Temperature and the Nernst Equation
The temperature dependency of the Nernst equation (and with it the temperature dependency of the theoretical slope of a pH sensor) has already been discussed in our prior article. The temperature dependency of the Nernst equation is easily calculated, and as a rule, only this temperature influence is considered by instrument manufacturers when they incorporate conventional manual or automatic temperature compensation in their pH meters. The adjacent graph illustrates the theoretical error which is compensated for by conventional temperature compensation.
The Position of the Isopotential Point
The position of the isopotential point has to be taken into consideration as well. Today electrode manufacturers try to produce pH measurement electrodes which have the isopotential point positioned as near as possible to the theoretical zero point (pH 7). This reduces the residual error of conventional temperature compensation. Nowadays, microprocessor-based pH meters and transmitters have the capacity to correct the position of the isopotential point (Uis) automatically during calibration.
The Temperature Dependency of the Measured Solution
The third factor is the pH/temperature dependency of the measured liquid solution which is referred to as the temperature coefficient of that solution. The dissociation of molecules is highly temperature dependent. Any change in temperature of the measured solution results in a change in the hydrogen ion concentration of that solution and therefore in its pH value. This pH change is a reality that is specific to the measured liquid media and cannot be described as a measurement error from the sensor itself.
The pH/temperature dependency of all acids and bases is not always known and it is therefore of utmost importance to state the related temperature when giving a pH value of a solution, otherwise the pH measurement is meaningless.