The Reaction Between Manganese Dioxide and Potassium Permanganate

Part 1

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 a statement of the earlier work in this line with references to the literature on the subject.

Johns Hopkins University 1892

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, 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 in 1858, ascribed the decomposition of potassium permanganate solutions to the presence of some potassium manganate.

Part 2

Comptes Rendues 42, 382

Jahresbericht 1858 p. 581

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.

Description of Apparatus.

The apparatus employed in this work consists of four parts:

“1) a flask, A, in which the reactions were conducted. A melting point bulb of about 40 cc. capacity, the diameter of its neck being 20 mm., was used for this purpose.

This apparatus was used by Allen and described by him. I wish to acknowledge my indebtedness to him for the illustration and description given here which is taken by permission word for word from his dissertation.

This was closed with a two-hole rubber stopper, through one hole of which passed:

2) A small glass tube, BB, running up from the flask about 40 cm., and then bent twice at right angles. One limb of this tube was surrounded by a small Liebig’s condenser, while the shorter turned down to meet--

3) a Schiff’s azotometer filled with mercury and connected at its top with the tube above mentioned. In practice it was found desirable to have a little water on the top of the mercury column.

4) Through the second hole of the rubber stopper closing the flask was passed a short piece of glass tubing bent obliquely just above the stopper and tightly connected with a rubber tube about 50 cm. long, DD. This tube was clamped near its lower end with a Mohr’s pinch-cock, while the upper was connected with the stem of a small funnel, F.

When an experiment was to be conducted, the azotometer C, was connected to the tube, B, and tied tightly, a piece of good rubber tubing being used for the connection. The reservoir of the azotometer, M, was then raised until the mercury had driven out all the air in C, and the stop-cock closed. Next the funnel F was filled with water, the pinch-cock closing D opened and the water allowed to flow down and drive out all the air in the tube. The latter was then clamped and thus kept full of water. The apparatus being now ready, the flask A, containing the substances in the desired quantity, was made fast to B and D by means of its stopper. The stop-cock of C opened. The flask A was heated in a water-bath for any desired time, the oxygen being collected in C as fast as it was given off.

At the end of the experiment, all the gases which remained in A and B were driven over into C by lowering the reservoir M and opening the pinch-cock at D. When this was accomplished the stop-cock of C was closed and the gases brought under atmospheric pressure by bringing the mercury in C and M to the same level.

It will be seen that in this gas volume was included the air which A and B contained at the beginning of the experiment.

The oxygen evolved from the contents of the flask was determined thus: After reading off the total volume of air and oxygen and reducing to normal conditions, the total volume of oxygen was obtained by absorption with phosphorus or pyrogallol, and from the residual nitrogen could be calculated from the volume of air to be deducted the total volume of gases. The remainder is the volume of oxygen sought.”

Action of manganese dioxide on potassium permanganate in acid solution.

Three sets of apparatus like the one described were used.

The flask of apparatus No. I contained manganese dioxide and dilute nitric acid.

The flask of apparatus No. II contained potassium permanganate and dilute nitric acid.

The flask of apparatus No. III contained potassium permanganate and dilute nitric acid and manganese dioxide.

The manganese dioxide used in these experiments was precipitated from a solution of potassium permanganate by a dilute solution of manganous sulfate in the manner described on page 34 and then washed and dried at 100°C.

An illustration of the proportions in which the different substances were brought together in each of the three flasks is to be found in the following statement of the quantities used in the first experiment.

The variations from these proportions occurring in subsequent experiments are noted in the table giving the summary of the results. Usually about 150 m.g. of manganese dioxide were used and the solutions in the three flasks were brought to the same volume by addition of distilled water.

Experiment No. I

Flask No. I Manganese dioxide 150 m.g. N/10 nitric acid 14.28 c.c. [= Mn. in KMnO₄ in No. II or No. III] Water, 27.12 c.c. [Total solution = 41.40 c.c.]

Flask No. II Potassium permanganate, 20 c.c. = 112.48 m.g. KMnO₄. N/10 nitric acid 7.14 c.c. [= K in KMnO₄] Water, 14.26 c.c. [Total solution = 41.40 c.c.]

Flask No. III Potassium permanganate, 20 c.c. = 112.48 m.g. KMnO₄. N/10 nitric acid 21.40 c.c. [= K + Mn in KMnO₄] Manganese dioxide 150 m.g.

The nitric acid in No. I is calculated to be the same as that remaining free after the potassium of the permanganate in No. III has been neutralized. It will be clear that the quantities of free nitric acid are the same in flask No. I and in flask No. III provided the potassium of the permanganate is appropriated by the nitric acid. Moreover each flask contains the same quantity of manganese dioxide and the volume of the liquid is the same in both.

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