Potassium bitartrate

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.

Approved by the FDA as a direct food substance, cream of tartar is used as an additive, stabilizer, pH control agent, antimicrobial agent, processing aid, and thickener in various food products. It is used as a component of baking powders and baking mixes, and is valued for its role in stabilizing egg whites, which enhances the volume and texture of meringues and soufflés. Its acidic properties prevent sugar syrups from crystallizing, aiding in the production of smooth confections such as candies and frostings. When combined with sodium bicarbonate, it acts as a leavening agent, producing carbon dioxide gas that helps baked goods rise. It will also stabilize whipped cream, allowing it to retain its shape for longer periods.

Potassium bitartrate further serves as mordant in textile dyeing, as reducer of chromium trioxide in mordants for wool, as a metal processing agent that prevents oxidation, as an intermediate for other potassium tartrates, as a cleaning agent when mixed with a weak acid such as vinegar, and as reference standard pH buffer. It has a long history of medical and veterinary use as a laxative administered as a rectal suppository, and is used also as a cathartic and as a diuretic. It is an approved third-class OTC drug in Japan^{[citation needed]} and was one of active ingredients in Phexxi, a non-hormonal contraceptive agent that was approved by the FDA in May 2020.

Potassium bitartrate was first characterized by Swedish chemist Carl Wilhelm Scheele (1742–1786). This was a result of Scheele's work studying fluorite and hydrofluoric acid.

Scheele may have been the first scientist to publish work on potassium bitartrate, but use of potassium bitartrate has been reported to date back 7000 years to an ancient village in northern Iran. Modern applications of cream of tartar started in 1768 after it gained popularity when the French started using it regularly in their cuisine.

In 2021, a connection between potassium bitartrate and canine and feline toxicity of grapes was first proposed. Since then, it has been deemed likely as the source of grape and raisin toxicity to pets.

Potassium bitartrate is naturally formed in grapes from the acid dissociation of tartaric acid into bitartrate and tartrate ions.

Potassium bitartrate has a low solubility in water. It crystallizes in wine casks during the fermentation of grape juice, and can precipitate out of wine in bottles. The rate of potassium bitartrate precipitation depends on the rates of nuclei formation and crystal growth, which varies based on a wine's alcohol, sugar, and extract content. The crystals (wine diamonds) will often form on the underside of a cork in wine-filled bottles that have been stored at temperatures below 10 °C (50 °F), and will seldom, if ever, dissolve naturally into the wine. Over time, crystal formation is less likely to occur due to the decreasing supersaturation of potassium bitartrate, with the greatest amount of precipitation occurring in the initial few days of cooling.