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Concept
Bisulfite
Summary
The bisulfite ion (IUPAC-recommended nomenclature: hydrogensulfite) is the ion HSO3−. Salts containing the HSO3− ion are also known as "sulfite lyes". Sodium bisulfite is used interchangeably with sodium metabisulfite (Na2S2O5). Sodium metabisulfite dissolves in water to give a solution of Na+HSO3−.
Na2S2O5 + H2O → 2 Na[HSO3]
The bisulfite anion exists in solution as a mixture of two tautomers. One tautomer has the proton attached to one of the three oxygen centers. In the second tautomer the proton resides on sulfur. The S-protonated tautomer has C3v symmetry. The O-protonated tautomer has only Cs symmetry.
There exist two tautomers of bisulfite. They interconvert readily but can be characterized individually by various spectroscopic methods. They have been observed by 17O NMR spectroscopy:
HSO3− SO2(OH)− K = 4.2
Solutions of bisulfite are typically prepared by treatment of sulfur dioxide with aqueous base:
SO2 + OH− → HSO3−
HSO3− is the conjugate base of sulfurous acid, (H2SO3).
HSO3− is a weak acidic species with a pKa of 6.97. Its conjugate base is sulfite, SO32−:
HSO3− ⇌ SO32− + H+
Attempted isolation of the common salts of bisulfite results in dehydration of the anion with formation of metabisulfite (S2O52−), also known as disulfite:
2 HSO3− ⇌ S2O52− + H2O
Because of this equilibrium, anhydrous sodium and potassium salts of bisulfite cannot be obtained. However, there are some reports of anhydrous bisulfites with large counter ions.
Bisulfite is a good reducing agent, especially for oxygen scrubbing:
2 HSO3− + O2 → 2 SO42− + 2 H+
Its reducing properties are exploited to precipitate gold from auric acid (gold dissolved in aqua regia) and reduce chromium(VI) to chromium(III). In water chlorination, sodium bisulfite is used to reduce the residual 'chlorine' which can have a negative impact on aquatic life.
In organic chemistry, "sodium bisulfite" is used to form adducts with aldehyde and with certain cyclic ketones. These adducts are α-hydroxysulfonic acids. This reaction is useful for the separation and purification of aldehydes.
Official source
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