| Three-hour determinations. | ||||||
|---|---|---|---|---|---|---|
| Flask I | Flask II | Flask III | ||||
| MnO₂ [150 m.g.] | N/10 HNO₃ No. of molecules | KMnO₄ 20 c.c. = 112.48 m.g. | N/10 HNO₃ No. of molecules | KMnO₄ 20 c.c. = 112.48 m.g. | N/10 HNO₃ No. of molecules | |
| 1 | 0.00 | 2 | 0.13 | 1 | 1.495 | 3 |
| 2 | 0.00 | ” | ? | 1 | 1.493 | ” |
| 3 | ” | 0.13 | 3 | 1.532* | ” | |
| 4 | 0.00 | ” | 0.21 | ” | 1.466 | ” |
| 5 | 0.03 | ” | 0.10 | ” | 1.467 | ” |
| 6 | 0.02 | ” | 0.25 | ” | ” | |
| 7 | 0.06 | ” | 0.08 | ” | 1.526* | ” |
| 8 | 0.04 | ” | 0.15 | ” | 1.527 | ” |
| 9 | 0.04 | ” | 0.11 | ” | 1.504 | ” |
| 10 | ” | 0.20 | ” | 1.504 | ” | |
| 11 | 0.03 | ” | 0.14 | ” | 1.516 | ” |
| 12 | 0.00 | ” | 0.18 | ” | 1.524 | ” |
| 13 | 0.03 | ” | 0.25 | ” | 1.559* | ” |
| 14 | 0.05 | ” | 0.26 | ” | 1.574 | ” |
| 15 | 0.02 | 1 | 0.32 | 2 | 1.552 | 2 |
| 16 | 0.03 | ” | 0.31 | ” | 1.583? | ” |
| 17 | 0.04 | ” | 0.31 | ” | 1.512 | ” |
| 18 | 0.01 | ” | 0.32 | ” | 1.466 | ” |
| 19 | 0.00 | ” | 0.18 | ” | 1.412 | ” |
| 20 | 0.01 | ” | 0.36 | ” | 1.411 | ” |
| 21 | 0.06 | ” | 0.22 | ” | 1.402 | ” |
| 22 | 0.00 | ” | 0.30 | ” | 1.495* | ” |
| 23 | 2 | 0.08 | 3 | 1.469 | 3 | |
| [5]Fifty minute determinations | ||||||
| 24 | 0.04 | 1 | 0.06 | 2 | 1.564 | 2 |
| 25 | 0.04 | ” | 0.11 | ” | 1.491 | ” |
| 26 | 0.05 | ” | ? | ” | 1.512 | ” |
| 27 | 0.03 | ” | 0.05 | ” | 1.532 | ” |
| [6]Fifteen minute determinations | ||||||
| 28 | 0.05 | 1 | 0.08 | 2 | 1.340 | 2 |
| 29 | 0.04 | ” | 0.05 | ” | 1.325 | ” |
| 30 | 0.06 | ” | 0.05 | ” | 1.335 | ” |
| 31 | 0.00 | ” | 0.00 | ” | 1.347 | ” |
| 32 | 0.03 | ” | 0.03 | ” | 1.412 | ” |
| 33 | 0.02 | 2 | 0.02 | 3 | 1.363 | 3 |
| 34 | 0.02 | ” | 0.06 | ” | 1.377 | ” |
| 35 | 0.00 | ” | 0.02 | ” | 1.363 | ” |
Inspection of the table will show that whether the oxygen were determined immediately after the subsidence of the oxides (i.e. after fifty minutes) or after three hours the results obtained are practically the same. The tendency of the potassium permanganate to lose strength made the frequent preparation of new samples advisable. Wherever a new sample has been used in the course of the experiments, the fact has been noted in the table by an asterisk. It will be noticed that where a new solution is used the figures for flask no. III are immediately larger and nearer to one and one half atoms of oxygen from each molecule of potassium permanganate.
A phosphorus gas-pipette was used for the absorption of the oxygen in all the experiments the results of which are embodied in the foregoing table. In subsequent experiments an alkaline solution of pyrogallol was employed. It is now known that the variation in the composition of the manganese oxide in use had some influence upon the results.
It has also been found that the manganese dioxide prepared by the reduction of permanganate by manganese sulphate is much less stable than was supposed at the time this work was begun. The dioxide prepared in this way begins immediately to lose oxygen spontaneously but recovers the same in the presence of an excess of potassium permanganate. In the light of these facts it is easy to understand why lower results were obtained when the oxygen was determined immediately after the disappearance of the color of the permanganate and before the suspended oxide had subsided. It appears that the manganese oxide employed in these experiments was not, as was supposed at the time, the dioxide but one containing a smaller proportion of oxygen. If such is the case the first action of the permanganate upon it would be to replace the oxygen which had been lost. The reduction of the remaining permanganate would then probably be in accordance with the equation,
2 KMnO₄ + 3 MnO₂ = 2 K₂O + 5 MnO₂ + 1½ O₂
At the time when the permanganate color disappears, all of the manganese is in the dioxide condition and the further evolution of oxygen, which is shown by the preceding experiments to take place during the subsidence of the suspended oxides, is due to a partial reduction of this manganese. Therefore the relation of the reduction of the manganese oxide below the MnO₂ condition before the treatment with permanganate to the reduction which follows the disappearance of the permanganate color will determine whether the oxygen evolved shall be more or less than one and one half atoms to each molecule of permanganate.
Neither variations in the quantities of nitric acid used (from two to three molecules in No. III) nor the very slight variations in the amount of manganese dioxide used, seem to affect appreciably the amount of oxygen obtained.
It appears that the action of manganese dioxide on potassium permanganate is the same as that of lead superoxide[7] in the presence of very dilute nitric acid. Both reduce it to manganese dioxide with the evolution of one and one half atoms of oxygen to each molecule of the permanganate.
The evolution of oxygen from flask No. I containing manganese dioxide and nitric acid is very slight. From flask No. II containing potassium permanganate and one equivalent nitric acid, it is also slight but usually greater than from flask No. I. The differences are much greater in the case of those determinations in which the heating of the flask was continued for three hours and in this fact is to be found further evidence of the reducing action of manganese dioxide on potassium permanganate.
The possibility of a reaction analogous to that between potassium permanganate and lead dioxide in the presence of strong nitric acid seems to be excluded by the fact that the higher oxides of manganese may be prepared in the presence of concentrated nitric acid.