t the values can be regarded as accurate to within one p a r t in 100,000 or better. This was actually a conservative use of the data in the Table of Nuclidic Masses, b u t the number of digits given is obviously adequate for any present demands of chemical work. The 21 mononuclidio elements arc listed in Table I, together with their 1961 atomic weights. I t is clear from Cameron's discussion in the preceding paper of this two-
I
REPORT
G. P. Baxter ( 1 8 7 6 - 1 9 5 3 ) was for several years associated with T. W. Richards and succeeded him as head of the atomic weights laboratory at Harvard. He introduced several new chemical techniques and also made excellent measurements of the den sities of neon and other gases
p a r t series t h a t fixing atomic weights with an accuracy of about one p a r t in 100,000, if isotopic abundance measure ments are required, presents a very different problem from the determina tion of nuclidic masses. I n his paper D r . Cameron has rendered a very useful service to chemists by so clearly analyz ing both the possibilities of the mass spectrometric technique and the nature and approximate magnitude of the errors t h a t are likely to be encountered in its use. The effect of these errors on the determination of an atomic weight varies with the number of isotopic species of which an element is com posed and with their relative abun dance. If only minor amounts of one or more isotopes are present, atomic weights can be calculated with ac curacies approaching those of the mononuclidie elements. Seven such elements may be noted. They are hydrogen, helium, carbon, nitrogen, oxygen, va nadium, and tantalum. (For the lighter of these elements the useful ac curacy is likely to be limited by natural
T. W. Richards ( 1 8 6 8 - 1 9 2 8 ) was Pro fessor of Chemistry at Harvard Uni versity. He was best known for re fining the procedure for determining atomic weights by comparing soluble chlorides and bromides with silver but also made other notable contri butions to chemistry. He received the Nobel Prize in 1914
variations in isotopic composition.) The accuracy of the evaluation of an atomic weight from isotope abundance measurements falls off with increasing complexity of isotopic composition. For about two thirds of the elements, the attainable accuracy of the atomic weight is markedly decreased unless the isotopic abundance measurements have been calibrated. Cameron has pointed out t h a t this is best done by compari son with similar measurements of care fully prepared synthetic isotopic mix tures closely approximating the natural element in composition. Thus far, iso topic abundance measurements so cali brated have been made for boron, nitrogen, chlorine, argon, chromium, silver, and uranium. F o r this group of elements and the seven mentioned pre viously as having only minor propor tions of isotopic forms, we thus have atomic weights of an assured high de gree of accuracy. These elements are listed in Table I I , together with bro mine, whose atomic weight has been determined by chemical methods with Ν
FOR
ANALYTICAL
CHEMISTS
O. Honigschmid ( 1 8 7 8 - 1 9 4 5 ) was Professor of Chemistry at the Univer sity of Munich. He worked for a time with Richards at Harvard and then returned to Munich. With his stu dents he conducted much experi mental work of very high accuracy. His last paper was published post humously in 1947
TABLE I The Mononuclidic Elements Element Aluminum Arsenic Beryllium Bismuth Cesium Cobalt Fluorine Gold Holmium Iodine Manganese Niobium Phosphorus Praseodymium Rhodium Scandium Sodium Terbium Thulium Thorium Yttrium
Atomic Weight 26.9815 74.9216 9.0122 208.Θ80 132.905 58.9332 18.9984 196.967 164.930 126.9044 54.9380 92.906 30.9738 140.907 102.905. 44.956 22.9898 158.924 168.934 232.038 88.905
VOL. 35, NO. 3, MARCH 1963
·
25 A
S'S
REPORT FOR ANALYTICAL CHEMISTS
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comparable accuracy (see the 1961 Report). T h e discussion to this point covers 35 elements whose assigned atomic weights can be considered, with a sub stantial degree of confidence, to have uncertainties ranging from not more t h a n three p a r t s to less t h a n one p a r t in 100,000. I t is much more difficult to make quantitative estimates of the reliability
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Polynuclidic Elements with Atomic W e i g h t s of H i g h A c c u r a c y Element Atomic Weight Argon 39.948 Boron 10.811 ± 0.003" Bromine 79.909 ± 0 . 0 0 2 » Carbon 12.01115 ± 0.00005" Chlorine 35.453 ± 0 . 0 0 1 " Chromium 51.996 ± 0.001'· Helium 4.0026 Hydrogen 1.00797 ± 0.00001" Nitrogen 14.0067 Oxygen 15.9994 ± 0.0001" Silver 107.870 ± 0 . 0 0 3 ' Tantalum 180.948 Uranium 238.03 Vanadium 50.942 " Natural isotopic variation. 6 Experimental uncertainty.
