It forms double salts with the sulphates of the metals of the alkalis, known as the alums (see Alum).
In a similar way potassium and ammonium cobalt alums have been obtained.
A somewhat similar process based on the varying solubilities of the corresponding alums has also been devised by Redtenbacher (Jour.
The bitartrates RbHC 4 H 4 0 6 and CsHC 4 H 4 O 6 have been employed, as have also the alums (see above).
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.
Such phenomena are well known in the alums - double sulphates of aluminium with another metal.
The iron alums are obtained by crystallizing solutions of equivalent quantities of ferric and an alkaline sulphate.