original reaction equation in the study: »X(s)-OH + H2AsO4<1-> = »X(s)-O-As(OH)O2<1-> + H2O with log K = 5.4±0.1, formation constant in aqueous solution: AsO4<3-> + 2H<1+> = H2AsO4<1-> logK = 18.35 (Schecher, W. D.; McAvoy, D. C. MINEQL+, V. 4.5, Users Manual, Hallowell, ME, 1998)