The reaction between manganese dioxide and potassium permanganate

by Arthur John Hopkins

The Reaction between
Manganese Dioxide
and
Potassium Permanganate.

Dissertation

presented for the degree of
Doctor of Philosophy
to the Board of University Studies
of the Johns Hopkins University

by
Arthur John Hopkins.
1893.

Acknowledgment——

The work recorded in this paper is the result of a suggestion given by Professor H. N. Morse. It has received his careful attention throughout its course. For his instruction and exact criticism, I wish to offer my acknowledgment and thanks. I wish also to express to Professor Ira Remson my appreciation of his interest and instruction and to offer to Dr. J. S. Ames my thanks for his instruction in Physics——

Table of Contents.

		Page.
I	Introduction	[ 1]
II	Description of Apparatus	[ 3]
III	Action of Manganese Dioxide on
	Potassium Permanganate in acid solution	[ 9]
IV	Action of a Black Oxide of Manganese on
	Potassium Permanganate	[24]
V	The Amount of Nitric Acid Neutralized	[31]
VI	The Stability of Manganese Dioxide	[34]
VII	The Effect of Varying Quantities of
	Manganese Oxide and Potassium Permanganate	[39]
VIII	The Reduction of Neutral Potassium Permanganate
	by Manganese Oxide at Ordinary Temperature	[45]
IX	Action of Potassium Permanganate upon a
	Manganese Oxide Obtained from Manganese
	Dioxide by Spontaneous Decomposition	[50]
	[Conclusion]

Introduction.

The usual laboratory solution of potassium permanganate must be frequently restandardized. A slight loss in strength may be detected after standing even a few days and this change becomes more rapid as the decomposition proceeds. When one looks for the cause of this increase in the rate of the decomposition of the permanganate, the attention is naturally directed to the brown manganese oxide which separates from the solution. A desire to ascertain whether the presence of this oxide influences the rate of the decomposition suggested the experiments here described.

The fact that potassium permanganate may react with certain so-called peroxides with evolution of oxygen, has long been known and it appeared possible that a similar reaction may take place between manganese dioxide and potassium permanganate.

Morse and Allen investigated the reaction between lead dioxide and potassium permanganate and there is embodied in Allen’s dissertation[1] a statement of the earlier work in this line with references to the literature on the subject.

They have shown that in the presence of a quantity of dilute nitric acid which is equivalent to the potassium in the potassium permanganate used, lead dioxide reduces the permanganate to manganese dioxide without itself suffering reduction, while in the presence of an excess of stronger nitric acid, e.g. normal to eight normal, the lead dioxide is also reduced.

The following equations represent the reactions referred to:

I, when the nitric acid is equivalent to the potassium in the potassium permanganate,

2 KMnO₄ + 3 PbO₂ = K₂O + 3 PbO₂ + 2 MnO₂ + 1½ O₂.

II, when the nitric acid is in excess,

2 KMnO₄ + 3 PbO₂ = K₂O + 3 PbO + 2 MnO₂ + 3 O₂.

The reducing action of manganese dioxide upon potassium permanganate was suspected from the observed increase in the rapidity of the decomposition of potassium permanganate solutions. But only a suggestion of such a reaction could be found in the literature. Thénard,[2] in 1856 states that manganese dioxide may act upon potassium permanganate either as a reducing agent, in which act the manganese dioxide is changed to a manganate, or its influence may be catalytic, causing the evolution of oxygen. Again Mulder[3] in 1858, ascribed the decomposition of potassium permanganate solutions to the presence of some potassium manganate.

It could not be ascertained that any investigation of this matter had ever been undertaken. It was therefore decided to study the question and it appeared that the most satisfactory evidence could be obtained by measuring the quantity of oxygen which is evolved when potassium permanganate and manganese dioxide are brought together.