Colours of Inorganic Salts

Abstract

No satisfactory explanation of the colours of inorganic salts in the vapour state, in solution, or in crystalline form, has yet been put forward, excepting certain suggestions by Fajans (“Handbuch der Physik”, Bd. 24, p. 564), that the colours are due to the deformation of the cation produced by surrounding anions and molecular complexes. The ideas of Fajans were rather vague, but the time has now come to put forward a more precise hypothesis. It is well known that salts like NaCl, CaCl₂, AlC₃, in which the electrons of the cation form closed shells (p⁶), are colourless or white. Herzfeld found from a study of dispersion of NaCl that there are three ultra-violet absorption bands, one at λ340 which was ascribed to Na⁺, another at about λ1500, which was ascribed to Cl⁻-ion. This last one has been experimentally obtained by Pfund (Phys. Rev., vol. 32) by the Reststrahlen method. The wave-length λ340 ascribed to Na⁺ agrees remarkably well with the resonance line of Na⁺ identified by K. Majumdar (Ind. Jour. Phys., 1927) and Bowen, though definite assignment of values of the absorption wave-length from dispersion data in this region is subject to certain uncertainties.