Tartaric acid

Ball-and-stick Stick Space-filling

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16 atoms, 15 bonds.

IUPAC name: (2R,3R)-2,3-dihydroxybutanedioic acid
Molar mass: 150.086(15) g/mol
Atoms: 16
SMILES: C(C(C(=O)O)O)(C(=O)O)O
InChIKey: FEWJPZIEWOKRBE-JCYAYHJZSA-N
PubChem CID: 444305

Made of

Structural formula of tartaric acid. Each unlabelled corner is a carbon atom, and the two hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Oxygen is bonded to every carbon in tartaric acid’s short, unbranched chain. The two end carbons each belong to a carboxyl group, –COOH. With two such groups it is a dicarboxylic acid, as the US National Cancer Institute (NCI) classes it. The two middle carbons each carry a hydroxyl group, –OH. Its IUPAC name, (2R,3R)-2,3-dihydroxybutanedioic acid, can be read part by part. It describes a four-carbon diacid with hydroxyl groups on carbons 2 and 3, and its prefix fixes how those two carbons are arranged in space.

Four atoms of carbon, six of hydrogen and six of oxygen add up to C₄H₆O₆, with a molar mass of 150.09 g/mol. The EU food-additive specification describes the pure acid as colourless or translucent crystals, or a white crystalline powder. It dissolves very readily: the International Chemical Safety Cards (ICSC) put its solubility in water at 1400 g per litre at 20 °C.

Structure and bonding

Four carbon atoms form tartaric acid’s spine. Each end carbon holds one O by a double bond, drawn in Ball-and-stick as two sticks, and a second O carrying an H. Each middle carbon holds one H and one –OH. Stick thins the frame to show all four O–H bonds, and in Space-filling the six oxygens make up most of the outline.

Both middle carbons are chiral centres, each bonded to four different groups, as the lactic acid page explains. The model draws the (2R,3R) form, L-tartaric acid, and the ChEBI database of biological molecules names D-tartaric acid as its mirror image. As flat formulas the two look alike, since according to Chem1 two-dimensional formulas hide what carbon’s three-dimensional bonding allows.

The model shows the un-ionised acid; dissolved in water, some molecules give up a carboxyl hydrogen and become tartrate ions. ChEBI calls L-tartaric acid the conjugate acid of L-tartrate(1−), the ion left after one such loss. Where an acid has more than one hydrogen to give, Chem1 explains, the second leaves less readily than the first. A proton is harder to pull from an ion that is already negatively charged.

Where it turns up

Tartaric acid occurs in many plants, tamarinds and grapes in particular, according to the NCI. The Merck Index classes it as a fruit acid found throughout nature. The LOTUS database of natural products also reports it in Camellia sinensis and Catunaregam spinosa.

In soft drinks and other foods it works as an acidulant, an ingredient added for acidity, according to the Merck Index. The EU counts it among food additives. Winemaking uses a related additive, metatartaric acid. According to JECFA, the food-additive panel that the UN’s Food and Agriculture Organization and WHO run together, enzymes in the body turn it into tartaric acid before absorption.

Outside the kitchen it serves cosmetics as a buffering agent (buffers are explained on the phosphoric acid page). It also appears on the International Fragrance Association’s list of fragrance ingredients. The US Environmental Protection Agency (EPA) permits it as an inert ingredient in pesticides not used on food.

Why it matters

The tartaric acid that goes into food comes from winemaking. The US Food and Drug Administration (FDA) regulates food-grade tartaric acid under 21 CFR 184.1099. That regulation records it as a by-product of wine manufacture, with the L configuration that the model draws. In food it works as a flavouring and as an anticaking, drying, firming and leavening agent, according to the FDA. The FDA also lists it as a humectant, which holds moisture in a food, and as a pH control agent. JECFA also classes L(+)-tartaric acid as a synergist, an additive that strengthens the effect of antioxidants.

Medicine pairs it with sodium bicarbonate for one X-ray examination. Swallowed together, the two produce carbon dioxide in the stomach. The gas swells the stomach and, according to the NCI, serves as a negative contrast medium in double-contrast radiography. A contrast medium is a substance that makes an organ stand out on the image.

Properties

Properties of tartaric acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 444305.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₄H₆O₆
Molar mass	150.086(15) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	4
Hydrogen atoms	6
Oxygen atoms	6
Bonds	15
Single bonds	13
Double bonds	2
PubChem molecular weight	150.09 g/mol	g/mol	PubChem

Safety

Pure tartaric acid powder can seriously damage the eyes, although the acid is a permitted food additive. In notifications from companies to the European Chemicals Agency (ECHA) it carries the signal word Danger. It is also classed there as irritating to the skin and airways and harmful if swallowed, and the ICSC call the solid combustible. The PubChem Laboratory Chemical Safety Summary sets out these hazards and the exposure limits that exist for it.

Questions

How many chiral centres does tartaric acid have?

Two, at carbons 2 and 3 in the middle of the chain. In the 3D model with Labels on, each bonds to one H, one O–H group, a carboxyl group and the other middle carbon: four different partners. The (2R,3R) in the IUPAC name records how both are arranged in L-tartaric acid, the form drawn, and the structural formula marks each with a wedge bond.

What does the plus sign in L(+)-tartaric acid mean?

The plus sign marks this form as dextrorotatory: a solution of it turns the plane of polarised light through a positive angle. The EU food-additive specification gives that angle, its specific rotation, as between +11.5° and +13.5° on a 20 per cent solution. The L is a separate label, naming which of the two mirror-image forms this is, as opposed to D-tartaric acid.

Is tartaric acid safe to eat?

It is a permitted food additive in the US and the EU. JECFA, the joint food-additive committee of the UN's Food and Agriculture Organization and WHO, re-evaluated L(+)-tartaric acid in 2017. It kept a group acceptable daily intake for the acid, its sodium, potassium and potassium sodium salts, and now metatartaric acid. The US Environmental Protection Agency's Safer Choice programme expects it to be of low concern. The pure powder, though, can seriously damage the eyes.

What is tartaric acid used for besides food?

It serves photography, tanning and ceramics and is used to make its salts, the tartrates, according to the Merck Index. Its diethyl and dibutyl esters go into lacquers and textile printing, and European chemical registrations place it in cleaning products and metal surface treatments, electroplating included. Researchers report recovering cobalt of 99.1 per cent purity from dissolved lithium-ion battery material by electrowinning with tartaric acid. Electrowinning deposits a metal from its solution with an electric current.

Made of the same elements

C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.