Sodium Bisulfite

Sodium bisulfite (NaHSO₃; CAS 7631-90-5) is the acidic sulfite — a preservative that acts simultaneously as an antioxidant and a mild acid buffer, lowering solution pH to roughly 4 while scavenging dissolved oxygen. This combination makes it the sulfite of choice in acid fixing baths and specific fixer-preservative formulations where both functions are wanted in a single reagent.^[1] The chemistry is closely related to sodium metabisulfite: sodium bisulfite partially dehydrates to metabisulfite on standing as a dry salt, which is why most suppliers stock the more stable metabisulfite instead.

Photographic uses

• Chrome-alum hardening fixer: 75–100 g/L NaHSO₃ plus sodium thiosulfate plus chrome-alum hardener, historically used for tropical processing where gelatin emulsions otherwise melted at high development temperatures.
• Acid fixer preservative / buffer: Small amounts (~10 g/L) in an acid fixer maintain working pH around 4.5, the range where thiosulfate remains maximally active.^[2]
• Reducing agent: Low concentrations function as mild silver reducers in some redevelopment toner formulas.
• Cr(VI) reducer (alternative to metabisulfite): Same chemistry as the metabisulfite dichromate-disposal workflow; less stable stock but interchangeable for reduction purposes.

Practical notes

Supplied as a white crystalline powder. Stability issues: sodium bisulfite crystals slowly lose water on storage and convert to sodium metabisulfite — most commercial "sodium bisulfite" is actually the metabisulfite labelled for the older name. In solution the two behave identically; for dry formulas, many contemporary sources specify metabisulfite explicitly to avoid this ambiguity.

Related compounds

Sodium sulfite is the neutral-pH preservative; sodium metabisulfite is the shelf-stable form essentially interchangeable with bisulfite in solution; potassium metabisulfite is the potassium analog used in some wine-chemistry crossover formulas.