oxalic acid

Oxalic acid
IUPAC name	ethanedioic acid
Identifiers
CAS number	144-62-7 Y
ATCvet code	QP53AG03
SMILES	OC(=O)C(O)=O
Properties
Molecular formula	C2H2O4 (anhydrous) C2H2O4·2H2O (dihydrate)
Molar mass	90.03 g/mol (anhydrous) 126.07 g/mol (dihydrate)
Appearance	white crystals
Density	1.90 g/cm³ (anhydrous) 1.653 g/cm³ (dihydrate)
Melting point	101-102°C (dihydrate)
Solubility in water	9.5 g/100 mL (15 °C) 14.3 g /100 mL (25 °C?) 120 g/100 mL (100 °C)
Acidity (pKa)	pKa1=1.27 pKa2=4.28
Hazards
MSDS	External MSDS
NFPA 704	1 3 0
Flash point	166 °C
Related compounds
Related compounds	oxalyl chloride disodium oxalate calcium oxalate phenyl oxalate ester
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Oxalic acid is the chemical compound with the formula C₂O₂(OH)₂ or HOOCCOOH. This colourless solid is a relatively strong carboxylic acid, being about 3,000 times stronger than acetic acid. The dianion, known as oxalate, is a reducing agent as well as a ligand for metal cations. Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, the primary constituent of the most common kind of kidney stones. Typically oxalic acid is obtained as the dihydrate.

Preparation

Oxalic acid is mainly manufactured by the oxidation of carbohydrates or glucose using nitric acid or air in the presence of vanadium pentoxide. A variety of precursors can be used including glycolic acid and ethylene glycol.^[1] A newer method entails oxidative carbonylation of alcohols to give the diesters of oxalic acid.

2 ROH + 2 CO + 0/5 O₂ → (CO₂R)₂ + H₂O

These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120M kg are produced annually.^[2]

Laboratory methods

Although it can be readily purchased, oxalic acid can be prepared in the laboratory by oxidizing sucrose using nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst.^[3]

The hydrated solid can be dehydrated with heat or by azeotropic distillation.^[4]

Of historical interest, Wöhler prepared oxalic acid by hydrolysis of cyanogen in 1824. This experiment may represent the first synthesis of a natural product.^[2]

Structure

Anhydrous oxalic acid exists as two polymorphs; in one the hydrogen-bonding results in a chain-like structure whereas the hydrogen bonding pattern in the other form defines a sheet-like structure.^[5] Because the anhydrous material is both acidic and hygroscopic (water seeking), it is used as in esterifications.

Reactions

Oxalic acid is relatively strong acid, despite being classified as a weak acid:

C₂O₄H₂ → C₂O₄H^- + H⁺; pK_a = 1.27

C₂O₄H^- → C₂O₄^2- + H⁺; pK_a = 4.28

Oxalic acid exhibits many of the reactions characteristic of other carboxylic acids. It forms esters such as dimethyloxalate (m.p. 52.5–53.5 °C).^[6]. It forms an acid chloride called oxalyl chloride.

Oxalate, the conjugate base of oxalic acid, is an excellent ligand for metal ions, e.g. the drug Oxaliplatin.

Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction.^[7]

Occurrence in nature

Oxalic acid and oxalates are abundantly present in many plants. It was first isolated from sorrel.

Applications

Oxalic acid's main applications include cleaning or bleaching. Most oxalic acid is used as a cleaning agent, especially for the removal of rust or removal of iron from minerals specimens. Many household chemical products contain oxalic acid, especially rustproofing treatments. Bar Keepers Friend is an example of a cleaner that is used in households and commercially that has oxalic acid as its active ingredient. About 25% of produced oxalic acid is used as a mordant in dyeing processes. It is used in bleaches, especially for pulpwood.^[2]

Extractive metallurgy

Oxalic acid is an important reagent in lanthanide chemistry. Hydrated lanthanide oxalates form readily in strongly acid solution in a densely crystalline easily filtered form, largely free from contamination by non-lanthanide elements. Lanthanide oxalates figure importantly in commercial processing of lanthanides, and are used to recover lanthanides from solution after separation. Upon ignition, lanthanide oxalates are converted to the oxides, which are the most common form in which the lanthanides are marketed. Upon heating, metal oxalates decompose to give the corresponding oxides.

Miscellaneous uses

Oxalic acid is used in the restoration of old wood. Its reducing properties are utilized in platinotype, the early photographic platinum/palladium printing process.

Vaporized oxalic acid, or a 6% solution of oxalic acid in sugar syrup, is used by some beekeepers as a miticide against the parasitic Varroa mite.

Safety

Oxalic acid is toxic to humans, with LD50 of 71 mg/kg. It has an extremely irritating taste. Prolonged handling of aqueous solutions cause joint pains. Humans excrete several milligrams in urine.^[2] Corrosive to concrete, wood and glass. ^[8]

References

External links

Wikimedia Commons has media related to: Oxalic acid

• International Chemical Safety Card 0529
• "Oxalic acid". ChemicalLand21.com. http://www.chemicalland21.com/arokorhi/industrialchem/organic/OXALIC%20ACID.htm. 
• Table: Oxalic acid content of selected vegetables (USDA)
• Alternative link: Table: Oxalic Acid Content of Selected Vegetables (USDA)
• About rhubarb poisoning (The Rhubarb Compendium)
• Low-Oxalate Diet (PDF)
• Calculator: Water and solute activities in aqueous oxalic acid