consistent subset of data compiled (Fig. S4–S12†) comprising 280 data points with a large range of variation in pH, initial AsO4 concentration, type and concentration of the background electrolyte, and Fh concentration, collected by 6 different research groups. ref. 39–43,51; protonated monodentate complex MH; Log K values for the monodentate species, formed from either ≡FeOH(a) or ≡FeOH(b), are kept equal