Kinetics of the crystallisation of calcium oxalate monohydrate

Abstract

The kinetics of precipitation of calcium oxalate monohydrate have been studied conductometrically at 298 K for both spontaneous and seeded growth systems. The rate of growth follows a quadratic dependence upon the relative supersaturation, which suggests a surface-controlled growth mechanism. This rate equation holds fairly well for the various solid/solution ratios used in the range of 2.35–10.18 relative supersaturation and for seed concentrations of 0–200 mg dm^–3. The effect of sodium tripolyphosphate and phosphonate additives on the precipitation kinetics of calcium oxalate in the absence and presence of well characterised seeds has been investigated at various levels of additive concentration. The inhibiting activity of these additives is discussed in relation to the surface characteristics of the inoculated seeds and structural features of the additive molecules. The fit of experimental data to the Langmuir adsorption isotherm supports a mechanism of inhibition through molecular adsorption of the foreign ions on the surface of the growing crystals.