ON THE POSSIBILITY OF CHARACTERIZING CALCIUM PHOSPHATES IN CALCAREOUS SOILS BY ISOTOPIC EXCHANGE

Abstract

SummaryCharacterization of calcium phosphates depended upon the nature and the amount of phosphate used to react with reagent‐grade CaCO3. Formation of octa‐calcium phosphate (OCP) was inferred from the solubility equilibria after reacting CaCO3 with KH2PO4 solutions. Isotopic exchange measurements confirmed the presence of OCP, when the amount of P retained exceeded 44 μrnoles/g CaCO3. The determined surface‐Ca to surface‐P molar ratios were close to the theoretical Ca/P ratio of 1.33 in OCP. As P retained on CaCO3 decreased the surface Ca/P ratio markedly increased because of interference from surface Ca of the CaCO3. When CaCO3 was reacted with monocalcium phosphate (MCP), solubility equilibria indicated the formation of dicalcium phosphate dihydrate (DCPD). Isotopic exchange measurements, however, showed an average Ca/P ratio of only 0.375. This value corresponds to the composition of the metastable triple point solution (MTPS) formed on MCP dissolution rather than to the Ca/P ratio in DCPD.MCP application decreased the measured surface‐Ca (exchangeable Ca) either for soil or Ca‐resin, because of blocking of the exchange sites by the MCP reaction products and, consequently, a lower rate of isotopic exchange. Surface phosphorus of the two investigated calcareous soils proved to be proportional to the lowering in pH initiated by MCP application. Characterization of MCP reaction products in calcareous soils may thus prove infeasible, in view of the unexpected reduction in surface‐Ca and the pH dependency of surface P.