potassium sodium tartrate

Potassium sodium tartrate
IUPAC name	Potassium sodium tartrate
Other names	E337
Identifiers
CAS number	[304-59-6]
EC number	206-156-8
Properties
Molecular formula	KNaC4H4O6·4H2O
Molar mass	282.1 g/mol
Melting point	75 °C
Boiling point	220 °C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Potassium sodium tartrate is a double salt first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, France. As a result the salt was known as Seignette's salt or Rochelle salt. Rochelle salt is not to be confused with rock salt, which is simply the mineral form of sodium chloride. Potassium sodium tartrate and monopotassium phosphate were the first materials discovered to exhibit piezoelectricity.^[1] This property led to its extensive use in "crystal" gramophone (phono) pick-ups, microphones and earpieces during the post-War 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 is a colorless to blue-white salt crystallizing in the orthorhombic system. Its molecular formula is KNaC₄H₄O₆·4H₂O. It is slightly soluble in alcohol but more completely soluble in water. It has a specific gravity of about 1.79, a melting point of approximately 75 °C, and has a saline, cooling taste. As a food additive, its E number is E337.

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, formerly used in the determination of reducing sugars in solutions.

In organic synthesis, it is used in aqueous workups to break up emulsions, particularly for reactions in which an aluminum-based hydride reagent was used.^[2]

It 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 of the Skylab

Rochelle salt (potassium sodium tartrate, NaKC₄H₄O₆) can be prepared from potassium bitartrate (KHC₄H₄O₆) and sodium carbonate (Na₂CO₃). A potassium bitartrate solution is heated and sodium carbonate is added to it in a 1:0.5 (KHC₄H₄O₆ : Na₂CO₃) mole ratio. The solution is then filtered while hot and then heated to dryness. The precipitate is small crystallites potassium sodium tartrate. Larger crystals of Rochelle salt have been grown under conditions of reduced gravity and convection aboard the Skylab.^[3]

References