Understanding the Properties of Sodium Bisulfite (Sodium Metabisulfite) for Industrial Applications
When sourcing chemical agents for water treatment, gold mining, or food preservation, the properties of sodium bisulfite are frequently the first technical checkpoint. In professional industrial procurement, “sodium bisulfite” almost always refers to sodium metabisulfite (Na₂S₂O₅), the anhydrous and more stable form that instantly generates bisulfite ions upon dissolution. A sound grasp of these properties—from molecular structure to storage behavior—empowers buyers to specify the correct grade, avoid handling pitfalls, and optimize process efficiency. At Hailei Chemical, we supply food-grade and industrial-grade sodium metabisulfite with 97–98% purity, backed by rigorous quality control and global logistics expertise.
Clarifying the Nomenclature: Sodium Metabisulfite vs. Sodium Bisulfite
Industrial buyers often encounter the term “sodium bisulfite” on tender documents and spec sheets, yet the shipped product is routinely sodium metabisulfite. This is not a discrepancy; it reflects the aqueous chemistry of the two compounds. Sodium metabisulfite (Na₂S₂O₅) is a stable, crystalline powder that, when dissolved in water, undergoes hydrolysis to yield two equivalents of sodium bisulfite (NaHSO₃):
Na₂S₂O₅ + H₂O → 2 NaHSO₃
In practice, the properties of sodium bisulfite as experienced in solution are realized from sodium metabisulfite. For this reason, the chemical industry uses the names interchangeably for many applications, although the dry storage and handling characteristics clearly belong to metabisulfite. Procurement professionals who understand this relationship can write clearer specifications and avoid confusion with true sodium bisulfite solutions, which are less stable and more expensive to transport.
Sodium Metabisulfite Molecular Formula and Structural Identity
Searching for the sodium metabisulfite molecular formula must return one definitive answer: Na₂S₂O₅. The substance carries CAS number 7681-57-4 and a molecular weight of 190.11 g/mol. Its structure centers on a disulfite ion (S₂O₅²⁻), where two sulfur atoms are linked by a direct S–S bond, each additionally bound to three oxygen atoms. This configuration imparts a high reducing potential and the ability to release sulfur dioxide (SO₂) under acidic conditions or heating.
The crystalline solid is white to yellowish-white and appears as free-flowing granules or powder in commercial grades. When evaluating supplier specifications, confirming the exact CAS and molecular formula is the first step toward verifying material identity—critical in regulated industries such as food processing and potable water treatment.
Physical Properties That Matter for Handling and Storage
Procurement managers and plant engineers need a clear profile of the physical properties of sodium bisulfite/metabisulfite to design safe receiving, storage, and feeding systems. Below are the key parameters that influence day-to-day operations:
- Appearance: White to light-yellowish crystalline powder or granules. Food-grade material often has a brighter whiteness due to lower iron and heavy metal content.
- Odor: Pungent, characteristic sulfur dioxide odor, especially upon opening containers. This is a result of gradual decomposition in contact with moisture.
- Bulk Density: Approximately 1.0–1.3 g/cm³ for powder; granular grades may have slightly higher tapped density, influencing silo and bag sizing.
- Solubility in Water: Freely soluble—about 540 g/L at 20 °C. The dissolution is endothermic, which can cause a noticeable temperature drop in solution preparation and may affect mixing in cold-weather operations.
- pH of Solution: A 5% w/w aqueous solution typically shows pH 4.0–5.0, indicating the acidic character of the bisulfite ion. This acidity is essential in applications like pulp bleaching adjustment and chromium reduction in tanning.
- Melting Point & Thermal Stability: Sodium metabisulfite does not have a sharp melting point. It decomposes above 150 °C, releasing SO₂ and leaving a sulfate residue. Storage must remain well below this temperature to prevent caking and off-gassing.
- Hygroscopicity: The material is moderately hygroscopic. In high-humidity environments without proper sealing, it will absorb moisture, clump, and gradually decompose. This underscores the need for moisture-barrier packaging and a dry, cool warehouse.
Chemical Reactivity and the True Properties of Sodium Bisulfite in Solution
Redox chemistry is the heart of sodium metabisulfite’s industrial value, and the properties of sodium bisulfite in an aqueous medium define its practical reactivity. Once dissolved, the bisulfite ion acts as a powerful reducing agent, participating in reactions that are central to water treatment, mining, and food preservation.
