Atomic Weight of Chromium

From a consideration of the vapour densities of volatile compounds of chromium, and from the application of Dulong and Petit's Law, it is obvious that the atomic weight of chromium is about 52 - that is, three times the chemical equivalent of chromium in chromic salts, or six times its combining weight in derivatives of chromium trioxide. Chromium thus exhibits di-, tri-, and hexa- valency in the chromous salts, chromic salts, and chromates and dichromates respectively.

The methods employed for the accurate determination of the atomic weight of chromium have been concerned chiefly with the analysis of chromates and dichromates, notably those of silver, potassium, and ammonium. In the case of chromium, methods involving precipitation of a halogen from a solution of a halide are at a serious disadvantage owing to the difficulty of complete precipitation. The earliest work of importance was that of Berzelius, who precipitated a known weight of lead nitrate as lead chromate, and deduced an atomic weight for chromium of about 56. This was high because the chromate precipitate carried down traces of alkali salts. Peligot, in 1844, questioned this result, and from analyses of the chlorides and of chromous acetate gave the value 52.5, but did not support it with sufficient experimental data. Many other investigators turned their attention to the subject, but most of the figures obtained are of no modern significance. The most reliable value is that obtained by Baxter and his collaborators, whose experimental work was carried out with all the precautions which modern investigations have proved to be necessary. The work consisted in the analysis of pure silver chromate and of pure silver dichromate. The weighed salt was dissolved in nitric acid and reduced either by sulphurous acid or hydrazine sulphate. The silver was then precipitated by dilute hydrochloric or hydrobromic acid and the halide compound weighed. The mean results obtained were:

From analysis of Ag2CrO4	Cr = 52.008
From analysis of Ag2Cr2O7	Cr = 52.013

Mean value 52.011.

From a consideration of these determinations the atomic weight of chromium is fixed by the International Committee (1925) as 52.01.