Potassium bitartrate, also known as potassium hydrogen tartrate, with formula KC4H5O6, is the potassium acid salt of tartaric acid (a carboxylic acid)—specifically, l-( + )-tartaric acid. Especially in cooking, it is also known as cream of tartar. Tartaric acid and potassium naturally occur in grapes; its crude precipitate form tartar or wine stone are byproducts of winemaking found deposited on top of must barrels which can be purified.