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SOIL CHEMISTRY

The Solubility Constant of Variscite

Dipartimento di Chimica, Università di Napoli "Federico II", via Cinthia 45, 80126 Napoli, Italy

^* Corresponding author (miuliano{at}unina.it).

Only a few studies have been made on the ion product of variscite, Al(III) phosphate dihydrate, AlPO₄·2H₂O, a parameter of fundamental importance in clarifying the fixation of soluble phosphates in acid soils. The solubility of variscite in H₃PO₄ solutions of molality ranging from 1 x 10^–4 to 0.1 mol kg^–1 was determined at 25°C. The data were explained with a value of the ion product equal to 10^–3.14 mol^–1 kg (at the infinite dilution reference state) and assuming the formation of complexes AlPO_4(aq), AlHPO₄⁺, and Al(H₂PO₄)₂⁺. The solubility product obtained in this work differs by orders of magnitude from the literature values. The discrepancies are mainly due to the use of solids of different composition as well as to the assumption of the Al³⁺ ion as the unique soluble Al species. On the basis of equilibrium constants, the fixation in acid soils of soluble phosphates implicates the formation of variscite, while only at higher concentrations does the fixation occur as strengite.