Environmental context. The behaviour of arsenic compounds is controlled by their solubility, which in turn controls both the forms and mobility of arsenic in the environment. Current knowledge on arsenic chemistry may be distorted because information available on the solubility of arsenic compounds usually does not include possible formation of metal–arsenate complexes. Our results show the formation of stable metal–arsenate complexes that have not been considered before, and this new data can be used to further examine the effect of these complexes on controlling the fate and transport of arsenic in the environment.

Abstract. The formation of solid metal arsenates could conceivably reduce the concentrations of arsenate and metal ions in natural and contaminated aqueous ecosystems, and possibly in human body fluids. In this study, solid metal arsenates were dissolved isothermally in solutions with different molar concentrations of arsenic acid. The saturated solutions were analysed and the results processed to derive the solubility products (K_sp) for solid phases and association constants (K) for metal arsenate ion-pairs. Ion chromatography was used to confirm the presence of ion-pairs, some of which had never before been considered. Association constants were determined for the following ion-pairs: FeHAsO₄⁺ (log K = 4.88), CoHAsO₄⁰ (log K = 1.50), ZnHAsO₄⁰ (log K = 3.28), SrH₂AsO₄⁺ (log K = 1.72), and Ag₂H₂AsO₄⁺ (log K = 4.50). The following metal ions apparently do not form stable complexes with HAsO₄^2–: Cd²⁺, Cr³⁺, Cu²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, and Sn²⁺. Standard state solubility products (K_sp°) were redetermined for the following compounds: Ag₃AsO₄, Cd₃(AsO₄)₂, Co₃(AsO₄)₂, CrAsO₄, Cu₃(AsO4)₂, FeAsO₄, Mg₃(AsO₄)₂, MnHAsO₄, NiHAsO₄, PbHAsO₄, Sn₃(AsO₄)₂, Sr₃(AsO₄)₂, Zn₃(AsO₄)₂·Zn₃(AsO₄)₂·8H₂O (koettigite), Cu₂Al₇(AsO₄)₄(OH)₁₃·12H₂O (ceruleite), and Pb₂CuAsO₄CrO₄OH (fornacite). Our results show the formation of ion-pairs for some metal arsenates and indicate that previous studies have overestimated the solubilities of many arsenates.