41 will give
that of metallic manganese.
When the quantity of manganese or of the dioxide to be determined is
small, it is not necessary to use 2 grams of iron; 1 gram, or even less,
may be taken. The iron may be used in the form of a standard solution of
ferrous sulphate and portions measured off, thus saving the labour of
weighing.
~Determination of Dioxide in a Manganese Ore.~--Weigh up 1 or 2 grams of
the finely-powdered ore[84] and an equal weight of pure iron wire,
dissolve the wire in 50 or 100 c.c. of dilute sulphuric acid, and, when
solution is complete, add the ore and warm till it too is dissolved.
Cool and titrate the remaining ferrous iron with the permanganate or
bichromate of potassium solution.
For example, 0.7560 gram of pyrolusite and 1.000 gram of iron were taken
and treated as above; 13.9 c.c. of "permanganate" (standard 100 c.c. =
0.4920 gram iron) were required; this indicates that 0.0684 gram of iron
was left unoxidised by the ore. The iron oxidised, then, was 0.9316 gram
(1.000 - 0.0684); multiplying this by 0.7768, we find that 0.7237 gram
is the quantity of manganese dioxide which was present. This is
equivalent to 95.77 per cent.;
0.7560 : 0.7237 :: 100 : 95.77.
IODINE METHOD.
It has been already stated that when dioxide of manganese is boiled with
strong hydrochloric acid chlorine is given off, and that the amount of
chlorine so liberated is a measure of the dioxide present.
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