Properties of Sodium Metabisulfite: A Technical Buyer’s Guide to Purity and Performance
For procurement managers and chemical engineers sourcing bulk reducing agents, a thorough understanding of the properties of sodium metabisulfite is the foundation of successful formulation, safe handling, and cost-effective supply chain management. From water dechlorination at municipal treatment plants to cyanide detoxification in gold mines and sulfite preservation in food processing, this versatile inorganic compound delivers reliable performance only when its chemical and physical characteristics align with end-use requirements.
At Weifang Hailei Fine Chemical Co., Ltd., we supply sodium metabisulfite (Na₂S₂O₅, CAS 7681-57-4) in both food-grade (FCC compliant) and industrial-grade (97–98% purity) to buyers across India, Southeast Asia, Africa, and the Middle East. This guide decodes every property you need to evaluate before issuing a purchase order, helping you avoid caking in storage, insufficient SO₂ release, or regulatory non-compliance.
What is Sodium Metabisulfite Made From? Chemical Composition and Manufacturing
Understanding what is sodium metabisulfite made from clarifies its inherent properties. Industrially, sodium metabisulfite is produced by saturating a sodium carbonate or sodium hydroxide solution with sulfur dioxide (SO₂). The exothermic reaction first forms sodium bisulfite (NaHSO₃) in solution, which is then carefully dehydrated through cooling and controlled crystallization to yield solid sodium metabisulfite:
2 NaHSO₃(aq) → Na₂S₂O₅(s) + H₂O
The product is a white to yellowish crystalline powder or granular material with a pungent SO₂ odor. Because its synthesis relies on high-purity SO₂ gas and a low-iron process environment, the quality of inputs directly determines the final product’s heavy metal profile and clarity in solution. For food-grade material, additional purification steps ensure compliance with FCC or E223 monographs, making it safe for use as a preservative in dried fruits, wines, and starch syrups.
Key Physical and Chemical Properties of Sodium Metabisulfite
A technical evaluation begins with the core properties of sodium metabisulfite. These data points influence dissolution rates, dosing precision, and compatibility with other process chemicals.
- Appearance: White or slightly yellow free-flowing crystalline powder or granules – color deviation signals iron contamination or oxidation.
- Odor: Sharp, characteristic SO₂ odor; becomes stronger when moist as acidification releases sulfur dioxide gas.
- Molecular weight: 190.11 g/mol (Na₂S₂O₅).
- Melting/decomposition point: Decomposes at temperatures above 150°C without a true melting point, releasing SO₂ and leaving sodium sulfate residue.
- Solubility in water: Freely soluble in water (approx. 54 g/100 mL at 20°C) and glycerol; practically insoluble in ethanol.
- pH (aqueous solution): A 10% solution typically exhibits a pH between 4.0 and 5.5, indicating the acidic nature due to hydrolysis into bisulfite ions (HSO₃⁻).
- Bulk density: Ranges from 1.1 to 1.4 g/cm³ for granular grades, important for container loading and silo capacity calculations.
- Redox potential: Acts as a strong reducing agent, with standard redox potential E° ≈ -0.93 V for the SO₄²⁻/SO₃²⁻ couple, enabling rapid dechlorination and metal ion reduction.
- Hygroscopicity: Moderately hygroscopic; absorbs moisture from air, leading to caking and gradual oxidation to sulfate if not sealed properly.
- Stability: Stable under dry, cool conditions. In solution, gradually oxidizes to sodium sulfate, especially when aerated or exposed to light. The dry product releases SO₂ slowly upon heating or contact with acids.
These parameters are not just laboratory curiosities—they directly affect real-world outcomes. For instance, the acidic pH of a metabisulfite solution makes it ideal for acidifying and preserving high-sugar food systems, while the decomposition temperature dictates safe storage away from heat sources.
How Purity Levels—Food Grade vs. Industrial Grade—Affect Performance
Sodium metabisulfite is traded primarily in two purity specifications: industrial grade (typically 97–98% Na₂S₂O₅) and food grade (≥98% plus strict limits on iron, arsenic, and heavy metals). The property differences may seem subtle, but they have outsized impacts in sensitive applications.
Industrial grade (97–98%): Optimized for water treatment, gold mining (cyanide destruction), and pulp & paper bleaching. A lower assay of 97% still supplies more than 65% equivalent SO₂ content, but the 1–2% inert salts or sulfate residues can affect clarity in closed-loop water systems. Buyers should monitor iron content (<0.003% typical), since dissolved iron can catalyze unwanted oxidation or stain pulp.
Food grade (FCC/E223): Must meet purity thresholds for arsenic (≤3 mg/kg), lead (≤2 mg/kg), and selenium. The absence of chloride by-products ensures no off-flavors in wine or dried fruit. At Hailei Chemical, our food-grade sodium metabisulfite is manufactured under rigorous HACCP-guided protocols to guarantee compliance with international food safety standards.
