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

Manganese 51.98 percent = 51.86 pr. ct.
51.74 ”
Oxygen 14.836 percent = 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

EightdaysgavearatioofMn :availableO= 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(?)

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  57 hours
one molecule KMnO₄ was reduced in
No. II containing two molecules of oxide and  35 hours
one molecule KMnO₄ was reduced in
No. III containing seven molecules of oxide  2¾ hours
and one molecule KMnO₄ was reduced in

C. In alkaline solution.

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  91¾ hours.
each molecule KMnO₄ was reduced in
No. II containing two molecules of oxide  55(?) hours.
for each molecule KMnO₄ was reduced in
No. III containing seven molecules of oxide for  10 hours.
each molecule KMnO₄ was reduced in

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

Experiment II

A repetition of experiment I. An analysis of the oxide after decolorization of the permanganate gave a ratio of manganese to available oxygen of

1.022 : 1.000

Experiment III

The conditions are the same as in the preceding experiments except that twice the amount of permanganate calculated for the conversion of the oxide into manganese dioxide was added.

An analysis of the residue after decolorization gave a ratio of manganese to available oxygen of

0.994 : 1.000

Experiment IV

A repetition of experiment III. An analysis of the residue gave a ratio of manganese to available oxygen of

0.995 : 1.000

Experiment V

A repetition of experiments III and IV. The ratio of manganese to available oxygen in the residue was

0.995 : 1.000.

The foregoing results indicate (experiments III, IV, and V) that the oxygen lost spontaneously by manganese dioxide prepared in the wet way, is fully recovered when the reduced oxide is treated with excess of potassium permanganate.

The ratio of the manganese to the available oxygen was determined in the following manner. The contents of the flask were filtered through asbestos and the flask and the precipitate washed with cold water. The contents of the filter were then returned to the flask and treated with an excess of a standard solution of oxalic acid and dilute sulphuric acid. After reduction the excess of the oxalic acid was determined by a standard solution of potassium permanganate.

The solution was then filtered, well washed and the manganese in the filtrate precipitated by the method of Gibbs. From the total amount of manganese found, there was deducted the quantity which was introduced in determining the excess of oxalic acid by potassium permanganate.