The partition of salts (i) between two immiscible solution phases and (ii) between the solid salt phase and its saturated salt solution

Abstract

Abstract The partition of salts between two polar immiscible solvents results from the partition of the cations and anions. Because electroneutrality rules in both phases, the partition of cations is affected by that of anions, and vice versa. Thus, the partition of a salt is determined by the chemical potentials of cations and anions in both phases, and it is limited by the boundary condition of electroneutrality. Whereas the partition of neutral molecules does not produce a Galvani potential difference at the interface, the partition of salts does. Here, the equations to calculate this Galvani potential difference are derived for salts of the general composition $$ {\text{Cat}}_{{\nu_{\text{Cat}} }}^{{(z_{\text{Cat}} ) + }} {\text{An}}_{{\nu_{\text{An}} }}^{{(z_{\text{An}} ) - }} $$CatνCat(zCat)+AnνAn(zAn)- and for uni-univalent salts $$ {\text{Cat}}^{ + } {\text{An}}^{ - } $$Cat+An-. The activity of a specific ion in a particular phase can thus be purposefully tuned by the choice of a suitable counterion. Finally, the distribution of a salt between its solid phase and its saturated solution is also presented, together with a discussion of the Galvani potential difference across the interface of the two phases. Graphical abstract