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 degrees C by measuring the total Fe(III), m(Fe(III)), dissolved while keeping strengite in contact with H3PO4 solutions. The acid concentration, mp ranged from 0.001 to 0.1 mol kg(-1). In solutions of m(P) < 0.01 mot kg(-1) m(Fe(III)), results were constant within the limits of experimental error. This was ascribed to the predominance of a soluble species FePO4(aq). Soluble complexes, mainly Fe(H(2)pO(4))(3)(aq) and FeH3(PO4)(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) met(-1) kg at the infinite dilution reference state.