Potentiometry - PH or concentration measurement

The first part of [P 2 115] describes the development of potentiometry into what it is today. Readers are invited to refer to this section.

The previous article dealt with the general principles of potentiometry. Here, we describe the two main measurements that can be made using potentiometry: pH and the concentration of an ion or molecule. Presented in this way, it might appear that the field of application of potentiometry is quite limited. This is clearly not the case.

The measurement of pH, at the origin of the development of potentiometry, still constitutes a privileged field of action (in aqueous media and solvents). First, we'll review the general principles of pH measurement, in addition to its very definition, and the means of measuring it. While glass electrodes are most often the ideal means of measurement, this is not always the case, particularly for complex or corrosive media that do not allow the use of glass. This is why, in addition to the hydrogen electrode, we also mention so-called second type electrodes (quinhydrone, antimony), which can be used as alternatives when the use of a glass electrode is not possible. The operating characteristics of glass electrodes will be described here, as they determine the reliability of pH measurement. While the use of a glass electrode combined with a device for automatic delivery of acid or base on either side of a set point can provide a means of continuously controlling the pH of a liquid medium, acidity buffers are more commonly used. The first case is more a matter of industrial processes (where the use of buffers is hardly conceivable), while the second is implemented in analysis and control laboratories. The operating characteristics of buffers and their composition according to the media concerned will be described.

Measuring the concentration of an ion or molecule greatly extends the possibilities of potentiometry. The principle of measurement and the general characteristics of ion indicator electrodes will be described. The important problem of selectivity and the impact on measurement of the presence of other ions will be discussed. The different types of ion indicator electrodes will be described, whether they exploit a redox potential or a membrane potential. Gas electrodes, a slightly different category, will also be described.