C. Fabiani. P. R. Danesi. G . Scibona. and B. Scuppa
2370
Electrical Liquid Membrane Potential. Biionic Isothermal Potential C. Fabiani,' P. R. Danesi, G. Scibona, and B. Scuppa Industrial Chemistry Laboratory. C.N.E.N.,CSN-Casaccia. Rome, ltaly (Received February 1, 1974; Revised Manuscript Recefved June 3, 7974) Publication costs assisted by the lndustrial Chemistry Laboratory
Biionic (Cl-,N0,3-) membrane potentials have been measured for liquid membranes formed by dissolving a tetraheptylammonium salt in chlorobenzene, o-dichlorobenzene, and nitrobenzene. T h e values of the physicochemical parameters required by the theoretical equations which describe the liquid ion-exchange membrane potential have been experimentally obtained by means of conductometric and ion distribution experiments. By using these parameters a rather good agreement between theoretical and experimental membrane potential values has been obtained for all the liquid membranes except nitrobenzene. I n fact for this solvent, due to the partial dissociation of t h e site-counterion ion pairs, a test of the theory was not possible.
Introduction
Equations Describing the Liquid Membrane Potential
Liquid ion-exchange membranes are formed by solutions of suitable organic salts or acids (e.g., long-chain alkylammonium salts; bis(2-ethylhexy1)phosphoric acid or salts) in low dielectric constant solvents. When these solutions are interposed between two aqueous electrolyte solutions of suitable composition a n electrical potential is generated. As far as the chemistry of these liquid membranes is concerned it has to be noted that the organic salt or acid can form free ions and associated species in the organic solution. Theoretical equations for the electrical potential of these liquid membranes have been derived under isothermal conditions by coupling ion distribution and ion diffusion proc e ~ s e s . l -T~h e same equations have been obtained from an irreversible thermodynamic treatment of the transport processes across a liquid membrane and extended to both the isothermal and nonisothermal conditions4 In the case of practical insolubility of the site in the aqueous solutions and coions exclusion, t h e theoretical treatment suggests the following: (i) the monoionic membrane potential values depend only on the aqueous activity of t h e counterion and are not affected by any change of the membrane charged site mobility (in other words t h e membrane is highly permselective); (ii) the biionic membrane potential values depend on the aqueous activity of t h e counterions, on the mobilities of t h e species present in t h e membrane phase, and on the ion-exchange constant, ion distribution constant, and ion pair formation constants. An experimental test of t h e theoretical predictions has been already performed in t h e case of liquid cation exchange membra ne^.^ This paper is instead devoted to the experimental analysis of t h e biionic liquid anion exchange membranes. To this purpose we have studied the biionic (chloride, nitrate couple) electrical membrane potential of liquid anion exchange membranes formed by tetraheptylammonium nitrate salt in chlorobenzene (dielectric constant t = 5.62), 0 - dichlorobenzene ( t = 9.931, and nitrobenzene ( t = 34). Further all the physicochemical parameters required by the theoretical equations have been experimentally determined.
We will summarize here the membrane potential equations derived in ref 2 and 4 for the case of two counterions (biionic potentials). T h e total membrane potential, Vo, is given by
The Journal of Physicai Chemistry. V o l . 78. No. 23 7974
ill
where u , are the membrane mobilities of the counterions, k , the single ion distribution constants, a, the aqueous activities of the counterions, and J1 accounts for the influence of the associated species. In the case of complete electrolyte dissociation in t h e membrane phase eq 1 becomes
In t h e case of strong association, z e , when (uscs/S,=ldu,,c,,)
