  In this paper we present a mathematical model for the electrochemical
deposition aimed at the production of inverse opals. The real system consists
of an arrangement of sub micrometer spheres, through which the species in an
electrolytic medium diffuses until they react to the electrode surface and
become part thereof. Our model consists in formulating convenient boundary
conditions for the transport equation, that somewhat resembles the real system
but is nevertheless simple enough to be solved, and then solve it. Similar
approach was taken by Nicholson [1, 2], except that, to avoid the difficulties
regarding the boundary conditions, he considered none whatsoever, and proposed
a modified diffusion coefficient for the porous medium instead. Apropos, our
model, with moving boundary condition pertain to the class of problems know as
The Stefan problem [3].
