Basic principles of pH registration.

Normally, pH-sensitive electrodes are used for measuring the acidity of an aqueous solution. A second electrode is required as reference and can be integrated in the pH-sensitive electrode (combined electrode); separate reference electrodes must be used, however, when combined ones are not available. Types of electrodes are: glass, antimony, ISFET and polymeric membranes; only glass and antimony electrodes are commercially available. Different factors determine the accuracy of a pH measurement, the most important being: (1) electrical and mechanical properties of the electrodes; (2) ionic strength, temperature and background activity of the test solution; and (3) electrical properties of the pH amplifier and display. The highest accuracy of about 0.1 pH units can be obtained with the glass electrode; the error of measuring systems equipped with antimony electrodes does not normally exceed 0.5 pH units. It is important to define the required accuracy prior to measurement. If, for example, long-term oesophageal pH monitoring is performed in newborns or infants, the best compliance is achieved with the relatively small and flexible antimony electrode. If, on the other hand, the effect of an allegedly weak drug on gastric acidity is to be tested in a volunteer study, only glass electrodes will be appropriate.