Apparent partition coefficient of methyl orange between H₂O and n-octanol at 25°C was found to be dependent on the concentrations of potassium or sodium ion in aqueous phase above pH 7.0, indicating that methyl orange is transferred from aqueous to organic phase via an ion-pair complex for mation with these cations. Potassium and sodium ions had the same affinity for methyl orange, and the hydrophobicity of potassium-methyl orange complex was approximately the same as that of sodium-methyl orange complex. However, in strong acidic region the partition of methyl orange was found to be independent on the potassium ion concentration. It would be possible that in this region only a neutral molecular species of methyl orange is transferred from aqueous to organic solvent phase.

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1. The equation for ion flux through liquid membrane interposed between two aqueous electrolyte solutions was derived on the basis of nonequilibrium thermodynamics. The flux equation was the same in form as that previously derived for the solid membrane. This indicates that the theoretical expressions for the electrical properties of membrane are identical with those for the solid membrane.
2. It was shown that the liquid membranes prepared by cetylpyridinium chloride CPC and di-n-dodecyl phosphate DDP behaved as an ideal membrane electrode for anion and cation, respectively.
3. The membrane potentials were measured with the systems, NaClCPCNaClO₄, NaClCPCNa₂SO₄ and CaCl₂|DDPMgCl₂. The results were treated by the theoretical equation
V_O X, Y =2RT/ Z_X+Z_Y F ln Z_YP_Ya_Y/Z_XP_Xa_X
where X and Y refer to counter ions, X and Y, respectively and Z, a and P, valency, activity and permeability coefficient, respectively. The bi-ionic potential B. I. P. V_O X, Y, was found to be linear against the logarithm of a_Y/a_X with a constant slope of 4.6 RT/ Z_X+Z_Y F. This implies that Z_YP_Y/Z_XP_X=K^pot._{X, Y} is constant.
4. The orders of K^pot_{X, Y} in various bi-ionic systems were ClO₄SCN NO₃Cl-, Mg²⁺Ca²⁺Sr²⁺Ba²⁺, Ca²Cs⁺Na⁺K⁺. These orders were consistent with lyotropic series except for that of alkali metal ions.
5. The additivity rule V_o (X, Y)=V_o (X, Z) +V_o Z, Y, was confirmed with any set of the hi-ionic systems. This could be reasonably explained by assuming that permeability coefficient, P, is proportional to partition coefficient.
6. The effect of the compositions of membrane as well as the concentration of external solution on the membrane potential was studied with the system, 10^-2 M NaCl CPC+CPX=5×10^-4 M10^-1-10^-3M NaX X=ClO₄-, SCN-, and Br-. When the composition of membrane was unaltered, the B. I. P. vs. concentration of external solution relationship was expressed with a constant value of K^pot._{X, Y}. On the other hand, K^pot._{X, Y}. showed strong dependence upon the composition of membrane mole fraction of CPC when it was altered.
7. It was pointed out that further studies on membrane permeability are necessary in order to elucidate the nature of K^pot._XY.

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