Laboratory preparation, properties and uses of Sulphurdioxide

Sulphur dioxide

• Sulphur dioxide was known from ancient times and Romans used it as a general cleansing agent.

• In 1770 , J.Priestly prepared the gas by the action of hot conc. Sulphuric acid on mercury and called it sulphurous acid.

• Lavosier studied the structure and proved that it was an oxide of sulphur.

Properties of sulphurdioxide

• Sulphur dioxide 'SO₂' a colorless gas with a pungent and suffocating odor. It is readily soluble in cold water, sparingly soluble in hot water, and soluble in alcohol, acetic acid, and sulfuric acid.

• It can be produced by the reaction of sulphur with oxygen, by burning sulphur in air, and it is often produced during the roasting of sulfide ores, e.g, zinc smelting.

• It is a non-combustible and non-supporter of combustion.

Magnesium burns in sulphur di oxide giving magnesium oxide and sulphur.

Potassium reacts with sulphur dioxide forming potassium sulphite and potassium thiosulphate.

• An aqueous solution of sulphur dioxide is acidic in nature

• Sulphur dioxide is called an “ acid anhydride of sulphurous acid”

• Sulphur dioxide dissolves in water to give unstable sulphurous acid

Sulphur dioxide reacts with alkali solution to give salt and water. [sulphites and bisulphites] When sulphur di oxide is

bubbled through sodium hydroxide solution no precipitate is formed since sodium sulphite is a soluble salt.

When sulphur dioxide is passed through Ca (OH)₂ solution, a white precipitate of insoluble calcium sulphite is

formed. If excess sulphur dioxide is passed, the precipitate disappears forming soluble calcium bi sulphite.

• Reacts with carbonates and liberates carbon dioxide.

• Reacts with basic oxide to give sulphites.

SO₂ Gas forms additional compounds with oxygen, chlorine and lead dioxide. Sulphur is hexavalent but in SO₂ only

four valencies of sulphur is satisfied by the two atoms of oxygen. The remaining two are unsatisfied and makes SO₂

an unsaturated compound.

• Sulphur dioxide is a strong oxidizing agent.

Oxidises magnisium to magnisium ion(Mg⁺²)

Oxidises hydrogen silphide to sulphur

• Sulphur dioxide is a strong reducing agent.

In the presence of moisture SO₂ liberated nascent hydrogen and reduction takes place by addition of hydrogen.

Example:

Potassium dichromate, potassium permanganate and nitric acid are reduced by the action of SO₂ by removal of

oxygen.

Example:

Sulphur dioxide gas exhibits bleaching properties in presence of moisture. It dissolves in water liberating nascent

hydrogen. Coloring matter is bleached by reaction with nascent hydrogen. Nascent hydrogen removes oxygen

atoms from the coloring matter (reduces coloring matter) and it loses its color. This bleaching is temporary

because the bleached product on exposure to atmospheric oxygen adds on oxygen atoms from air and regains its

original color.

Uses of sulphur dioxide

The various uses of sulphur dioxide are:

1) In the manufacturing of sulphuric acid, sulphites, and hydrogen sulphite.

2) In the sugar industry for refining and decolorizing sugar.

3) For refining kerosene, and other petroleum products.

4) As a disinfectant.

5) As a fumigant.

6) For bleaching delicate articles.

7) As antichlor, to remove the excess chlorine from substances that have been bleached by chlorine.

8) As a solvent for glue.

9) As a refrigerant in household refrigerators.

10) As a preservative for wines, meat, dry fruits etc.

The uses of sulphur dioxide are summarized in the figure 7.8.