In this study we investigated the kinetics of the coagula formation reaction of polymer latex to solve the unsteady equation of diffusion involving the moving boundary. To investigate the kinetics of solid fluid reactions when the pseudo-steady state approximation is not held, we tried to apply the orthogonal collocation method to an unsteady equation of diffusion involving the moving boundary. This model had good agreement with the reasonable and constant effective diffusion coefficient without the dependence on the coagulant concentration.