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Solubility Product of Iron Phosphate¹

ABSTRACT

The solubility product of iron phosphate was determined by both dissolution and precipitation methods in dilute salt solutions. Solution pH values slightly below the determined isoelectric pH of 2.75 for iron phosphate were employed to get determinable amounts of iron in the solution. The pK_sp value was calculated from the ionic activities of Fe³⁺ and H₂PO₄^- in the equilibrium solution by the following equation, pK_sp = pFe³⁺ + pH₂PO₄^- + 2pOH^- which was found to coordinate the data most satisfactorily. The pK_sp value of iron phosphate increased with a 1000-fold decrease of the solid-to-solution ratio from 33.6 to 35.1. It increased from 33.0 to 33.2 over a 50-fold decrease of the concentration of the precipitation reagents. A medial value of 33.5 was approached by the two procedures; however, for low iron phosphate-solution ratios, such as those commonly employed for soil extraction, the applicable pK_sp value agrees with the most dilute solid-solution value of 35 or more. Similarly for aluminum phosphate, the applicable pK_sp value extends as high as 32 for dilute suspensions. The variation in apparent solubility product with amount of solid per unit volume of solution is negligible compared to the magnitude of the pK_sp which is the reciprocal of a billion trillion trillion. The calculations from solubility products show that aluminum phosphate and iron phosphate should precipitate and accumulate even in neutral soils, as observed experimentally.

¹ This work was supported in part by the Research Committee of the University of Wisconsin through a grant of funds from the Wisconsin Alumni Research Foundation. Presented before Div. II, Soil Science Society of America, Nov. 15, 1956, at Cincinnati, Ohio.

² Now Professor of Soils, National Taiwan University, Taipei, Taiwan (Formosa), China, formerly Graduate Assistant, University of Wisconsin, and Professor of Soils, University of Wisconsin, respectively.

Received for publication November 16, 1956. Accepted for publication December 17, 1956.