Tartaric acid

Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds, and is one of the main acids found in wine. It is added to other foods to give a sour taste, and is used as an antioxidant. Salts of tartaric acid are known as tartrates. It is a dihydroxyl derivative of succinic acid.

Tartaric acid was first isolated from potassium tartrate, known to the ancients as tartar, c. 800 by the alchemist Jabir ibn Hayyan. The modern process was developed in 1769 by the Swedish chemist Carl Wilhelm Scheele.

Tartaric acid played an important role in the discovery of chemical chirality. This property of tartaric acid was first observed in 1832 by Jean Baptiste Biot, who observed its ability to rotate polarized light. Louis Pasteur continued this research in 1847 by investigating the shapes of ammonium sodium tartrate crystals, which he found to be chiral. By manually sorting the differently shaped crystals under magnification, Pasteur was the first to produce a pure sample of levotartaric acid.

Stereochemistry
Naturally occurring tartaric acid is chiral, meaning that it has molecules that are non-superimposable on their mirror-images. It is a useful raw material in organic chemistry for the synthesis of other chiral molecules. The naturally occurring form of the acid is L-(+)-tartaric acid or dextrotartaric acid. The mirror-image (enantiomeric) form, levotartaric acid  or D-(−)-tartaric acid, and the achiral form, mesotartaric acid, can be made artificially. Note, that the dextro and levo prefixes are not related to the D/L configuration (which is derived rather indirectly from their structural relation to D - or L -glyceraldehyde), but to the orientation of the optical rotation, (+) = dextrorotatory, (−) = levorotatory. Sometimes, instead of capital letters, small italic d and l are used. They are abbreviations of dextro- and levo- and, nowadays, should not be used. Levotartaric and dextrotartaric acid are enantiomers, mesotartaric acid is a diastereomer of both of them.

A rarely occurring optically inactive form of tartaric acid,  DL -tartaric acid is a 1:1 mixture of the levo and dextro forms. It is distinct from mesotartaric acid and was called racemic acid (from Latin racemus – "a bunch of grapes"). The word racemic later changed its meaning, becoming a general term for 1:1 enantiomeric mixtures – racemates.

Tartaric acid is used to prevent copper(II) ions from reacting with the hydroxide ions present in the preparation of copper(I) oxide. Copper(I) oxide is a reddish brown solid, and is produced by the reduction of a Cu(II) salt with an aldehyde, in an alkaline solution.

Derivatives
Important derivatives of tartaric acid include its salts, cream of tartar (potassium bitartrate), Rochelle salt (potassium sodium tartrate, a mild laxative), and tartar emetic (antimony potassium tartrate). Diisopropyl tartrate is used as a catalyst in asymmetric synthesis.

Tartaric acid is a muscle toxin, which works by inhibiting the production of malic acid, and in high doses causes paralysis and death. The median lethal dose (LD50) is about 7.5 grams/kg for a human, ~5.3 grams/kg for rabbits and ~4.4 grams/kg for mice. Given this figure, it would take over 500 g to kill a person weighing 70 kg, and so it may be safely included in many foods, especially sour-tasting sweets. As a food additive, tartaric acid is used as an antioxidant with E number E334, tartrates are other additives serving as antioxidants or emulsifiers.

When cream of tartar is added to water, a suspension results which serves to clean copper coins very well, as the tartrate solution can dissolve the layer of copper(II) oxide present on the surface of the coin. The resulting copper(II)-tartrate complex is easily soluble in water.

Tartaric acid in wine
Tartaric acid may be most immediately recognizable to wine drinkers as the source of "wine diamonds", the small potassium bitartrate crystals that sometimes form spontaneously on the cork. These "tartrates" are harmless, despite sometimes being mistaken for broken glass, and are prevented in many wines through cold stabilization. The tartrates that remain on the inside of aging barrels were at one time a major industrial source of potassium bitartrate.

However, tartaric acid plays an important role chemically, lowering the pH of fermenting "must" to a level where many undesirable spoilage bacteria cannot live, and acting as a preservative after fermentation. In the mouth, tartaric acid provides some of the tartness in the wine, although citric and malic acids also play a role.