THE LIQUID JUNCTION CONTRIBUTION TO EMF

The contribution to the emf of an electrochemical cell from the liquid junction is the topic of this paper. Different kinds of liquid junctions between KCl solution of 4 kmol m-3 and dilute solutions of HCl were investigated experimentally and theoretically. Our method of theoretical calculations involves only operationally defined quantities. This permits a detailed analysis of the different contributions and a separation of the approximations introduced. The experimental emf of a cell with linear concentration profiles in the liquid junction was compared to the theoretical emf. The variation in activity coefficients over the junction contributes several mV to the emf, and the difference in ionic mobilities also contributes substantially. A liquid junction with restricted diffusion and the liquid junction of a commercial reference electrode were compared experimentally to the junction with linear concentration profiles. There was a significant difference in emf, showing that the Henderson approximation of linear concentration profiles is not applicable when accurate values are wanted. One implication of the results is that one cannot expect a perfect Nernstian behaviour in an electrochemical cell designed for analytical purposes, not even for dilute solutions.