Potassium sodium tartrate

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.

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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.

It is also an ingredient in the Biuret reagent which is used to measure protein concentration.

Preparation

Rochelle salt (potassium sodium tartrate, NaKC₄H₄O₆) can easily be prepared from potassium bitartrate (KHC₄H₄O₆) and sodium carbonate (Na₂CO₃). First heat a potassium bitartrate solution. Add sodium carbonate to the still hot solution. Add sodium carbonate until no more reacts (Effervescence will occur). Filter the solution while hot and then heat to evaporate the water. After continued heating potassium sodium tartrate will precipitate.

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