The thirst part of the kinetic sorption curve at short times (and short square root of time) has got a tangent straight with a characteristic slope wich is related to the diffusion coefficient. At the half-sorption time the kinetic curve is almost linear. At this moment one may notice that the concentration profile is positive in each part of the membrane. At the legal sorption time which is four times the half-sorption time and which corresponds to the intersection of the initial tangent straight with the equilibrium plateau, the absorbed mass is only equal to 85% of the equilibrium absorbed mass Moo.
Example 2, following the sorption kinetics with a square root of time scale :