In electrodialysis and the other separation processes using ion exchange membranes, transport of components generally occurs under the driving forces of both concentration and electric potential (voltage) gradients. However, because the two types of ion present, anions and cations, move in opposite directions under an electric potential gradient, ion exchange membrane processes are often more easily treated in terms of the amount of charge transported than the amount of material transported. Consider, for example, a simple univalent–univalent electrolytesuch as sodium chloride, which can be considered to be completely ionized in dilute solutions. The concentration of sodium cations is then c+, and the concentration of chloride anions is c−. The velocity of the cations in an externally applied field of strength, E, is u (cm/s), and the velocity of the anions measured in the same direction is −v (cm/s). Each cation carries the protonic charge +e and each anion the electronic charge of −e, so the total amount of charge transported per second across a plane of 1 cm2 area is
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