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Phosphatoiron (III) and Phosphatoaluminum Complexes in Dilute Solutions¹

ABSTRACT

The phosphate complexes of iron(III) and aluminum in dilute solutions were studied by ultraviolet spectrophotometry, voltammetry, and cation exchange resins. The stability constant of the FeH₂PO₄²⁺ ion was found to be 4(± 0.8) x 10³ at ionic strength of 0.1. Although phosphatoaluminum complexes were not directly detectable, their stability constants could be estimated as 10³ for AlH₂PO₄²⁺ and 10⁷ for AlHPO₄⁺. These results indicate that, over the range of ion concentrations likely to be found in soil solutions, phosphatoiron(III) complexes are the predominant phosphate species in solutions of pH < 1.3; phosphatoaluminum complexes are the predominant phosphate species in the range of pH 1.3–4.3; and aquated phosphate ions are the predominant species in the range of 4.3–7.2. Published equilibrium constants for phosphatocalcium and phosphatomagnesium complexes and ion pairs revealed that these species are the predominant phosphate species at pH > 7.2.

¹ Joint contribution from the Department of Agricultural Chemistry and Soils, University of Arizona, Tucson, and the Department of Agronomy, Cornell University, Ithaca, N.Y. Taken from a Ph.D. thesis submitted by H. L. Bohn to the Graduate School of Cornell University, August 1963. This work was carried out in cooperation with the Tennessee Valley Authority, Muscle Shoals, Ala.

² Associate Professor and Professor of Soils, respectively.

Received for publication November 12, 1968. Accepted for publication July 14, 1969.