Constant composition dissolution kinetics studies of human dentin

The constant composition (CC) method has been used to study the dissolution kinetics of whole powdered human dentin as a function of calcium phosphate concentration at relative undersaturations with respect to hydroxyapatite (sigma HAP), ranging from +0.8 to -2.8, ionic strength from 0.05 to 0.30 mol/L-1 in sodium chloride or potassium nitrate, pH 4.00 to 5.50, and molar calcium/phosphate ratio in the reaction solutions from 0.05 to 11.1. The results suggest that human dentin behaves as a mixture of at least two calcium phosphate phases, HAP-like and octacalcium phosphate-like, OCP-like. Significant dissolution took place in solutions that were even supersaturated with respect to HAP, and the rates exhibited a striking insensitivity to relative undersaturation, while influenced by ionic strength, pH, and molar calcium/phosphate ratio in the reaction solutions. Although the dissolution was retarded in the presence of magnesium ion, the reaction rate showed the same insensitivity to undersaturation with respect to calcium phosphate.