[Footnote: Read before the American Chemical Society, Dec. 16, 1881]
By NELSON H. DARTON.
The element manganese having many peculiarities in its reactions
with the other elements, is now extensively used in the arts, its
combinations entering into and are used in many of the important
processes; it is consequently often brought before the chemist in his
analysis, and has to be determined in most cases with considerable
accuracy. Many methods have been proposed for this, all of them of more
or less value; those yielding the best results, however, requiring a
considerable length of time for their execution, and involving so large
an amount of manipulatory skill as to render them fairly impracticable
to a chemist at all pressed for time, and receiving but a mere trifle
for the results.
As I have had to make numerous estimations of manganese in various
compounds, as a public analyst, I have been induced to investigate the
volumetric methods at present in use to find their comparative values,
and if possible to work out a new one, setting aside one or more of the
difficulties met with in the use of the older ones. This paper is a part
summary of the results. First, I will detail my process of estimation,
then on the separation.
From all compounds of manganese, excepting those containing cobalt and
nickel, the manganese is precipitated as binoxide; those containing
these two elements are treated with phosphoric acid, or as noted under
Separation.
A.--The Estimation. The binoxide of commerce, as taken from the mine, is
well sampled, powdered, and dried at 100°C. 0.5 gramme of this is taken
and placed in a 250 c.c. flask; in analysis the binoxide on the filter,
from the treatments noted under separation is thoroughly washed with
warm water; it is then washed down in a flask, as above, after breaking
the filter paper; sufficient water is added to one-third fill the flask,
and about twice the approximate weight of the binoxide in the flask of
oxalate of potassa; these are agitated together. A twice perforated
stopper is fitted to this flask, carrying through one opening a 25 c c.
pipette nearly filled with sulphuric acid, sp. gr. 1.4, the lower point
of which just dips below the mixture in the flask, and the upper end,
carrying a rubber tube and pinch cock to control the flow of acid.
Through the other opening passes a glass tube bent at an acute angle
and connected by a short rubber tube to an adjoining flask, two-thirds
filled with decinormal baryta solutions. These connections are all made
air tight. Sulphuric acid is allowed in small portions at a time to
flow into the mixture. Carbonic acid is evolved, and, passing into
the adjoining flask, is absorbed by the baryta, precipitating it as
carbonate. To prevent the precipitate forming around or choking up the
entrance tube, the flask must be agitated at short intervals to break it
off. The reaction so familiar to us in other determinations is expressed
thus:
MnO_{2}+KO,C_{2}O_{3}+2SO_{3} = MnO,SO_{3}+KO.SO_{3}+2CO_{2},
When no more carbonic acid is evolved, another tube from this last flask
is connected with the aspirator, the pinch-cock of the pipette open, and
air drawn through the apparatus for about half a minute, and thus all
the carbonic acid evolved absorbed, or the flasks may be slightly
heated. If danger of more carbonic acid being absorbed from the air is
feared, and always in very accurate analysis, a potassa tube may be
connected to the pipette before drawing the air through. The precipitate
formed is allowed to settle, 50 c.c. of the supernatant solution
is removed with a pipette and transferred to a beaker; 50 c.c. of
decinormal nitric acid and some water is added with sufficient cochineal
tincture. It is then titrated back with decinormal soda; from this is
now readily deducted the amount of carbonic acid, and from that the
MnO_{2}, holding in view that 44 parts of carbonic acid is equivalent to
43.5 of MnO_{2} or 98.87 per cent, and that 1 c.c. of the N/10 baryta
solution is equivalent to 0.0022 grm. of CO_{2}.
If a carbonate, chloride, or nitrate, be present in the native binoxide,
it must be removed with some sulphuric acid. This is afterward
neutralized with a little caustic soda. This method yields the
following results for its value in amount of manganese to 100:
99.91-99.902-99.895, and can be executed in about twenty minutes.
Fifteen determinations can be carried on at once without loss of time,
this, however, depending on the operator's skill. I have made many
assays, and assays by this method with similarly excellent results.
Of the other methods, Bunsen's is acknowledged to be the most accurate,
but is, of course, too troublesome to be used in technical work,
although it is used in scientific analysis. Ordinary samples are not
sufficiently accurate to allow the use of this method.
