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.
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 purgative but in more recent years its piezoelectric properties have been more important and it has found usage in phonograph pickups and other sensing devices. 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.