A Re-evaluation of the Solubility and Stability Regions of Calcium Arsenites and Calcium Arsenates in Aqueous Solution at 25°C

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Abstract
The systems Ca(II)-As(III)-H2O and Ca(II)-As(V)-H2O were studied by equilibrating mixtures of calcium oxide and arsenious or arsenic acid solution at 25°C for one month. Where the pH of the final solution was likely to be greater than 8, care was taken to exclude atmospheric carbon dioxide. Chemical analysis of the final solid phases and the associated solution from each experiment for calcium and arsenic, together with X-ray diffraction analysis of the solids and an inspection of graphed relationships, indicated the existence of the compounds Ca(AsO2)2 and CaAsO2OH 1/2H2O in the system Ca(II)-As(III)-H2O; and Ca(H2AsO4)2, CaHAsO4 H2O, Ca3(AsO4)2 4H20 and Ca2AsO4OH 2H2O in the system Ca(II)-As(V)-H2O. The solubility and stability regions for these compounds were assessed as a function of pH, and solubility constants and conditional free energies of formation for each compound calculated from the solubility data obtained. A brief historical review of literature in which calcium arsenites and calcium arsenates have been reported is included in this paper.

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