Gradient-pH Permeability Equation

If surfactant is added to the acceptor wells, then in general, P A D and P 0 - are not the same under iso-pH conditions. The acceptor-to-donor permeability needs to be solved by performing a separate iso-pH assay, where the surfactant is added to the donor side, instead of the acceptor side. The value of Pe is determined, using Eq. (1), and used in gradient-pH cases in place of PeA D as described in the preceding paragraph. The gradient-pH calculation procedure is also iterative. 

In iso-pH surfactant-free solutions, the concentration of the sample in the acceptor wells cannot exceed that in the donor wells. With gradient-pH conditions, this limitation is lifted. At very long times, the concentrations in the donor and acceptor chambers reach equilibrium values, depending on the pH gradient  

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