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.
On the other hand flask No. III contains permanganic acid. This
arrangement was selected in order to determine how far the evolution
of oxygen was influenced by the action of nitric acid on the manganese
dioxide or by the spontaneous decomposition of permanganic acid. It
was thought that if more oxygen should be obtained from flask No. III
containing a mixture of permanganate, manganese dioxide and nitric
acid, than from mixtures No. I and No. II, the larger volume must be
due to the reduction of the permanganate by the manganese oxide.
The three flasks after having been filled as described were attached to
the apparatus and then all submerged to the same depth in the boiling
water of a single glass water-bath. The stop-cocks of the azotometers
were then opened and the mercury allowed to fall to within about fifty
millimetres of its level in the reservoirs. This difference in level
was maintained as nearly as practicable by lowering the reservoirs
commenserably with the increase in gas volume. No action could be
observed in flasks No. I and No. II during the whole course of the
experiment. The contents of flask No. III however gave off bubbles of
oxygen immediately, the mercury fell in the azotometer and the dark
purple of the permanganate solution in flask No. III became rapidly
lighter. Within about five minutes it was reduced to a delicate pink
but this tint persisted for about ten minutes when it also disappeared.
Immediately after the disappearance of the permanganate color the
solution was filled with a brown oxide which remained for about
thirty five minutes when it subsided leaving the supernatant liquid
clear. Notwithstanding the fact that the color of the permanganate
had entirely disappeared and the suspended oxide had subsided within
an hour after beginning the experiment, the flasks in the earlier
experiments were allowed to remain for two hours longer. By admitting
distilled water through the tube =D= [see figure] the gas was then
forced over into the azotometer. Finally, the oxygen was determined in
the manner previously described.
The following illustration from the first experiment may serve to show
the method of calculating the results.
I II III
Total air and oxygen = 42.60 c.c. 52.40 c.c. 60.60 c.c.
Nitrogen after absorption
of oxygen = 34.00 c.c. 40.90 c.c. 37.80 c.c.
Oxygen = 8.60 c.c. 11.50 c.c. 22.80 c.c.
Air = 79.8% nitrogen
20.2% oxygen
Oxygen of air 20.2
-------------- × ---- = 8.60 c.c. 10.353 c.c. 9.57 c.c.
Nitrogen found 79.8
Oxygen obtained = 0.0 1.147 c.c. 13.23 c.c.
Temperature = 21°C.
Barometer = 737 mm.
Oxygen obtained at Normal = 0 1.033 c.c. 11.913 c.c.
The 20 c.c. KMnO₄ = 112.48 m.g.
1 atom of oxygen = 7.967 c.c.
Atoms oxygen found I II III
from the molecule KMnO₄ = 0 0.129 1.496
As at the end of the preceding calculation the results are expressed
in the table following in terms of oxygen atoms obtained from each
molecule of potassium permanganate [KMnO₄] reduced. Thus an equivalent
of one atom of oxygen from one molecule of potassium permanganate
would equal 15.96/157.67 of the weight of potassium permanganate used
in the experiment. This weight divided by the weight of one cubic
centimetre of oxygen gives the number of cubic centimetres of oxygen at
normal, equivalent to one atom of oxygen from one molecule of potassium
permanganate. The results obtained are tabulated as follows:
Three-hour determinations.
-------------------+-------------------------+----------------------
Flask I | Flask II | Flask III
-------------------+-------------------------+----------------------
MnO₂ N/10 HNO₃| KMnO₄ N/10 HNO₃ | KMnO₄ N/10 HNO₃
[150 m.g.] No. of | 20 c.c. = No. of | 20 c.c. = No. of
molecules | 112.48 m.g. molecules | 112.48 m.g. 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 ”
[5] This marked the time when the oxides first settled, leaving a
colorless supernatant liquid.
[6] This marked the time when the solution was first decolorized.
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,