As illustrated in Fig, 1, when two solutions with different pH values are separated by a pH-sensitive glass membrane, a potential difference develops that can be measured by chemical half-cells as a voltage change across the membrane. The reference half-cell, usually composed of mercury/mercurous chloride (calomel) housed in a compartment filled with potassium chloride solution, supplies a constant reference voltage. The measuring half-cell is a Sanz electrode, usually composed of a silver/silver chloride substance embedded within a pH 6.840 buffer chamber. A contact bridge consisting of a potassium chloride solution completes the electronic circuit.
Fig. 1 Basic principles of the pH electrode: (a) a voltage develops across pH-sensitive glass when pH is unequal in the two solutions; (b) chemical half-cells used as measuring and reference electrodes; (c) the pH electrode system completed by a device measuring voltage changes. (Reproduced with permission from Shapiro et al. (.,19.94),)
The relationship between the potential difference Eu - Ek and the pH values of the known (pHk) and unknown (pHu) solutions is defined by the Nernst equation:
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