The
nitrates of the metals, on heating, leave the oxide of the metal. It is
as yielders of oxygen that nitrates are so largely used in the
manufacture of explosives. Gunpowder contains from 65 to 75 per cent. of
potassium nitrate (nitre).
Nitrates are best detected and determined by their yielding nitric oxide
when treated with sulphuric acid and a suitable reducing agent, such as
ferrous sulphate, mercury, or copper. Nitric oxide is a colourless gas
very slightly soluble in water. It combines at once with oxygen, on
mixing with the air, to form brown "nitrous fumes," and dissolves in a
solution of ferrous sulphate, producing a characteristic blackish-brown
colour. It is this colour which affords the best and most easily-applied
test for nitrates. The substance suspected to contain nitrates is
dissolved in about 1 c.c. of water, and treated with an equal volume of
strong sulphuric acid. After cooling, a solution of ferrous sulphate is
poured on its surface, so as to form a layer resting on it. On standing,
a brown or black ring is developed where the liquids join, if any
nitrate or nitrite is present. Nitrites are distinguished from nitrates
by effervescing and yielding brown fumes when treated with a little
dilute sulphuric acid.
The separation of nitrates is in many cases difficult. Generally, on
treating the substance with water, the nitrate will be in the solution,
and is filtered off from any insoluble matter.
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