Calorimetric investigations of calcium phosphate precipitation in relation to solution composition and temperature

Abstract

The kinetics of formation and conversion of calcium phosphate under different initial solution compositions and temperature have been studied by calorimetric methods. Experimental values of enthalpy of calcium phosphate formation at various Ca:P molar ratios, initial pH of solution and temperatures are given. It is found that in the initial pH range 5.8 < pH < 11.79 the formation of calcium phosphate from relatively high supersaturated solutions is associated with a strong endothennic effect. Under a wide range of Ca : P ratios (0.33–2.79) and with an initial pH of 7.40 the formation of calcium phosphate is associated with the repeated heat effect. This effect, which is connected with the transformation of the first precipitated phase, is revealed clearly and occurs in two stages. This transformation may be explained by considering that a mixture of Ca4H(PO4)3 · 2.5H2O-like and Ca5(PO4)3OH-like phases is formed during the amorphous-to-crystalline transition.