Sodium Sulfite Anhydrous

PreservativeNa2SO3CAS: 7757-83-7
Sodium Sulfite Anhydrous
Image: Smokefoot — CC BY 4.0

Physical Properties

Also known as: Anhydrous Sodium Sulfite, Dry Sulfite

Sodium sulfite anhydrous (Na₂SO₃; CAS 7757-83-7) is the water-free form of sodium sulfite — the canonical photographic-grade supply that most modern formulas assume when they say "sodium sulfite" without qualification. See the primary sodium sulfite entry for the full chemistry, photographic uses, and practical-notes discussion.

Weight conversion

Anhydrous vs heptahydrate conversion: anhydrous sodium sulfite is roughly twice the effective sulfite content per gram as the heptahydrate Na₂SO₃·7H₂O, which is about 50% water by weight (MW 252 vs 126). To substitute the heptahydrate in a recipe that specifies anhydrous, roughly double the weight. If a published formula is unambiguous about "anhydrous" or "heptahydrate", honour the specification; if it simply says "sodium sulfite" and dates from the last 30 years, assume anhydrous.

Related compounds

Sodium sulfite — the primary page covering the chemistry and uses in detail. Sodium bisulfite is the acid-form variant. Sodium metabisulfite is the shelf-stable anhydride.

Safety Notes

Same hazard profile as sodium sulfite — see that page for full acute hazards, storage/handling, and first aid. Summary:

  • Mild skin and eye irritant; respiratory irritant / asthma trigger in dust form or on acidification.
  • PPE: gloves, safety glasses, dust mask when weighing.
  • First aid: flush with water; escalate for respiratory symptoms.

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[1]

References

  1. WEB Sigma-Aldrich (Merck KGaA). Sigma-Aldrich Safety Data Sheets Sigma-Aldrich. https://www.sigmaaldrich.com/US/en/search/safety-data-sheets ↩

Reference databases

Last reviewed 2026-04-22