Oxalic Acid
Properties
| State | Solid at room temperature |
| Color | White crystalline powder |
| Solubility | Soluble in water (14.3 g/100 mL at 25 °C) |
| Melting Point | 189 °C (decomposes) |
About Oxalic Acid
Oxalic acid is the simplest dicarboxylic acid, written HOOC-COOH, with a molar mass of 90.034 g/mol. The two carboxyl groups sit directly on each other, which makes the first ionization unusually strong for an organic acid (pKa1 = 1.25, comparable to bisulfate) and gives the dianion (oxalate) excellent bidentate chelating geometry — a five-membered ring closes when oxalate binds a metal through both carboxylate oxygens. This chelation is why oxalic acid eats rust: it pulls Fe(III) out of iron oxide and ferries it away as the soluble tris(oxalato)ferrate(III) complex. Plants store oxalic acid as the calcium salt — those needle-like raphide crystals in spinach, rhubarb leaves, and dieffenbachia are calcium oxalate, and the same insoluble salt is what kidney stones are mostly made of. In a chemistry teaching lab, oxalic acid is the primary standard for KMnO4 titrations: the reaction 5 C2O4^2- + 2 MnO4- + 16 H+ yields 10 CO2 + 2 Mn^2+ + 8 H2O is self-indicating because the purple permanganate color disappears as soon as it's reduced, returning at the endpoint when oxalate is exhausted. The reaction needs to run hot (about 60 °C) and slowly at first because it is autocatalyzed by Mn(II).
Where you'll encounter it
If you've ever stripped iron stains from a porcelain sink, restored an old wooden deck, or watched a beekeeper sublime acid crystals into a hive to kill Varroa mites, you've used oxalic acid. Wood-deck cleaners sold at hardware stores are often 5-10 percent oxalic acid solutions because the chelation chemistry pulls tannins and iron stains out of cedar and redwood without bleaching the lignin. In analytical labs, sodium oxalate (Na2C2O4, NIST SRM 40) is still the gold-standard primary reference for permanganate standardization in undergraduate quant courses. Industrial chemists exploit oxalate's bidentate chelation in rare-earth separation, where oxalate precipitation is one of the cleanest ways to remove lanthanides from solution before calcining the oxalates to mixed oxides for further processing.
Common Uses
- Rust and iron-stain removal from porcelain, wood decks, and stone via Fe(III) chelation
- Bleaching and brightening agent for wood pulp, restored furniture, and textile fibers
- Primary standard for standardizing potassium permanganate solutions in analytical chemistry
- Reducing agent in classic redox titrations and in Tollens-type silver-mirror experiments
- Sublimed in beehives at low doses for Varroa mite control by hobbyist and commercial beekeepers
Safety Information
Toxic if ingested. Lethal oral dose in adults is around 15-30 g. Once absorbed, oxalate precipitates with serum calcium as insoluble calcium oxalate, causing hypocalcemia, kidney damage, and tubular obstruction. Corrosive to skin and mucous membranes. GHS: H302 (harmful if swallowed), H312 (harmful in contact with skin), H318 (causes serious eye damage). The airborne dust carries a low occupational exposure limit. Wear nitrile gloves, splash goggles, and dust mask when weighing dry crystals, and never mouth-pipette solutions.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.