Key Properties and Common Applications of Sulfurous Acid
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 - 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).
FAQs on Sulfurous Acid (H₂SO₃): Meaning, Structure & Uses
1. What is sulfurous acid (H₂SO₃)?
Sulfurous acid is a weak, unstable acid with the chemical formula H₂SO₃. It is not typically isolated in its pure form, as it exists in equilibrium in an aqueous solution of sulfur dioxide (SO₂). It is also known as sulfur dioxide solution or trioxosulfuric acid.
2. How is sulfurous acid formed?
Sulfurous acid is formed when sulfur dioxide (SO₂) gas dissolves in water (H₂O). This reaction is reversible, and the equilibrium largely favours the reactants, which is why H₂SO₃ is unstable. The chemical equation is: SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq).
3. What is the difference between sulfurous acid (H₂SO₃) and sulfuric acid (H₂SO₄)?
While both are oxoacids of sulfur, they differ in several key aspects:
- Formula and Oxidation State: Sulfurous acid is H₂SO₃, where sulfur has an oxidation state of +4. Sulfuric acid is H₂SO₄, with sulfur in its highest oxidation state of +6.
- Acid Strength: Sulfurous acid is a weak acid that does not fully ionise in water. Sulfuric acid is a very strong acid that dissociates completely.
- Stability: Sulfurous acid is unstable and only exists in solution. Sulfuric acid is highly stable and can be isolated as a dense, oily liquid.
4. What are the main uses of sulfurous acid?
Due to its properties as a reducing agent, sulfurous acid and its salts (sulfites and bisulfites) have several important applications, including:
- As a mild bleaching agent for materials sensitive to chlorine, like paper products and textiles.
- As a disinfectant to prevent bacterial growth, particularly in winemaking.
- As a reducing agent in various chemical processes.
- As an intermediate in the manufacturing of other chemicals.
5. Is sulfurous acid a strong or weak acid, and why?
Sulfurous acid (H₂SO₃) is considered a weak acid. This is because when dissolved in water, it does not completely dissociate into its ions. The equilibrium SO₂ + H₂O ⇌ HSO₃⁻ + H⁺ shows that only a fraction of the molecules release a proton (H⁺), unlike strong acids like sulfuric acid which ionise fully.
6. Why can't sulfurous acid be isolated in its pure form?
Sulfurous acid cannot be isolated as a pure compound because it is highly unstable. The molecule H₂SO₃ only exists transiently in an aqueous solution of sulfur dioxide. The equilibrium strongly favours the reverse reaction, breaking it down into sulfur dioxide and water. Therefore, any attempt to remove the water to isolate H₂SO₃ simply results in the evolution of SO₂ gas.
7. What is the structure of the sulfurous acid molecule?
The sulfurous acid (H₂SO₃) molecule has a trigonal pyramidal geometry. The central sulfur atom is bonded to one oxygen atom via a double bond and to two hydroxyl (-OH) groups via single bonds. The presence of a lone pair of electrons on the sulfur atom causes the molecule to adopt this shape.
8. Is sulfurous acid (H₂SO₃) a monoprotic or diprotic acid?
Sulfurous acid is a diprotic acid. The term 'diprotic' means it has two acidic protons (hydrogen ions) that it can donate. It releases these protons in two successive steps: first to form the hydrogen sulfite ion (HSO₃⁻), and second to form the sulfite ion (SO₃²⁻).
9. How does sulfurous acid act as a reducing agent?
Sulfurous acid acts as a good reducing agent because the sulfur atom is in an intermediate oxidation state of +4. It can easily be oxidised to the more stable +6 oxidation state (found in sulfate, SO₄²⁻). In a chemical reaction, by donating its electrons and becoming oxidised itself, it causes the other substance to be reduced.
10. What are the safety risks associated with sulfurous acid?
Sulfurous acid is a corrosive and toxic substance. Direct contact can cause severe irritation and burns to the skin and eyes. Inhaling its vapours (which is primarily sulfur dioxide gas) can irritate the nose, throat, and lungs, potentially leading to respiratory issues like bronchitis with repeated exposure.
