Sodium Metabisulphite: Formula, Uses, Grades and Safety

The short answer

Sodium metabisulphite (also written sodium metabisulfite, and sold as SMBS) is Na₂S₂O₅, molecular weight 190.11 g/mol, CAS 7681-57-4. It is a white crystalline powder that dissolves in water to form sodium bisulphite and releases sulphur dioxide, which makes it a preservative, an antioxidant and a reducing agent in one inexpensive solid.

As a food additive it is E223. In industry its value is measured by one number above all others: how much sulphur dioxide it actually delivers. That is why good specifications quote a minimum SO₂ content, not just a purity.

Sodium metabisulphite formula and identifiers

Where the 190.11 comes from: two sodium at 22.99, two sulphur at 32.06 and five oxygen at 16.00 — 45.98 + 64.12 + 80.00 = 190.10, or 190.11 with the unrounded atomic weights. The 67.4 % figure follows from the fact that one mole of Na₂S₂O₅ can yield two moles of SO₂ (2 × 64.07 = 128.14, divided by 190.11).

The name confuses people because “meta” suggests a variant of sodium bisulphite. It is better thought of as bisulphite with the water taken out: two bisulphite ions joined into the disulphite ion S₂O₅²⁻. That is why “sodium disulfite” and “sodium pyrosulfite” appear on some safety data sheets for exactly the same product.

What happens when it dissolves

In water, sodium metabisulphite hydrolyses to sodium bisulphite:

Na₂S₂O₅ + H₂O → 2 NaHSO₃

The bisulphite is in equilibrium with dissolved sulphur dioxide, and the lower the pH, the more free SO₂ there is. That single fact explains most of its behaviour:

• It is acidic. Solutions are mildly acidic, which is why sulphite preservation works best in acidic foods and drinks such as wine, juice and pickles.
• It is a reducing agent. Bisulphite consumes oxygen and chlorine and reduces metal ions, which is the basis of its antioxidant, dechlorinating and metal-recovery uses.
• It releases SO₂ gas with acid. Mixing with strong acid drives off sulphur dioxide quickly, which is useful in controlled processes and dangerous by accident.
• It oxidises in air. Moist powder and open solutions slowly turn to sulphate, losing strength. Stored powder that has caked or smells weak has usually lost SO₂.

Physical properties

Grades and what to specify

• Technical or industrial grade. For water treatment, textiles, leather, mining and pulp and paper. Specified mainly on SO₂ content, iron and water-insoluble matter.
• Food grade (E223). Meets food additive specifications such as FCC, JECFA or the relevant national standard, with limits on arsenic, lead, selenium and iron. See our sodium metabisulphite (SMBS) food grade page.
• Pharmaceutical grade. Meets the IP, BP, Ph. Eur. or USP-NF monograph for use as an antioxidant excipient. See sodium metabisulphite pharma grade.
• Photographic and reagent grades. Tighter limits on thiosulphate, heavy metals and insolubles.

Whatever the grade, write these points into the order: minimum SO₂ content (or assay as Na₂S₂O₅), iron, heavy metals and arsenic, water-insoluble matter, the monograph if a pharmacopoeial grade is needed, and the particle form. Two lots that both say “sodium metabisulphite” can differ by several points of SO₂, and the dose on the plant is set by SO₂, not by kilograms of powder.

Uses of sodium metabisulphite

• Food and beverages. A preservative and antioxidant that stops browning and microbial spoilage in dried fruit, fruit juices and concentrates, wine and cider, pickles, and crustaceans such as shrimp, where it prevents black spot. It also conditions dough in some biscuit and cracker manufacture.
• Water treatment. Removes residual chlorine before reverse osmosis membranes, which chlorine would damage, and dechlorinates treated effluent before discharge. The theoretical dose is about 1.34 mg of sodium metabisulphite per mg of chlorine; plants usually dose above that. Dilute solutions are also used to preserve RO membranes during storage.
• Pharmaceuticals. An antioxidant in injectable and oral liquid formulations, protecting oxidation-sensitive actives.
• Textiles. An antichlor to neutralise residual chlorine after bleaching, and a reducing agent in dyeing and finishing.
• Mining and metal refining. Precipitates gold from chloride leach and aqua regia solutions, and serves as the sulphur dioxide source in the SO₂/air process for destroying cyanide in tailings.
• Photography. A preservative in developer solutions and a component of acid fixing baths.
• Leather, pulp and paper, and chemicals. Bleaching, pH adjustment and a reducing agent in chemical synthesis.

Is sodium metabisulphite safe in food?

Within legal limits, yes. It is a permitted additive in the EU, the United States, India and most other markets, with maximum levels set food by food and expressed as SO₂. JECFA has set a group acceptable daily intake for sulphites of 0 to 0.7 mg per kg of body weight, expressed as SO₂.

The real issue is sulphite sensitivity. A minority of people, particularly some asthmatics, react to sulphites. For that reason EU rules require sulphites to be declared as an allergen above 10 mg/kg or 10 mg/L as SO₂, the US requires declaration at 10 ppm or more, and the US does not allow sulphites on fresh fruit and vegetables meant to be eaten raw. Sulphites also destroy thiamine (vitamin B1), so they are not used in foods that are an important source of it.

Sodium vs potassium metabisulphite

Both do the same chemical job. Sodium delivers more SO₂ per kilogram; potassium avoids adding sodium to the product. We supply both: sodium metabisulphite and potassium metabisulphite. Where a sulphite without the acidic character is needed, for example as an oxygen scavenger, sodium sulphite anhydrous is the usual alternative.

Handling, storage and export documents

• Hazards. Harmful if swallowed and can cause serious eye damage. Contact with acids liberates toxic sulphur dioxide gas. Wear eye protection and gloves, and avoid breathing dust.
• Keep it away from acids and oxidisers. Store separately from acids, and from oxidising agents such as nitrates, chlorates and hypochlorite.
• Keep it dry and closed. Moisture and air slowly convert it to sulphate and drive off SO₂. Reseal part-used bags and store in a cool, dry, ventilated place.
• Packing. Usually 25 kg multi-wall paper or HDPE woven bags with a polyethylene liner, palletised; jumbo bags for large users.
• Transport. Generally not classified as a dangerous good for sea freight; confirm the position for your destination and pack with the forwarder and the safety data sheet.

An export consignment should carry a certificate of analysis stating SO₂ content or assay, iron, heavy metals and appearance; a 16-section GHS safety data sheet; the FCC, JECFA or pharmacopoeial compliance statement where claimed; commercial invoice and packing list; and a certificate of origin. For more on how pharmacopoeial and food grades are specified, see our guide to citric acid anhydrous vs monohydrate, and for another sodium salt often confused with it, sodium sulphate and Glauber’s salt.

Enquire with a specification

Scapex Exports supplies sodium metabisulphite from India in technical, food and pharmaceutical grades, to an agreed specification, with a certificate of analysis and a 16-section GHS safety data sheet on every batch and full export documentation.

Send the grade, the minimum SO₂ content, any monograph, the pack size, the quantity and the destination port, and you will get a quotation written against that specification rather than a generic offer.

Frequently asked questions