If a solution of ferric oxide and manganese ammonium oxalate is
submitted to electrolysis, without the previous addition of ammonium
oxalate, the characteristic color of permanganic acid immediately makes
its appearance, and the peroxide gradually precipitates itself on the
positive, while the iron is deposited on the negative electrode. When
the examination is made in the above manner, it is impossible to
separate the two metals, for the peroxide will bring down with it a
considerable quantity of ferric hydrate. The separation of the two
metals is only possible when the precipitation of the manganese peroxide
is prevented, until the greater portion of the iron has been deposited.
This result may be attained by adding sodium phosphate, or, better
still, by the addition of ammonium oxalate in great excess. In both
cases the characteristic coloration from permanganic acid is developed
by the action of the current at the positive pole; this, however,
disappears in the direction of the negative electrode. After the greater
portion of the ammonium oxalate has been converted into carbonate, the
coloration and necessarily the formation of manganese peroxide begins.
Ammonium oxalate is added to the solution, and heat applied; then three
or four grammes more of ammonium oxalate are dissolved in the liquid,
which is then immediately submitted to electrolysis. When the amount of
manganese is small, the separation of the two elements takes place very
rapidly, and the results are accurate. If the amount of manganese is
more than double that of iron, the separation of the latter will take a
much longer time. Then, in order to effect a complete separation of the
two elements, it is necessary to redissolve the deposited manganese in
oxalic acid (the acid is added, without interrupting the current, until
the liquid becomes red), and the current is allowed to continue its
action.
It was found desirable, in effecting this separation, not to employ
too strong a current (two Bunsen elements will suffice), and only
to increase the strength of the current when it is necessary, in
consequence of a large amount of manganese being present, to redissolve
the peroxide.
When the process is completed, it is not advisable to allow the current
to act any longer, for otherwise some of the peroxide may adhere firmly
to the iron, and the latter (after previously having poured off the
liquid) must be redissolved in oxalic acid, that is to say, the
electrolysis must be repeated. As has been already mentioned in the
determination of manganese as peroxide, its precipitation from ammonium
oxalate is not complete. The solution which contains the greater portion
of manganese, suspended as peroxide, must first, therefore, be boiled to
decompose the ammonium carbonate; the remainder of the ammonium oxalate
is neutralized with nitric acid, and the manganese converted into the
sulphide by ammonium sulphide. The manganese sulphide is then ignited in
a current of hydrogen, and weighed as such.