and it may be employed for estimating the content of manganese by
titration. As follows from the formula two equivalents of permanganate
of potash are required for the titration of three equivalents of
protoxide of manganese, which has also been established by direct
experiments, as well as that the escape of carbonic acid indicated
by the formula actually takes place. The precipitate of manganese is
dissolved either in water to which 0.5 per cent. of hydrochloric acid
has been added, or in boiling nitric acid. When manganese occurs along
with iron, which in general is the case, we must take care that the iron
in the solution is in the state of peroxide, which is precipitated on
the addition of the bicarbonate of soda, and is allowed to remain as a
precipitate, because it does not affect the titration injuriously. The
removal of this precipitate by filtering would be more loss than gain,
partly because there would be a risk of losing manganese in this way,
partly because the precipitate of manganese, which occurs immediately on
the addition of the chameleon solution, proceeds both more rapidly and
with greater completeness in the presence of the iron precipitate than
otherwise. This appears to be caused by the iron precipitate as it
were inclosing, and mechanically drawing down the light manganese
precipitate, provided a weak chemical union between the two precipitates
does not even take place, depending on the tendency of peroxide of
manganese to behave toward bases, as, for instance, hydrate of lime
as an acid. Hence it thus follows that it ought to be arranged that
a sufficient quantity of iron[1] (at least the same quantity as of
manganese) be present in the liquid at titration, also that time be
given for the precipitate to fall, so that the color of the solution may
be observed between every addition of chameleon solution.
[Footnote 1: For this in case of need a solution of perchloride of iron
free of manganese may be employed.]
When the content of manganese is large, it is sometimes rather long
before the solution is ready for titration. The reason of this appears
to be that a part of the manganese is first precipitated as hydrated
sesquioxide, which is afterward oxidized to hydrated peroxide, for the
upper portion of the liquid may sometimes be colored by chameleon, while
the lower portion, which is in closer contact with the precipitate,
is less colored or absolutely colorless. From this we also see how
advisable it is to stir the liquid frequently during titration. Toward
the close of it, it is also advantageous, when the contents of manganese
are large, to warm the solution to about 50° C., because the removal of
color is thereby hastened. When the fluid, which is well stirred after
each addition of chameleon, has obtained from it a perceptible color,
which does not disappear after several stirrings, the whole of the
manganese is precipitated and the color of the solution remains almost
unchanged after the lapse of at least twelve hours.
When the content of manganese is large the solution may be divided into
two equal portions, one of which is first to be roughly titrated to
ascertain its content approximately, after which the whole is to be
mixed together and the titration completed, which can thus be performed
with greater speed and certainty. If too much chameleon has been added,
one may titrate back with an accurately estimated solution of manganese,
which is prepared most easily by evaporating fifteen cubic centimeters
chameleon solution down to two or three cubic centimeters, boiling with
two to three cubic centimeters hydrochloric acid so long as the smell
of chlorine is observed, and then diluting the solution to ten cubic
centimeters, when one cubic centimeter of it corresponds to the same
measure of chameleon.
With respect to the delay which must take place during the titration in
order to give the precipitate time to fall, it is advantageous, in order
to save time, to work with several samples; but it is, in such a case,
desirable to have a separate burette for each sample, in order to avoid
noting every addition of the chameleon solution and afterward adding
them up. If burettes are wanting, and one must be used for several
samples, a Mohr's burette with glass cock is the most convenient to
use. For the titration of iron with chameleon solution, the latter is
commonly used of such a strength that 0.01 gramme of iron corresponds to
about one cubic centimeter of chameleon solution, which is obtained
by dissolving 5.75 grammes permanganate of potash in 1,000 cubic
centimeters water. The titration is determined by means of iron, a salt
of iron or oxalic acid. A drop of such a solution, corresponding to
about one-twentieth cubic centimeter, or 0.0001 gramme Mn, is sufficient
to give a perceptible reddish color to 200 cubic centimeters of water.
