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quinhydrone electrode
1. An electrode in the electric potential is generated by the relative proportions of quinone (a class of aromatic compounds found widely in plants, especially the yellow crystalline form used in making dyes, tanning hides, and photography) and quinhydrone (dark green, crystalline, slightly water-soluble solid used in a solution, together with a platinum wire, as an electrode) which are present.
2. One of several oxidation-reduction electrode's in which the ratio of the two forms (quinone-quinhydrone), determined by the hydrogen ion concentration, sets up a potential that can be measured and converted to a pH value (fails above pH 8).
3. Quinhydrone electrode is a redox electrode (inert electrode; such as, platinum, gold, carbon) used for measuring pH (measure of the acidity/alkalinity of a solution).

An inert metal (usually platinum) is immersed into the solution to be analyzed and a small amount of quinhydrone crystals is added to the solution.

Quinhydrone is slightly soluble in water, dissolving to form a mixture of two substances, with each of the two substances easily oxidized or reduced to the other.

The potential at the inert electrode depends on the ratio of the concentrations of two substances, which, in turn, depends on the pH.

This entry is located in the following unit: electro-, electr-, electri- (page 99)