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

Five strong aq. binary electrolytes - one sym. (CsCl) and four unsym. (Li2SO4, K2SO4, Rb2SO4, Cs2SO4) - have been examd., for possible use as salt bridges for the minimization of liq. junction potentials (EL), up to the highest concns. 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 sulfates where Me = Li, K, Rb and Cs. CsCl, K2SO4, Rb2SO4, and Cs2SO4, prove to belong to the class obeying close equality of transference nos. for their ions, i.e., t+ = |t-| = 0.5, over the whole concn. range (namely, from infinite diln. up to satn.). This result qualifies aq. CsCl as an unrivalled salt bridge, whose equitransference is obeyed more stringently than any other salt. This is now demonstrated exptl. over the whole molality range, the satn. molality being as high as 11.30 mol kg-1 at 25°. The obsd. property t+ = |t-| = 0.5 excludes K2SO4, Rb2SO4, and Cs2SO4, as possible salt bridges because the equitransference conditions for minimization of EL's are τ+ = |τ-| = 1/(z+ + |z-|) = 0.333, i.e., t+ = 0.333 and t- = 2t+ = 0.667. Finally, Li2SO4, though behaving quite differently from the other three sulfates studied, does not sufficiently approach the required conditions, contrary to what one might have hoped from its known infinite-diln. transference nos.