Junction Potentials in Galvanic Cells

The Nernst equation, derived under the assumption of thermodynamic equilibrium, calculates the electromotive force (emf) as the sum of potential differences at phase boundaries in a reversible cell without a liquid junction. However, in irreversible cells such as the Daniell cell, an additional potential difference named the liquid-junction potential (E_J) arises across the interface of two electrolyte solutions due to different ion diffusion rates. This E_J represents the potential difference between the right and left half-cell electrolyte solutions. This means that the total emf of a cell with a liquid junction equals the sum of E_J and the emf from the Nernst equation.

Although junction potentials are small, they are significant for accurate work and can be minimized—but not eliminated—by connecting electrolyte solutions with a salt bridge. The success of the salt bridge lies in the fact that if the ions dissolved in the agar jelly have similar mobilities, the liquid junction potentials at both ends are largely independent of the concentrations of the two dilute solutions, nearly canceling each other out.