To measure pH in-situ, so-called pH testers (for a rough estimate of the pH value) and pocket (portable) pH meters are used (Fig. 7.9). Periodic calibration of the instrument is required.
The determination of pH is very fast and reliable when a combined glass electrode is used. It enables an automatic measurement over long time intervals with the accuracy of ± 0.01 pH units. The glass electrode can be used even in strongly acidic and alkaline solutions, and also in the presence of oxidizing or reducing substances. It must be constantly immersed in water. With time, all glass electrodes deteriorate due to alkali leaching from the surface layers.
When collecting a sample of water, certain principal variables that are prone to more or less rapid change upon sample storage must be measured in-situ. These variables characterize the status of the water at the time of sampling. These variables commonly include electrical conductivity, pH, temperature, redox potential, and sometimes also total hardness, turbidity, salinity and dissolved oxygen. Among the in-situ variables, electrical conductivity determination gives the most important information about water quality since it gives an indication of the salts dissolved in water.
Ion selective electrodes provide, in principle, a method for users to determine the concentrations of many ions. However, the instruments need careful (and often repeated) calibration to reference concentrations and washing in a buffer solution between successive readings. This makes their routine use on-site somewhat problematic and prevents their sensible use as remote instrumentation. For this reason further details of these instruments are given in Section 7.6.2.
