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.
[7] Allen’s thesis.
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.