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, $a_{H^{+}}$ , of hydrogen ions in the solution.

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]

References

^ ^a ^b Bates, Roger G. Determination of pH: theory and practice. Wiley, 1973, pp 246-252
^ Rossotti, F. J. C.; Rossotti, H. (1961). The Determination of Stability Constants. McGraw-Hill., p 135
^ 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.
^ Pietrzyk, DONALD J.; Frank, CLYDE W. (1979-01-01), Pietrzyk, DONALD J.; Frank, CLYDE W. (eds.), "Chapter Thirteen - Ion-Selective Electrodes", Analytical Chemistry, Academic Press, pp. 291–319, doi:10.1016/b978-0-12-555160-1.50017-4, ISBN 978-0-12-555160-1, retrieved 2022-11-17
^ 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. S2CID 97012644.

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[Bates-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", Analytical Chemistry, Academic Press, pp. 291–319, doi:10.1016/b978-0-12-555160-1.50017-4, ISBN 978-0-12-555160-1, 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. S2CID 97012644.

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