Grade Capacity meq/g Standard 20 0.3 (Epichlorohydrin tnethanol amine) 40 Separation and purification of viruses.
of the atomic weights assigned to the remaining 48 elements. Such an esti mate was made for iron in the 1961 R e port. For this element the value given is 55.847, with an uncertainty of ± 0 . 0 0 3 , which is probably justified b y the agreement between the results of chemical and mass spectrometric meas urements. There are fourteen other elements for which existing chemically and physically derived values are in rather good agreement. T h e mutual support given by the results of the two procedures suggests t h a t the last digit of the officially adopted value can be accepted with some confidence. These elements are listed in Table I I I . For the remaining 33 elements, which are listed in Table IV, the accuracy of the values chosen for the 1961 Table is somewhat more uncertain and probably variable. Differences between the re sults of available chemical and physical measurements were, in general, larger t h a n the a p p a r e n t uncertainty of either. Choices therefore had to be made between them. For six of t h e elements—cadmium, lithium, selenium, tellurium, tin, and zinc—(here were de terminations by at least two inde pendent chemical methods that agreed with each other more closely t h a n with the physically derived value. In t h e case of lead, most isotopic abundance work has been aimed at studying varia tions in the composition of the element. The chemical value was chosen as the
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ANALYTICAL CHEMISTRY
Physical and Chemical Values in Good Agreement* Element
Chemical Value
Physical Value
1061 Table
Antimony Barium * Calcium Cerium Copper Gallium Iron Lanthanum Lutetium Mercury Rubidium Strontium Sulfur Thallium Zirconium
121.75 137.35 40.082 140.114 63.537 69.72 55.845 138.91 174.97 200.59 85.473 87.63 32.064 204.37 91.21
121.76 137.34 40.078 140.12 63.548 69.72 55.847 138.905 174.97 200.61 85.468 87.616 32.064 204.38 91.22
121.75 137.34 40. OS 140.12 63.54 69.72 55.847 ± 0.003» 138.91 174.97 200.59 85.47 87.62 32.064 ± 0.003» 204.37 91.22
* There are also a number of instances of good agreement between chemical and physical values among the mononuclidic elements. " Natural isotopic variation. b Experimental uncertainty.
