The electromotive force of a cell consisting of two hydrogen electrodes used to define the pH value would be too difficult to handle in practice, which is why other types of pH electrodes have been (and still are) designed. However, there are also different measurement errors and application limits with the different principles and designs .
Construction methods and measuring principles
- Glass electrodes with gel or liquid electrolyte filling or solid discharge ( polymer , metal)
- Reversible hydrogen electrode combined with a fixed reference electrode e.g. B. Silver-silver chloride electrode
- Enamel electrodes
- Ionophore electrodes
- Antimony and bismuth electrodes
- Metal oxide electrodes (e.g. ruthenium (IV) oxide )
- Ion-sensitive field effect transistors (ISFET)
- Quinhydrone electrode
Before the actual measurement, it is necessary to check and adjust the pH electrode. In practice, this process is referred to as calibration , in fact it is a calibration with subsequent adjustment . For this purpose, the pH electrode is immersed in one, two or more different buffer solutions with a defined pH value and the value displayed on the measuring device is set to the target value. With the 2-point calibration, the asymmetry of the electrode is corrected first with a buffer with pH = 7.0 and then the slope is corrected with another buffer (preferably in the expected pH range) .
Due to the temperature dependence of the pH value, the temperature of the buffer must be observed.
The measurement is then carried out by dipping the electrode into the sample to be examined. Here, too, the temperature must be taken into account or specified in the measured value determined. Particularly with highly viscous samples and with older pH electrodes, it is often necessary to wait a certain amount of time for the measured value to stabilize.