Sodium Potassium Tartrate
The short version
Rochelle salt — a mixed-cation tartrate buffer; also famous for its piezoelectric crystals.
It's widely considered safe at the amounts used in food, so we mark it green. The Safety section below has the details.
Where it comes from
Discovered in 1675 by Pierre Seignette of La Rochelle, France (the historic name). Produced by neutralizing tartaric acid with mixed sodium and potassium hydroxides. The crystalline form is famously piezoelectric — used in early phonograph pickups and pressure sensors.
How it's used
Used as a buffer in processed cheese (where the mixed-cation form gives a softer melt than pure sodium or potassium citrate), in jellies, and in instant puddings. Outside food, the Fehling and biuret reagents for sugar and protein assays use Rochelle salt to keep copper soluble in alkali.
Safety profile
ADI of 0–30 mg/kg body weight (combined tartrates). Same safety profile as the other tartrate salts. Mild laxative at high doses, like other tartrates.
Physical & chemical properties
Colorless crystalline powder, very soluble in water. Mildly alkaline solution. Hygroscopic. Forms piezoelectric crystals when grown slowly.
Regulation worldwide
Permitted globally without restrictive limits.
Educational information only. Not medical or dietary advice.