Potentiometric Endpoint Methods Electrochemical Titrations

The basis of potentiometry is the Nernst equation, which relates the concentration of electroactive species at the surface of an electrode (C) to the potential of that electrode (E); that is, for the reaction O + e = R where E is the formal redox potential of the electron transfer reaction. In a potentiometry experiment, the open-circuit potential (i.e., there is no measurable current through the cell) is measured between two electrodes: the indicator electrode and the reference electrode. The potential of the indicator electrode is sensitive to the concentration of the analyte in solution, and the reference electrode (typically the saturated calomel or the silver/silver chloride electrode) provides a stable reference potential for measurement of the potential of the indicator electrode. Therefore, the potential of this electrochemical cell depends upon the analyte concentration. There are a number of different indicator electrodes. The simplest one is an inert metal such as platinum, which can be used to measure the ratio of the concentrations of the oxidized and reduced species. These are therefore suitable for redox titrations (the curve for the titration of iron(II) by cerium(IV) recorded by an epsilonTM prototype using a platinum indicator electrode is shown in F1). Alternatively, if the Mn+ + ne= M half-reaction is reversible, then an M electrode can be used for the determination of Mn+ ions. Electrochemical Titrations