Sodium metabisulfite (Na2S2O5, CAS 7681-57-4) is one of the most versatile inorganic chemicals in industry — a food preservative, a water treatment dechlorinator, a bleaching agent and a reducing agent. Its ability to release sulfur dioxide (SO2) under controlled conditions is the key to most of its applications. This article covers the manufacturing process, the specifications, the applications and the buying considerations for sodium metabisulfite.
1. What Is Sodium Metabisulfite?
Sodium metabisulfite (also called sodium pyrosulfite) is a white to yellowish crystalline powder with a characteristic sulfurous odor. On dissolution in water it hydrolyzes to sodium bisulfite (NaHSO3) and releases SO2 — this is the active species behind its antioxidant, preservative and reducing properties. It is supplied in technical, food and photo grades.
2. Manufacturing Process
2.1 The industrial route: SO2 absorption
Sodium metabisulfite is produced by absorbing sulfur dioxide into a sodium carbonate (soda ash) or sodium hydroxide solution:
Na2CO3 + 2 SO2 + H2O → 2 NaHSO3 + CO2
The sodium bisulfite solution is then concentrated and crystallized, and the bisulfite is converted to metabisulfite by controlled dehydration:
2 NaHSO3 → Na2S2O5 + H2O
Process steps:
- SO2 generation: sulfur is burned (or SO2 recovered from smelting/roasting off-gas) and purified.
- Absorption: the SO2 gas is absorbed into the soda ash solution under controlled pH and temperature; the pH window is critical — too high yields sulfite, too low releases SO2.
- Crystallization: the bisulfite solution is concentrated and crystallized to metabisulfite.
- Separation and drying: crystals are centrifuged and dried carefully (excess heat decomposes the product).
2.2 Quality-defining points
- SO2 gas purity (traces of metals and other impurities carry into the product)
- Absorption pH/temperature control (determines sulfite vs bisulfite balance)
- Drying temperature (decomposition risk)
- Packing (moisture-proof; the product absorbs moisture and slowly releases SO2)
3. Specifications
| Parameter | Typical value | Significance |
|---|---|---|
| Assay (as Na2S2O5) | 96-98% min (grade-dependent) | Active content |
| SO2 content | ~65-67% (equivalent) | Directly correlates with preservative/reducing power |
| Iron | Low (e.g. <0.005%) | Color and purity for food/photo uses |
| Water-insolubles | Low | Purity |
| Heavy metals | Low (food grade must meet limits) | Food safety |
| Appearance | White to slightly yellow crystalline powder | Visual QC |
4. Applications
4.1 Food preservation and antioxidant
Sodium metabisulfite is a widely used food additive (E223) as a preservative and antioxidant: it inhibits microbial growth and enzymatic browning in dried fruit, wine (sulfite), juices, pickles and seafood. Sulfite sensitivity in some consumers is regulated — labeling requirements apply in most regions.
4.2 Water treatment
It is the standard dechlorination agent in water treatment: it rapidly neutralizes residual chlorine and chloramine in municipal and industrial water, protecting RO membranes and downstream processes.
4.3 Bleaching and textiles
Used for bleaching in textile, paper pulp and straw processing, and as a reducing agent in dyeing (vat dyes).
4.4 Mining and industrial
As a reducing agent it is used in mineral processing (e.g. in some flotation circuits) and in photographic and rubber applications.
5. Safety and Handling
- SO2 release: contact with acids releases sulfur dioxide gas — toxic and irritating; never mix with acids.
- Allergen/sensitizer: sulfites can trigger asthma in sensitive individuals; handle with dust protection.
- Storage: keep sealed, dry, cool; moisture degrades the product and releases SO2.
- PPE: gloves, goggles, dust mask.
6. FAQ
Is sodium metabisulfite the same as sodium sulfite?
No. Sodium metabisulfite (Na2S2O5) is the bisulfite form; sodium sulfite (Na2SO3) is the normal sulfite. Metabisulfite releases more SO2 per unit and is the common commercial form for preservation and dechlorination.
Is sodium metabisulfite safe in food?
It is an approved food additive (E223) in many regions with usage limits; sulfite-sensitive individuals must be warned via labeling.
How does it remove chlorine from water?
It reduces free chlorine (and chloramine) to chloride, typically at a dose of about 1.5-3 mg/L of metabisulfite per mg/L of chlorine, depending on the form.
What grade should I buy?
Food grade for food applications; technical grade for water treatment, textiles and industrial use; photo grade for photographic applications. Confirm the COA limits.
7. References and Further Reading
- Peer-reviewed literature on sulfite chemistry and food preservation
- Industry literature on water dechlorination and bleaching
- Food additive regulations (E223, sulfite labeling)
This article is published for industrial and technical reference by PolyblueChem. Specifications are typical industry values; always verify against the supplier’s COA.
PolyblueChem supplies sodium metabisulfite (CAS 7681-57-4) in food and technical grades with COA and MSDS. Contact us for specifications and quotations.