  Diffusion in a two-phase system is a classical problem discussed in the
literature. The general solution of the one-dimensional case to this problem is
revisited and a detailed derivation is proposed. The solution is discussed in
relationship with ion exchange in silicate glasses. The general solution is
compared with the complementary error function (erfc) solution and the
assumptions under which the general solution reduces to the erfc are
identified. Interface or glass surface issues related to ion exchange are
discussed in terms of the ratio between the diffusion coefficients of the
diffusing alkali ions in the glass and in the ion reservoir phases
respectively. Another relevant issue can be identified in the assumption of
instantaneous equilibrium at the two-phase interface once the contact is
established. Even though this last assumption is somehow unphysical it is the
common approach used in the technological application of ion exchange in
silicate glass which is justified when the contact time (immersion time) of
glass with an ion reservoir is much larger than the time to achieve
thermodynamic equilibrium at the surface.
