The region in the vicinity of an electrode where the concentrations are different from their value in the bulk solution. The definition of the thickness of the diffusion layer is arbitrary because the concentration approaches asymptotically the value c0 in the bulk solution (see diagram).
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See also: Nernst's diffusion layer
Source:
PAC, 1981, 53, 1827. 'Nomenclature for transport phenomena in electrolytic systems' on page 1837 (https://doi.org/10.1351/pac198153101827)