UNEXPECTED PH-DEPENDENCE OF DISSOLUTION KINETICS OF DICALCIUM PHOSPHATE DIHYDRATE

The dissolution kinetics of dicalcium phosphate dihydrate (CaHPO4.2H(2 )O), an important biological mineral, has been studied using the const ant composition method at 37 degrees C over a range of pH and relative undersaturations (sigma). At sigma = 0.5, the dissolution rate, contr olled mainly by volume diffusion, has an unexpected minimum at pH 5.0. This minimum occurs at higher pH values at lower driving forces where the rate is determined to a greater extent by surface processes. The pH dependence observed at sigma = 0.5 can be quantitatively interprete d by assuming independent desorption and diffusion of various ions in the presence of high concentrations of supporting electrolytes. Our th eoretical model allows for the assessment of the relative importance o f volume diffusion and adsorption solely from the kinetics data, which has been difficult in the past because both processes yield first-ord er kinetics. The rate minima at lower undersaturations may be attribut ed to the combination of proton catalysis at lower pH and solution sto ichiometry under more basic conditions.