Solubility of Gypsum and Dicalcium Phosphate Dihydrate in the System CaO‐P2O5‐SO3‐H2)O and in Soils

Abstract

Abstract The solubility isotherm (25°C.) for gypsum was determined in the concentration range from 1.5 × 10 ‐2 to 6.0 × 10 ‐2 mols of S per liter by equilibrating CaSO 4 · 2H 2 O with H 2 SO 4 solutions of varying concentrations. The solubility product of gypsum calculated from these data was 2.45 × 10 ‐5 . The effect of gypsum upon the solubility of dicalcium phosphate dihydrate (DCPD) was related to the pH. The percentage of calcium in solution which was derived from DCPD was 80% and 3% at pH 3.6 and 6.3, respectively. The data from exchange resin and solubility studies are consistent with a hypothesized complex in solution of the form Ca(H 2 PO 4 ) + . Equilibrations of DCPD and gypsum in soil systems indicated that apparent supersaturation with respect to both salts was obtained at pH values above 5.8. This anomalous behavior was explained by the complexing of Ca by soluble organic matter. The research findings are discussed in the light of soil and fertilizer chemistry.