Characterization of lithium sulfate as an unsymmetrical-valence salt bridge for the minimization of liquid junction potentials in aqueous-organic solvent mixtures.

The transference numbers t of lithium sulfate in acetonitrile/water and methanol/water solvent mixtures have been studied by measuring the emfs of such transference cells as Pb(Hg)|PbSO(4)|Li(2)SO(4) (m(2))||Li(2)SO(4) (m(1))|PbSO(4)| Pb(Hg), Hg|Hg(2)SO(4)|Li(2)SO(4) (m(2))||Li(2)SO(4) (m(1))|Hg(2)SO(4)|Hg, and Li(x)Hg(1-)(x)|Li(2)SO(4) (m(1))||Li(2)SO(4) (m(2))|Li(x)Hg(1-)(x), in view of characterizing Li(2)SO(4) as an unsymmetrical salt bridge for the minimization of liquid junction potentials in potentiometric applications. In water, Li(2)SO(4) is nearly as good a salt bridge as the popular KCl one. Its effectiveness has been verified through the operational pH-metric cell using various aqueous pH(S) standards. In acetonitrile/water solvents of 0.09 mass fraction of acetonitrile at 298.15 K, Li(2)SO(4) shows exact ionic equitransference, obeying the general condition t(+)/z(+) = t(-)/|z(-)|. The Li(2)SO(4) salt bridge can be used either for simple insertion between two different electrolyte solutions to minimize the intervening liquid junction potentials, as the outer component of a double-bridge arrangement of any commonly available reference electrode, or, obviously, for structural incorporation in sulfate-reversible reference electrodes as the appropriate supporting and bridge electrolyte. The operational potential of the Hg| Hg(2)SO(4)|2 m Li(2)SO(4) reference electrode in aqueous solutions is 0.6326 V at 298.15 K.