Citric acid

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21 atoms, 20 bonds.

IUPAC name: 2-hydroxypropane-1,2,3-tricarboxylic acid
Molar mass: 192.123(21) g/mol
Atoms: 21
SMILES: C(C(=O)O)C(CC(=O)O)(C(=O)O)O
InChIKey: KRKNYBCHXYNGOX-UHFFFAOYSA-N
PubChem CID: 311

Made of

Structural formula of citric acid. Each unlabelled corner is a carbon atom, and the four hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Biologists name a whole metabolic pathway after this acid: the citric acid cycle, also called the TCA cycle or the Krebs cycle. Chemists name the molecule 2-hydroxypropane-1,2,3-tricarboxylic acid, C₆H₈O₇, with a molar mass of 192.12 g/mol. The name lists its parts. A carboxyl group is a carbonyl, a carbon double-bonded to oxygen, whose carbon also holds an –OH by a single bond, and citric acid carries three of them.

The pure solid forms colourless, odourless crystals with an acid taste, and it melts at 153 °C. Heated further, it decomposes rather than boiling. It dissolves readily in water: 59.2 per cent of a solution at 20 °C is citric acid by mass. From hot, concentrated solution it crystallises in one form. From cold water it forms a monohydrate, with one water molecule per acid molecule, which effloresces, losing that water, in dry air.

Structure and bonding

Citric acid is built from six carbon, eight hydrogen and seven oxygen atoms. Three carbons form an open backbone with no ring. Each end carbon carries a carboxyl group; the middle carbon carries the third one and an alcohol O–H, the “2-hydroxy” of the name.

Like other larger molecules, it has no single central atom. OpenStax treats such molecules as chains of interior atoms, each with its own local geometry, so no one shape word fits. Stick view shows three C=O double bonds, 1.225 Å long in the model, beside carboxyl C–O singles of 1.355 to 1.359 Å. In Space-filling the atoms merge into one lumpy outline. Its four O–H bonds each put hydrogen on oxygen, OpenStax’s condition for hydrogen bonding, a strong attraction between molecules.

The model draws the neutral acid with no charges, every carboxyl group still holding its hydrogen. In water some of those hydrogens leave as ions; the pKa, a measure of how readily an acid gives up a hydrogen ion, is 2.79. Only a carboxyl hydrogen reacts with bases, according to OpenStax. In cells the molecule goes by the name of its anion, citrate.

Where it turns up

Citrus fruits contain it, and makers add it for tartness to drinks, jellies, jams, preserves and sweets. The pure crystals taste pleasant, sweet and tart. Cheese processing uses it as well, as a synergistic antioxidant, an ingredient that strengthens the effect of other antioxidants.

In the consumption pattern compiled by the US Hazardous Substances Data Bank, 16 per cent of the acid goes into medicines, mainly effervescent antacid powders and tablets. Cleaning is another outlet. Bathroom products and cleaners for dairy and food processing equipment contain it, and it is an active ingredient in registered disinfectants and sanitisers. The US Environmental Protection Agency registered the first products containing it in the early 1970s.

Further afield it serves as a mordant that brightens dyed colours, in special inks, in electroplating, and as a sequestering agent that removes trace metals. A US consumer product database lists 2,599 products containing it.

Why it matters

In the matrix of mitochondria, the Krebs cycle begins by making citrate. An acetyl group from acetyl CoA, the carrier that delivers two-carbon acetyl groups, joins a four-carbon oxaloacetate molecule to give six-carbon citrate. OpenStax Biology calls the step irreversible because it is highly exergonic, giving out much free energy. It slows when ATP, the molecule cells use to carry energy, is plentiful and speeds up when ATP runs short. Citrate then loses one water molecule and gains another, becoming its isomer, a molecule with the same atoms arranged differently, isocitrate.

For each acetyl group fed in, the cycle releases two molecules of carbon dioxide. Hans Krebs worked out the steps in the 1930s.

The same molecule is also a bulk chemical. US figures reported to the Environmental Protection Agency put 2023 volume between 250 million and under 400 million pounds. World output around 1990 was roughly 550,000 metric tons. The same breakdown puts 11 per cent into metal cleaning and finishing and into esters and salts for detergents and plasticisers.

Properties

Properties of citric acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 311.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₆H₈O₇
Molar mass	192.123(21) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	6
Hydrogen atoms	8
Oxygen atoms	7
Bonds	20
Single bonds	17
Double bonds	3
PubChem molecular weight	192.12 g/mol	g/mol	PubChem

Safety

As a food additive in drinks, sweets and jams, citric acid is affirmed by the US Food and Drug Administration as generally recognised as safe. The concentrated solid is another matter; its EU classification carries the signal word Warning for serious eye irritation and possible respiratory irritation from dust. The International Chemical Safety Card adds that long-term or repeated exposure may erode the teeth. The PubChem Laboratory Chemical Safety Summary is the sheet for anyone handling the pure acid.

Questions

Is citric acid a strong acid?

No. In water only a small fraction of it gives up its hydrogen ions, which makes it a weak acid, as most organic acids are. The Hazardous Substances Data Bank gives its pKa, a measure of how readily an acid gives up a hydrogen ion, as 2.79; a lower value means a stronger acid. OpenStax says only a carboxyl hydrogen reacts with bases.

Why is citric acid called a tricarboxylic acid?

Its IUPAC name, 2-hydroxypropane-1,2,3-tricarboxylic acid, counts the groups: three carboxyl groups on a three-carbon propane backbone, plus one hydroxy group on the middle carbon. OpenStax Biology counts six carbons in citrate and three carboxyl groups among them. The same label gives the Krebs cycle its other name, the TCA cycle, because citrate and isocitrate are both tricarboxylic acids.

How is citric acid made?

Commercial production uses a mould, Aspergillus niger, fed on beet or cane molasses or on corn sugar, and it began in 1923. The older method grows the mould in trays; deep tank fermentation is replacing it because large tanks cost less in labour and equipment than small pans. A full fermentation can run for up to 15 days. A chemical synthesis was reported in 1880 but never proved commercially feasible.

Why is citric acid added to donated blood?

Blood collected for transfusion is kept from clotting with a citrate solution, usually mixed with glucose. Haz-Map, quoting Dorland's medical dictionary, states that citrate chelates, or binds up, the calcium ions in blood and so prevents stored blood from clotting. Citrate also serves as an anticoagulant for red cells and blood specimens.

Made of the same elements

C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 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.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single 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.