BERZELIUS-PIONEER CHEMIST
ATOMIC WEIGHT
WILLIAM MARSHALL MacNEVIN The Ohio State University, Columbus, Ohio
BERZELIUS made the determination of atomic weights the principal work of his life. To the modern analytical chemist this appears a monotonous, uninterestinz task. But when one surrounds Berzelius' work on atomic weight vith the fertile and rapidly growing atmosphere of fundamental chemical theory which depended upon atomic weights, one sees in Berzelius' activity a research task that would certainly challenge an alert mind. A similar interest claimed t,he major attention of the American chemists T. W. Richards and G. P. Baxter. According to Berzelius' own report, he first became interested in atomic weights in about 1807. Like many professors, he felt the necessity for writing a textbook. In it he discusssd the latest work of Jeremias Benjamin Richter (1762-18071, proposing the idea of definite proportions. Richter's work was severely criticized. Berzelius, feeling that Richter's ideas were nevertheless correct, set forth to obtain more extensive analytical data. He ssys: "I determined then to prove them [Richter's ideas1 factually hv correct analvses. This i&estigation became the dasisfor the direction of my scientific endeavours during the greater part of my most active y e u s of life, i. e., for my work on chemical proportion.'' Some idea of Berzelius' zeal for atomic weight determinations is found in his autohiogaphical sketch ( I ) , where he tells about his first experiments. .
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My first experiments .' . . did not turn out well. I had as yet no experience either of the great accuracy necessary in the methods of weighing or how the great accuracy might he obtained. I also lacked instruments and tools far delicate experimentation. There wasonly one platinum crucible in Sweden, which was owned by Hisinger. I n friendship he placed it a t my disposal but i t was too heavy for the balance. I frequently was misled because of the of bringillg an insoluble precipitated compound to its correct eaturntion point. 1 had to repeat analyses many times in order to find the method which would lead to correct results: in short I must, through my own mistakes, be led into procedures which are now so generally known that all immediately turn to them, and I must stacd by the principle, that of so choosing my analytird method that the result would depend as little ss ~ossiblean the ooerator's skill in mrtnioulation.
In order to appreciate the significance of Berzelius' work it is necessary to remind the reader that Dalton had deduced his atomic theory about the years 180208. He had propounded the idea of combining weights or relative atomic weights based on hydrogen as unity. Richter, some years before, had arrived a t his "law of progression" which attempted to establish a relation between combining weights of bases and acids. Richter established a series of "ratios" which he proposed to
use for the calculation of combining amounts. Berzelius was unfamiliar with the Daltonian developments until the time of their publication. I t has already been pointed out that Berzelius did know about Richter's work and it was this that influenced Berzelius to pursue his independent study of atomic weights and their significance to chemical theory. I t should he pointed out that the Daltonian system of symbols for designating atoms and molecules was in vogne at this time. I t is to Berzelius that we owe the suggestion of the present system of symbols consisting of abbreviations of Latin names. Also lacking at the time was a clear understanding of the source of the mass in a molecule. The old phlogiston theory still held sufficient grip on men's minds that they were willing to concede that a large part of the mass of a molecule might be energy. I t was only when Dalton made a firm statement that the molecular weight is the svm of the atomic veights that real progress began. EARLYATOMIC
