The most common mistake buyers make with sodium metabisulfite is ordering an industrial-grade lot for a food application, or paying food-grade prices for a dechlorination line that never touches food. The chemistry is identical - Na₂S₂O₅, CAS 7681-57-4 - but the specification, documentation and price are not.
What the Two Grades Actually Differ On
- Assay: both grades are typically sold at 97% minimum (65% minimum as SO₂), but food grade carries tighter limits on heavy metals - lead, arsenic and selenium are specified item by item on the COA.
- Iron content: food grade caps Fe at a far lower level, because iron discolors dried fruit and wine.
- Documentation: food grade shipments should be supported by a food-additive-compliant COA and MSDS; industrial shipments need neither, which is where the price difference comes from.
- Packaging hygiene: food grade uses 25 kg PP woven bags with food-contact liner; the 1000 kg jumbo bag option is mostly chosen for industrial water treatment.
Match the Grade to the Application
- Water treatment (dechlorination, heavy-metal reduction): industrial grade. The chemical is dosed into waste streams; paying a 15-20% premium for food-grade metal limits buys nothing.
- Dried fruit, candied fruit, wine and starch processing: food grade only. Import authorities at destination frequently check heavy-metal certificates, not just assay.
- Textile bleaching and printing auxiliaries: industrial grade works, but if the mill also handles export apparel, check whether the customer restricts sulfur residues on fabric.
- Leather, mining and pulp bleaching: industrial grade across the board.
Practical Supply Details
DSC Trading supplies both grades under the DSC, YINQIAO, KAIANG and MALU brands, 25 kg PP bags or 1000 kg jumbo bags. A 20' FCL loads 27 MT without pallets or 25 MT palletized - pallets are worth the capacity loss for food consignments, where warehouse handling at destination tends to be more automated and less forgiving of torn bags.
Every lot is SGS-inspected before shipment, and the COA lists assay, SO₂ content, Fe, heavy metals and water-insoluble matter. If you are unsure which grade your process requires, send us the application and destination market - we will quote both grades side by side so the decision is a price decision, not a guess.