Potentiometers - Definitions and general principles

Of all electrochemical methods, potentiometry is certainly the most frequently used. It is the evolution of instrumentation that has enabled it to develop (the demonstration of the existence of a membrane potential for glass is over one hundred years old). In the case of membrane electrodes, the method exploits extremely small variations in potential, which only very high impedance millivoltmeters are capable of discriminating. It was only when such devices could be built, thanks to the parallel development of electronics, that this method became available. The availability to laboratories of instrumental pH measurement by potentiometry thus represented, some sixty years ago, a considerable advance in terms of convenience, speed and precision.

A physico-chemical analysis method, since it can measure the activities of real species in solution, potentiometry was the subject of a major new development some thirty years ago, this time on the electrode side. In response to the initial demand from biologists to be able to measure calcium activities, and thus discriminate between free Ca ²⁺ and complexed forms, a calcium-indicating liquid membrane electrode was built. At the same time, the development of solid-state chemistry led to the development of a fluoride-indicating membrane electrode, based on a single crystal of lanthanum fluoride doped with europium. From these two electrodes, a whole range of electrodes was gradually and rapidly developed, whose initial processing disadvantages (in the case of liquid membrane electrodes) were gradually overcome by the development of disposable membranes.

The recent development of information technology has enabled potentiometry to cross a new threshold by greatly extending its possibilities, in terms of data acquisition, processing, storage and measurement automation, through the design of appropriate, high-performance software. At the same time, equipment was also undergoing a major evolution, with the miniaturization of devices, their adaptation to field measurements, and the integration of all necessary functions into a potentiometric analysis chain, often automatic.

Allowing speciation of species in solution as a result of physico-chemical measurement, suitable for in-situ measurement and on-line control, potentiometry has continued to develop ever since, and is now used in all sectors of activity, from laboratory analysis (chemical, biochemical, pharmaceutical, etc.) to industrial analysis (for process control, water monitoring, warning devices, etc.).

This article is devoted to the general principles governing potentiometry. It begins with a reminder of the relationship between activities and concentrations, insofar as potentiometry...