The methods of reducing with iron and titrating this with chromate of
potassa, etc., have given a constant average of from 98.60-99.01. These
results are fair, but hard to obtain expeditiously.
Of the methods of precipitating the compounds of the protoxide and
estimating the acid, that of the phosphate is by far the most accurate,
titrating with uranium solution; 99.82 is a nearly constant average
with me, much depending on the operator's familiarity with the uranium
process.
The methods of Lenssen, or ferricyanide of potassium method, yields very
widely differing results. I have found the figures of Fresenius about
the same as my own in this case; that is from 98.00-100.10.
B.--On the Separation. First, from its soluble simple combinations with
the acids or bases containing no iron or cobalt; if they are present, it
is treated as is noted later. If sulphuric acid is present it must be
separated by treating the solution of the compound with barium chloride
and filtering. A nearly neutral solution is prepared in water or
hydrochloric acid and placed in a flask. Here it is treated with
chlorine by passing a current of that gas through it as long as
it causes a precipitate and for some time afterward. It is then
discontinued, the mixture allowed to deposit for a few moments, and
about two-thirds of the supernatant solution decanted; it is mixed with
some more water, and these decantations repeated until they pass away
without reaction, or by filtering it and washing on the filter; it is
then dissolved in hot hydrochloric acid, this nearly neutralized, a
solution of sesquichloride of iron is added, and again treated with an
excess of chlorine. After washing it is transferred to the flasks of
the apparatus mentioned in the first part of this paper, and estimated.
Myself and several others have found this always to be a true MnO_{2},
and not a varying mixture of protosesquioxide and binoxide, and will
thus yield accurate results. This reprecipitation may sometimes be
dispensed with by adding the iron salt before the first precipitation,
but it of course depends upon the other elements present.
From Compounds containing Cobalt, Cobalt and Nickel, Iron and group
III., together or with other elements.--Group III. and sesqui. iron are
separated by agitation with baryta carbonate, some chloride of ammonia
being added to prevent nickel and cobalt precipitation traces, and
filtering. If cobalt is present we treat this filtrate with nitrite
of potassa, etc., to separate it (that is, if it and nickel are to
be separated and estimated in the same sample; but if they are to be
estimated as one, or not separated, the treatment with nitrite, etc.,
is not used). The filtrate from this last is directly treated with
chlorine. If nickel and cobalt are not to be estimated in this sample,
the solution, as chlorides, is mixed with some chloride of ammonium
and ammonia, then with a fair excess of phosphoric acid, a sufficient
quantity more of ammonia to render the mixture alkaline. The precipitate
formed is transferred to the filter and well washed with water
containing NH_{3}Cl and NH_{4}O, then dissolved in hydrochloric acid
and reprecipitated with ammonia, filtering and washing as before. It is
again dissolved in HCl and titrated with uranium solution, or decomposed
by tin, as noted below, and the manganese precipitated as binoxide with
chlorine, and determined. The latter method is hardly practicable, and
I never have time to use it, as the titration and all together yields a
value of 99.80 in most cases, if accurately executed.
From the bases of groups V. and VI. these are separated by hydrogen
sulphide, from iron in alloys, ores, etc., and in general the iron is
separated as basic acetate, and the manganese afterward precipitated
with chlorine. Bromine is generally used in place of chlorine, the use
of which chemists claim as troublesome; but in a number of examinations
I have found it to yield more satisfactory results than bromine, which
is much more expensive.
From the acids in insoluble and a few other compounds, chromic, arsenic,
and arsenious acids, by fusion with carbonate of soda in presence of
carbonic acid gas; borate of manganese is readily decomposed when the
boracic acid is to be determined by boiling with solution of potassa,
dissolving the residue in hydrochloric acid and precipitating the
manganese as binoxide. This boiling, however, is seldom needed, as the
borate is soluble in HCl.
From phosphoric acid I always use Girard's method of treatment with
tin, using it rasped, and it yields much more accurate results with but
little manipulation. When the other acids mentioned above are present in
the compound, we treat it as directed there.
From silicic acid, by evaporation with hydrochloric acid.
From sulphur or iodine, by decomposing with sulphuric acid and
separating this with baryta chloride.
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