When specifying your purchase, always request a Certificate of Analysis (COA) detailing assay, SO₂ content, pH of 10% solution, iron, heavy metals, and water-insoluble matter. The sodium metabisulfite product page provides typical Hailei COA ranges to simplify your comparison.
Sodium Bisulfite and Sodium Metabisulfite: A Comparative Property Analysis
Many industrial buyers wonder about the interchangeability of sodium bisulfite and sodium metabisulfite. While both provide SO₂ in solution, their chemical properties lead to different dosing behaviors and storage considerations.
| Property | Sodium Metabisulfite (Na₂S₂O₅) | Sodium Bisulfite (NaHSO₃) |
|---|---|---|
| Physical form | Dry crystalline powder/granules | Often supplied as solution (approx. 40%) or crystalline solid (less stable) |
| SO₂ equivalent | Approx. 65–67% by weight | Approx. 22–24% in commercial solutions; solid bisulfite contains 60–65% but is hygroscopic |
| pH of solution | 4.0–5.5 (10% solution) | 2.5–4.0 (solution), more acidic |
| Stability in storage | Excellent when kept dry; >12 months shelf life | Solution prone to oxidation by air; solid form absorbs moisture rapidly |
| Shipping | Dry bulk; cost-effective long-distance shipping | Solution requires tankers or IBCs; higher freight per active SO₂ |
| Typical applications | Water dechlorination, gold mining, food preservative, pulp bleaching, textile | On-site chlorine neutralization, reducing agent in chemical synthesis, leather tanning |
The choice often boils down to logistics and pH buffering. For intercontinental shipments where freight cost and product stability matter, dry sodium metabisulfite is the preferred form. Upon dissolution in water, it forms sodium bisulfite ions, delivering the same reducing action. However, if your process demands immediate solution without handling dry powder, a pre-made bisulfite solution might be convenient—though it costs more per kilogram of SO₂ delivered. Hailei supplies both dry sodium metabisulfite and can direct you to logistics partners for sodium bisulfite solutions where needed.
How to Use Sodium Metabisulphite Based on Its Chemical Properties
Knowing how to use sodium metabisulphite safely and efficiently starts with respecting its reactivity profile. The compound’s core property is its ability to release sulfur dioxide in acidic conditions or when heated, making it a potent oxygen scavenger and antimicrobial agent.
Water Dechlorination
In municipal and industrial water treatment, sodium metabisulfite instantly neutralizes free chlorine and chloramines. The stoichiometric reaction requires 1.34 parts of pure Na₂S₂O₅ per part of chlorine. A typical dosing rate is 2–3 mg/L of a 10% metabisulfite solution per mg/L residual chlorine, adjusted for mixing efficiency. Because the reaction drops pH slightly, systems with low alkalinity may need buffering.
Gold Mining Cyanide Detoxification
The SO₂/air process (INCO method) uses metabisulfite as the SO₂ source to oxidize cyanides (CN⁻) to cyanate (OCN⁻), catalyzed by copper. The reducing property here must be carefully controlled: a solution pH between 8.5 and 9.5 optimizes cyanide destruction while minimizing off-gassing of HCN. Miners often dose metabisulfite at 2–4 grams per gram of WAD cyanide.
Food Preservation
As an E223 additive, sodium metabisulfite inhibits enzymatic and non-enzymatic browning in dried fruits, frozen seafood, and wine. The effective SO₂ concentration for antimicrobial action is 50–400 ppm, depending on the product. Its ability to lower pH helps stabilize ascorbic acid and color. Importantly, residual sulfite must meet local labeling laws—typically <10 ppm SO₂ in final product.
Pulp and Paper Bleaching
In mechanical pulping, metabisulfite acts as a reducing bleach to brighten lignin-rich pulps without degrading cellulose. The property of preserving fiber strength while increasing brightness by 5–8 ISO points makes it valuable for newsprint and tissue grades. Typical application: 1–3% on oven-dry pulp at pH 5–6.
Textile Anti-Chlorine Treatment
After chlorine bleaching, fabrics carry residual chlorine that can yellow or weaken fibers. A 0.5–2 g/L sodium metabisulfite bath, based on its rapid dechlorination property, eliminates this risk. The slightly acidic pH helps close fiber cuticles, improving dye uptake in subsequent steps.
For detailed application recipes and custom formulation support, refer to our comprehensive sodium metabisulfite application guide.
Quality Specifications and Testing Standards: What a Technical Buyer Must Verify
Procuring sodium metabisulfite without verifying properties against recognized standards can lead to inconsistent batch performance, rejected shipments, or even plant shutdowns. A rigorous supplier evaluation framework should include the following analytical benchmarks.
