Two caustic soda offers can both say "99%" and still not be the same product. The difference lives in the COA's impurity lines - and knowing what each one does downstream is what separates a smooth shipment from a claim.
The Four Lines That Matter (GB/T 209-2018)
- NaOH content: ≥99.0% for superior grade, ≥98.0% for first-class. Below spec means carbonation or dilution has already happened.
- Na₂CO₃ (sodium carbonate): ≤0.5% superior, ≤0.8% first-class. Carbonate forms when NaOH meets air CO₂ - it is a freshness marker as much as a purity marker.
- NaCl (sodium chloride): ≤0.03% superior, ≤0.05% first-class. Chloride rides in from brine electrolysis and never leaves the product.
- Fe₂O₃ (iron oxide): ≤0.005% superior, ≤0.008% first-class. Iron is picked up from contact with unsuitable equipment and shows as discoloration.
What Each Impurity Does in Your Process
- Alumina (Bayer process): carbonate consumes caustic that should be dissolving bauxite and upsets precipitation seeding - high-Na₂CO₃ lots cost you yield per ton of caustic.
- Soap and detergent: carbonate weakens lye strength and leaves specks; iron tints white noodles grey-yellow. Both impurities are visible in the finished bar.
- Textile mercerization: iron deposits on fabric as colored specks after caustic treatment - dyers reject lots that a washing plant would accept.
- Water treatment: chloride raises the corrosion burden on dosing lines; for most neutralization duty, first-class material is fully adequate.
How to Verify Before You Pay
- Insist on a per-lot COA with batch number matching the bags - not a brand brochure.
- Check the Na₂CO₃ trend: a lot produced recently but showing carbonate near the limit signals poor storage before shipment.
- Ask for third-party inspection (SGS or equivalent) on first orders with a new supplier - it costs a fraction of one rejected container.
- Match grade to use: superior grade for textiles, soap, alumina and food-adjacent plants; first-class is often enough for neutralization and pulp.
Our Recommendation
We recommend buying caustic soda from DSC Trading with specification confirmed up front: 99% min flakes (Zhongtai, Tianye, China Salt, Yuanlong) and 99% min pearls (TIANYE), each lot supplied with COA listing all four items above, optional SGS inspection before loading, and moisture-proof UN 1823 Class 8 packaging. Send your process and destination port - we will recommend the right grade tier and return CIF/FOB pricing within 12 hours.
Buyers who read the COA lines rather than the headline number almost never file caustic soda claims. Make the four lines part of your purchase order, and hold your supplier to them.