Grades and active content
Sodium hydrosulfite (sodium dithionite, Na₂S₂O₄, CAS 7775-14-6) is a powerful reducing agent used mainly in vat dyeing and reduction clearing. It is graded by active content, and the range is wider than buyers often assume.
| Grade | Active content | Form | Typical use |
|---|---|---|---|
| Technical | ≥85% | Powder | General textile bleaching, paper pulp |
| Industrial high-purity | ≥88–90% | Powder or granular | Vat dyeing, reduction clearing, kaolin |
| Stabilised | ≥85% with stabiliser | Powder | Longer shelf life, remote or seasonal supply |
| Food (FCC) | ≥85% | Powder | Regulated food uses |
| Liquid | 10–15% solution | Solution | Continuous dosing on large operations |
The gap between 85% and 90% matters more than it looks, because the balance is largely inert salts that consume dosing capacity and add to effluent load without contributing reducing power. Compare on cost per kilogram of active Na₂S₂O₄.
Production route affects stability
| Parameter | Zinc process | Sodium formate process |
|---|---|---|
| Feedstock | Zinc dust, SO₂, caustic soda | Sodium formate and SO₂ |
| Typical purity | 85–88% | 88–92% |
| Waste | Zinc hydroxide sludge | None comparable |
| Product stability | Moderate | Higher |
| Status | Being phased out | Industry standard |
Formate-process material is both purer and more stable in storage, and it avoids the zinc contamination that creates an effluent problem for the mill. Where your effluent discharge is monitored against NEQS limits, specifying formate-process material is worth doing explicitly.
Key properties
| Property | Value |
|---|---|
| Chemical formula | Na₂S₂O₄ |
| CAS number | 7775-14-6 |
| Molecular weight | 174.11 g/mol |
| Appearance | White to pale yellow crystalline powder |
| Odour | Faint sulphurous |
| pH (1% solution) | 4.0–6.0 |
| Solubility in water | ~18 g/100 mL at 20 °C |
| Reduction potential | −1.12 V — strongly reducing |
The storage hazard, and how to buy around it
This is the section that matters most. Sodium hydrosulfite reacts with moisture and atmospheric oxygen, the reaction is exothermic, and the heat produced accelerates it further. In a confined, damp mass it can self-heat to ignition and release sulphur dioxide.
- Store in a cool, dry, well-ventilated area, off the floor and away from any water source, steam line or wet process
- Keep bags sealed; reseal any opened bag immediately and use opened stock first
- Reject damaged, damp or previously wet bags at the gate — this is not a cosmetic issue
- Never use water on a hydrosulfite fire; water feeds the reaction. Smother with dry sand or an appropriate dry agent and evacuate for SO₂
- Segregate from oxidisers, acids and combustibles
- Rotate stock strictly first in, first out, and keep quantities matched to consumption rather than stockpiling
- Buy smaller, more frequent deliveries in humid coastal conditions even at higher freight cost
Stabilised grades exist precisely for buyers who cannot control storage tightly — remote sites, seasonal users and anyone with a humid warehouse. They cost more and are frequently the right answer.
Where it is used
- Vat and sulphur dyeing — reducing the insoluble dye to its soluble leuco form so it can penetrate the fibre, then reoxidising to fix it
- Reduction clearing after polyester dyeing — stripping unfixed disperse dye from the fibre surface to improve fastness
- Textile stripping — removing faulty dyeings for reprocessing
- Paper pulp brightening, particularly mechanical pulp
- Kaolin clay brightening, by reducing iron impurities
- Water treatment — reduction of hexavalent chromium and other oxidised contaminants
What to put on the RFQ
- Active Na₂S₂O₄ content minimum, not just "hydros"
- Production route — specify formate process where effluent zinc is a concern
- Whether a stabilised grade is required, based on your storage conditions
- Packaging integrity requirements — moisture-barrier lining, and drum rather than bag where storage is humid
- Quantity per delivery and delivery frequency, since frequent smaller deliveries reduce risk
- Delivery city and confirmation of dry, ventilated, segregated storage
- Documentation — batch COA reporting active content, MSDS, and manufacturing date
What drives the price
- Sulphur dioxide and sodium formate feedstock costs
- Active content, which is what you are actually buying
- Production route, with formate-process material commanding a premium over zinc-process
- Stabilised grades, which cost more but reduce loss and risk in poor storage
- Packaging, since moisture-barrier packs cost more and are often worth it
- Freight, delivery city and delivery frequency
- Textile season demand, which is significant and predictable in Pakistan
Sourcing considerations in Pakistan
- Supply is largely imported, so confirm lead time and plan around the textile season
- It is classified as a self-heating substance for transport, so packaging, segregation and vehicle requirements are specific and non-negotiable
- Ask for the manufacturing date — active content declines with age, and a long-stored consignment is both weaker and less stable
- Where your effluent is monitored, specifying formate-process material avoids adding zinc to the discharge and is easier than treating it later