~Citric Acid~ (H_{3}[=C=i] or C_{6}H_{8}O_{7}.H_{2}O) is an organic acid
which occurs in colourless crystals, soluble in less than their weight
of water. The solution must be freshly prepared, as it gets mouldy when
kept. It forms a comparatively unimportant class of salts (citrates). It
is used in the determination of phosphoric acid, chiefly for the purpose
of preventing the precipitation of phosphates of iron and alumina by
ammonia, and in a few similar cases. The commercial crystals are used;
they should be free from sulphuric acid and leave no ash on ignition.
~Hydrochloric Acid~, HCl in water, (sp. gr. 1.16. It contains 32 per
cent. of hydrogen chloride).--It is sometimes called "muriatic acid,"
and when impure, "spirit of salt." The acid solution should be
colourless and free from arsenic, iron, and sulphuric acid. It forms an
important family of salts, the chlorides. It is the best acid for
dissolving metallic oxides and carbonates, and is always used by the
assayer when oxidising agents are to be avoided. The acid is used
without dilution when no directions are expressly given to dilute it. It
has no action on the following metals: gold, platinum, arsenic, and
mercury; it very slightly attacks antimony, bismuth, lead, silver, and
copper. Tin is more soluble in it, but with difficulty; whilst iron,
zinc, nickel, cobalt, cadmium, and aluminium easily dissolve with
evolution of hydrogen and the formation of the lower chloride if the
metal forms more than one class of salts.
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