Oxalic Acid

About This Product

Oxalic acid, ethanedioic acid, has the formula C2H2O4 and CAS number 144-62-7 for the anhydrous form, and is usually supplied as the dihydrate in white crystals or powder. It occurs naturally in rhubarb leaves, sorrel and many other plants. It is unusual in combining two properties: it is a comparatively strong acid for an organic compound, with a first pKa near 1.25, and it is a chelating agent that binds metal ions, particularly iron, into stable soluble complexes. That combination is what makes it the standard treatment for rust and iron staining. Where a simple acid would only dissolve the surface of an iron oxide deposit, oxalic acid converts the iron into a soluble ferrioxalate complex that rinses away completely, taking the stain with it rather than redistributing it. The same chemistry bleaches the dark iron-tannin stains in timber and lifts rust marks from stone and textiles. Its toxicity is a genuine constraint rather than a formality: absorbed oxalate precipitates as calcium oxalate in the kidneys, and it is hazardous by ingestion and skin absorption. It is used in rust and stain removal, wood bleaching, stone and marble restoration, metal cleaning, textile processing and as an analytical reagent. Bio Green Chem supplies oxalic acid to specification on assay, sulfate, chloride, iron, ash, insolubles and heavy metals.