Chromium Arsenates

Chromic Orthoarsenate, CrAsO₄, is formed as an apple-green precipitate when potassium dihydrogen orthoarsenate is added to a solution of a chromic salt.

Chromium Pyroarsenate, Cr₄(As₂O₇)₃, is produced when a small quantity of chromium sesquioxide is fused at a low temperature with sodium or potassium dihydrogen arsenate. It yields green transparent prisms, sometimes elongated, sometimes flattened, insoluble in dilute acids.

Double arsenates of chromium with the alkali metals have been described. When aqueous arsenious acid is added to a solution of potassium chromate, the liquid becomes green and soon coagulates to a tremulous jelly, which when dried at 100° C. yields a substance whose empirical formula is 4K₂O.3Cr₂O₃.3As₂O₅.10H₂O. The double arsenates K₃Cr₂(AsO₄)₃ and Na₃Cr₂(AsO₄)₃ have been prepared by the addition of chromium sesquioxide to the fused alkali metarsenate. Crystallisation is accelerated by the addition of alkali chloride. Both compounds yield green transparent crystals. In the case of the potassium salt more than 7 per cent., and of the sodium salt more than 8 per cent., of the sesquioxide must be employed or a pyroarsenate is produced.

In the presence of alkali hydroxide, arsenic pentoxide forms with chromic hydroxide complex arsenates, analogous to the ferriphosphates, which have been formulated NaH₂[Cr(AsO₄)₂].H₂O and KH₅[Cr(AsO₄)₃].7 and 12H₂O.

By allowing solutions containing molecular proportions of alkali arsenate and chromic oxide, or of arsenic pentoxide and alkali chromate or dichromate, to crystallise, the following complex salts have been obtained:

2K₂O.As₂O₅.4CrO₃.H₂O
2(NH₄)₂O.As₂O₅.4CrO₃.H₂O
3(NH₄)₂O.As₂O₅.8CrO₃.H₂O

These compounds may be regarded as salts of two heteropoly-acids, and may be represented, respectively, according to the co-ordination theory by the formulae:

R₂'AsO₂(CrO₄)₂]
and
R₃'[As(CrO₄)₄].0.5H₂O