The Isopotential Point of a pH Sensor

The isopotential point is another case of disagreement between theory and practice. In theory, all temperature dependent slope lines intersect the theoretical zero point (0 mV/pH 7). When an asymmetry potential is present – and that is always the case – this intersection shifts either to the right or to the left of the zero point, as can be seen from the diagram to the right.

All voltage potentials contained within the pH sensor assembly vary with temperature. The temperature dependency of each individual potential cannot be accurately defined, but it will shift the resultant intersection point of all temperature slope lines away from the theoretical zero point and away from the asymmetry potential. This intersection point is then known as the Isopotential point (U_is = isotherm potential).

In order to perform accurate pH measurement, the position of the isopotential point has to be established. Two buffer solutions are required. The position of the isopotential point can then be determined by measuring the potential difference of the pH sensor assembly against various temperatures (heated buffer solutions). The mV outputs of the sensor are then plotted against their pH value on graph paper and thereby the position of the isopotential point is established. Modern microprocessor-based pH meters/transmitters have the capacity to compensate for the voltage and polarity of the U_is potential.

New pH sensors from Hamilton show a maximum compensation error of 0.1 pH when calibrated at 25°C and thereafter measuring in a solution having 60°C.