Rochelle salt

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Rochelle salt,

colorless to blue-white orthorhombic crystalline salt with a saline, cooling taste. It is also called Seignette salt after Pierre Seignette, an apothecary of La Rochelle, France, who was the first to make it (c.1675). Chemically, it is potassium sodium tartrate, KNa (C4H4O6)·4H2O. It is soluble in water and slightly soluble in alcohol, melts at about 75°C;, has specific gravity 1.79, and exhibits double refraction. It is used in medicine as a mild purgative, often in the form of Seidlitz powders. It is an ingredient of Fehling's solutionFehling's solution
, deep-blue, alkaline solution used to test for the presence of aldehydes (e.g., formaldehyde, HCHO) or other compounds that contain the aldehyde functional group, -CHO.
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. It is used in silvering mirrors. Crystals of Rochelle salt are easily grown and are used in piezoelectric devices, e.g., crystal microphones and phonograph pickup cartridges (see piezoelectric effectpiezoelectric effect
, voltage produced between surfaces of a solid dielectric (nonconducting substance) when a mechanical stress is applied to it. A small current may be produced as well. The effect, discovered by Pierre Curie in 1883, is exhibited by certain crystals, e.g.
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).

Rochelle salt

[rō′shel‚sȯlt]
(inorganic chemistry)