The conditions were the same as under B, except that five molecules of
potassium hydroxide were added for each molecule of the permanganate.
No. I {containing one molecules of oxide for
{each molecule KMnO₄ was reduced in
91¾ hours.
No. II {containing two molecules of oxide
{for each molecule KMnO₄ was reduced in
55(?) hours.
No. III {containing seven molecules of oxide for
{each molecule KMnO₄ was reduced in
10 hours.
It appears that potassium permanganate is less easily reduced by
manganese oxide in neutral than in acid solutions and in alkaline that
in neutral solutions.
The Reduction of Neutral Potassium Permanganate by Manganese Oxide at
Ordinary Temperature.
In these experiments, the manganese oxide used was prepared in the same
manner as that in which the proportion of manganese to available oxygen
was found immediately after its preparation to be
1.018 : 1.000
It was dried for several days at 65°C., but for almost a month after
its preparation was not used. Hence, what the ratio of the manganese
to the oxygen at the time when the permanganate was treated with it,
is quite uncertain. It however appears probable from the subsequent
experiments on the instability of manganese dioxide at 65°C. that the
ratio of manganese to available oxygen must have been at about 1.05:
1.00 corresponding to a formula MnO·20MnO₂.
Two solutions of permanganate were made, filtered through glass wool
and standardized.
Solution A.
A cubic centimetre of this was found to be equivalent to 5.278 m.g.
iron in the ferrous condition.
Solution B.
A cubic centimetre of this was found to be equivalent to 14.379 m.g.
iron in the ferrous condition.
Three 250 c.c. portions of each of the solutions A and B were placed in
clean glass-stoppered bottles. These bottles were labelled “a”, “a′”
and “a″” “b”, “b′” and “b″”.
“a” contained 250 c.c. potassium permanganate each cubic centimetre of
which was equivalent to 5.278 m.g. Fe″. It was closed and put away in
the dark and allowed to stand from June fourteenth until the fourteenth
of the following October. Its strength was then determined and found to
have declined 0.72 percent.
“a′” contained the same quantity of the same permanganate solution as
“a” and it was closed on the same date and placed in mildly diffused
light until October fourteenth. An analysis on the latter date showed
that its strength had also declined 0.72 percent.
“a″” contained the same quantity of the same permanganate solution as
“a” and “a′” and to this was added 0.5 gram of the manganese oxide
previously referred to. The bottle was then closed and placed in
diffused light beside “a′”. An analysis, October fourteenth showed that
its strength had declined 34.62 percent.
“b” contained potassium permanganate each cubic centimetre of which was
equivalent to 14.379 m.g. Fe″. It was closed and put away in the dark
beside “a”. An analysis October fourteenth showed that its strength had
declined 1.71 percent.
“b′” contained the same quantity of the same permanganate as “b”. It
was closed and placed in mildly diffused light beside “a′”. An analysis
October fourteenth showed a decline in strength of 2.41 percent.
“b″” contained the same quantity of the same permanganate as “b” and
“b′”. To it was added 0.5 grams of the manganese oxide. The bottle
was closed and placed in diffused light beside “a′”, “a″”, and “b′”.
An analysis, October fourteenth showed a decline in strength of 78.86
percent.
The conclusions to be drawn from the foregoing results are (1) that
dilute solutions of potassium permanganate are more stable both in
diffused light and in darkness than the more concentrated ones, and (2)
that the presence of manganese oxides hasten to an enormous degree the
reduction of potassium permanganate even at summer temperatures. It
will also be observed that this reducing action of the manganese oxide
is greater in the stronger than in the more dilute solutions. It should
be remarked that the summer of 1892 in which these experiments were
made, was one of unusual heat.
Action of Potassium Permanganate upon a Manganese Oxide Derived from
Manganese Dioxide by Spontaneous Decomposition.
The oxide used in these experiments was one which had been prepared
in the manner heretofore described by adding manganese sulphate to an
excess of potassium permanganate. Soon after its preparation the ratio
of manganese to available oxygen in it was found to be
1.018 : 1.000
At the time when it was used in these experiments, the ratio of
manganese to available oxygen was found to be
1.198 : 1.000
Corresponding very nearly to the ratio of manganese to available oxygen
in MnO·5MnO₂.
Experiment I.
230 m.g. of the oxide was treated with the quantity of permanganate
calculated to be necessary for its oxidation to manganese dioxide.
The mixture was kept at a temperature of 22°C. until the color of the
permanganate disappeared. This required about ten hours.
An analysis of the oxide then gave a ratio of manganese to available
oxygen
1.027 : 1.000