The kinetics of dissolution of calcium oxalate hydrates. II. The dihydrate.

We studied the kinetics of calcium oxalate dihydrate (COD) dissolution over a range of temperature and ionic strength. The rate of dissolution is proportional to the relative undersaturation expressed in terms of ionic activities. Subsequent transformation to the thermodynamically more stable monohydrate (COM) takes place by a secondary nucleation process. This transformation is markedly dependent upon the COD specific surface area and is appreciably slower than for the corresponding trihydrate transformation of comparable area. In the presence of polyphosphate, which retards the transformation COD leads to COM, the dissolution of COD seems to be controlled by a surface reaction. Differences in dissolution rates and adsorption behavior among the three calcium oxalate hydrates may play a part in determining the nature of the mineral phases that form in vivo.