Sodium Oxalate
Properties
| State | Solid (white crystalline powder, monoclinic) |
| Color | White |
| Solubility | Sparingly soluble in water (37 g/L at 20 °C); insoluble in ethanol |
| Melting Point | 260 °C (decomposes to Na2CO3 and CO) |
| Boiling Point | Decomposes before boiling |
About Sodium Oxalate
Sodium oxalate (Na2C2O4, MW 133.999) is the disodium salt of oxalic acid and one of the half-dozen classic primary standards in analytical chemistry, alongside potassium hydrogen phthalate, sodium carbonate, and sodium tetraborate. The reason it earned that status is a rare combination of practical virtues: it's available at 99.95+% purity from any chemical supplier as NIST SRM 40h, it doesn't pick up water from the air (non-hygroscopic), it's stable for years on the shelf, the molecular weight of 134 is high enough that weighing errors are small relative to the moles delivered, and it reacts cleanly and stoichiometrically with potassium permanganate in hot dilute sulfuric acid: 5 Na2C2O4 + 2 KMnO4 + 8 H2SO4 -> 2 MnSO4 + 5 Na2SO4 + K2SO4 + 10 CO2 + 8 H2O. The reaction is autocatalytic in Mn2+, slow at first then accelerating, and it's self-indicating because the deep purple permanganate becomes colorless as it's reduced; the endpoint is the first faint persistent pink. The titration has to be run at 70-80 °C and the Na2C2O4 has to be added slowly to avoid forming MnO2 from incomplete reduction. Beyond its role as a permanganate standard, sodium oxalate chelates Ca2+ as insoluble calcium oxalate (Ksp = 2.32 x 10-9), which is why it was used historically as a clinical-lab anticoagulant (the gray-top tube before fluoride/oxalate became standard) and why oxalate-rich foods like spinach, rhubarb, and beet greens reduce dietary calcium absorption and contribute to calcium oxalate kidney stones. In metallurgy and rare-earth processing, oxalate precipitation separates lanthanides as their insoluble oxalates before calcination to oxides.
Where you'll encounter it
If you've ever standardized a permanganate solution in undergraduate analytical chemistry by titrating against a weighed portion of Na2C2O4 in 1 M H2SO4 at 70 °C, drawn a fluoride-oxalate gray-top tube for a fasting glucose, or watched a urologist analyze a kidney stone composition, you've seen sodium oxalate chemistry. In an analytical lab, the standard prep is to dry Na2C2O4 at 105 °C for 1-2 hours, weigh out exactly 0.2 g into a 250 mL Erlenmeyer, dissolve in 100 mL of 1 M H2SO4, heat to 70-80 °C, and titrate against the KMnO4 solution to be standardized; the calculation gives molarity to four significant figures and a precision of about 0.1%. In a rare-earth refinery (Mountain Pass, Lynas, etc.), oxalate precipitation is the gate that separates rare-earth elements from non-rare-earth contaminants before the lanthanide oxalates are roasted at 800-900 °C to mixed oxides for further separation by solvent extraction. In a clinical chemistry context, calcium oxalate makes up about 75% of all kidney stones, and the spiky envelope-shaped crystals are immediately recognizable under polarized light microscopy.
Common Uses
- NIST primary standard (SRM 40h) for standardizing KMnO4 and Ce(IV) oxidants
- Reference titrant in iron and calcium determinations by redox titration
- Calcium chelator in legacy gray-top blood collection tubes (paired with NaF)
- Precipitation reagent for rare-earth element separation prior to oxide calcination
- Reducing agent in textile and leather processing (color stripping, tanning bath cleanup)
- Source of oxalate ion for synthesizing metal oxalate complexes and MOF precursors
- Calibration material for atomic absorption sodium and calcium determinations
- Analytical reagent for the classical Mohr/Volhard chloride end-point support
Safety Information
Toxic by ingestion: oxalate chelates serum Ca2+ to form insoluble calcium oxalate, causing acute hypocalcemia (tetany, QT prolongation, seizures) and renal tubule blockade by precipitating crystals. Estimated human lethal oral dose is 5-15 g of free oxalic acid equivalent. The classical antidote is oral calcium (milk, calcium gluconate) to precipitate oxalate in the gut before absorption. Oxalate's most important chronic effect is contribution to calcium oxalate nephrolithiasis (kidney stones), the most common stone type. OSHA PEL for oxalic acid is 1 mg/m3 (8-hour TWA) and ACGIH TLV is the same; sodium oxalate dust is treated equivalently. GHS classifications: H302 (harmful if swallowed), H312 (harmful in contact with skin), H318 (causes serious eye damage). Pictograms: Corrosion (eye damage), Exclamation mark. Standard PPE: nitrile gloves, splash goggles, dust mask when weighing dry powder. Avoid contact with strong oxidizers (will ignite) and do not heat in glassware that will be reused for sensitive Ca2+ work, since Na2C2O4 thermal residues can perturb later assays.
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.