Part 4

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.

Action of a Black Oxide of Manganese on Potassium Permanganate.----

As an analysis of the manganese dioxide showed the probable presence of some manganese in the manganous condition, a sample was treated on the water-bath for some days with dilute nitric acid while distilled water was added from time to time to keep the dilution nearly constant. This was done for the purpose of determining whether the manganous manganese might not be extracted in the form of manganese nitrate. The light brown oxide which was used in the previous experiments had been prepared by adding a dilute solution of manganous sulphate to a hot solution containing potassium permanganate in excess and dilute nitric acid. No analysis of the oxide thus prepared was made at the time of its preparation, but it has been shown by the work of others that the oxide must have had at that time a composition as regards the relation of manganese to oxygen, very nearly equivalent to that of MnO₂. During the course of the experiments previously recorded, a determination of the ratio of the manganese to the available oxygen in the compound was made and found to be as

1.03 : 1.00

The substance gave 54.69 percent manganese and 15.44 percent available oxygen.

A second analysis made some weeks later gave a ratio of manganese to available oxygen of 1.06: 1.00.--

The substance gave 54.76 percent manganese 15.02 ” available oxygen.

The manganese was determined by the method of Gibbs and the available oxygen by oxalic acid and potassium permanganate.

Silliman’s Am. J. Science (2) 44.216

It was this oxide which was treated with nitric acid in the manner described. In the course of this treatment the light brown color changed to black and the oxide became heavier. The dilute nitric acid in which the oxide had been digested was found to contain manganous manganese, but analysis of the black product showed that the oxide had suffered a still further reduction. The ratio of manganese to available oxygen was found to have risen from 1.06: 1.00 to 1.157: 1.000. The analytical data are given below:--

The substance gave { 60.49 percent manganese } { 60.46 ” ” } and { 15.23 percent oxygen } { 15.19 ” ” }

This ratio would correspond very nearly to a compound having the composition MnO·6MnO₂ but it is by no means certain that the reducing action of nitric acid was finished. The action of this oxide on potassium permanganate was determined under the same conditions as in the case of the brown oxide as described on page 14. The following illustration of the proportions used is taken from the first experiment.

Experiment No. I

Flask No. I Manganese oxide, black, = 150 m.g. N/10 nitric acid = 14.93 c.c. [= Mn in KMnO₄ in No. II or No. III] Water = 27.46 c.c. [Total solution = 42.39 c.c.]

Flask No. II Potassium permanganate = 20 c.c. = 117.71 m.g. N/10 nitric acid = 22.39 c.c. [= K and Mn in KMnO₄] [Total solution = 42.39 c.c.]

Flask No. III Potassium permanganate = 20 c.c. = 117.71 m.g. N/10 nitric acid = 22.39 c.c. [= K and Mn in KMnO₄] Manganese oxide, black = 150 m.g. [Total solution = 42.39 c.c.]

The first two results are from experiments in which the action was stopped at the first loss of the color of potassium permanganate. The last two are the results from experiments in which the action was stopped after the oxide had settled leaving a clear colorless supernatant liquid. The amount of oxygen given off in any one case is expressed in the table as the number of atoms of oxygen derived from each molecule of potassium permanganate, as follows:

A = N/10 nitric acid. No. of molecules. ------------------+-------------------------+-------------------- Flask I | Flask II | Flask III ------------------+-------------------------+-------------------- Black | Potassium | Potassium oxide A |permanganate A | permanganate A 150 m.g. | 20 c.c. = | 20 c.c. | 117.71 m.g. | = 117.71 m.g. ----------------------+-------------------------+---------------- 1 2 hours. 0.00 2 0.18 3 1.337 3 2 2¼ hours 0.02 ” 0.24 ” 1.335 ” 3 2½ hours 0.02 ” 0.44 ” 1.402 ” 4 4½ hours 0.00 ” 0.55 ” 1.362 ”

The volume of the oxygen evolved is considerably smaller than that obtained when the brown oxide was used. It is nevertheless larger than could have been evolved if all of the manganese had been left in the dioxide condition at the close of the experiment. This fact indicates that there was a slight reduction of the manganese dioxide after the disappearance of the color of the permanganate. That such a reduction of manganese dioxide does take place will be made apparent by a comparison of the results of the first two experiments in which the oxygen was measured soon after the color of permanganate had disappeared, with the results of the last two experiments in which the oxygen was determined after a longer interval. It will be noticed in this as in the series of experiments with the brown oxide that the rate of the evolution of oxygen liberated in flask No. II increases with the time of heating.

The Amount of Nitric Acid neutralized.

A few experiments were made to determine how much nitric acid is neutralized under the conditions which prevailed in the foregoing experiments.

I. The flask contained brown oxide 150 m.g. N/10 nitric acid 6.98 c.c. and was heated for three hours in the water-bath.

Acid neutralized = 4.51 c.c. = 64.60 percent.

II. The flask contained brown oxide 150 m.g. N/10 nitric acid 6.98 c.c. and was heated for 50 minutes in the water-bath.

Acid neutralized = 3.09 c.c. = 44.20 percent.

III. The flask contained brown oxide 150 m.g. N/10 nitric acid 7.46 c.c. and was heated for fifty minutes in the water-bath.

Acid neutralized = 3.16 c.c. = 42.30 percent.