Characterization of Lithium Sulfate as an Unsymmetrical-Valence Salt Bridge for the Minimization of Liquid Junction Potentials in Aqueous−Organic Solvent Mixtures

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Abstract
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)|PbSO4|Li2SO4 (m2)||Li2SO4 (m1)|PbSO4| Pb(Hg), Hg|Hg2SO4|Li2SO4 (m2)||Li2SO4 (m1)|Hg2SO4|Hg, and LixHg1-x|Li2SO4 (m1)||Li2SO4 (m2)|LixHg1-x, in view of characterizing Li2SO4 as an unsymmetrical salt bridge for the minimization of liquid junction potentials in potentiometric applications. In water, Li2SO4 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 pHS standards. In acetonitrile/water solvents of 0.09 mass fraction of acetonitrile at 298.15 K, Li2SO4 shows exact ionic equitransference, obeying the general condition t+/z+ = t-/|z-|. The Li2SO4 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| Hg2SO4|2 m Li2SO4 reference electrode in aqueous solutions is 0.6326 V at 298.15 K.