«Sodium sulfite (sodium sulphite) is a soluble sodium salt of sulfurous acid with the chemical formula Na2SO3. It is a product of sulfur dioxide scrubbing, a part of the flue-gas desulfurization process. It is also used as a preservative to prevent dried fruit from discoloring, and for preserving meats, and is used in the same way as sodium thiosulfate to convert elemental halogens to their respective hydrohalic acids, in photography and for reducing chlorine levels in pools.»I looked for this substance without success. So I decided to prepare it. At an agriculture medicine shop I found sulfurous acid at 6%.
Using the chemical equation:
Na2CO3 (105g) + H2SO3 (82g)→ Na2SO3 (126g)+ CO2 (44g)+ H2O (18g)
One kilogram of the sulfurous acid solution has 60g of H2SO3. Making the proportion I putted 77 g of Na2CO3 .Weighting the Sodium carbonate to a large bottle, I poured the acid slowly, it makes a lot of bubbles of CO2 and at the end we have about 100g sodium sulfite pro liter, about 10% solution of sodium sulfite.
Sodium sulfite is very useful to prepare photographic baths, from developers to fixers and clearing baths.