Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical compound with the chemical formula NaHSO3. Sodium bisulfite is a food additive with E number E222. This salt of 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 generate heat and form sodium bisulfate and sodium chloride.
Sodium hydrogen sulfite
E222, sodium bisulphite
3D model (JSmol)
|E number||E222 (preservatives)|
CompTox Dashboard (EPA)
|Molar mass||104.061 g/mol|
|Odor||Slight sulfurous odor|
|Melting point||150 °C (302 °F; 423 K)|
|Boiling point||315 °C (599 °F; 588 K)|
|42 g/100 mL|
Refractive index (nD)
|R-phrases (outdated)||R22 R31|
|S-phrases (outdated)||(S2), S25, S46|
|NFPA 704 (fire diamond)|
|NIOSH (US health exposure limits):|
|TWA 5 mg/m3|
IDLH (Immediate danger)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
- SO2 + NaOH → NaHSO3
- SO2 + NaHCO3 → NaHSO3 + CO2
- HSO3− ↔ SO32− + H+
The theoretical fully protonated species is sulfurous acid (H2SO3); however this does not exist in solution and treatment of sodium bisulfite with acids will result in decomposition with telltale effervescence of sulfur dioxide, e.g.,
- NaHSO3 + HCl → NaCl + H2O + SO2
This reaction is useful for purification procedures. Contaminated aldehydes in a solution precipitate as the bisulfite 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, 2-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 because it can reduce strongly coloured oxidizing agents, 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 and 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 that uses sodium bisulfite as a reagent.
Uses in food and FDA ban
While the related compound, sodium metabisulfite, is used in almost all commercial wines to prevent oxidation and preserve flavor, sodium bisulfite is sold by some home winemaking suppliers for the same purpose. 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 allergic reactions.
On July 8, 1986, sodium bisulfite was banned from use by the FDA on fresh fruits and vegetables in the United States following the deaths of 13 people and many illnesses, mainly among asthmatics.
Bisulfite DNA sequencing
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.
Sodium bisulfite is a common reducing agent in the chemical industry, as it readily reacts with dissolved oxygen:
- 2 NaHSO3 + O2 → 2 NaHSO4
It is usually added to large piping systems to prevent oxidative corrosion. In biochemical engineering applications, it is helpful to maintain anaerobic conditions within a reactor.
In drinking water treatment, sodium bisulfite is often added after super chlorination, to reduce the residual chlorine before discharging to the service reservoir.
In wastewater treatment, sodium bisulfite is often added following disinfection with chlorine prior to discharging the effluent to the receiving water. Residual chlorine can have a negative impact on aquatic life.
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- The Many Uses Of Sodium Bisulfite
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