REPORT FOR ANALYTICAL CHEMISTS
one representing the lead most likely to be encountered in normal laboratory work. Two series of isotopic abundance measurements on molybdenum differed too much to justify displacement of the previously accepted chemical value. The Commission also decided that the chemically derived values for titanium and germanium should be retained, principally on the basis of not displacing previously accepted values without convincing evidence. Cameron has discussed the germanium case in detail in his paper. Neon is the only element in this group whose atomic weight is based on gas density measurements, at one time the only method applicable to the noble gases. Of the remaining 22 elements there are only 5—europium, magnesium, neodymium, potassium, and ytterbium—whose atomic weights were based on physically derived values for the first time in the 1961 Table. This change has been made for the other 17 in earlier revisions of the Table. Among these 17 were a number whose chemically derived atomic weights had large errors, most probably resulting either from undetected impurities in the substances used or unsuspected abnormalities in their stoichiometric composition. One of the important contributions of mass spectrometry in its earlier years was to reveal errors of this kind. Figure 1 of Cameron's pa-
per shows discrepancies between chemical and physical values in the range of 0.1 to 0.3 percent for nine elements. None of these remained for correction in the 1961 revision. MEANS OF IMPROVEMENT
It is not possible to make a general estimate of the uncertainties in the atomic weights of the elements listed in Table IV beyond saying that they probably do not exceed 0.05 percent and in a number of cases may be no more than 0.01 percent. Whether such uncertainties are important depends on the point of view. One can hardly argue that every atomic weight should be known to an accuracy of 1 part in 100,000, or even that it matters very much if some of them are uncertain to the extent of several parts in 10,000. However, the general progress of science depends in no small measure on progressively better knowledge of physical constants. For this reason it would be desirable to seek better values for the atomic weights of the elements listed in Tables III and IV. The remainder of this article will deal mainly with the conditions that must be met if significant improvements are to be realized for these elements. Cameron has said that "the comparatively ready availability of separated isotopes makes it possible to correct for the errors inherent in iso-
TABLE IV Elements with Atomic Weights of Lower Accuracy Element Cadmium Dysprosium Krbium Europium Gadolinium Germanium Hafnium Indium Iridium Krypton Lead Lithium Magnesium Molybdenum Xeodymium Neon Nickel
Atomic Weight 112.40 162.50 167.26 151.96 157.25 72.59 178.49 114.82 192.2 83.80 207.19 6.939 24.312 95.94 144.24 20.183 58.71
Source* C M M M M C M M M M C C M C M D M
" Natural isotopic variation * Chemical Determination—C Mass Spectrometry—M (ias Density—I)
THE ICE AGE IS OVER...
Element Osmium Palladium Platinum Potassium Rhenium Ruthenium Samarium Selenium Silicon Tellurium Tin Titanium Tungsten Xenon Ytterbium Zinc
Atomic Weight 190.2 10G.4 195.09 39.102 186.2 101.07 150.35 78.96 28.086 ± 0.001" 127.60 U S . 69 47.90 183.85 131.30 173.04 65.37
Source M M M M M M M C M C C C M M M C
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topic abundance measurements by the use of synthetic isotopic mixtures having very nearly the composition of the natural element." He adds that this "procedure . . . is, in many respects, hardh7 less exacting than the determination ot an atomic weight by chemical methods." At the risk of being presumptuous this precautionary statement may be expanded somewhat. One of the great advantages of mass spectrometry is that the measurements are not likely to be affected by chemical impurities. Impurities, other than those that are isobaric with the nuclides being measured, are separated and discarded by the instrument. For the same reason the stoichiometry of the substances used is of little importance. However, in synthesizing an isotopic mixture the composition of the compounds used in the mixture must be accurately known, both chemically and stoichiometrically. Thus it is necessary to make sure that pure isotopic preparations have adequate chemical purity. It is necessary also to convert them to reliable weighing forms in order to make sure that the synthesized mixture will contain the isotopes in the desired proportions. If separated isotopes of requisite isotopic purity are available, and if the proper preparation of the synthetic mixture can be assured, improvements in atomic weights can probably be realized most easily by mass spectrometry. The usefulness of the procedure has been recently demonstrated with silver (3> and chlorine, two elements that had been much studied chemically but whose atomic weights were nevertheless somewhat in doubt. The costs of separated isotopes are usually high, especially if the preparations have a high level of isotopic purity. They must be available in sufficient quantities for the necessary chemical purification, followed by conversion into weighing forms suitable for synthesizing a mixture of accurately known isotopic proportions. The discrepancy between the chemical and physical values for the atomic weight of germanium is of particular interest in view of Dr. Cameron's detailed analysis of the results of several isotopic abundance measurements for the clement, from which he concludes that the value 72.63 cannot reasonably be in error by as much as the difference between this and the chemical value, 72.59. The writer would not presume to question this conclusion, especially since the averages of five series of completely independent abundance measurements are reasonably concordant. However it is also difficult to under-
