It must be dried and
ignited, the filter paper being burnt separately and its ash added. When
further ignition ceases to cause a loss of weight, the residue is ferric
oxide (Fe_{2}O_{3}), which contains 70 per cent. of iron. The weight of
iron therefore can be calculated by multiplying the weight of oxide
obtained by 0.7.
The presence of ammonic chloride causes loss of iron during the
ignition, and organic matter causes an apparent loss by reducing the
iron to a lower state of oxidation. When the iron in the solution much
exceeds 0.2 gram the volumetric determination is generally adopted, as
the bulkiness of the precipitate of ferric hydrate makes the gravimetric
method very inconvenient.
VOLUMETRIC METHODS.
As already explained these are based on the measurement of the volume of
a reagent required to bring the whole of the iron from the ferrous to
the ferric state (oxidation), or from the ferric to the ferrous
(reduction). Ferrous compounds are converted into ferric by the action
of an oxidising agent in the presence of an acid. Either permanganate or
bichromate of potash is generally used for this purpose.[65]
Ferric compounds are reduced to ferrous by the action of:--
(1) Stannous chloride;
(2) Sulphuretted hydrogen;
(3) Sodium sulphite; or
(4) Zinc.[66]
The processes, then, may be divided into two kinds, one based on
oxidation and the other on reduction.
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