Quinhydrone electrode

The quinhydrone electrode may be used to measure the hydrogen ion concentration (pH) of a solution containing an acidic substance.^[1][2]

Principles and operation

Quinones form a quinhydrone cocrystal by formation of hydrogen bonding between ρ-quinone and ρ-hydroquinone.^[3] An equimolar mixture of ρ-quinones and ρ-hydroquinone in contact with an inert metallic electrode, such as antimony, forms what is known as a quinhydrone electrode. Such devices can be used to measure the pH of solutions.^[4] Quinhydrone electrodes provide fast response times and high accuracy. However, it can only measure pH in the range of 1 to 9 and the solution must not contain a strong oxidizing or reducing agent.  

A platinum wire electrode is immersed in a saturated aqueous solution of quinhydrone, in which there is the following equilibrium

C
₆H
₆O
₂ ⇌ C
₆H
₄O
₂ + 2H⁺ +2e⁻.

The potential difference between the platinum electrode and a reference electrode is dependent on the activity, ${\displaystyle a_{H^{+}}}$, of hydrogen ions in the solution.

${\displaystyle E=E^{0}+{\frac {RT}{2F}}\ln a_{H^{+}}}$ (Nernst equation)

Limitations

The quinhydrone electrode provides an alternative to the most commonly used glass electrode.^[5] however, it is not reliable above pH 8 (at 298 K) and cannot be used with solutions that contain a strong oxidizing or reducing agent.^[1]