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pH meters are used to determine the pH of a water sample or solution. There are four pH meters in the engineering laboratories. pH meters use electrodes to determine the difference in electrical potential (voltage) between the reference electrode and the internal or ion selective electrode. The reference electrode maintains a constant electrical potential based on the internal electrode buffer solution, independent of the sample composition and temperature. In contrast, the internal electrode or hydrogen ion selective electrode (ISE) determines an electrical potential based on hydrogen (H+) ion activity in the sample solution. Based on the difference between electrode potentials pH is calculated by the pH meter.