~Sulphurous Acid~, H_{2}SO_{3}.--The reagent used may be regarded as a
saturated solution of sulphur dioxide in water. It may be purchased, and
keeps for a long time. It may be made by heating copper with sulphuric
acid and passing the gas formed into water. The heat should be withdrawn
when the gas is coming off freely. It is used as a reducing agent, and
should not be diluted.
~Sulphuric Acid~, H_{2}SO_{4}. (Sp. gr. 1.84, containing 96 per cent. of
real acid, H_{2}SO_{4}.)--This acid forms insoluble sulphates with salts
of lead, strontium, and barium. It has a high boiling point, 290° C.,
and, when evaporated with salts of the more volatile acids, converts
them into sulphates. When nitrates or chlorides are objectionable in a
solution, evaporation with sulphuric acid removes them. In working with
this acid caution is necessary, since, on mixing with water, great heat
is evolved; and, if either the acid or water has been previously heated,
a serious accident may result. In diluting the acid it should be poured
into cold water. Glass vessels containing boiling sulphuric acid should
be handled as little as possible, and should not be cooled under the
tap. The action of diluted sulphuric acid on metals closely resembles
that of dilute hydrochloric acid. Magnesium, aluminium, iron, zinc,
nickel, cobalt, manganese, and cadmium dissolve, with evolution of
hydrogen, in the cold acid, or when warmed.
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