Field Notes on Oxalic Acid: Specs, Uses, and What Buyers Keep Asking Me

When I first toured a metal finishing line in Hebei, the maintenance supervisor joked that their secret sauce wasn’t fancy at all—it was Oxalic acid. To be honest, I shrugged. Then I saw the before-and-after on heat exchangers and stone panels. Since then, I’ve been tracking suppliers, test data, and real-world outcomes. Here’s the straight talk—light on hype, heavy on what actually matters.

What it is (and why users like it)

Oxalic acid (CAS 6153-56-6, dihydrate) is a white, transparent crystalline solid. Formula: H2C2O4·2H2O (sometimes written as HOOCCOOH·2H2O). It melts at ≈101–102°C and sublimates around 100°C. Easily soluble in water and alcohol, specific gravity ≈1.653. In practice, customers choose it for controlled-scale removal, wood and textile bleaching, passivation before electroplating, and as a reliable precipitant in rare-earth processing. It’s strong but predictable—if you respect the SDS.

At-a-glance specs

Process flow and QC (short version)

Typical industrial routes (yes, there are variations): carbohydrate oxidation to calcium oxalate; acidification to free oxalic acid dihydrate; controlled crystallization; filtration; washing; drying under 100°C to protect hydration state; screening. QC includes:

- Assay by redox titration (KMnO4), moisture by Karl Fischer, heavy metals by ICP-OES.
- pH of 1% solution (ASTM/ISO lab methods), insolubles, color index, particle size (laser diffraction).
- COA issued by ISO 9001–certified QA; third-party verification (e.g., SGS) on request.

Applications that keep coming up

- Metal cleaning/descaling: heat exchangers, stainless pre-passivation lines.
- Stone and wood: rust/tannin stain removal, wood bleaching (test patches first, always).
- Textile and leather: controlled bleaching; dye intermediates.
- Chemical synthesis: rare-earth separation, catalysts prep, pharmaceuticals intermediates (non-pharma grade unless specified).
- Household/industrial cleaners: specialty blends; do not mix with hypochlorite bleach—ever.

Vendor snapshot (what buyers compare)

Customization, handling, and field feedback

Custom options usually include tighter iron limits, anti-caking control, and particle size tuning for faster dissolution. Packaging: 25 kg laminated bags or 1,000 kg big bags with PE liners. Service life is solid if you keep oxalic acid dry and sealed; humidity is the enemy. Safety-wise, it’s corrosive—use gloves, goggles, and local exhaust; follow GHS/OSHA labels and your SDS. Many customers say they get the best results with a 2–10% solution, warm water, and thorough rinsing—especially on stone.

Mini case notes

- Textile mill (South Asia): switched to oxalic acid bleaching step; reported ≈7% fewer re-dyes after adjusting pH and rinse protocol.
- Stainless shop (EU): pre-passivation clean saw 15–20% cycle-time reduction; verified by surface iron tests.
- Stone fabricator (US): rust stain removal on quartzite; success at 5% solution, 15-minute dwell, neutralize and rinse—spot test prevented etching surprises.

Note: Data above are typical values from lab and customer reports; always validate on your line and substrate.

Standards and compliance checklist

- ISO 9001–certified quality systems for production and COA issuance.
- REACH/CLP and GHS-aligned SDS; OSHA HazCom labeling for US sites.
- Testing per ISO/ASTM lab methods; third-party audits available on request.

Final buying tips

- Ask for current CoA with Fe, assay, moisture, insolubles, and particle size.
- Confirm packaging, liner type, and palletization—humidity control matters.
- Pilot a small batch; real-world use may vary by water chemistry and substrate.

Authoritative sources

1. ECHA, Substance Information: Oxalic acid, regulatory properties and classification.
2. NIST Chemistry WebBook, thermophysical data for oxalic acid dihydrate.
3. NIOSH/CDC Pocket Guide to Chemical Hazards: Oxalic acid safety guidance.
4. ISO 9001:2015 Quality management systems—Requirements (for production QA frameworks).