In a similar way potassium and ammonium cobalt alums have been obtained.
The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, whilst caesium and rubidium alums are only sparingly soluble.
Recoura, Annales de, Chimie et de Physique, 1895 (7), 4, p. 505.) Chromic sulphate combines with the sulphates of the alkali metals to form double sulphates, which correspond to the alums. Chrome alum, K2S04 Cr2(S04)3.24H20, is best prepared by passing sulphur dioxide through a solution of potassium bichromate containing the calculated quantity of sulphuric acid, K 2 Cr 2 O 7 -}-3SO 2 +H 2 S04 = H20+K2S04 4-Cr2(S04) 3.
Such phenomena are well known in the alums - double sulphates of aluminium with another metal.
A somewhat similar process based on the varying solubilities of the corresponding alums has also been devised by Redtenbacher (Jour.
The iron alums are obtained by crystallizing solutions of equivalent quantities of ferric and an alkaline sulphate.