Sodium bisulfite (sodium hydrogen sulfite) is a compound of formula NaHSO3. Sodium bisulfite is a food additive for E number E222. It is a white crystalline solid that turns yellow in solution. It is acidic and therefore corrosive. Sodium bisulfite in contact with chlorine bleach (aqueous solution of sodium hypochlorite) generates heat and forms sodium bisulfate and sodium chloride.
This salt of bisulfite can be prepared by bubbling sulfur dioxide in a solution of sodium carbonate in water. Aqueous sodium hydroxide, sodium bicarbonate, sodium carbonate, or sodium sulfite solution are treated with sulfur dioxide to produce sodium metabisulfite solution. In one operation, the mother liquor from the previous batch is reinforced with additional sodium carbonate, which need not be totally in solution, and then is treated with sulfur dioxide. In some plants, the reaction is conducted in a series of two or more stainless steel vessels or columns in which the sulfur dioxide is passed countercurrent to the alkali. The solution is cooled and the sodium metabisulfite is removed by centrifuging or filtration. Rapid drying, eg, in a stream-heated shelf dryer or a flash dryer, avoids excessive decomposition or oxidation to which moist sodium metabisulfite is susceptible.
In fruit canning, sodium bisulfite is used to prevent browning (caused by oxidation) and to kill microbes. In the case of wine making, sodium bisulfite releases sulfur dioxide gas when added to water or products containing water. The sulfur dioxide kills yeasts, fungi, and bacteria in the grape juice before fermentation. When the sulfur dioxide levels have subsided (about 24 hours), fresh yeast is added for fermentation. It is later added to bottled wine to prevent the formation of vinegar if bacteria are present, and to protect the color, aroma and flavor of the wine from oxidation, which causes browning and other chemical changes. The sulfur dioxide quickly reacts with oxidation by-products and prevents them from causing further deterioration. Sodium bisulfite is also added to leafy green vegetables in salad bars and elsewhere, to preserve apparent freshness, under names like LeafGreen. The concentration is sometimes high enough to cause severe allergic reactions.
It is used as a reducing agent in the manufacture of many organic compounds. It is used for decolorization agent in many purification processes. It can efficiently remove traces amounts of chlorine, bromine, iodine, hypochlorite salts, chromium trioxide and potassium permanganate.
It is used in large piping systems to prevent oxidative corrosion. It helps to maintain anaerobic conditions inside biochemical reactors. It is used on water treatment to remove residual chlorine after super chlorination. It is a strong reducing agent in wastewater treatment, textile dye preparation and film development. It is also used to bleach pulp in the paper industry.