Reducing Potential and Dechlorination
In water and wastewater plants, bisulfite is employed to neutralize free chlorine and chloramine residuals. The reaction is rapid and stoichiometric:
NaHSO₃ + Cl₂ + H₂O → NaHSO₄ + 2 HCl
About 1.46 kg of pure sodium metabisulfite is required to remove 1 kg of chlorine. Understanding this ratio allows operators to size dosing equipment accurately. The ability to achieve undetectable chlorine residuals without introducing toxic byproducts makes the material indispensable in reverse osmosis pretreatment and effluent dechlorination.
Cyanide Detoxification in Gold Mining
One of the most critical safety applications of sodium metabisulfite is the destruction of free cyanide in gold leach tailings. The bisulfite ion, together with an oxygen source (usually air), oxidizes cyanide to the far less toxic cyanate:
CN⁻ + S₂O₅²⁻ + O₂ + H₂O → CNO⁻ + 2 SO₄²⁻ + 2 H⁺
Mine site laboratories regularly monitor ORP (oxidation-reduction potential) to confirm that enough metabisulfite has been added. The dose depends on the cyanide concentration but typically ranges from 2 to 8 grams of sodium metabisulfite per gram of free cyanide. Buyers in this sector place a premium on high purity and low heavy metal levels to meet stringent environmental compliance.
Sulfite Preservative Action in Food and Beverages
The food industry relies on the sodium metabisulphite in food as a preservative (E223) with antioxidant and antimicrobial functions. In solution, the bisulfite ion scavenges oxygen and inhibits enzymatic browning in dried fruits, fruit juices, and wine. The active species reacts with carbonyl compounds to form stable sulfonates, preserving color and freshness. The permitted residual SO₂ level in food products varies by regulation—for example, the EU allows up to 200 mg/kg in certain dried fruits—so food processors must select a certified food-grade powder with guaranteed purity and minimal arsenic, lead, and selenium content.
Role in Winemaking
Sodium metabisulfite uses in winemaking deserve special attention. Winemakers prefer potassium metabisulfite for routine sulfite additions, but sodium metabisulfite is occasionally used for cleaning and sanitizing equipment, and in some regions for must sulfiting where potassium buildup is not a concern. The bisulfite ion releases free SO₂, which acts as an antimicrobial agent against wild yeasts and bacteria and as an antioxidant to prevent oxidative spoilage. The amount needed depends on pH—lower pH requires less free SO₂ to achieve the same molecular SO₂ concentration. A winery procurement team sourcing sodium metabisulfite must demand a purity of at least 97% and ensure the product is free from foreign odors that could taint wine.
Pulp Bleaching and Textile Anti-Chlorine Treatment
In pulp mills, sodium metabisulfite serves as a reducing bleach assistant, especially in mechanical pulping where it brightens lignin-rich pulp without degrading fiber strength. The bisulfite ion reduces chromophoric groups, enhancing brightness. Similarly, in textile finishing, a mild bisulfite rinse removes residual chlorine from fabrics after hypochlorite bleaching, preventing yellowing and fiber degradation. The properties of sodium bisulfite as a chlorine scavenger make it a cost-effective alternative to activated carbon in many post-bleach baths.
Quality Specifications and Grades: What Procurement Teams Must Verify
A thorough understanding of commercial specifications prevents costly non-conformances. At Hailei Chemical, we supply industrial and food grades with the following typical parameters :
| Parameter | Industrial Grade | Food Grade (E223) |
|---|---|---|
| Purity (as Na₂S₂O₅) | 97.0% min | 97.5–98.0% min |
| SO₂ Content | 65.0% min | 65.0% min |
| Iron (Fe) | ≤ 0.005% | ≤ 0.001% |
| Heavy Metals (as Pb) | ≤ 0.002% | ≤ 0.001% |
| Arsenic (As) | ≤ 0.0002% | ≤ 0.0002% |
| Selenium (Se) | — | ≤ 0.003% |
| pH (5% solution) | 4.0–5.0 | 4.0–5.0 |
| Water Insoluble | ≤ 0.05% | ≤ 0.02% |
Buyers serving regulated markets—especially beverage, dairy, or municipal drinking water—should request a Certificate of Analysis (CoA) for each lot and verify compliance with FCC, JECFA, or EU standards as appropriate. Our quality management system ensures batch traceability from production to shipment.
