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 whichseparates 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 KMnOâ‚„ + 3 PbOâ‚‚ = Kâ‚‚O + 3 PbO + 2 MnOâ‚‚ + 3 Oâ‚‚.
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 hadever 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.