Sodium sulfite

Sodium sulfite (CAS 7757-83-7), also known as sodium sulphite and approved as food additive E221, is an inorganic reducing agent and antioxidant.

Where is Sodium sulfite used?

• Food and beverage: preservative and antioxidant in dried fruits, wines and starch products, inhibiting browning and microbial growth
• Water treatment: dechlorination of process water and boiler feedwater, neutralizing residual chlorine
• Pulp and paper: delignification and brightness control in kraft and sulfite pulping
• Textile and leather: reducing agent and stripping agent to correct uneven dyeing
• It preserves product quality and controls oxidative degradation in aqueous systems and dry formulations.
• Used in the thermomechanical defibration of wood for the production of medium-density fiberboard (MDF).
• In the photographic industry, protects developer solutions from oxidation and, as a hypo clearing solution, preserves the fixing agent (sodium thiosulfate) from film and photographic paper emulsions.
• Primarily used as an oxygen scavenger in water-based oil drilling fluid systems.

Formulation notes

• In water treatment, used at a dosage of 10-50 ppm depending on oxygen/chlorine levels.
• In food applications, used at a dosage of 10-100 ppm depending on the application.
• In paper and pulp industry, used at a dosage of 5-15% of the cooking liquor.
• Treatment with strong acids releases sulfur dioxide.

Typical specification

• Also transported as the heptahydrate form Na2SO3·7H2O.
• Sold in anhydrous form.

Regulatory status

• In 2016, EFSA set a temporary group ADI of 0.7 milligrams per kilogram body weight per day for sulfites.
• Prohibited for use in meats or foods containing vitamin B1.
• ECHA separately evaluated sulfur dioxide under the EU biocides regulation, including for use in disinfecting wine barrels and as a preservative to prevent microbial growth in shoe boxes during storage and transport.

Storage, packaging and handling

• Should be stored in a well-closed container in a cool, dry place.
• Should be stored separately from strong oxidants and acids.
• Should be stored in an area without drains or sewer access.

Frequently asked questions about Sodium sulfite

At what pressures is sodium sulfite unsuitable for oxygen scavenging?

It should not be used for oxygen scavenging at pressures above 122 atm (12.4 MPa or 1.8 ksi) because it decomposes into acidic gases at high temperatures associated with high pressures.

In which forms is sodium sulfite generally available?

It is generally available in powder, crystal, and tablet forms.

Technical content is compiled and updated by the SoleChem product team from public technical sources; the certificate of analysis (COA) governs product-specific specifications.