Sodium Sulfite Anhydrous — Frequently Asked Questions

Because of pH. Boiler water is deliberately held alkaline to protect steel from acid corrosion, and sodium bisulfite and metabisulfite both drive pH downward as they dissolve and react. Sodium sulfite scavenges oxygen just as effectively while giving a mildly alkaline solution, so the carefully maintained boiler pH is not disturbed.

Sodium sulfite containing a cobalt accelerator, which greatly increases the rate of the reaction with dissolved oxygen. Uncatalysed sulfite reacts too slowly at low temperature to be effective in a cold or ambient deaerator, whereas the catalysed grade scavenges oxygen fast enough for those conditions. The catalyst is present at very low concentration.

Because sulfite destroys thiamine, vitamin B1. Its use is therefore not permitted on foods regarded as a significant dietary source of that vitamin, since the additive would negate the nutritional contribution. This is separate from the allergen declaration requirement, which applies to all sulphites above 10 mg/kg as SO2.

Dechlorination of water, reduction of hexavalent chromium in effluent treatment, as a reducing agent in dye chemistry, as a component of photographic developer where it prevents oxidation of the developing agent, and in the sulfite pulping process. All of these rest on the same reducing chemistry.