The process
is continued in this way until two tests have given no reaction of
protoxide of iron, when the solution is diluted with water; but no
dilution should take place until the oxidation is complete, because
in the course of it the solution ought to be kept as concentrated as
possible. Silica, and graphite when it is present, need not be removed
by filtration, if it is not intended to estimate them, or there be no
fear that the graphite is accompanied by any humous substance, or that
any oily, viscous compound has been deposited on the sides of the
beaker. In the last mentioned case the solution should be transferred
into another beaker, and filtered, if graphite be present. When the
solution is evaporated to dryness, the remainder has five cubic
centimeters hydrochloric acid added to it, and the liquid is then
brought to boiling in order that the perchloride of manganese
possibly formed during the evaporation to dryness may be reduced to
protochloride, after which the solution is diluted with water till it
measures about 100 cubic centimeters. To this is now added in small
portions and with constant stirring as much of a saturated solution of
bicarbonate of soda (thirteen parts water dissolve one part salt), that
all the iron is precipitated, after which, when the escape of carbonic
acid has ceased, the solution is diluted with water till it measures 200
cubic centimeters and is then ready for titration.
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