Shift of pH-absorption curves

The conditions for deviations of thepH-absorption curve from the course predicted by the simplepH partition theory are investigated theoretically. The deviations are an elevation of the asymptotic section usually approaching zero and/or a shift of the intermediate section, more exactly of the inflection point, in the direction of the abscissa and/ or the ordinate. In the absence of a specialpH at the surface of the barrier (microclimatepH), the elevation of the asymptotic section can be attributed to a permeability of the barrier to the ionized form of the permeating substance. A shift of the inflection point to the right for acids and to the left for bases can be explained by a significant unstirred layer at the surface of the barrier or-only when the concentration in the well- stirred bulk phase is decreasing with time-by a high distribution ratio of the substance between barrier and bulk phase. A variable microclimatepH influenced by bulk phasepH can also produce the described deviations.