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 species 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

C6H6O2 ⇌ C6H4O2 \+ 2H+ +2e−.

The potential difference between the platinum electrode and a reference electrode is dependent on the activity, aH+, of hydrogen ions in the solution.

E=E0+RT2Fln⁡aH+ (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]

## References

1. ↑ 1.0 1.1 Bates, Roger G. Determination of pH: theory and practice. Wiley, 1973, pp 246-252
2. ↑ Rossotti, F. J. C.; Rossotti, H. (1961). The Determination of Stability Constants. McGraw-Hill. , p 135
3. ↑ Sakurai, T. (1968). "On the refinement of the crystal structures of phenoquinone and monoclinic quinhydrone". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry 24 (3): 403–412. doi:10.1107/S0567740868002451.
4. ↑ Pietrzyk, DONALD J.; Frank, CLYDE W. (1979-01-01), Pietrzyk, DONALD J.; Frank, CLYDE W., eds., "Chapter Thirteen - Ion-Selective Electrodes" (in en), Analytical Chemistry (Academic Press): pp. 291–319, doi:10.1016/b978-0-12-555160-1.50017-4, ISBN 978-0-12-555160-1, https://www.sciencedirect.com/science/article/pii/B9780125551601500174, retrieved 2022-11-17
5. ↑ Vonau, W.; Guth, U (2006). "pH Monitoring: a review". Journal of Solid State Electrochemistry 10 (9): 746–752. doi:10.1007/s10008-006-0120-4.

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