Quinhydrone electrode
The quinhydrone electrode may be used to measure the hydrogen ion concentration (pH) of a solution containing an acidic substance.Bates, Roger G. Determination of pH: theory and practice. Wiley, 1973, pp 246-252{{cite book|first1=F. J. C.|last1=Rossotti|first2=H.|last2=Rossotti|title=The Determination of Stability Constants|publisher=McGraw-Hill|date=1961}}, p 135
Principles and operation
Quinones form a quinhydrone cocrystal by formation of hydrogen bonding between ρ-quinone and ρ-hydroquinone.{{cite journal |doi=10.1107/S0567740868002451 |title=On the refinement of the crystal structures of phenoquinone and monoclinic quinhydrone |date=1968 |last1=Sakurai |first1=T. |journal=Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry |volume=24 |issue=3 |pages=403–412 }} 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.{{Citation |last1=Pietrzyk |first1=DONALD J. |title=Chapter Thirteen - Ion-Selective Electrodes |date=1979-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780125551601500174 |work=Analytical Chemistry |pages=291–319 |editor-last=Pietrzyk |editor-first=DONALD J. |publisher=Academic Press |language=en |doi=10.1016/b978-0-12-555160-1.50017-4 |isbn=978-0-12-555160-1 |access-date=2022-11-17 |last2=Frank |first2=CLYDE W. |editor2-last=Frank |editor2-first=CLYDE W.|url-access=subscription }} 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
:{{chem| C|6|H|6|O|2}} {{eqm}} {{chem| C|6|H|4|O|2}} + 2H+ +2e−.
The potential difference between the platinum electrode and a reference electrode is dependent on the activity, , of hydrogen ions in the solution.
: (Nernst equation)
Limitations
The quinhydrone electrode provides an alternative to the most commonly used glass electrode.{{cite journal |last1=Vonau |first1=W. |last2=Guth |first2=U |title=pH Monitoring: a review |journal=Journal of Solid State Electrochemistry |date=2006 |volume=10 |issue=9 |pages=746–752 |doi=10.1007/s10008-006-0120-4|s2cid=97012644 }} 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.
References
{{reflist}}
Category:Electroanalytical chemistry devices
{{electrochem-stub}}
{{analytical-chem-stub}}