Characterization of Potassium Chloride as an Equitransferent “Intersolvental” Salt Bridge

Abstract

The electromotive forces (emf) E _A and E _C of the following concentration cells with transference: ${\text{Ag}}\left| {{\text{AgCl}}} \right|{\text{KCl (}}m_2 {\text{)}}\left\| {{\text{KCl(}}m_1 {\text{)}}\left| {{\text{AgCl}}} \right|{\text{Ag and K}}_{\text{x}} {\text{Hg}}_{{\text{1}} - {\text{x}}} |{\text{KCl(}}m_1 {\text{)}}} \right\|{\text{KCl(}}m_2 {\text{)|K}}_{\text{x}} {\text{Hg}}_{{\text{1}} - {\text{x}}} ,$ respectively, together with the emfs E _MAX of the corresponding double cell without transference: ${\text{K}}_{\text{x}} {\text{Hg}}_{{\text{1}} - {\text{x}}} \left| {{\text{KCl(}}m_1 {\text{)}}} \right|{\text{AgCl}}\left| {{\text{Ag}} - {\text{Ag}}} \right|{\text{AgCl}}\left| {{\text{KCl(}}m_2 {\text{)}}} \right|{\text{K}}_{\text{x}} {\text{Hg}}_{{\text{1}} - {\text{x}}}$ have been measured at KCl molalities m (m ₁ fixed and m ₂ varied, with m ₂>m ₁) approximately up to the KCl solubility limit in 12 solvent mixtures for the three aqueous–organic solvent systems (ethylene glycol+water), (acetonitrile + water), and (1,4-dioxane + water) up to 0.8 mass fraction of organic component. For all the cases explored, the E _A vs. E _MAX relation is linear over the whole KCl molality range. The ionic transference numbers t of KCl determined therefrom show a curvilinear dependence on the mass fraction of the organic component of the relevant solvent mixture and are found to fall in the range 0.52–0.48, viz., within ±4% of exact equitransference (t ₊ = t ₋ = 0.5). In particular, KCl becomes exactly equitransferent (i.e., an ideal salt bridge) in aqueous mixtures with the following mass fractions of organic component: 0.4 ethylene glycol and 0.09 acetonitrile, as well as 0.12 methanol, and 0.08 and 0.34 ethanol from our recent work. Even if use of KCl as a salt bridge would be somewhat restricted by its limited solubility in high mass fractions of dioxane and acetonitrile and pending extension of investigation to other mixed-solvent systems, the above figures characterize KCl as a fairly good “intersolvental” salt bridge in electrochemistry, electroanalysis, and corrosion science.