A measurement of the unstirred aqueous boundary layer in a Franz diffusion cell

The unstirred aqueous boundary layer (ABL) thickness was determined for modified Franz diffusion cells using three test compounds-parathion, 4-hexylresorcinol, and 2,4-dinitrochlorobenzene (DNCB). Steady-state fluxes through one, two, and three dialysis membranes in series were achieved in donor-saturated, infinite-dose permeation experiments. These results were then used to calculate the total diffusive resistance for each case, as well as an extrapolated zero membrane value which was used in conjunction with an estimate of the aqueous diffusivities to calculate the ABL thickness. The resulting value (mean +/- SE) was 0.070 +/- 0.004 cm.