Whether you source chemicals for a municipal water treatment plant, a gold mine in India, or a food processing facility, understanding the properties of sodium metabisulfite is critical to making safe, cost-effective, and performance-driven procurement decisions. Sodium metabisulfite (Na2S2O5, CAS 7681-57-4) is a versatile inorganic sulfite used globally for dechlorination, cyanide detoxification, food preservation, pulp bleaching, and textile anti-chlorine treatment. Yet many buyers overlook how the product’s chemical identity, solubility, stability, and purity directly impact dosage, handling, and final application outcomes. This guide breaks down the essential physical and chemical properties, explains how sodium metabisulfite is made, compares it with sodium bisulfite, and provides practical usage directions—all from the perspective of a procurement manager or process engineer sourcing industrial-grade material.
To use the product effectively, you must first know its core properties. Sodium metabisulfite is a white to yellowish crystalline powder with a pungent sulfur dioxide odor. It is freely soluble in water (54 g/100 mL at 20°C) and slightly soluble in alcohol. The aqueous solution is acidic, with a pH around 4.0–5.5 for a 5% solution, due to the release of sulfurous acid. On a molecular level, sodium metabisulfite is the disodium salt of disulfurous acid (H2S2O5), meaning each molecule contains two sulfur atoms bridged through an S–S bond, which distinguishes it from simple sulfites.
Key physical constants: molecular weight 190.11 g/mol; melting point >150°C with decomposition; apparent density approx. 1.1–1.3 g/cm³. When heated above 150°C, it decomposes, releasing sulfur dioxide (SO2) and leaving sodium sulfite (Na2SO3) and sulfate residues. This thermal behavior is important if your process involves drying or exposure to high temperatures.
The properties of sodium metabisulfite that matter most in industrial use are its strong reducing action in aqueous media and its ability to generate SO2 under acidic conditions. In water, it dissociates into sodium (Na⁺) and metabisulfite ions (S2O52−), which further hydrolyze to bisulfite (HSO3−) and sulfite (SO32−) species. This equilibrium gives it excellent reactivity for removing chlorine, oxygen, and heavy metal oxidants. The high assay (97–98% Na2S2O5 for Hailei Chemical’s food and industrial grades) ensures low iron (≤10 ppm), heavy metals (≤5 ppm as Pb), and minimal insoluble matter, meeting EU, FCC, and USP standards for critical applications.
Understanding the production pathway helps buyers evaluate supply chain consistency and impurity profiles. Sodium metabisulfite is manufactured by reacting sulfur dioxide (SO2) with a sodium base—typically sodium carbonate (soda ash) or sodium hydroxide (caustic soda)—under controlled aqueous conditions. The process can be summarized in two main steps:
High-quality producers—like Hailei Chemical—use food-grade sulfur and purified water, ensuring the final product meets both industrial and food-grade specifications. The choice of sodium source influences residual alkalinity: soda ash yields a product with slightly higher sodium carbonate traces, while NaOH route gives a purer bisulfite intermediate but requires tighter process control. Knowing this helps you request the right certificate of analysis (CoA) for your specific purity and particle size needs.
A common point of confusion is the relationship between sodium bisulfite (NaHSO3) and sodium metabisulfite. While both are sulfite-based reducing agents, their chemical structure and practical behavior differ. Sodium bisulfite is a monoprotic salt of sulfurous acid, typically existing only in aqueous solution or as a labile solid. Sodium metabisulfite, on the other hand, is the dehydrated dimer of sodium bisulfite: two bisulfite ions lose one water molecule to form the S–S bonded metabisulfite. When dissolved in water, sodium metabisulfite hydrolyzes to sodium bisulfite and then to sulfite species, so the active chemistry is often similar. However, there are key practical differences:
For most procurement scenarios involving solid sulfite chemicals, sodium metabisulfite offers better shelf life and concentration. Always verify which form your process actually requires; switching from a liquid bisulfite to dry metabisulfite can cut logistics costs significantly but may require dissolving equipment.
Proper usage hinges on matching the properties of sodium metabisulfite to your process chemistry. Below we cover general guidelines for four key sectors.
