Potassium sodium tartrate

Sodium potassium L(+)-tartrate tetrahydrate^[1][2]

Names
IUPAC name Sodium potassium L(+)-tartrate tetrahydrate
Other names E337; Seignette's salt; Rochelle salt
Identifiers
CAS Number	304-59-6 Y
3D model (JSmol)	Interactive image
ChemSpider	8031536 Y
ECHA InfoCard	100.005.598
EC Number	206-156-8
E number	E337 (antioxidants, ...)
PubChem CID	9855836
UNII	QH257BPV3J Y
InChI InChI=1S/C4H6O6.K.Na/c5-1(3(7)8)2(6)4(9)10;;/h1-2,5-6H,(H,7,8)(H,9,10);;/q;2*+1/p-2 Y Key: LJCNRYVRMXRIQR-UHFFFAOYSA-L Y InChI=1/C4H6O6.K.Na/c5-1(3(7)8)2(6)4(9)10;;/h1-2,5-6H,(H,7,8)(H,9,10);;/q;2*+1/p-2 Key: LJCNRYVRMXRIQR-NUQVWONBAG
SMILES [K+].[Na+].O=C([O-])C(O)C(O)C([O-])=O
Properties
Chemical formula	KNaC4H4O6·4H2O
Molar mass	282.1 g/mol
Appearance	large colorless monoclinic needles
Odor	odorless
Density	1.79 g/cm³
Melting point	75 °C (167 °F; 348 K)
Boiling point	220 °C (428 °F; 493 K) anhydrous at 130 ℃; decomposes at 220 ℃
Solubility in water	26 g / 100 mL (0 ℃); 66 g / 100 mL (26 ℃)
Solubility in ethanol	insoluble
Structure
Crystal structure	orthorhombic
Related compounds
Related compounds	Acid potassium tartrate; Aluminum tartrate; Ammonium tartrate; Calcium tartrate; Metatartaric acid; Potassium antimonyl tartrate; Potassium tartrate; Sodium ammonium tartrate; Sodium tartrate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Potassium sodium tartrate tetrahydrate, also known as Rochelle salt, is a double salt of tartaric acid first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, France. Potassium sodium tartrate and monopotassium phosphate were the first materials discovered to exhibit piezoelectricity.^[3] This property led to its extensive use in "crystal" gramophone (phono) pick-ups, microphones and earpieces during the post-World War II consumer electronics boom of the mid-20th Century. Such transducers had an exceptionally high output with typical pick-up cartridge outputs as much as 2 volts or more. Rochelle salt is deliquescent so any transducers based on the material deteriorated if stored in damp conditions.

It has been used medicinally as a laxative. It has also been used in the process of silvering mirrors. It is an ingredient of Fehling's solution (reagent for reducing sugars). It is used in electroplating, in electronics and piezoelectricity, and as a combustion accelerator in cigarette paper (similar to an oxidizer in pyrotechnics).^[2]

In organic synthesis, it is used in aqueous workups to break up emulsions, particularly for reactions in which an aluminium-based hydride reagent was used.^[4] Sodium Potassium tartrate is also important in the food industry. ^[5]

It is a common precipitant in protein crystallography and is also an ingredient in the Biuret reagent which is used to measure protein concentration. This ingredient maintains cupric ions in solution at an alkaline pH.

Preparation

Large Rochelle salt crystal grown aboard Skylab

The starting material is tartar with a minimum tartaric acid content 68 %. This is first dissolved in water or in the mother liquor of a previous batch. It is then saponified with hot caustic soda to pH 8, decolorized with activated charcoal, and chemically purified before being filtered. The filtrate is evaporated to 42 °Bé at 100 °C, and passed to granulators in which Seignette's salt crystallizes on slow cooling. The salt is separated from the mother liquor by centrifugation, accompanied by washing of the granules, and is dried in a rotary furnace and sieved before packaging. Commercially marketed grain sizes range from 2000 μm to < 250 μm (powder).^[2]

Larger crystals of Rochelle salt have been grown under conditions of reduced gravity and convection on board Skylab .^[6]

Piezoelectricity

In 1824, Sir David Brewster demonstrated piezoelectric effects using Rochelle salts,^[7] which led to him naming the effect pyroelectricity.^[8]

References