CID 971

H₂C₂O₄

Oxalic acid

Q: Is oxalic acid a strong acid?

No. Its pKₐ values are 1.46 for losing the first proton and 4.40 for losing the second, according to the Hazardous Substances Data Bank (HSDB); the lower the value, the stronger the acid. Even so, water does not take the first proton completely, which makes it a weak acid. The second proton comes off less readily because it has to leave an ion that already carries a negative charge.

Ball-and-stick Stick Space-filling

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8 atoms, 7 bonds.

IUPAC name: oxalic acid
Molar mass: 90.0340(84) g/mol
Atoms: 8
SMILES: C(=O)(C(=O)O)O
InChIKey: MUBZPKHOEPUJKR-UHFFFAOYSA-N
PubChem CID: 971

Made of

H C O

Structural formula of oxalic acid. Each unlabelled corner is a carbon atom. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Bonded carbon to carbon, two carboxyl groups, –COOH, make up the whole of an oxalic acid molecule. That makes it a dicarboxylic acid. It contains only carbon, hydrogen and oxygen, and one mole of the dry acid has a mass of 90.03 g. The pure substance is an odourless white solid, in colourless crystals or a white powder. It is hygroscopic, drawing water out of the air.

About 9 to 10 g dissolves in 100 mL of water at 20 °C. The acid dissolves very well in ethanol, only slightly in ether and not in benzene. Warmed, it sublimes, going from solid directly to gas, most readily at 157 °C. At higher temperatures it breaks apart into carbon dioxide, carbon monoxide, formic acid and water.

Structure and bonding

Oxalic acid’s two carbons sit at the centre, bonded to each other. Each carbon holds one O by a double bond and a second O that carries an H. Turned edge-on, the model shows all eight atoms in a single plane, the two C=O bonds pointing to opposite sides. In Ball-and-stick each carbon’s three bonds spread flat, an arrangement explained on the ethylene page. Stick thins it to a frame with two oxygens splayed from each end of the C–C bond, and in Space-filling the four oxygens make up most of the outline.

The drawing is the neutral acid, HOOC–COOH. In water it gives up the H from one –OH to become the hydrogen oxalate ion, then the H on the other carbon’s –OH to become oxalate. Its pK_a values, a measure explained on the phosphoric acid page, are 1.46 and 4.40 for the two steps. Commercial crystals are a dihydrate, two water molecules to each acid molecule, which they give off as they melt at 101–102 °C. The dihydrate’s molar mass is 126.1 g/mol, against the 90.03 g/mol of the molecule drawn.

Where it turns up

Rhubarb, parsley and spinach all contain oxalic acid, and it has been found in the tea plant, Camellia sinensis, among many other plants. Several plant species contain oxalate salts, some soluble in water and some insoluble. With calcium the acid forms calcium oxalate, which comes out of solution as a solid. If you have drunk milk with a rhubarb pudding and noticed a strange feeling in your mouth afterwards, that was calcium oxalate precipitating.

Moulds make the same solid without any plant: several species of Penicillium and Aspergillus turn sugar into calcium oxalate. The human body makes oxalic acid too, from glyoxylic acid among other starting points, and passes it out in the urine without breaking it down. At home it sits in cleaners for aluminium, silver, copper and copper alloys, and in some bathroom sanitisers.

Why it matters

Oxalic acid grabs divalent metal ions, those carrying a 2+ charge, and binds them. That makes it a chelating agent, a compound that holds metal ions this way. Many such ions, calcium among them, are nutrients the body needs, so on Chem1’s account this binding can harm health.

Beyond the home it strips rust, grease and wax from metal, lifts ink stains, cleans car radiators and bleaches straw hats and leather. Dyers use it to bind dye to fibre, photographers to tint prints and in platinum printing, and laboratories to analyse other substances. Blood drawn for chemical analysis is sometimes kept from clotting with oxalic acid or its salts.

In 1983 textiles took 33 per cent of the acid, metals and making oxalates, its salts, 20 per cent each, leather 7 and other uses the last 20. By then most US producers had stopped, and imports, chiefly from China and Brazil, met the demand. Commercial acid comes from oxidising starch, sugar or ethylene glycol with nitric acid, or as a co-product of fermenting molasses into citric acid.

Properties

Properties of oxalic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 971.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₂H₂O₄
Molar mass	90.0340(84) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	2
Carbon atoms	2
Oxygen atoms	4
Bonds	7
Single bonds	5
Double bonds	2
PubChem molecular weight	90.03 g/mol	g/mol	PubChem

Safety

Oxalic acid is corrosive and harmful if swallowed or in contact with the skin. As a dust or in solution it can severely burn the eyes, skin and mucous membranes, and swallowing it may disturb the body’s calcium balance. NIOSH, the US National Institute for Occupational Safety and Health, names the eyes, skin, respiratory system and kidneys as its target organs. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card for oxalic acid set out the hazards and the workplace limits.

Questions

What is oxalic acid used for?

The Hazardous Substances Data Bank (HSDB) lists oxalic acid for scouring in textile finishing and in metal polishes. It also removes paint or varnish, cleans wood, serves in rare-earth processing and goes into blue ink. In soil it is used to make phosphate fertilisers more available, and Haz-Map adds disinfecting swimming pools and drainage systems.

Is oxalic acid a strong acid?

No. Its pK_a values are 1.46 for losing the first proton and 4.40 for losing the second, according to the Hazardous Substances Data Bank (HSDB); the lower the value, the stronger the acid. Even so, water does not take the first proton completely, which makes it a weak acid. The second proton comes off less readily because it has to leave an ion that already carries a negative charge.

Why are rhubarb leaves poisonous?

The oxalic acid in them is the usual explanation. The Hazardous Substances Data Bank states that its toxicity is believed to come from locking up calcium, upsetting the balance of calcium and potassium in critical tissues. According to Haz-Map, swallowed oxalic acid or soluble oxalate causes a sudden fall in blood calcium and leaves crystals of calcium oxalate in the brain, heart, kidneys and other organs. Insoluble oxalates from plants irritate and swell the mouth, throat and oesophagus.

Does oxalic acid cause kidney stones?

The International Chemical Safety Cards (ICSC) list kidney stones among the possible results of long-term exposure to oxalic acid, together with slow-healing ulcers and black fingernails. The Hazardous Substances Data Bank reports that long-term treatment with the drug piridoxilate can make the body overproduce oxalic acid. The result can be kidney stones of calcium oxalate, the solid the acid forms with calcium.

Made of the same elements

H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.

All 51 compounds made of hydrogen, carbon and oxygen

C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 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.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single 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.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 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.