All the metallic oxides,
except a few of the native and rarer oxides, are dissolved by it with
the formation of chlorides of the metal and water.
~Dilute Hydrochloric Acid~ is made by diluting the strong acid with an
equal volume of water. This is used for dissolving precipitates obtained
in the general course of analysis and the more easily soluble metals.
~Hydrofluoric Acid, HF.~--A solution in water may be purchased in
gutta-percha or lead bottles. It is of variable strength and doubtful
purity. It must always be examined quantitatively for the residue left
on evaporation. It is used occasionally for the examination of
silicates. It attacks silica, forming fluoride of silicon, which is a
gas. When the introduction of another base will not interfere with the
assay, the substance may be mixed in the platinum dish with fluoride of
ammonium, or of potassium, or of calcium, and hydrochloric acid,
instead of treating it with the commercial acid. It is only required in
special work. The fumes and acid are dangerous, and, of course, glass or
porcelain vessels cannot be used with it.
~Iodine, I.~--This can be obtained in commerce quite pure, and is often
used for standardising. It is very slightly soluble in water, but
readily dissolves in potassium iodide solution. It closely resembles
chlorine and bromine in its properties, and can be used for dissolving
metals without, at the same time, attacking any oxide which may be
present.
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