In municipal and industrial water treatment, sodium metabisulfite is used to neutralize free chlorine and chloramines before discharge or membrane protection. The stoichiometric reaction is: Na2S2O5 + 3 Cl2 + 5 H2O → 2 NaHSO4 + 6 HCl. Typically, 1.34 mg of pure Na2S2O5 removes 1 mg of Cl2. Because commercial product is ~97% pure, a safety factor of 1.1–1.2 is used. A 10% solution is prepared and dosed via metering pump. Stirring and contact time of 5–15 minutes assure complete dechlorination. Online ORP sensors can automate the feed rate.
In gold leaching operations, residual cyanide must be destroyed to meet tailings discharge limits. Sodium metabisulfite provides SO2 gas in the well-known INCO process, oxidizing cyanide to cyanate. A typical usage rate is 5–10 kg metabisulfite per ton of ore depending on cyanide concentration. A 20% solution is injected into tailings slurry along with copper sulfate catalyst, at pH 8–9, with vigorous air sparging. The technical-grade sodium metabisulfite with high purity and low iron is essential to avoid precipitates that clog nozzles.
As an E223 additive, sodium metabisulfite inhibits enzymatic browning in dried fruits, wines, and vegetable preserves. Dosage varies: 200–500 ppm SO2 equivalent in wine must, or dipping solutions of 1–2% w/v for sliced apples and potatoes. Always use food-grade material that meets FCC/USP standards, free from arsenic and heavy metals. Important: sodium metabisulfite is an allergen; final products must declare sulfite content if above 10 ppm.
In pulp and paper mills, metabisulfite solutions (5–10%) remove residual chlorine-based bleach from pulp before papermaking, preventing yellowing and fiber damage. In textile finishing, it serves as an anti-chlor agent after hypochlorite bleaching to neutralize excess chlorine before dyeing. For both, a gentle reducing environment (pH 4–6, ambient temperature) works best.
India is one of the largest import markets for sodium metabisulfite due to its booming water treatment, mining, and food sectors. If you are searching for a sodium metabisulfite supplier in India, you need a partner who combines consistent product quality with logistics expertise. Hailei Chemical exports bulk sodium metabisulfite to major Indian ports (Nhava Sheva, Chennai, Mundra) in 25 kg woven bags, 1000 kg supersacks, or custom packaging. Our 97–98% purity with low heavy metals and iron aligns with BIS and food additive standards, while our logistical support—including full customs documentation, REACH compliance data, and pre-shipment samples—simplifies your import process.
When qualifying any international supplier, request a detailed CoA showing not just Na2S2O5 assay but also iron, arsenic, selenium, and chloride content. Scrutinize the product’s physical properties like bulk density and particle size distribution (typically 40–120 mesh) if your process uses dry feeders. Hailei Chemical’s technical team can provide specification sheets tailored to your application, ensuring the properties of sodium metabisulfite you receive precisely match your needs.
It is an acidic salt. A 5% aqueous solution has a pH of about 4.0–5.5 due to the formation of sulfurous acid. This mild acidity makes it ideal for reducing environments without drastically altering process pH.
Yes, in many applications. Sodium metabisulfite releases sulfite ions upon dissolution. The main advantage is higher SO2 equivalent per kilogram and better dry stability. However, for some food or pharmaceutical processes requiring precise sodium sulfite, direct substitution should be validated.
Store in a cool, dry, well-ventilated area away from acids and oxidizers. Sealed bags prevent moisture absorption, which would trigger decomposition and caking. Under proper conditions, shelf life exceeds 12 months without significant loss of SO2 content.
A simple iodometric titration determines available SO2 content. If the assay has dropped below your required threshold (e.g., 97%), the product may have degraded due to moisture ingress. Hailei Chemical uses quality-controlled packaging to maintain assay throughout transit.
When sourcing a bulk supply of sodium metabisulfite, having a deep grasp of its properties—from solubility and pH to redox behavior and impurity limits—empowers you to optimize processes and avoid costly misapplications. Whether you need food-grade powder for fruit preservation or industrial-grade crystals for cyanide destruction, our sodium metabisulfite product page provides full specifications and order options. To discuss your volume, packaging, or documentation requirements with our technical sales team, request a competitive quote today.