How to solve a diffusion problem with precipitation reaction

Hi,

I want handle a dilute diffusion problem. During diffusion, precipitaion reaction may occure and compounds would form if concentration on the element is high enough.

I am not sure how to solve this problem. Maybe I could link COMSOL to Matlab to determine if the reaction would occure during each time step on all elements and calculate concentration after reaction? Or maybe I could apply some constraint on the domain?

thanks a lot.

Jiang