Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium hydroxide : NaOH
sodium sulfite : Na2SO3
potassium ferricyanide : K3 [Fe(CN)6]
vanadyl sulfate : VOSO4.5H2O
Class:
------
elem=V,Fe
redox
precipitation
Summary:
--------
The vanadyl salt of ferricyanide does not dissolve. The differences between
the ferricyanide salt and ferrocyanide salts are not very clear. In alkaline
solution apparently ferricyanide and vanadate (IV) can coexist.
Description:
------------
Add a solution of potassium ferricyanide to a slightly acidic solution of
vanadyl sulfate: Formation of a dark green precipitate.
The liquid with the dark green precipitate is called (I).
Sequence 1:
------------
Add an excess amount of a solution of NaOH to (I): The precipitate dissolves
and the resulting solution turns red/brown and does become completely clear.
When the NaOH is added in small amounts and the liquid is shaken well in
between, then one can observe that each little amount of NaOH contributes
to a part of the dissolving of the precipitate.
Sequence 2:
------------
Add some solid Na2SO3 to (I): The Na2SO3 dissolves, odour of SO2. The preci-
pitate turns lighter from dark green to green/yellow/brown.
Add an excess amount of a solution of NaOH to the liquid, containing the
lighter green/yellow/brown precipitate: The result is very similar to that
obtained by adding NaOH to (I). The final solution again is red/brown and
is not completely clear.
Remark:
--------
Apparently, vanadyl gives a dark green precipitate with ferricyanide and a
lighter green/yellow/brown precipitate with ferrocyanide. The sulfite
reduces the vanadyl ferricyanide to vanadyl ferrocyanide.