Here R is the gas constant, T the absolute temperature (K) and F the Faraday constant.
The sensitivity RT/F is the same factor as kT/q as known from solid state physics. Ion
concentrations ci are noted in terms of activities ,ai=fi.ci, with fi being the activity
coefficient. In diluted electrolytes fi =1. This equation shows that, providing that at
one side of the interface the activity ai of the ion of interest is kept constant, the
electrode potential is a direct logarithmic function of the ion activity on the other side.
Thus, for instance, a metal electrode in its “own solution”, for instance copper in a
copper sulphate solution, will result in a stable defined potential, provided that no
interference reaction will occur. In practice this type of ion sensor will hardly appear,
especially not in biomedical measurements. A well-known electrode makes use of a
membrane of conducting glass, which buffers the ion of interest in a thin surface layer
of the membrane. pH, pNa, pK etc. sensitive glass compositions have been developed,
which are shaped in a bulb size and melted to a glass shaft. The internal volume of the
bulb with its shaft is filled with a liquid of known, constant composition. This means
that now in fact a cell is constructed with a constant potential drop at the inner surface
of the glass membrane and a “sense potential” at its outer surface, both according to
the Nernst equation. This can only be measured by contacting both, the internal solution as well as the external solution in which an ion concentration has to be
measured, by reference electrodes. A reference electrode is in fact nothing else than a
contact between a metal wire and an aqueous solution, to determine the electrical
potential of this solution. In practice a reference electrode consists of a chloridated
silver wire (silver with a coating of insoluble silver chloride) in a potassium chloride
solution with a constant concentration. The electrochemical couple thus formed
results in a constant potential according to the Nernst equation. The inner solution of a
reference electrode makes contact with the solution of which the electrical potential
has to be measured by means of a barrier, the so-called frit. Often glass membrane
electrodes are combined with reference electrodes to one system.
Various performances of such rather bulky electrode assemblies, both separate pH
glass electrodes and reference electrodes, as well as combined systems, are in daily
use in laboratories of chemical analysis, process industry etc. A typical performance
of a combined pH glass electrode is shown in figure 1.