Packaging, Storage, and Stability Considerations
The physical and chemical properties of sodium metabisulfite dictate specific packaging and storage protocols to preserve product integrity. Standard packaging includes 25 kg woven polypropylene bags with an inner polyethylene liner, 1000 kg FIBC bulk bags, and drums for smaller quantities. The liner must provide a robust moisture barrier because even slight humidity ingress triggers decomposition, leading to caking and SO₂ loss.
Warehouse conditions should maintain temperatures below 30 °C and relative humidity below 60%. Stacking should allow air circulation, and opened bags must be re-sealed promptly. Under these conditions, the shelf life ranges from 12 to 24 months depending on initial purity and packaging integrity. In tropical and monsoon climates—such as those in Bangladesh where interest in sodium metabisulfite price in bangladesh is growing—suppliers must offer packaging with enhanced moisture protection and possibly arrange container liners for sea freight to prevent deterioration during transit.
Addressing Regional Buying Concerns: The Sodium Metabisulfite Price in Bangladesh
Chemical importers in South Asia closely watch the sodium metabisulfite price in bangladesh due to the country’s expanding textile, leather, and food processing sectors. Prices are influenced by international sulfur and soda ash feedstock costs, shipping freight, and local duties. A reliable long-term supplier like Hailei Chemical can provide stable pricing through direct factory contracts, flexible CIF Chittagong or Mongla terms, and volume flexibility from FCL to break-bulk shipments. When evaluating price quotations, buyers should consider the total delivered cost, including moisture-proof packaging and demurrage risks, not just the EXW price per metric ton.
Sustainability, Safety, and Regulatory Compliance
Responsible buyers integrate EHS (Environment, Health, Safety) criteria into sourcing decisions. Sodium metabisulfite is classified as an irritant, and its solid form can cause eye and respiratory irritation. Proper PPE includes dust masks, goggles, and gloves. Under REACH and GHS, it carries H318 (causes serious eye damage) and H302 (harmful if swallowed) hazard statements. Our Safety Data Sheets (SDS) are always supplied with shipments and available for review before purchase.
On the environmental side, bisulfite ions oxidize to sulfate in wastewater, a benign species at typical discharge concentrations. When used in cyanide destruction, the resulting cyanate further hydrolyzes to ammonia and carbonate, dramatically reducing ecotoxicity. This green chemistry aspect makes sodium metabisulfite a preferred choice over chlorine-based cyanide oxidizers, which can form toxic cyanogen chloride.
How to Specify Sodium Metabisulfite on Your Purchase Order
A well-written purchase order prevents costly discrepancies. Based on the properties of sodium bisulfite outlined, include these details:
- Chemical name: Sodium metabisulfite (also referred to as sodium pyrosulfite)
- CAS number: 7681-57-4
- Grade: Industrial grade or food grade (E223)
- Purity: Minimum 97% as Na₂S₂O₅
- Packaging: 25 kg bags/1 MT big bags with moisture barrier
- Acceptable heavy metal limits (especially for food grade)
- Required documentation: CoA, SDS, fumigation certificate, GMP certificate if applicable
- Incoterms: CIF, FOB, or EXW as negotiated
Our team at Hailei Chemical will provide a detailed technical data sheet for each shipment, aligning with these parameters to make your incoming inspection seamless.
Why Choose Hailei Chemical as Your Sodium Metabisulfite Supplier
With over a decade of experience exporting to more than 40 countries, Weifang Hailei Fine Chemical Co., Ltd. combines manufacturing control with supply chain reliability. Our product consistency is validated by ISO-certified management systems, while our logistics team handles the complexities of hazardous goods documentation (Class 8 for some concentrations) and customs clearance. We offer sample evaluation, contract manufacturing, and monthly production capacity of 3,000 MT to support large-scale industrial demands.
Whether you need sodium metabisulfite uses in winemaking or high-tonnage deliveries for a gold mining operation, our technical service engineers are available to assist with application-specific recommendations. We understand that the sodium metabisulfite molecular formula on paper must translate into consistent real-world performance.
Get a Personalized Quote Today
Choosing the right sodium metabisulfite grade begins with a clear understanding of its physical and chemical properties. Let our experts help you match those specifications to your process requirements. Request a competitive quotation for industrial or food-grade sodium metabisulfite, including tailored packaging and delivery to your port. We look forward to powering your success with reliable chemistry and global reach.