ABSTRACT
A redox reaction is any reaction between two species where electrons are transferred from one species to another. The species losing electrons is said to be oxidized; as it is giving electrons away and reducing the other reactant it is also called a reducing agent or reductant. The species gaining electrons is reduced and acts as an oxidizing agent or oxidant, as it causes oxidation. In electrochemistry, this transfer of electrons between chemical species is accomplished via an external electrical circuit by using an electrochemical cell. This consists of two half-cells connected together. The simplest half-cell consists of a metal M (or electrode) dipping into an aqueous solution of its metal ion, Mz+ (the electrolyte), which has the half-cell reaction:
The electrons in this reaction are deposited on the metal and their concentration and energy, controlled by the relative stability of the metal and metal ion, determines the electrode potential. A salt bridge, usually consisting of a solution or gel containing saturated KCl in a glass tube, connects the two half-cell solutions (Fig. 1a). An alternative method of separating the two half-cell solutions is a porous glass frit or porous ceramic (Fig. 1b), which also allows the passage of ions without mixing the solutions and enables the half cells and solutions to be combined into one container (see Topic E5). This is experimentally simpler, but introduces an extra cell voltage due to the liquid junction in the frit, and is thus avoided when making thermodynamic measurements.
