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Electromotive force measurements at 25 °C of the transference cells Ag|AgCl|MeCl (m 2)|MeCl (m 1)|AgCl|Ag and Me x Hg1 - x |MeCl (m 1)|MeCl (m 2)|Me x Hg1 - x (where Me = Li, Na, K, and Rb and Me x Hg1 - 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 Hg1 - 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.

The electromotive force E of the amalgam cell & I e p g , --x I MeCl (m) I AgCl I AgJ, where Me = K or Rb, has been measured at the temperature 298.15 K as a function of the molality rn of MeCl in (methanol+water) mixed solvents up to 0.8 mass fraction of methanol, with relative permittivities ~2 4 0 , and as a function of the mole fraction x of Me in amalgam. The respective standard electromotive forces E; have been determined, together with the mean molal activity coefficients y * of the relevant MeCl as a function of its molality. For interpolation purposes the E; values can be expressed as a function of the methanol mass fraction wM by the following polynominals, valid for 0s wM 50.8: (for Me = K): Ek/V = 2.19781 -0.22826 w M +~. i~6~~( W M ) 2 -~.~~~~~( W~) 3 (for Me = Rb): E;/v = 2.19221 -0.24057 wM+O.20271 (wM)2-o.23~(wM)3These results for standard e.m.f.s, interpreted in terms of primary medium effects, lead to primary hydration numbers nhydr = 6.6 for KCI, and = 6.7 for RbCI, in good agreement with previous results using different methods.