New developments in coulometric titrations: Non-isolated counter electrodes

A non-isolated counter electrode for coulometric titrations has been developed. The idea is to render the mass transport toward the non-isolated electrode negligibly small by modifying the factors involved. In practice, a miniature non-isolated inert electrode is positioned in the upper part of the solution, away from the generating electrode. This procedure is applicable in the following cases, which normally requires the use of an isolated counter electrode: (i) the electrogenerated titrant can react at the counter electrode, (ii) the analyte can be reformed at the counter electrode, and (iii) the hydrogen evolved at a counter anode can be oxidized at the generating electrode or by the titrant. This approach is analyzed theoretically and proven experimentally to provide an excellent solution for several types of coulometric titrations: Fe2+ with Ce4+; As(III) with I2; Cl− with Hg2+2. There are cases in which the non-isolated mini-counter electrode is not suitable. For example, titration of Cl− with Ag+, where silver dendrites formed on the counter electrode, increase its area and reduce the titration efficiency. The coulometric electrode assembly with non-isolated counter electrode is simple and increases the suitability of the coulometric method, especially when automation and miniaturization are required.