The pH is normally measured by chemical indicators or by pH meters. The final color of chemical indicators depends on the hydrogen ion concentration; their accuracy is only 0.1 to 0.2 pH units. For indication of acid, alkali, or neutral water, litmus paper is used; it turns pink when acidic, blue when alkaline, and stays white if neutral.
A pH sensor normally consists of a sensing electrode and a reference elec- trode immersed in the test solution which forms an electrolytic cell, as shown in Fig. 9.10a. One electrode contains a saturated potassium chloride (alkaline) solution to act as a reference; the electrode is electrically connected to the test solution via the liquid junction. The other electrode contains a buffer which sets the electrode in contact with the liquid sample. The electrodes are connected to a differential amplifier, which amplifies the voltage difference between the electrodes, giving an output voltage that is proportional to the pH of the solu- tion. Figure 9.10b shows the pH sensing electrode.