On the Solubility Constant of Strengite

Strengite, FePO 4 ·2H 2 O, commonly occurs in soils and represents a source of P for plants. The solubility of strengite is thus considerably important in the field of agriculture and environmental geochemistry, but few solubility data are available. Using absorption spectrophotometric techniques, the solubility equilibrium was studied at 25°C by measuring the total Fe(III), m Fe(III) , dissolved while keeping strengite in contact with H 3 PO 4 solutions. The acid concentration, m P , ranged from 0.001 to 0.1 mol kg −1 In solutions of m P ≤ 0.01 mol kg −1 m Fe(III) , results were constant within the limits of experimental error. This was ascribed to the predominance of a soluble species FePO 4 (aq). Soluble complexes, mainly Fe(H 2 PO 4 ) 3 (aq) and FeH 3 (PO 4 ) 2 (aq), were responsible for the increased solubility at m P ≥ 0.02 mol kg −1 , as evidenced in previous investigations. Using the constants evaluated in these investigations, the solubility data can be explained with the ion product equal to 10 −6.70 mol −1 kg at the infinite dilution reference state.