Jw. Zhang et Gh. Nancollas, UNEXPECTED PH-DEPENDENCE OF DISSOLUTION KINETICS OF DICALCIUM PHOSPHATE DIHYDRATE, Journal of physical chemistry, 98(6), 1994, pp. 1689-1694
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.