This unit highlights the
preparation of sulphur dioxide in the laboratory and on a large industrial scale.
Discusses the properties of sulphur dioxide and its uses such as bleaching of
straws and sponges, fumigation of our houses to kill insects (i.e. black ants),
preservation of some liquids (i.e. orange juice). Sulphur dioxide is also used
in the manufacture of sulphuric acid. Sulphuric acid is widely utilized to make
paints, plastics, detergents (i.e. Omo, Nomi, Aerial, etc.) and fertilizers.
Sulphur dioxide could be obtained on a large scale from burning sulphur that is
itself obtained from natural gas, a component of crude petroleum. In Uganda, the
discovery of oil in the western part of the country will in the near future
become a source of sulphur.
Sulphur dioxide is a major pollutant of atmospheric air. It is toxic, causes respiratory
ailments, damages plant life and corrodes metals. The gas is soluble in water
forming what is known as ‘acid rain’. Acid rain erodes marble and
limestone. In industries where this gas is evolved it is recommended to bubble
the gas through an alkali to form sulphites and thus control its polluting effect
on the environment.
Sulphites are useful as mild oxidising agents but more often they act as reducing
agents. They are oxidised to sulphates by substances such as potassium manganate
(VII), potassium dichromate (VI), chlorine, bromine, hydrogen peroxide, oxygen
and iron (III) chloride.
A mixture of sodium sulphite and sulphur when heated react to form sodium thiosulphate.
A solution of a ‘fixer’ containing sodium thiosulphate is added to
developed films to remove unreduced silver ions to form a soluble complex, which
is washed away. This prevents unreduced silver ions from becoming reduced and
darkening slowly over time. This is useful in photography, which is a very vital
AND PROPERTIES OF SULPHUR DIOXIDE
This sub-topic deals with
Methods of preparing sulphur dioxide in the laboratory and on a large scale.
- The physical and chemical
properties /reactions of sulphur dioxide gas.
Uses of sulphur dioxide
Preparation and properties of sulphite salts.
(a) Laboratory methods of preparing sulphur dioxide
Sulphur dioxide in the laboratory is prepared by:
(i) Reacting dilute hydrochloric
acid or dilute sulphuric acid with sodium sulphite crystals
Diagram of the
Equation of reaction
Heating a mixture of copper turnings and concentrated sulphuric acid
of Apparatus used is the same as the one above except a source of heat must be
Equation of reaction taking place
of sulphur dioxide:
Sulphuric acid used to make soap, paints and pigments
(i) Soap and Detergents
(iii) Bleaching agent for wool, straws and sponges.
(iv) Preservative for some liquids e.g. orange juice.
(v) Preparation of calcium hydrogen sulphite used for bleaching wood-pulp in the manufacture
(g) Preparation of sulphite salts for example sodium sulphite and zinc sulphite.
the end of this sub-section a student (you) should be able to:
(a) Describe the two methods by which sulphur dioxide is prepared in the laboratory.
(b) State possible methods of preparation of sulphur dioxide gas on a large scale.
(c) State the method of collection of sulphur dioxide gas in the laboratory.
(d) State the physical and chemical properties of sulphur dioxide.
(e) Describe a confirmatory test for sulphur dioxide.
(f) State the observations made in the oxidising and reducing properties of sulphur
Write equation(s) for the reaction(s) of sulphur dioxide with various substances.
(h) State the uses of sulphur dioxide.
(i) Describe the preparation of sodium sulphite and zinc sulphite in the laboratory
(j) State chemical properties of sulphite salt.