Sulfurous Acid - H₂SO₃

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What is H₂SO₃ ?

Sulfurous Acid is defined as a chemical compound having the chemical formula as H2SO3. H2SO3 is chemically known as Sulfurous Acid.

Sulfurous acid is also referred to as Sulfur dioxide solution or trioxosulfuric acid or di-hydrogen trioxosulfate. It is an intermediate species that forms acid rain from sulfur dioxide (which is SO2).

Trioxosulfuric acid is described as a liquid without having colour, and it has a pungent burning sulfur smell and is corrosive to metals and tissue. It is a sulfur oxoacid, the conjugate acid of a hydrogen sulfite, and tautomer of sulfonic acid.

H2SO3 Structure

Let us look at the structure of sulphurous acid represented as follows.

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Properties of Sulfurous Acid – H2SO3

Let us look at the important properties of sulfurous acid as given below:


Sulfurous acid

Molecular weight of H2SO3

82.07 g/mol

Monoisotopic mass of Sulfurous Acid

81.97 g/mol

Number of hydrogen bond acceptor


Number of hydrogen bond donor


Sulfurous acid - A Reducing Agent or Oxidizing Agent

Sulfurous acid (which is also called sulphurous acid) is the chemical compound having the formula H2SO3. There is no evidence that sulfurous acid exists in the solution, but this molecule has been detected in the gaseous phase.

However, the conjugate bases of this elusive acid are common bisulfite (or hydrogen sulfite), anions, and sulfite. Whereas, weak and unstable acid, is formed when sulfur dioxide dissolves in water. The fact that sulfur dioxide compound actually exists in the solution cannot be explained exactly, but the molecules of this substance have been detected in the gaseous phase.

It is a reducing, and a bleaching agent too.

The sulfurous acid compound only forms in the aqueous solution, and it is thus not isolated in its pure state.

Reasons Behind Some of the Acids being Oxidizing and Some Non-oxidizing

All the Bronsted Lowry acids (which are proton donors) are said to be oxidizing agents because H+ is reduced to hydrogen gas H2.

However, acids that form an anionic form of oxygen, such as sulphuric H2SO4, nitric HNO3, chromic H2CrO4 (or H2Cr2O7), chloric HClO3, and perchloric HClO4 tend to be strong oxidizing acids.

Moreover, phosphoric acid due to its high affinity for the oxygen compound is a poor oxidizing acid.

The oxidizing acids which are mentioned above can react with the less reactive metals like copper, where the H+ cation is not reduced to hydrogen gas.

For example, the reaction of dilute nitric acid with copper metal and its reaction is given below:

3 Cu + 8 HNO3+ → 3 Cu2+ + 2 NO + 4H2O + 6 NO3−

Reactions of Sulfurous Acid

According to SO2 Raman spectra solution, it represents that the signal intensities are consistent with the equilibrium given as below:

SO2 + H2O ⇌ HSO−3 + H+

where , pKa = 1.81 and Ka = 1.54 × 10−2.

H2SO3 Uses (Sulfurous Acid)

  • This compound is used as a reducing agent.

  • Sulfurous acid can be used as an intermediate in industries.

  • It is used in paper products manufacturing.

  • It is also used as a disinfectant.

Health Hazards

Sulfurous acid is a corrosive, non-combustible, and toxic compound. Ingesting or inhaling or skin contact with the compound solution causes a severe injury leading to death. Sulfurous acid, in its molten form, can cause severe burns to skin and eyes. Thus, it is advised to avoid skin contact with this compound. It also liberates toxic, irritating, and corrosive gases.

Let us look at more information about various types of health effects when exposed to sulfurous acid.

Chronic Health Effects

The following chronic (which are long-term) health effects can take place at some time after the exposure to Sulfurous Acid and may last for months or even years: 

  • Cancer Hazard - As per the presently available information to the New Jersey Department of Health & Senior Services, sulfurous acid has not been tested on its ability to cause cancer in animals. 

  • Reproductive Hazard - Sulfurous acid compound has not been tested on its ability to affect reproduction. 

  • Other Long-Term Effects - Sulfurous acid can also irritate the lungs. Repeated exposure can cause bronchitis to develop with phlegm, cough, and/or shortness of breath.

Medical Testing

If there is suspicion of overexposure symptoms, it is recommended to consider a chest x-ray after acute overexposure. 

Any evaluation should include a careful history of previous and current symptoms with the examination. Medical tests which look for any damage already done are not a substitute to control the exposure.

Solubility of Sulphurous Acid

Sulfurous acid is a colourless gas having a high solubility in water. It hydrates to sulphuric acid (H2SO3) in its solution, which in turn dissociates to produce form ions of bisulfite (and sulfite (SO2–3).

FAQ (Frequently Asked Questions)

1. Is Sulfurous Acid a Strong or Weak Acid?

Answer: Unlike sulfuric acid (H2SO4), the sulfurous acid (H2SO3) is defined as a weak acid; it means, aqueous sulfurous acid does not dissociate completely into bisulfite and H+ (H3O+) ions, which means that the bisulfite ion is comparatively stronger in proton maintenance when there is a base, such as water.

2. What is Sulfurous Acid Used For?

Answer: The sulfuric acid is a dibasic, and solid acid. It reacts with bases to produce salts of sulfite and bisulfite.

Its uses are given as: sulphuric acid and its salts can be used as disinfectants and effective reducers. This is used as a mild bleaching agent in applications, where chlorine sensitive materials are included.

3. Explain if Sulfur is a Monoprotic Acid?

Answer: None of the monoprotic acids exists. In the starting state, there exist two hydrogen protons, sulphuric acid (H2SO4). In other terms, the sulfate molecule is bound to particular acidic protons. It is also noted that monoprotic acids will contribute simply one acidic proton, not two as in this acid.

4. Is sulfurous Acid Dangerous?

Answer: Sulfurous acid may affect us by getting inhaled and moving through the skin. Sulphurous acid is also a corrosive substance; hence, its contact with eyes can seriously damage and irritate and cause a burning sensation. Sulfuric Acid breath can also irritate the throat and neck.