- Assay (Na₂S₂O₅ purity): Titration with iodine or iodometry per ISO 3625. Minimum 97.0% for industrial, ≥98.0% for food grade.
- SO₂ content: Calculated from assay; should be 65–67% w/w. Some buyers request direct SO₂ determination via distillation and oxidation.
- Iron (Fe): ≤0.003% by AAS or ICP; critical for food and pulp applications.
- Heavy metals (as Pb): ≤0.001% for food grade.
- Water-insoluble matter: ≤0.02%, indicating minimal contamination from silica or filter aid.
- Arsenic (As): ≤3 mg/kg for food grade.
- pH (10% solution): 4.0–5.5. A deviation may signal presence of excess sodium carbonate or sulfate.
- Particle size distribution: 95% passing 20–100 mesh, or as agreed, to guarantee dissolution rate and flowability.
Request a batch-specific COA with every shipment. Third-party testing by SGS, Bureau Veritas, or Intertek adds an extra layer of trust. Hailei Chemical routinely arranges pre-shipment inspections against these parameters to ensure that the properties of sodium metabisulfite you order are exactly what you receive.
Storage, Handling, and Stability: Protecting the Properties of Sodium Metabisulfite
The shelf life and performance of sodium metabisulfite depend critically on storage conditions. Exposure to moisture and heat degrades its reducing power and turns the powder into a hard, unworkable mass.
Key recommendations:
- Store in a cool (below 30°C), dry, well-ventilated area away from direct sunlight and heat sources.
- Keep containers tightly sealed when not in use. Use original 25 kg polyethylene-lined woven bags or supersacks; transfer opened product into airtight bins.
- Avoid contact with acids, oxidizing agents (nitrates, peroxides), and water. Incompatible materials can trigger rapid SO₂ release, posing toxic gas hazards.
- Monitor first-expiry-first-out (FEFO) inventory: shelf life is typically 12–24 months under optimal conditions. After 12 months, assay may drop by 0.5–1.0% due to slow oxidation.
- For bulk storage in silos, ensure the vessel is equipped with desiccant breathers and the conveying air is dry to prevent caking on walls.
Understanding these environmental sensitivities allows you to plan inventory turnover and avoid waste, directly protecting your investment in each purchased ton.
Sourcing a Sodium Metabisulfite Supplier in India and Global Markets: Logistics Considerations
India is one of the world’s largest importers of sodium metabisulfite, driven by expansive water treatment programs, a massive gold import sector, and a growing food processing industry. If you are seeking a sodium metabisulfite supplier in India, working directly with a chemical exporter in China can often offer cost advantages of 12–18% compared to local distributors, provided you manage shipping and quality inspection efficiently.
Hailei Chemical serves Indian buyers through Nhava Sheva, Mundra, Chennai, and Kolkata ports. Our typical packaging in 25 kg HDPE bags with inner liner is palletized and shrink-wrapped for tropical climates. We also offer 1000–1250 kg big bags and custom private labeling. For oil & gas or mining tenders, we provide full documentation: COA, MSDS, Certificate of Origin, and fumigation certificates as required by Indian customs (HS Code 28321010).
The same logistics expertise applies to buyers in the Middle East (Jebel Ali, Dammam), Southeast Asia, and Africa. Our in-house team coordinates FCL and LCL shipments, transit times, and demurrage avoidance. By consolidating volumes, we help you optimize landed costs without compromising the high purity and consistent properties of sodium metabisulfite you require.
Why Hailei Chemical is Your Trusted Partner for High-Purity Sodium Metabisulfite
Since 2005, Weifang Hailei Fine Chemical Co., Ltd. has been manufacturing and exporting sulfur-based chemicals under strict ISO 9001 and industry-specific certifications. Our sodium metabisulfite is produced in a dedicated facility with advanced crystallization controls, ensuring every batch meets the precise physical and chemical properties detailed in this guide. We maintain ready stock in our warehouse, enabling 7–15 day lead times for standard FCL orders.
When you partner with Hailei, you gain:
- Guaranteed purity: 97–98% industrial grade, FCC-compliant food grade with traceable COA.
- Technical support: Application-specific advice on dissolution, dosing, and compatibility.
- Flexible logistics: FOB Qingdao, CIF to your port, or door-to-door DDP solutions.
- Sample policy: Pre-shipment sample dispatch for your lab evaluation.
Don’t leave your procurement to chance. If you need a reliable supply of sodium metabisulfite that matches the property specifications critical to your operation, request a quote today. Visit our sodium metabisulfite product page to download the latest COA, MSDS, and application brochures, and let us customize a shipment that meets your exact volume and grading needs.