For a barrier membrane (thickness h) that is sandwiched between external media (both donor and receiver sides), the surface concentrations within the barrier ordinarily are not known but can be replaced by the membrane-to-medium partition coefficient K multiplied by the concentrations in the medium of donor side Cd or receiver side Cr. In some cases it is not possible to determine D, K, or h independently and thereby to calculate a hybrid constant, permeability or permeability coefficient P. With these considerations, Fick's first law (Eq. 4.1) may be written

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