Transference Numbers of Alkali Chlorides and Characterization of Salt Bridges for Use in Methanol + Water Mixed Solvents

Electromotive force measurements at 25 °C of the transference cells Ag|AgCl|MeCl ( m 2 )|MeCl ( m 1 )|AgCl|Ag and Me x Hg 1 - x |MeCl ( m 1 )|MeCl ( m 2 )|Me x Hg 1 - x (where Me = Li, Na, K, and Rb and Me x Hg 1 - x denotes a flowing Me-amalgam electrode at Me mole fraction x ) have been made at various molalities m 2 > m 1 (with m 1 fixed and m 2 varied) in methanol + water solvent mixtures with methanol mass fractions w M up to 0.8. Supplementary emf measurements have been made of the cell Pt|Li x Hg 1 - x |LiCl ( m 1 )|AgCl|Ag|Pt to obtain the required activity coefficients for LiCl at methanol mass fractions w M = 0.2. The general trend of the ionic transference numbers of each MeCl is a t ° Me + increase with w M, which is much more pronounced for those Me + 's whose primary hydration sheaths are bigger (namely, Li + and Na + ). In particular, KCl becomes exactly equitransferent ( t ° K + = t ° Cl − = 0.5, i.e. an ideal salt bridge) at w M ≈ 0.1, but at w M > 0.6 the KCl solubility becomes insufficient for a salt bridge function. The same drawback occurs also for RbCl, which is known to be the most closely equitransferent salt in water ( t ° Rb + = 0.5007). NaCl, which is quite unproposable as a salt bridge in water, may be useful at high methanol concentrations, as its ionic transference numbers would approach 0.5 at w M ≥ 0.8.