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This article in SSSAJ

  1. Vol. 33 No. 6, p. 873-876
    Received: Nov 12, 1968
    Accepted: July 14, 1969

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Phosphatoiron (III) and Phosphatoaluminum Complexes in Dilute Solutions1

  1. H. L. Bohn and
  2. Michael Peech2



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) × 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.

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