In
such a case the rule for dissolving is, one gramme iron (more if the
content of silica is small) is dissolved in a mixture of two cubic
centimeters sulphuric acid of 1.83 specific gravity and twelve cubic
centimeters of water in the way described above, and boiled until salt
of iron begins to be deposited on the bottom of the beaker. Five cubic
centimeters hydrochloric acid are now added, and the solution tested
with red prussiate of potash for protoxide of iron, and the boiling
continued till near dryness, when all the nitric acid is commonly driven
off. Should nitrous acid still show itself, some more hydrochloric acid
is added and the boiling continued.
As in dissolving in hydrochloric acid and oxidizing with nitric acid the
solution ought to be twice tested for protoxide of iron, even although
at the first test none can be discovered. The silica is taken upon
a filter, dried, ignited, and weighed. The filtrate is treated with
bicarbonate of soda, and titrated with chameleon solution in the way
described above. If the content of manganese is small (under 0.5 per
cent.) it is not necessary to warm the liquid before titration; but
in proportion as the content of manganese is larger there is so much
greater reason to hasten the removal of color by warming and constant
stirring toward the close of the titration.
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