Quinhydrone electrode

The quinhydrone electrode is a type of redox electrode which can be used to measure the hydrogen ion concentration (pH) of a solution in a chemical experiment. It provides an alternative to the commonly used glass electrode in a pH meter.

The electrode consists of an inert metal electrode (usually a platinum wire) in contact with quinhydrone crystals and a water-based solution. Quinhydrone is slightly soluble in water, dissolving to form a mixture of two substances, quinone and hydroquinone, with the two substances present at equal concentration. Each one of the two substances can easily be oxidised or reduced to the other.

The potential at the inert electrode depends on the ratio of the activity of two substances (quinone-hydroquinone), and also the hydrogen ion concentration. The electrode half-reaction is:

Hydroquinone ↔ Quinone + 2H⁺ +2e^-

Because the electrode half-reaction involves hydrogen ions, the electrode potential depends on the activity of hydrogen ions. From the Nernst equation:

$E= E^0 %2B \frac{RT}{nF} \ln a_{H^{%2B}}$

For practical pH measurement, a second pH independent reference electrode (such as a silver chloride electrode) is also used. This reference electrode does not respond to the pH. The difference between the potential of the two electrodes depends (primarily) on the activity of H⁺ in the solution. It is this potential difference which is measured and converted to a pH value.

The quinhydrone electrode is not reliable above pH 8. It is also unreliable in the presence of strong oxidising or reducing agents, which would disturb the equilibrium between hydroquinone and quinone. It is also subject to errors in solutions containing proteins or high concentrations of salts.

Other electrodes commonly used for measuring pH are the glass electrode, the hydrogen electrode, the antimony – antimony oxide electrode, and the ion-sensitive field effect transistor ISFET electrode.