Studies on liquid junction potentials in concentrated chloride solutions and determination of ionic activities and hydration numbers by the emf method

Abstract

The validity of the Henderson equation for calculating liquid junction potentials in concentrated salt solutions is examined by comparing the emf of cells Pt|H 2, HCl (0.1–6M)‖saturated KCl|SCE and Ag|AgCl (saturated), NaCl, CaCl 2 or MgCl 2 (0.1–6M)‖saturated KCl|SCE with the emf values calculated from single ion activities of H + and Cl − determined by the acidity function or hydration treatment. An excellent agreement and Nernstian response is obtained when the emf is corrected for E J (Henderson) in up to 4 M HCl or 5 M NaCl, MgCl 2 and CaCl 2 test solutions. This is despite large changes in ionic activities and water activity at the liquid junction boundary and assumptions of ideal behaviour of the Henderson equation. Measurements of ionic activities of H +, Mg 2+, Ca 2+, and Cl − measured by emf in solutions of varying water activity provide a rational set of ionic hydration numbers. The hydration numbers of H + and Cl − were found to be 7 and 1, respectively.