Acid neutralized = 38.10 percent.
XI. The flask contained black oxide 150 m.g. potassium permanganate 20
c.c. = 177.71 m.g., N/10 nitric acid 22.39 c.c. (= 2 molecules for each
molecule of KMnO₄) and was heated four and one half hours in the water
bath.
Acid neutralized = 100.00 percent.
No definite conclusions can be drawn from these neutralization
experiments since it is well known that manganese oxides prepared
as were those used in these experiments always contain some, though
probably not a constant quantity of alkali. It is stated by Bemmelen[9]
that manganese dioxide decomposed the salts of the alkalies with
liberation of acid.
[9] J. für Prak. Ch. (2) 23, 324-379.
The Stability of Manganese Dioxide.
Having found in the course of the work already described that
manganese dioxide prepared in the wet way is much less stable than was
supposed, it was decided to make some experiments upon the spontaneous
decomposition which it undergoes. To this end a fresh sample was
prepared in the following manner.
10 grams of potassium permanganate were dissolved in 500 c.c. of
distilled water and the solution allowed to settle for one day. The
liquid was then filtered through glass wool, heated to 65°C. and
treated with 320 c.c. N/10 nitric acid, having also a temperature
of 65°C. 20.5 grams of manganous sulphate dissolved in 2.5 litres of
water heated to 65°C, were now added, with stirring, to the acidified
solution of potassium permanganate. The precipitate was allowed to
settle and the supernatant liquid which still retained the permanganate
color, decanted. The residue was then treated with water, stirred and
allowed to settle. The water was decanted and the oxide filtered. The
filter used consisted of two platinum cones between which was placed a
small paper filter which did not quite reach to the edge of the outer
cone. This filter was placed in the bottom of a glass funnel and the
oxide poured upon it. The oxide was repeatedly washed with distilled
water and then transferred to a porous plate. It was afterwards
heated for several hours to a temperature of 65°C. At this temperature
it was found impracticable to bring the oxide to a constant weight,
but this fact was no serious obstacle in the way of the subsequent
work since it was only desired to ascertain what changes take place in
the ratio of the manganese to the available oxygen. The manganese was
determined by the method of Gibbs and the available oxygen by oxalic
acid and potassium permanganate. An analysis of the oxide made shortly
after its preparation in the manner described, gave
{51.98 percent}
Manganese { } = 51.86 pr. ct.
{51.74 ” }
{14.836 percent}
Oxygen { } = 14.837 ”
{14.839 ” }
showing a ratio of manganese to available oxygen of 1.018 : 1.000.
The above analysis was made May 1, 1892. The oxide was then placed in
a glass-stoppered weighing bottle and allowed to stand until November.
An analysis made on the tenth day of the latter month gave a ratio of
manganese to available oxygen of
1.198 : 1.000
corresponding very closely to a composition MnO·5MnO₂, in which the
calculated ratio is
1.200 : 1.000
Six month later (April 20, 1893) an analysis of the same material
showed a ratio of manganese to available oxygen of
1.300 : 1.000
This result was confirmed Mʳ Walker who made repeated analyses of the
substance at the same time.
Another sample of manganese oxide was prepared in the same manner
as that used in the previous experiments and placed under water. An
analysis made after
Eight days gave a ratio of Mn : available O = 1.013 : 1.000
Twenty ” ” ” ” ” ” ” ” = 1.037 : 1.000
Forty three ” ” ” ” ” ” ” ” = 1.040 : 1.000
The results here recorded clearly indicate that the brown oxide of
manganese which is prepared by the reduction of potassium permanganate
by manganese sulphate undergoes a spontaneous decomposition involving
a loss of oxygen. This loss of oxygen or the effect of varying
conditions upon this decomposition of manganese dioxide is now being
investigated by Mʳ Walker of this laboratory.
The Effect of Varying Quantities of Manganese Oxide on Potassium
Permanganate.
The following experiments were made for the purpose of determining the
effect of varying quantities of manganese oxide on the time required
for the reduction of potassium permanganate.
A. In the presence of nitric acid. The ratio of manganese to available
oxygen in the material used for these experiments was determined and
found to be
1.018 : 1.000
The experiments were conducted after the same manner as those
previously described.
Different portions of the manganese oxide were weighed out and treated
in the flasks used in the previous experiments with the quantities
of permanganate which were found by calculation to have the desired
molecular ratios to the oxide.
A quantity of N/10 nitric acid equivalent to the potassium of the
potassium permanganate was added. The flasks were then immersed in the
bath of boiling water and the time required for the disappearance of
the permanganate color noted.
The ratios of the oxide to the permanganate and the times required for
the reduction of the latter are embodied in the following table.
Molecular Ratio }
Oxide to KMnO₄ } 1:1 2:1 3:1 4:1 5:1 6:1 7:1
Time of decolorization
in minutes
1 ” 120 68
2 ” 110 65
3 ” 150 65
4 ” 150 80
5 ” 120 75
6 ” 120 75
7 ” 175(?) 78 53
8 ” 160 79 54
9 ” 145 41 30
10 ” 120 50 36
11 ” ? 47 32
12 ” 110 37 30
13 ” 64 [10]{20(?)
{20(?)
14 ” 72 5(?) 5(?)
15 ” 85 5(?) 5(?)
[10] Decolorized before time was taken.
The results appearing in the table on the same horizontal lines are
those of experiments made simultaneously, that is, the flasks in these
experiments were immersed at the same time and in the same water-bath.
These, that is the results appearing on the same horizontal line,
and these only are comparable, since it was found that very slight
differences in the temperature of the bath were of great influence on
the length of time required for the reduction of the permanganate.
It is clear that the rate of the reduction of the permanganate is
greatly increased by increasing the quantity of the manganese oxide.
After reduction of the permanganate, the contents of the flasks were
in every case filtered and the filtrate tested by Crum’s reaction
for the presence of manganese. No manganese was found. The absence of
manganese in the filtrate shows that the reduction of the permanganate
was not due to the formation of manganese nitrate, though it will be
clear from the results of subsequent experiments that the presence of
nitric acid exerts an influence on the rate of the reduction.
B. In neutral solution.
The condition were the same as under A, except that the nitric acid was
omitted. Those flasks containing potassium permanganate and one, two
and seven molecular equivalents respectively of the manganese oxide
were immersed in the water-bath as before.
No. I {containing one molecule of oxide and
{one molecule KMnO₄ was reduced in 57 hours
No. II {containing two molecules of oxide and
{one molecule KMnO₄ was reduced in
35 hours
No. III {containing seven molecules of oxide
{and one molecule KMnO₄ was reduced in
2¾ hours