Mass balance equations at the electrode surface, for the case of time-dependent geometry of the depletion layer, that are valid for any stoichiometry and any value of diffusion coefficients, are formulated and evaluated. Analysis of the errors identifies a simple, general mass balance equation. Errors due to application of the general mass balance equation for mixed diffusion within the entre depletion layer are determined also. The focus is on mixed planar and spherical diffusion.