The theory of the reverse diffusion of ions in mixed electrolyte solutions and experimental results on the HCl-CaCl2-H2O ternary system.

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Reverse diffusion is the migration of solutes in the direction opposite to that expected from the concentration gradient, that is, from a portion with a lower concentration to that with a higher concentration. This type of diffusion of ions in mixed electrolyte solutions was studied theoretically on the basis of nonequilibrium thermodynamics. For a solution containing two kinds of cations and one kind of anion, it has been concluded that the reverse diffusion occurs when the mobilities of the coexisting ions differ largely, because the effect of the diffusion potential is pronounced in such a case. It has also been concluded that the practical diffusion coefficient of an ion varies linearly with the ratio of the concentration gradient of a coexisting ion to that of the diffusing ion when the average concentration ratio of the ions is fixed. Moreover, the range of the concentration-gradient ratio, where the practical diffusion coefficient is negative, has been determined. These theoretical conclusions were tested experimentally for the HCl–CaCl<SUB>2</SUB>–H<SUB>2</SUB>O ternary system. The results satisfactorily verified that reverse diffusion actually occurred and that the relationship between the diffusion coefficient and the concentration-gradient ratio was linear, as has been expected.
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