The quantitative separation of iron from aluminum, which presented many
difficulties according to the older methods, may be easily performed
by electrolysis. If a solution of iron ammonium oxalate and aluminum
oxalate, to which an excess of ammonium oxalate has been added, be
submitted to the action of the electric current, the iron will be
deposited as a firmly adhering coat on the negative electrode, while
the aluminum oxide remains in solution, just so long as the quantity
of ammonium oxalate is in excess of the quantity of ammonium carbonate
produced. When, finally, a precipitation of aluminum oxide takes place
the liquid is almost free from iron. From time to time, the solution, in
which the aluminum oxide is suspended, is tested for iron by ammonium
sulphide, and the current is interrupted when no further reaction is
observed. The best method of procedure is to add ammonium oxalate in
excess to a neutral, a slightly acid solution, or to one which has been
neutralized by the addition of ammonium hydrate (a hydrochloric acid
solution is not well adapted for this purpose); then as much more solid
ammonium oxalate is added until for every 0.1 gramme there is 2 to
3 grammes of the oxalate present. The hot solution is then directly
submitted to the action of the electric current. After the iron has been
precipitated, it is best to stop the action of the current before all
the aluminum oxide is thrown down, for otherwise a portion of the latter
may adhere firmly to the iron, and be difficult to remove.
In such a case, as was mentioned previously in the separation of iron
from manganese, it is necessary to redissolve the iron (after previously
having poured off the liquid) in oxalic acid, and then the electrolysis
is continued.
In order to effect the complete precipitation of the aluminum oxide from
the solution which was poured from the iron, ammonium hydrate is added,
and the solution boiled for some time, and then the aluminum oxide is
determined in the usual manner. When the quantity of aluminum is less
than that of iron, this method may be relied upon to give exact results.
With the reverse (_i. e._, an excess of iron) the precipitate
of aluminum oxide must be dissolved in oxalic acid (without the
interruption of the current), and the electrolysis continued.--_Berichte
der Deutschen Chemischen Gesellschaft_, 14, 1662.
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