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Atomic Weight of Calcium, History

Approximate Atomic Weight

The equivalent of calcium is approximately 20. The most appropriate position for calcium in the Periodic Table indicates a valency of two, and, therefore, an atomic weight of about 40. The specific heat determinations of Eastman and Rodebush give, a little over 0.16 for calcium at the ordinary temperature, and assuming, according to Dulong and Petit's Law, an atomic heat of about 6.4, an atomic weight of 40 is again obtained.

Exact Atomic Weight

Berzelius, in 1809, obtained 7.75 grm. of silver chloride from 3.01 grm. of fused calcium chloride, giving 40.44 as the atomic weight of calcium. In 1841 Baup analysed a series of organic salts of calcium, and obtained a mean value of 249.845 (O = 100), or 39.98 (O = 16). Dumas calcined very pure Iceland spar, and compared the ratio calcium carbonate to calcium oxide, obtaining as percentage of oxide 56.12, 56.04, and 56.06, which gave a mean value of 40.17 as the atomic weight of calcium. The same method was adopted by Erdmann and Marchand in their first publication. A precipitated carbonate was also calcined, and the value 40.0 obtained. At the same time pure Iceland spar was transformed into sulphate, and from the change in weight gave 40.021 as the atomic weight.

Most of the later investigators adopted modifications of these processes, and their results are summarised on the following page: -

The work on which depends the value at present accepted for the atomic weight of calcium is that of Richards and Honigschmid. As early as 1899 Richards communicated to the American Association for the Advancement of Science his first experiments in this direction, and published the work in 1902. He obtained anhydrous calcium chloride by fusing in a current of dry hydrochloric acid, dissolved it in water, and weighed the chlorine as silver chloride. The mean value of the ratio 2AgCl: CaCl2 from five experiments was 100: 38.7195, giving an atomic weight of 40.084.

In 1910 and 1911 Richards and Honigschmid published the results of experiments carried out with precautions calculated to ensure a very high degree of accuracy. The original papers should be read. The ratios measured were: calcium bromide to the silver required to precipitate it, determined by titration with silver nitrate; calcium bromide to the weight of silver bromide obtained from it; and calcium chloride to the silver required to precipitate it. The salts were prepared by the action of hydrobromic or hydrochloric acid on calcium carbonate. The purest initial materials were employed. The final stages in the purification of the products were conducted in platinum or quartz vessels, and during the operations of transference and evaporation the substances were very carefully protected from all dust or contaminating vapours.

Six measurements of the ratio CaBr2: 2Ag gave a mean value of 92.6502: 100, and hence the atomic weight of calcium is 40.0701±0.0007. Six determinations of the ratio CaBr2: 2AgBr gave a mean value 53.22323: 100, the atomic weight is therefore 40.0699.

The authors suggested 40.07 (Ag = 107.880) as a minimum value for the atomic weight of calcium owing to the instability of calcium bromide and the tendency to form a basic salt. The greater stability of the chloride was the reason for the selection of the third ratio CaCl2: 2Ag. Seven experiments gave a mean value = 51.44054: 100, resulting in an atomic weight 40.077, and probable error 0.0004. They suggested 40.075 as the most reliable value. The International Commission on Atomic Weights for 1925, however, have adopted the value

Ca = 40.07.

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