We have studied the precipitation of calcium oxalate monohydrate (COM) from supersaturated saline solutions both as an in where AP is the activity product, the product of the free vitro model of the formation of human kidney stones and more calcium (Ca 2/ ) and oxalate (C 2 O 20 4 ) ion activities; thus, generally as a model of particle-size enlargement during precipitation from solution. In any precipitation system there are two mechanisms for size enlargement: growth, by which is meant the deposi-AP Å a Ca 2/ a C 2 O 20 4 . [3] tion of ionic or molecular species on crystal surfaces, and aggregation, the process by which crystals collide, adhere to each other, The activities are related to concentrations by an activity and form new, stable particles. In this paper we report batch coefficient, g { , for example, precipitation experiments conducted at constant initial supersaturation and varying ratios of calcium to oxalate ions. By comparison with a simple diffusion-reaction model we are able to draw sub-
stantial inferences on the processes that control aggregation. We conclude that at constant temperature and agitation, the aggregafor calcium ions. In Eq. [2], K sp is the solubility product, tion rate constant depends only on the cementing rate, which in the value of the activity product at equilibrium. The rate turn depends on the solution composition and an appropriately expression, Eq. [1], and this definition of supersaturation is formulated Thiele modulus only. Thus, for the first time, it is applicable, whatever the ratio of calcium to oxalate ions in possible to propose and evaluate a physico-chemical model for the solution. kinetics of aggregation during precipitation from solution. ᭧ 1998 By contrast, very little is known about the effect of Academic Press
solution composition on the rate of aggregation. In non-