In potentiometric pH measurements ion selective electrode is used. Potential of such electrode is usually given by a simplified Nernst equation:
E = E0' - 0.0591/n pa
where n is a charge of an ion and pa is minus logarithm of its activity. In the case of H+ ions this equation takes the simplest possible form:
E = E0' - 0.0591 pH
In practice both constants present in the equation (E0' and 0.0591) are not as constant as we would like them to be and they can change with the electrode age and temperature. However, usually dependence between the electrode potential and solution pH is linear in the wide range of pH and stable in a few hours time. Thus it is enough to calibrate the electrode using two buffer solutions of known pH before measurements, to obtain reliable results.
It is important to remember that potential of pH electrode is measured not as an absolute value, but against a reference electrode of known potential. Most often used reference electrodes are silver chloride electrode and saturated calomel electrode (the latter being phased out, as it uses mercury). Depending on the details of pH electrode construction reference electrode can be closed in the same housing or can be separated. This is covered in much more details in the pH electrode section.