Thus we see that Berzelius began his work a t a time which was difficult because of the uncertainties in what are now simple and accepted truths. On the other hand, the time was ripe for the firm establishment of the new ideas, but it was very apparent to Berzelius that this could only he accomplished on the basis of sound analytical measurements. Attempts were made before Berzelins to set up tables Of atomic ~~l~~~~~ ($1 (1808) mere really cornbilling weights based on hydrogen as unity. W0llast0n (51 in 1813 published a more elaborate table in which some of the errors of Dalton's fignres were reduced. ~t is interes+,ingto note that wollaston introduced the term equivalent weight in connection with this table. Wollaston used oxygen as a basis and called it 10. Berzelius' values were more extensive and more exact. ~h~~ represented also a development in chemical Wollaston had "'led the atomic weight hvdrozen - - that weight which combined with 10 oarts of oxygen. I n the meantime Gay-Lussac had published his work on the combining volumes of gases. Berzelius was therefore aware of the fact that two volumes of hydrogen combine with one of oxygen and reasoned that two atoms of hydrogen combine with one of oxygen. He therefore halved the value which Wollaston had given to hydrogen. This is said to be the point in historical development a t which the distinction between atomic and equivalent weight was first recognized. Berzelius' use of the gas-volume theory led to erroneous conclusions
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in some cases. For example, he observed that one volume of chlorine combines with one volume of hydrogen to produce two volumes of hydrogen chloride. Berzelius thought he was getting a single molecule of hydrogen chloride and therefore was puzzled over its apparent double size. Beraelius used the term "atomic weight" to include what we now mean by molecular weight. He spoke of the "atomic weight of barium sulfate." Much of his so-called atomic weight work dealt with the composition of compounds. For example, from an analysis of barium sulfate for SOs there conld be calculated the atomic weight of barium if one assumed values for sulfur and oxygen. The volume 01 work performed by Berzelius in this k i d of determination of combining proportions is prodigious. Data exist for about 2000 such investigations. Some of Berzelius' experiments had as a definite purpose the determination of atomic weights of the elements. Approximately 45 research papers on atomic weights appear in the literature, although most of the 2000 researches in one way or another bear upon the evaluation of atomic weights. Some qualities of Berzelius are apparent from a study of his atomic-weight work. He gave much thought t o the planning of his experiments, so much that rarely did he feel it was necessary to carry out more than two or
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~ a n 1 . r1 Atomic Weights of Hydrogen Based on O/H Ratio nulonennd Rpndina -~.---.-
1x21 . .-.
Dumas Erdmanand Marchand Thornsen Coake and Richards Keiser Keiser Rayleigh Noyes Dittmar and Henderson Morlev Thomien Keiser Noyes
1842 1842 1870 1887 1887 1888 1889 1890 1890 1895
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1907
1 00RR7
1.00783
three determinations, frequently only one. The selection of the proper method of analysis seemed far more important to him than the frequent repetition of the measurement common today. Much preliminary experimental work went into the testing of various methods and the selection of the proper one for the final measurement. Berzelins was thoroughly confident of the reliability of his work. He seldom repeated any of it once completed and was ready to defend its reliability. If the spirit of Berzelius is still abroad it must certainly be much disturbed by the statement that practically none of Berzelius' atomic weight values agree well with modern ones. They are acceptable, however, in view of the pioneer character of his work. I n many cases they had great effect upon the new chemical theory and these effects would not have been any d i e r e n t had the resnlts been more accurate.
Berzelins apparently did not apply vacuum corrections. The lack of high precision in his results, owing partly to the absence of numerous repetitions, makes vacuum corrections less significant. These are important to remember, however, when one attempts to correlate his with modem data. REACTIONS USED BY BERZELIUS
The reactions used by Berzelius as a basis for his atomic weight calculations can be arranged in several groups. 1. Reduction of Oxides. The metal oxide was reduced with hydrogen. The ratio of metal to metal oxide was thereby ohtained and the atomic weight of the metal could be referred to oxygen. In this way, the atomic (or combining) weights of copper (4), lead (6),tungsten (6), and iron (7) were determined. Vanadic oxide (8) was also reduced to vanadous state. The results ohtained indicate the probable impurity of phosphoric acid in the original vanadium salt. It is interesting to note that Beraelius conld not have tested for impurities of phosphate because the molybdate test had not yet been discovered. The reduction of metal oxides was also used for establishing the ratio between oxygen and hydrogen. Beraelius, together with Dulong (9),made the first important measurements of the O/H ratio. They used the incomplete reduction of copper oxide with hydroeen. The oxvzen eonaled the loss inweight of the CODper oxide. %e i a t e r formed was collected a i d weighed. Three experiments were performed. The O/H ratio found wa8 16.031 t o *0.05?. Berzelius did not apply vacuum corrections. F. W. Clarke (10) has since made these corrections and recalculated the Dulong and Berzelius ratio a t 15.894 i 0.057. It is interesting t o note that this value is far closer to the present value than anyone was able to get for quite a few years. Table 1 shows that Berzelius was definitely ahead of his time in the reliability of his results. 