Sodium metabisulfite (CAS 7681-57-4) is a simple-looking chemical, but buying it right means checking the grade, the assay, the SO2 content and the COA carefully — especially because food, water and technical applications demand different grades. This purchasing guide covers the specification sheet, the COA, packaging and storage, and the supplier checklist for sodium metabisulfite.
1. Reading the Specification
| Parameter | Typical value | Why it matters |
|---|---|---|
| Assay (as Na2S2O5) | 96-98% min | Active content |
| SO2 content | ~65-67% equivalent | Directly drives preservative/dechlorination power |
| Iron | Low (e.g. <0.005%) | Purity, color |
| Heavy metals | Strict for food grade | Food safety |
| Water-insolubles | Low | Purity |
2. What a Complete COA Should Contain
- Product name and CAS number (7681-57-4)
- Batch number and production date
- Assay and SO2 (measured)
- Iron, heavy metals (food grade: As, Pb, Hg limits)
- Appearance and water-insolubles
- Grade declaration (food/technical)
- Test methods and quality manager signature
3. Packaging, Storage and Shelf Life
- Packaging: typically 25 kg bags, or 50 kg drums; keep sealed.
- Storage: cool, dry, sealed; moisture degrades the product and releases SO2.
- Shelf life: typically 12-24 months when stored properly.
- Handling: avoid dust inhalation; the sulfurous odor indicates SO2 release — ventilate.
4. Food Grade vs Technical Grade
The two grades differ mainly in impurity limits and certification:
- Food grade (E223): strict heavy-metal limits, food-additive certification, used in food and beverage.
- Technical grade: for water treatment, textiles, bleaching and industrial use — lower cost, looser impurity limits.
- Never use technical grade in food applications.
5. How to Verify Quality on Arrival
- Check packaging and labels against the COA batch number.
- Visual check: white to yellowish crystalline powder, free-flowing.
- Verify assay (iodometric titration is standard) against the COA.
- For food grade, confirm heavy-metal values on the COA.
- Keep a retained sample per batch.
6. Supplier Checklist
- Manufacturer vs trader — manufacturers control grade consistency.
- Consistent COA with measured assay/SO2 on every batch.
- Both food and technical grades available if needed.
- Stable supply, reasonable lead time, export documentation.
- Price comparison on a per-kg basis at the same grade.
7. FAQ
What is the difference between food and technical grade?
Food grade has strict heavy-metal limits and food-additive certification; technical grade is for industrial use. Never substitute.
Why does my metabisulfite smell of sulfur?
It is releasing SO2, usually from moisture or age — check storage and the COA batch; a strong odor suggests degradation.
How should I compare supplier quotes?
On price per kg at the same grade and assay; check the SO2 content, which is the true active measure.
Is sodium metabisulfite a dangerous good?
It is generally not classified as a dangerous good for transport in solid form, but confirm the SDS and shipping classification with your forwarder.
8. References and Further Reading
- Food additive standards (E223) and purity criteria
- Standard analytical methods for metabisulfite assay
This article is published for industrial and technical reference by PolyblueChem. Specifications are typical; verify against the supplier’s COA.
PolyblueChem supplies sodium metabisulfite (CAS 7681-57-4) in food and technical grades with COA and MSDS. See our sodium metabisulfite guide. Contact us for samples and quotations.