It is soluble in water, this liquid
dissolving at the ordinary temperature about three times its volume of
the gas. The solution is only useful for testing. In separations, a
current of the gas must always be used. It is best prepared in an
apparatus like that shown in fig. 32, by acting on ferrous sulphide with
dilute hydrochloric acid. When iron has to be subsequently determined in
the assay solution, the gas should be washed by bubbling it through
water in the smaller bottle; but for most purposes washing can be
dispensed with. The gas is very objectionable, and operations with it
must be carried out in a cupboard with a good draught. When the
precipitation has been completed, the apparatus should always be washed
out. The effect of this acid on solutions of the metals is to form
sulphides. All the metallic sulphides are insoluble in water; but some
are soluble in alkaline, and some in acid, solutions. If sulphuretted
hydrogen is passed through an acid solution containing the metals till
no further precipitation takes place, a precipitate will be formed
containing sulphides insoluble in the acid. On filtering, adding ammonia
(to render the filtrate alkaline), and again passing the gas, a further
precipitate will be obtained, consisting of sulphides insoluble in an
alkaline solution, but not precipitable in an acid one; the filtrate may
also contain sulphides not precipitable in an acid solution, which are
soluble in an alkaline one; these will be thrown down on neutralising.
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