stand why the average of the eight series of chemical determinations should be in error by as much as 0.04. Both the chloride and the bromide of germanium were compared with silver and the respective silver halides. The work was done in two laboratories, using totally different starting materials, and showed an extreme range of 0.027. Review of the publications did not disclose likely sources of error large enough to account for the discrepancy. If the discrepancy is to be resolved by absolute abundance measurements, there will have to be provided at least two, and preferably all five, of the natural isotopes of germanium at a high level of isotopic purity. This will be a formidable and costly task. Of the ten elements for which atomic weights arc based on chemical determinations, seven besides germanium exist in nature in from 5 to 10 isotopic forms. For this group as a whole there would seem to be little prospect of getting better values soon from mass spectrometry. It is therefore desirable to consider whether new and useful evidence can be obtained from chemical determinations or, alternatively, from measurements of the density and lattice dimensions of crystals (see later discussion). At the outset, it seems reasonable to say that in any new work on atomic weights the accuracy target should bebetter that 1 part in 10,000 and, if possible, approach 1 part in 100,000. Such accuracy was attained in some of the earlier chemical work. Present-day chemical procedures have numerous refinements and other advantages over those in use a generation ago. Perhaps the greatest advantage is knowing that the atomic weights of the 21 mononuclidic elements now provide a network of reliable landmarks in a territory where once nothing was completely fixed except the defined atomic weight of oxygen. Among others may be mentioned more powerful techniques for preparing pure substances and for evaluating their purity by physical methods and by trace analysis. In reviewing the earlier publications one does not find reference to good fractionating columns, to cryometric methods for measuring purity, to zone refining, to the preparations of solids in the form of single crystals, or to the examination of solids for stoichiometric imperfections by the techniques now employed in research on the solid state. To be as sure as possible of attaining a sufficiently high level of purity, the earlier workers often were forced to much more time-consuming and laborious operations than would be necessarv todav
to accomplish the same ends. Because of these considerations, there is every reason to believe that the quality of the classical work on atomic weights could be equalled or exceeded today. THE SILVER RATIOS
The chemical procedure that has been used most commonly for deriving atomic weights is to determine the equivalent weights of silver and ionizable chlorides or bromides. This was done by mixing solutions containing nearly equivalent amounts of the reactants and titrating to the equivalence point. This was often followed by col lecting and weighing the precipitated silver halide. Frequently both the chloride and the bromide of an element were used. This practice provided a useful check on the purity of the com pounds, since it was unlikely that sig nificant amounts of impurities in one or both would cause errors that would compensate in such a way as to yield identical values for the atomic weight. The silver comparison had its origin well back in the nineteenth century. Early in the twentieth century T. W. Richards, at Harvard, pioneered in its refinement. One of the important re finements was the use of an ingenious "bottling apparatus" which permitted substances to be transferred for weigh ing without access to the atmosphere. The other was a specially designed "ncphelometer" for measuring the rela tive turbidities produced when equal portions of the mother liquor resulting from the precipitation of silver chloride or bromide were treated with small, equivalent amounts of silver and halide ions, respectively. Unequal turbidities showed that the mother liquor con tained either unreacted halide or silver ions and permitted a close estimate of the imbalance. After the requisite amount of deficient ion had been added to the reaction vessel and time had been allowed for a new approach to equilib rium, the nephelometric procedure was repeated. Equal opalescence in the two test portions was considered to mark the attainment of exact equivalence of the reactants. We owe the highest esteem to the skill, learning, and dedication the great figures in atomic weight work brought to their task. Richards' work was so highly regarded that he received the Nobel Prize for Chemistry in 1914. Richards' principal associate was G. P. Baxter who even more than Richards devoted his whole career to atomic weights. Richards also had a dis tinguished German disciple in the per son of O. Honigschmid, who came to the United States to learn the "Harvard
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method" and then returned to Munich to spend the remainder of a very pro ductive life in the same career. A. F. Scott
