Tin and antimony are converted into insoluble oxides, while
the other metals (with the exception of those already mentioned)
dissolve as nitrates. During the solution of the metal red fumes are
given off, which mainly consist of nitrogen peroxide. The solution is
often coloured brown or green because of dissolved oxides of nitrogen,
which must be got rid of by boiling. Generally some ammonium nitrate is
formed, especially in the cases of zinc, iron, and tin, when these are
acted on by cold dilute acid. Sulphur, phosphorus, and arsenic are
converted into sulphuric, phosphoric, and arsenic acids respectively,
when boiled with the strong acid.
~Dilute Nitric Acid.~--Dilute 1 volume of the strong acid with 2 of
water.
~Oxalic Acid~, H_{2}[=O] or (H_{2}C_{2}O_{4}.2H_{2}O.)--This is an organic
acid in colourless crystals. It forms a family of salts--the oxalates.
It is used in standardising; being a crystallised and permanent acid, it
can be readily weighed. It is also used in separations, many of the
oxalates being insoluble. For general use make a 10 per cent. solution.
Use the commercially pure acid. On ignition the acid should leave no
residue.
[Illustration: FIG. 32.]
~Sulphuretted Hydrogen.~ Hydrosulphuric acid, SH_{2}.--A gas largely
used in assaying, since by its action it allows of the metals being
conveniently classed into groups.
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