As what takes place in the titration of iron with chameleon is indicated
by the following formula,
10FeO + 2KMnO_{4} = 5Fe_{2}O_{3} + K_{2}O + 2MnO_{2},
it appears, on making a comparison with the formula given above, that
ten equivalents of iron correspond to three equivalents of manganese,
and that there is thus required for three equivalents manganese as
much chameleon solution as for ten equivalents iron. When we know the
titration of the chameleon solution for iron, that for manganese is
obtained by multiplying the former by (3 x 55)/(10 x 56) =0.295. If, for
instance, one cubic centimeter chameleon solution corresponds to 0.01
gramme iron, the figure for manganese is 0.01 x 0.295 = 0.00295 gramme
per cubic centimeter.
We can of course also determine the titration for manganese in a
chameleon solution with the greatest certainty by titrating a compound
of manganese with an accurately estimated content of it, for instance, a
spiegeleisen or ferromanganese; the test is carried out in the following
way: The substance, which is to be examined for manganese, is dissolved
by means of hydrochloric acid. If the manganese, as in slags, be
combined with silica, it is frequently necessary first to fuse the
specimen with soda. Iron ores and refinery cinders may indeed, if they
are reduced to a very fine state of division, be commonly decomposed by
boiling with hydrochloric acid with or without the addition of sulphuric
acid, but the undissolved silica is generally rendered impure by
manganese, which can only be removed by fusion with soda.
The dissolving of the fused mass in hydrochloric acid does not need to
be carried to dryness for the separation of the soluble silica, but the
boiling, after the addition of a little nitric acid, is only kept
up until the iron passes into perchloride and the manganese into
protochloride. The quantity, which ought to be taken for the test,
depends on the accuracy with which it is desired to have the manganese
estimated.
Of ferromanganese and other very manganiferous substances, in which the
manganese need not be determined with greater exactness than to 0.1 per
cent., only 0.01 gram. is taken for a test; but of common pig, wrought
iron, steel, iron ore, slags, etc., there is taken 0.5 to 1 gramme
according to the supposed content of manganese and the desired exactness
of the estimation. For instance one gramme iron, which has passed
through a metal sieve with holes half a millimeter in diameter, is
placed in a beaker 125 mm. in height and 60 mm. in diameter, and has
added to it twenty cubic centimeters of hydrochloric acid of 1.12
specific gravity, which, with a well-fitting glass cover, is boiled for
half an hour, in order that the combined carbon may be driven off in the
shape of gas. After at least the half of the hydrochloric acid has been
boiled away, there are added at least five cubic centimeters nitric acid
of 1.2 specific gravity, partly to bring the iron to peroxide, partly to
destroy the organic matters formed from the carbon, which might possibly
be remaining and might tend to remove the color of the chameleon
solution. The boiling is now continued till near dryness, when five
cubic centimeters hydrochloric acid are added, after which the solution
is boiled as long as any reddish-yellow vapors of nitrous acid are
observed. When these have disappeared a drop of the liquid taken up on
a small glass rod is tested with an newly prepared solution of red
prussiate of potash (2 grammes in 100 cubic centimeters water), to
ascertain whether there is any protoxide of iron remaining. First, when
no indication of blue or green is visible, the test shows a pure yellow,
it is certain that there are no reducing substances in the solution.
If a trace of protoxide of iron remains in the solution another cubic
centimeter of nitric acid ought to be added and the boiling continued so
long as any reddish-yellow vapors are visible, more hydrochloric acid
also being added to keep the solution from being dried up. 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.
A large excess of bicarbonate ought to be avoided, because in a solution
of pure protochloride of manganese it renders the liquid milky and
turbid; the addition of more water, however, makes it clear. The
solution of bicarbonate must be free from organic substances which may
tend to remove the color of the chameleon solution. To ascertain this,
the latter is added to the former drop by drop so long as the color is
removed.
If it be desired to estimate the silica in the same test, the iron, as
when it is analyzed for silica, may be also dissolved in sulphuric acid,
and afterward oxidized with nitric acid, after which the solution is
boiled to near dryness, so that the organic substances are completely
destroyed. In order afterward, to drive off the nitric acid and get the
manganese with certainty reduced to protoxide, the solution is boiled
with a little hydrochloric acid. In this way the solution goes on
rapidly and conveniently, but the titration takes longer time than when
the iron is dissolved in hydrochloric acid, because the iron precipitate
is more voluminous, and, in consequence, longer in being deposited. To
diminish this inconvenience the solution ought to be made larger. 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.
* * * * *