2. Oxidation of Metals. Berzelius tried a few estimations of atomic weights by determining the gain in weight of a metal upon oxidation. .Tin (4), antimony (6), tellurium (11), and tungsten (6) were studied. The data on tungsten are interesting. One experiment was done in which 676 parts by weight of tungsten produced 846 parts of oxide. The ratio of W/W03 found was 0.79905. Upon reduction of W08, the ratio found was 0.79644. From the mean of these two ohservations he calculated W = 189.324. The present-day value is 183.92 and subsequent investigators have eoneluded that Berzelins probably had an alkali-contaminated sample. These results are included here because they illustrate the rather large errors that occur in some of Berzelius' values. 3. Decomposition by Ignition. This has long been a popular method for getting the proportions of oxygen in an alkali chlorate. From such data the atomic weight of the metal or of the chlorine can be calculated. Earlier estimates of the ratio KC1/KCIO1 had been attempted hut Beraelius was able t o show that none
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APRIL, 1954
was reliable because some KC1 was always carried away in the vapors. Berzelius recovered and corrected for i t and found (6) in four experiments a ratio for KCl/KClOa = 0.60851, which agrees fairly well with the present accepted value of 0.608365. It is interesting to note his precision in the four analyses: 0.60854, 0.60850, 0.60850, and0.60851. The decomposition of metal sulfates was also used. Aluminum sulfate (4) was ignited t o the oxide. A single experiment was performed. Ten grams of aluminum sulfate gave 2.9934 grams of oxide, from which he calculated Al = 27.30. The present value is 26.97. The atomic weight of boron (6) was also calculated from data on the dehydration of borax. Three determinations of the per cent of water gave 47.10,47.10 and 47.10, from which boron was calculated t o he 11.014. The present value is 10.82. 4. Gavimetrie Precipitations. Berzelius made frequent use of the reaction between silver and chloride. He determined the ratio AgCI/Ag in three experiments (19). I n the first he dissolved 20 g. of silver in nitric acid, precipitated with hydrochloric acid, filtered, dried, and weighed. I n the second and third the silver chloride suspension was evaporated to dryness, fused, and weighed without transfer. The ratios AgCl/Ag found were 1.32700, 1.32780, and 1.32790; average 1.32757. The modern value is 1.328640. Berzelius (6) also determined the ratio KCI/Ag. One experiment gave the ratio KCl/AgCl = 0.51997. Richards and Staehler (13) much later found 0.520118. Berzelius made use of this reaction to determine the atomic weights of a number of metals in metal chloride salts. Excess silver nitrate was added and the AgCl filtered and weighed. In this way values for barium (4), calcium (I$), and manganese (16) were determined. In the manganese work, two experiments were done and the ratio MnC12/2AgCI found t o be 0.43945 and 0.43950, from which Mu was calculated to be 55.07 (present value 54.93). It is interesting to note here that much later Baxter and Hines (16) got 0.43898 for this same ratio. The precipitation of barium sulfate was also used as the basis of several determinations. The atomic weight of barium (4) was first estimated a t 135.60 from two experimental determinations of the ratio BaSOJBaCI,. Lead chromate was also used as a precipitating form in the evaluation of chromium (17). Berzelius encountered much difficulty with the coprecipitation of alkali chromates and with the solubility of lead chromate. 5. Displacement Reactions. Berzelius (18) converted silver iodide to chloride by heating it in chlorine. The ratio found by him for AgI/AgCl is 1.63326. Subsequent values by other investigators show a higher value, which indicates the incomplete displacement of the iodide. Another displacement reaction used was that between calcium fluoride (6) and sulfuric acid. Three experiments were done and fluorine was calculated to he 18.86 (present value 19.00). 6. Addition Reactions. Berzelius first determined
