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The red and blue colours of the cobaltous compounds and the change from one to the other have attracted much attention. As the result of an investigation of the absorption spectra of a number of red and blue cobaltous pigments of known crystal structure and also of a number of red and blue cobaltous compounds in solution, it has been suggested that both in the crystal and in solution the cobalt atom is in association with six other atoms or groups when the colour is red and with four when the colour is blue. Subsequent determinations of the crystal structures of a variety of compounds have confirmed this view. The red colour of an aqueous solution of cobalt chloride is attributed to the ion [Co(H2 O)6 ] and the blue colour with the same substance in presence of concentrated hydrochloric acid to the ion [CoCl4 ]". The transition from red to blue has been followed by measuring the densities, viscosities, refractive indices, surface tensions and electrical conductivities of a series of solutions containing the same amount of cobalt chlorids with increasing concentration of hydrochloric acid and of an exactly similar series containing no cobalt. The differences for corresponding members show marked infection at the same concentration of acid (about 5N) for all the properties (except surface tension, since the concentration in the surface is different from that in the bulk), and clearly indicate that the change is practically complete when the concentration of acid is about 9N. The present investigation confirms these earlier observations and the general conclusions drawn from them, and throws much new light on the changes occurring in solution.

The change in the absorption spectrum of cobalt chloride in aqueous solution with increasing concentration of hydrochloric acid