Characterization and use of aqueous caesium chloride as an ultra-concentrated salt bridge

Five strong aqueous binary electrolytes — one symmetrical (CsCl) and four unsymmetrical (Li₂SO₄, K₂SO₄, Rb₂SO₄, Cs₂SO₄) — have been examined, for possible use as salt bridges for the minimization of liquid junction potentials (E_L), up to the highest concentrations practicable, by the method of homoionic transference cells: Pt–Ir | Cl₂ | CsCl (m₂) CsCl (m₁) | Cl₂ | Pt–Ir and Hg | Hg₂Cl₂ | CsCl (m₂) CsCl (m₁) | Hg₂Cl₂ | Hg for CsCl, and Hg | Hg₂SO₄ | Me₂SO₄ (m₂) Me₂SO₄ (m₁) | Hg₂SO₄ | Hg for the Me₂SO₄ sulphates where Me=Li, K, Rb and Cs. CsCl, K₂SO₄, Rb₂SO₄, and Cs₂SO₄, prove to belong to the class obeying close equality of transference numbers for their ions, that is,t₊= |t_–|=0.5, over the whole concentration range (namely, from infinite dilution up to saturation). This result qualifies aqueous CsCl as an unrivalled salt bridge, whose equitransference is obeyed more stringently than any other salt. This is now demonstrated experimentally over the whole molality range, the saturation molality being as high as 11.30 mol kg^–1 at 25°C. The observed propertyt₊=|t_–|=0.5 excludes K₂SO₄, Rb₂SO₄, and Cs₂SO₄, as possible salt bridges because the equitransference conditions for minimization ofE_L's are ₊ = |_–| = l/(z₊ + |z_–|) = 0.333, i.e.,t₊=0.333 andt_–=2t₊=0.667. Finally, Li₂SO₄, though behaving quite differently from the other three sulphates studied, does not sufficiently approach the required conditions, contrary to what one might have hoped from its known infinite-dilution transference numbers.