Quinhydrone electrode is one of several oxidation-reduction electrode's used for measuring hydrogen ion concentration or pH. It consists of an inert metal wire (usually platinum) 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 concentrations 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 concentration of hydrogen ions.
For practical pH measurement, a second pH independent reference electrode (such as a silver - silver chloride electrode)is also used, and the potential difference between the two electrodes 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. Other electrodes commonly used for measuring pH are the antimony - antimony oxide electrode and the glass electrode.