Bismuth Chromate

The neutral salt is not known. The metal shows an exceptionally marked tendency to form double salts, and a very large number of basic chromates have been described. It has been shown by Cox, however, from an investigation of the conditions under which the chromates of bismuth are capable of existing in solutions of chromic acid, that in accordance with the phase rule only two such compounds, Bi₂O₃.4CrO₃ and Bi₂O₃.2CrO₃, definitely exist. The former yields orange-scarlet monoclinic prisms, decomposed by water and by heat; the latter is an orange-yellow powder, which is sometimes called bismuthyl dichromate, (BiO)₂Cr₂O₇.

It must be assumed, therefore, that most of the basic salts described in the literature are not true compounds, or at least that they do not exist in the system Bi₂O₃-CrO₃-H₂O; however, some may exist in such a system as K₂O-Bi₂O₃-CrO₃-H₂O; for example, the compounds 3Bi₂O₃.2CrO₃ and 3Bi₂O₃.CrO₃ appear to be stable in alkaline solutions.