The differential equation for dilute species transport next to a planar interface within the hydrodynamic boundary layer is transformed into a differential equation for the diffusion boundary layer. This differential equation contains two coefficients, which depend explicitly on the concentration profile. We show that these coefficients can be taken as identical constants, in a number of limiting cases. Using these constants, we show that this equation reproduces very well the temporal and spatial profile of the diffusion boundary layer determined from numerical simulations in some more...