Sodium bisulfite
Sodium hydrogen sulfite or sodium bisulfite is a chemical compound with the chemical formula NaHSO3. Sodium bisulfite is a food additive with E number E222. Sodium bisulfite can be prepared by bubbling sulfur dioxide in a solution of sodium carbonate in water. Sodium bisulfite in contact with chlorine bleach (aqueous solution of sodium hypochlorite) will release harmful fumes.

# Uses in chemistry
In organic chemistry sodium bisulfite has several uses. It forms a bisulfite adduct with aldehyde groups and with certain cyclic ketones to a sulfonic acid.
This reaction has limited synthetic value but it is used in purification procedures. Contaminated aldehydes in a solution precipitate as the bisulfate adduct which can be isolated by filtration. The reverse reaction takes place in presence of a base such as sodium bicarbonate or sodium hydroxide and the bisulfite is liberated as sulfur dioxide.
Examples of such procedures are described for benzaldehyde, tetralone, citral, the ethyl ester of pyruvic acid and glyoxal. In the ring-expansion reaction of cyclohexanone with diazald, the bisulfite reaction is reported to be able to differentiate between the primary reaction product cycloheptanone and the main contaminant cyclooctanone.
The other main use of sodium bisulfite is as a mild reducing agent in organic synthesis in particular in purification procedures. It can efficiently remove traces or excess amounts of chlorine, bromine, iodine, hypochlorite salts, osmate esters, chromium trioxide and potassium permanganate.
A third use of sodium bisulfite is as a decoloration agent in purification procedures obviously because it can reduce strongly colorized oxidizing agents and conjugated alkenes and carbonyl compounds.
Sodium bisulfite is also the key ingredient in the Bucherer reaction. In this reaction an aromatic hydroxyl group is replaced by an aromatic amine group en vice versa because it is a reversible reaction. The first step in this reaction is an addition reaction of sodium bisulfite to an aromatic double bond. The Bucherer carbazole synthesis is a related organic reaction.

# Uses in food
Sodium bisulfite is used in almost all commercial wines, to prevent oxidation and preserve flavor. 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 oxidation (which makes vinegar), and to protect the color of the wine from oxidation, which causes browning. The sulfur dioxide displaces oxygen in the bottle and dissolved in the wine. Oxidized wine can turn orange or brown, and taste like raisins or cough syrup.

# Bisulfite DNA sequencing
Sodium bisulfite is used in the analysis of methylation status of cytosines in DNA.
In this technique, sodium bisulfite deaminates cytosine into uracil, but does not affect 5-methylcytosine, a methylated form of cytosine with a methyl group attached to carbon 5.
When the bisulfite-treated DNA is amplified via polymerase chain reaction, the uracil is amplified as thymine and the methylated cytosines are amplified as cytosine. DNA sequencing techniques are then used to read the sequence of the bisulfite-treated DNA. Those cytosines that are read as cytosines after sequencing represent methylated cytosines, while those that are read as thymines represent unmethylated cytosines in the genomic DNA.