Bisulfite ion is the ion HSO₃⁻. Salts containing the HSO₃⁻ ion are termed bisulfites also known as sulfite lyes. For example, sodium bisulfite

Sodium bisulfite

Sodium bisulfite 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...

 is NaHSO₃.

Reactions

Bisulfite salts are typically prepared by treatment of alkaline solutions with excess sulfur dioxide

Sulfur dioxide

Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

:

SO₂ + NaOH → NaHSO₃

HSO₃⁻ is the conjugate base of sulfurous acid

Sulfurous acid

Sulfurous acid is the chemical compound with the formula H2SO3. There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase...

, H₂SO₃:

H₂SO₃ HSO₃⁻ + H⁺

Sulfurous acid is not an isolable compound and does not appear to exist in solution either. An equilibrium that is much more consistent with spectroscopic

Spectroscopy

Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 evidence is given :

SO₂ + H₂O HSO₃⁻ + H⁺

HSO₃⁻ is a weakly acidic species with a pK_a

Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...

 of 6.97. Its conjugate base is the sulfite

Sulfite

Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...

 ion, SO₃²⁻:

HSO₃⁻ SO₃²⁻ + H⁺

Bisulfites are reducing agents, as are all sulfites and sulfur dioxide, which contains sulfur in the same oxidation state (+4).

Structure

Most evidence suggests that the proton in bisulfite ion is located on sulfur, giving rise to C_3v symmetry

Molecular symmetry

Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...

. There is, however, some evidence from ¹⁷O

Oxygen-17

Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

 NMR spectroscopy to suggest that two tautomeric forms of HSO₃⁻ exist in dynamic equilibrium, one having C_3v symmetry (protonated at sulfur) and other C_s symmetry (protonated at oxygen). The C_3v structure is supported by X-ray crystallography

X-ray crystallography

X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

 and, in aqueous solution, by Raman spectroscopy

Raman spectroscopy

Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...

 (ν(S–H) = 2500 cm⁻¹).

Medicine

Bisulfite salts are common additives to the drug epinephrine

Epinephrine

Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...

 in order to prevent its oxidation to adrenochrome

Adrenochrome

Adrenochrome, chemical formula C9H9NO3, is a pigment obtained by the oxidation of adrenaline . The derivative carbazochrome is a hemostatic medication.-Chemistry:...

 and resulting inactivation. Bisulfites can sometimes cause an allergic reaction. This is different from the common sulfa drugs allergy.