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the atomic weight of calcium (14) from the ratio CaC12/ 2AgCI. The value ohtained proved to be erroneous and in 1843 he sought a new reaction for this purpose. He converted CaO to CaSOI and ohtained (19) the ratio SOs/CaO = 1.423998, from which he calculated Ca = 40.225. 7. Reduction of Noble Metal Complexes. Berzelius carried out a large amount of work on the metals of the platinum group. He had already reduced platinons chloride with hydrogen and was the first (4) to report an atomic weight for platinum (194.69). Later ($0) he reduced the double salt, K2PtC16 in hydrogen. He determined the HC1 volatilized, the KC1 remaining and the weight of platinum metal. From these results he calculated P t = 197.10. He applied this same reaction to determinations of osmium (?I), iridium ($$), rhodium (%), palladium (B),and gold ($4). These results are summarized and compared with modern values in Table 2. TABLE 2 Berzelius' Atomic Weights of Noble Metals Element
Berrelius' findinos
Values in 1949
8. Miscellaneous Reactions. Berzelius made use of an ingenious reaction in the determination of the atomic weight of gold. A weighed amount of mercury was allowed to react with an excess of gold chloride ($4) until all the mercury had reacted. The weight of gold deposited was equivalent to the mercury dissolved. Phosphoms was also used (25) in place of mercury as the reducing agent. Other elements whose atomic weights were determined by Berzelius were Li, S, Mg, Ti, Mo, G1, Zn, Si, Zr, Th, As, Ta, U, and Se. From this discussion it will he wen that Berzelius' contribution was enormous. Much of its significance lay in the better establishment of chemical theory. Some of his analytical determinations, especially the O/H, Ag/Cl, Mn/Cl, and Au/C1 ratios agree remarkably well with modern values. In other cases, the agreement is very poor. His recognition of the difference between equivalent and atomic weights is outstanding. The reports of his work in the literature stimulated others, and men like Marignac, Pront, Davy, Wollaston, Gay-Lnssac, Dulong, Mitscherlich, and Dumas went on t o make their contribution to chemical science through atomic weight determinations. Berzelius has been vigorously criticized by many who followed. Dumas (96) was the most vigorous because his value for carbon, 6.0, disagreed by so much with Berzelius' value, 6.12. It is easy to condemn Berzelius' values. But we must realize that so much chemical theory which we take for granted today was either
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unknown or was uncertain and the subject of controversy. Consequently, it was hard to get a starting point. We must therefore give much credit to the man for his clear insight into the significance of his work and for his recognition of the true basis of chemical proportions. LITERATURE CITED (1) BERZELIUS, J. J., "Autobiographical Notes," trans. by OMF LAMELL,Williams & Wilkins Co., Baltimore, 1934, p. 63. (2) DALTON,J., "A New System of Chemical Philosophy," publi~hedby S. Russel for R. Bickerstaff, London, 1808. , H., Ann. Chim., 90, 1138 (1814). (3) W O L U S ~ NW. (4) BERZELIUS, J. J., Poggend. Annala, 8 , 177 (1826). (5) BERZELIUS, J. J., "Lehrbuch der Chemie," 5th ed., Vol. 3, Arnold Leipzig, 1845, p. 1218. (6) BERZELIUS, J. J., Poggend. Annalen, 8, 1 (1826). (7) BERZELIUS, J. J., Bern. Jahresberickt, 25, 43 (1844); Ann. Chem. Phmm., 30,432 (1839). (8) BEnzeL~us,J. J., Poggend. Annalen, 22, 14 (1831). (9) DUMNG,P. L., AND J. J. BERZELIUS, Annals of Philosophy, Thomson, 16, 50 (1820).
JOURNAL OF CHEMICAL EDUCATION (10) CLARKE, F. W., Phil. Mag., (3), 20, 341. (11) BERZELIUS, J. J., Poggend. Annalen, 28, 395 (1833). (12) BERZELIUS,J. J., Annals of Philosophy, Thomson, 15, 89 (1820). . . (13) RICHARDS, T. W., AND A. STAEHLER, Carnegie Inst. Wash. Publ., No. 69, 1907, p. 7. (14) BERZELIUS, J. J., Poggend. Annalen, 8,189 (1826). (15) BERZELIUS, J. J., "Lehrbuch der Chemie," 5th ed., Vol. 3, 1845, p. 1224. (16) BAXTER,G. P., A N D M. A. HINES,J . Am. Chem. Soc., 28, 1560 (1906). J. J., Schweigg. J., 22, 53; Poggend. Annalen, (17) BERZELIUS, 8.22(1826). (18) B E ~ L I U J S ,J., Ann. Chim. Pkgs. (2),40,430 (1829). J. J., J . prakt. Chem., 31, 263 (1844); Ann. (19) BERZELIUS, Chem. Pharm., 46,241 (1843). (20) BERZELIUS, J. J., Pogyend. Annalen, 13, 468 (1828). (21) Ibid, p. 530. (22) Ibid., p. 435. (23) Ibid., p. 454. (24) BERZELIUS, J. J., "Lehrbuch der Chemie," 5th ed., Val. 3, 1845, p. 1212. (25) Ibid., p. 1188. (26) DUMAS, J. B., AND J. S. STAS,Ann., 38, 141 (1841).
