Potentiometric Methods of Analysis
In potentiometry the potential of an electrochemical cell is measured under static conditions. Because no current, or only a negligible current, flows while measuring a solution’s potential, its composition remains unchanged. For this reason, poten- tiometry is a useful quantitative method. The first quantitative potentiometric ap- plications appeared soon after the formulation, in 1889, of the Nernst equation re- lating an electrochemical cell’s potential to the concentration of electroactive species in the cell.
When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discov- ered that a potential difference exists between the two sides of a thin glass mem- brane when opposite sides of the membrane are in contact with solutions contain- ing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on mem- brane potentials are called ion-selective electrodes, and their continued develop- ment has extended potentiometry to a diverse array of analytes.