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 (m2)|MeCl (m1)|AgCl|Ag and MexHg1-x|MeCl (m1)|MeCl (m2)|MexHg1-x (where Me = Li, Na, K, and Rb and MexHg1-x denotes a flowing Me-amalgam electrode at Me mole fraction x) have been made at various molalities m2>m1 (with m1 fixed and m2 varied) in methanol+water solvent mixtures with methanol mass fractions wM up to 0.8. Supplementary emf measurements have been made of the cell Pt|LixHg1-x|LiCl (m1)|AgCl|Ag|Pt to obtain the required activity coefficients for LiCl at methanol mass fractions wM = 0.2. The general trend of the ionic transference numbers of each MeCl is a t°Me(+) increase with wM, 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 wM0.1, but at wM>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 wM≥0.8.