Sulfurous Acid

Introduction

Sulfurous acid is highly “toxic, corrosive, and non-combustible” in nature that affects human health if inhaled in an excess amount. The individual can lead to death if the acid encounters their skin, or if it is “inhaled or ingested” in a higher amount. It can cause severe skin burns and affect vision if gets into the eyes. Sulfurous acid also liberates “irritating, corrosive, and toxic gases”.

What is Sulfurous Acid?

Sulfurous acid is determined as an “unstable and weak acid” that is derived when “sulphur dioxide” are dissolved in water. The formula of Sulfurous acid can be written as “$\mathrm{H_{2}SO_{3}}$” and has a molecular weight of nearly “82.07 g/mol”. The acid has a neutral acidity rate as its acidic nature is not very strong or weak. “$\mathrm{H_{2}SO_{3}}$” is a clear liquid that has a strong pungent smell.

Figure 1 − Structure of Sulfurous Acid

The acid is generally used as a disinfectant, as a “reducing agent”, and in the production of paper products. It can be observed from the “Raman spectra of $\mathrm{SO_{2}}$ solutions” that signal intensities of Sulfurous acid are “consistent with equilibrium”. This phenomenon can be denoted by the reaction mentioned below.

$$\mathrm{“SO_{2}\:+\:H_{2}O\:\leftrightarrows\:H_{2}SO-_{3}\:+\:H^{+}"\:,\:where\:"K_{a}\:=\:1.54\times\:10^{-2}"\:and\:"pK_{a}\:=\:1.81"}$$.

Sulphurous acid is a “diprotic acid” as it tends to yield “2 protons (H+)/molecule”. The acid is dissolved in water and release a certain amount of heat energy, as it is “corrosive to metals and tissue”.

Properties of Sulfurous Acid

The “chemical and physical properties” of Sulfurous acid are listed below.

Properties of Sulfurous Acid
“Chemical formula”	$\mathrm{“H_{2}SO_{3}”}$
“Number of hydrogen bond donor”	“2”
“Molecular weight”	“82.07 g/mol”
“Boiling point”	“-60°C”
“Density”	“1.03 g/cm3 (20°C)”
“Melting point”	“10°C”
“Number of hydrogen bond acceptor”	“4”
“Monoisotopic mass”	“81.97 g/mol”
“Vapour Pressure”	“1740 kPa at about 21°C”
“Solubility”	“Highly soluble in water”
Uses of “Sulfurous acid”	The acid is widely used as a “disinfectant, reducing agents, and moderate bleaching agent”.

Table 1: Properties of Sulfurous acid

Some common chemical properties of $\mathrm{“H_{2}SO_{3}"}$ are stated below.

• The conjugated bases of Sulfurous acid include “bisulfite (or hydrogen sulfite) and sulfite ions”. The major examples of frequent anions of Sulfurous acid are “$\mathrm{HSO_{3}}$ and $\mathrm{SO_{2}^{-3}}$”.

• According to the experiment of “17O NMR (Nuclear Magnetic Resonance) spectroscopy”, it has been understood that the solution of “sulphurous acid and protonated sulfites” consists of a “combination of isomers”. The isomers usually remain in a “dynamic equilibrium” with each other.

• The acid is highly unstable in nature when it is formed and is decomposed into parts. This phenomenon can be stated in reaction −

$$\mathrm{“H_{2}SO_{3}(aq)\:\rightarrow\:H_{2}O(I)\:+\:SO_{2}(g)"}$$

• The acid can be converted to “sulfuric acid or sulphate” when it absorbs the atom of oxygen at a higher rate.

Preparation of Sulfurous Acid

Various processes highlight the formation of Sulfurous acid as discussed below.

• The “aqueous solution of sulphur dioxide” can be determined as Sulfurous acid that is prepared as the sulphur gets dissolved in water. This can be denoted by a reaction −

$$\mathrm{“SO_{2}\:+\:H_{2}O\:\longleftrightarrow\:H_{2}SO_{3}"}$$

• Another process of preparation of “H2SO3” is the derivation of “sulphur dioxide and sodium bisulfite salt”. The compounds are then introduced to the “aqueous solution of sulphur dioxide” to form Sulfurous acid that can be denoted by the reaction −

$$\mathrm{“Na_{2}SO_{3}\:+\:H_{2}O\:+\:SO_{2}\:\rightarrow\:2NaHSO_{3}"}$$

Uses of Sulphurous Acid

The major uses or application of “$\mathrm{H_{2}SO_{3}}$” is stated below.

• Sulfurous acid generally works as a “bleaching and reducing agent”.

• The acid is highly utilised as an intermediate in several chemical industries across the world.

• “$\mathrm{H_{2}SO_{3}}$” is used as an insecticide and disinfectant in houses and medical sectors.

• Sulfurous acid also helps in the production of paper products.

Effects of Sulfurous Acid

“$\mathrm{H_{2}SO_{3}}$” is highly toxic and corrosive in nature that can affect the health of humans in various forms. The acid can cause severe burns on the skin and eye damage that can lead to “loss of vision”. An excessive rate of “inhaling and ingesting” this acid can lead to irritation and disease like “Bronchitis with phlegm, cough, shortness of breath, respiratory tract, heart and lung”. In order to prevent hazards “$\mathrm{H_{2}SO_{3}}$” can be stored at “+15°C to +25°C” or it will affect human health by liberating “toxic, irritating, and corrosive gases”.

Conclusion

Sulfurous acid can also be identified by other names “Sulphur dioxide solution, dihydrogen trioxosulphate or trioxosulphuric acid”. The acids generally act as an intermediate during the production of “acid rain from sulphur dioxide $\mathrm{(SO_{2})}$”. Trioxosulphuric acid is determined as an oily liquid with a clear appearance and a “pungent burning sulphur smell”. The acid is highly corrosive in nature to “tissue and metals” and has a density of “1.03 g/cm3 (20°C)”. It can also be referred to as “sulphur oxoacid, sulfonic acid tautomer, and conjugate acid of a hydrogensulfite”.

FAQs

1. What is the preparation process of Sulfurous acid?

Sulfurous acid forms as an intermediary of acid rain and can be created both naturally and artificially in labs. It forms when “sulphur dioxide combines with moisture” present in the atmosphere.

2. What are the major health hazards caused by Sulfurous acid?

Sulfurous acid highly affects the reproduction process and is “inhaled or ingested” at a higher rate. The acid can also cause irritation to the lungs, and respiratory tract by developing diseases such as “Bronchitis with phlegm, cough, and shortness of breath”.

3. How can you understand if sulphurous acid is a “strong or weak acid”?

Sulfurous acid $\mathrm{(H_{2}SO_{3})}$” is denoted as a weak acid unlike “sulphuric acid $\mathrm{H_{2}SO_{4}}$” as in an aqueous state it never dissociates into “$\mathrm{H+\:(H_{2}O+)}$ and bisulphite ions”. This phenomenon means that the ion of bisulphite is quite strong in proton maintenance when there is a water base.