The electric potential and reaction rates of ions hitting the chemically active surfaces of microcrystals in an asymmetric electrolyte are computed analytically. Following ideas of Debye we start by solving the Poisson-Boltzmann equations and by determining the electric potential of the transport equations, We find distinct deviations when comparing our result with the Gouy-Chapman formula. In a simple model approximating a situation in which lead and hydrogen ions can react at goethite surfaces we compute analytically the currents of ions diffusing to the surfaces of microcrystals where they undergo a chemical reaction. We compute the reaction rates that can be controlled either by chemical reactions at the surface of the microcrystals or by diffusional transport. For realistic parameters of our model we find that the diffusional transport is the rate determining step.