When a suitable pair of electrode probes are introduced into a solution, a small potential difference is produced. This potential, which is pH sensitive, is amplified and measured on a scale which is calibrated to be read in either millivolt or pH units. Figure 2.1 shows the CHEM-MATE pH meter with a sketch of the electrode system.

Electrodes In Figure 2.2 the electrodes which are most common in pH determination are sketched and briefly described.

Glass Electrode - A potential will develop across a special composition glass membrane when that membrane is used to separate two solutions. The solution inside the glass bulb is held at a constant pH and therefore any change in potential is due to the change in pH of the unknown solution. This electrode does not permit electron transfer but establishes an ion exchange equilibrium between hydrogen ions in the solution being measured and positive ions from the glass. Reference Electrode - The reference electrode completes the electrical circuit by a liquid junction with a slow-flowing filling solution. This electrode also provides a stable and reproducible reference potential with an internal element. The most common internals are the calomel and the silver-silver chloride . These internal half cells remain constant since the filling solution about the internal is constant.

Normally, a saturated potassium chloride filling solution is used. When used with a calomel internal, the electrode is referred to as a SCE (saturated calomel electrode).

Combination Electrode - A combination electrode is the combined parts of a glass and reference built into a single probe.