Transference numbers of alkali chlorides and characterisation of salt bridges for use in methanol + water mixed solvent

Electromotive force measurements at 25 degrees C of the transference cells Ag\AgCl\MeCl (m(2))\MeCl (m(1))\AgCl\Ag and MexHg1-x\MeCl (m(1))MeCl (m(1))\Me,Hg1-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 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\LixHg1-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 degrees(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 degrees(K+) = t degrees(Cl-) = 0.5, i.e. an ideal salt bridge) at w(M) approximate to 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 degrees(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) greater than or equal to 0.8.