Part 7

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.