The acid from the Glover tower is
purified by special treatment--for example, the arsenic may be removed,
after precipitation with sulphuretted hydrogen, in the form of insoluble
arsenic sulphide,--and the purified acid is concentrated by heating in
glass or platinum vessels.
A considerable amount of sulphuric acid is now made by the so-called
"contact process," in which sulphur dioxide and oxygen unite to form
sulphuric acid in presence of a heated "contact" substance, usually some
form of finely-divided platinum.
_Nitric Acid._--This acid is usually prepared by distilling a mixture of
sodium nitrate and vitriol in cast-iron retorts or pots, the nitric acid
being collected in stoneware vessels connected one with another, or, as
is more generally the case at the present time, in condensing apparatus
consisting of stoneware pipes or coils cooled by water. The effluent
gases are passed through a scrubber in order to free them from the last
traces of acid before discharging them into the atmosphere.
_Hydrochloric Acid.
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