log K for background cation from [D78] assuming K+ behaves like Na+; best-fit constants determined with model simulations of the experimental data; original equation in the study: »Fe-OH + H3AsO3 = »Fe-HAsO3<1-> + H2O + H<1+>, logK = -3.1; formation constant in aqueous solution: H3AsO3 = AsO3<3-> + 3H<1+> with logK = -34,7 (DA88]