Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ammonium thiocyanate : NH4SCN
sulphuric acid : H2SO4
hydroxyl amine sulfate : (NH3OH)2 SO4
sodium hydroxide : NaOH
potassium ferricyanide : K3 [Fe(CN)6]
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
Class:
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elem=Fe
coordination
redox
Summary:
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Hydroxylamine is capable of reducing ferricyanide to ferrocyanide. On heating,
however, a yellow compound is formed, which apparently is not ferricyanide.
With thiocyanate a pale rose-purple solution is formed on standing. Probably
oxygen from the air also takes part in the reaction.
Description:
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Sequence 1:
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Add a solution of potassium ferricyanide to an excess amount of a solution of
hydroxylamine sulfate and sodium hydroxide: The yellow color of the solution
of potassium ferricyanide quickly disappears on mixing with the NaOH/hydroxyl-
amine solution. It becomes pale yellow. There also is evolution of a colorless
gas, when the ferricyanide is added.
This result can be interpreted as reduction of ferricyanide to ferrocyanide.
Sequence 2:
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Boil a solution of potassium ferrocyanide and sodium hydroxide and shake a
little with air-contact: The liquid becomes slightly more yellow. The yellow
color, however, still remains very pale.
Add a small pinch of hydroxylamine sulfate to the still very hot light yellow
solution: The yellow of the solution now becomes much more intense. The yellow
is not completely the same as the yellow of a ferricyanide solution, it is a
little bit more golden yellow.
Let liquid stand overnight: The yellow color has faded somewhat, but it still
is more intense than a simple ferrocyanide solution.
The golden-yellow compound probably is not ferricyanide. The formation of this
golden-yellow compound probably cannot be attributed to oxidation of the
ferrocyanide to ferricyanide.
Sequence 3:
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Prepare a solution of potassium ferrocyanide, sodium hydroxide and a small
amount of hydroxylamine sulate: The liquid is very pale yellow.
Divide the liquid in two parts and boil one part of the liquid: The heated
part clearly has a stronger yellow color than the other part.
Add some ammonium thiocyanate to both parts of the liquid: In both liquids,
the ammonium thiocyanate dissolves without causing any change of color. Hence,
the golden-yellow color in the heated solution is not free Fe3+, capable of
forming a bloodred complex with thiocyanate.
Let both liquids stand for a few hours: The heated liquid has turned pale
rose-purple, while the other liquid still has its original very pale yellow
color.
Boil the original pale yellow liquid for a while: The color of the liquid
deepens somewhat, its color also shifts to brown a little bit. After heating,
let stand for a few minutes. It still looks pale yellow/brown. When the
liquid is shaken, then it looks brown-pink, just after a short time of
shaking. It looks as if the shaking, which gives intense air contact for a
while, caused the quick change of color. After even more shaking, the liquid
almost has the same pale rose-purple color as the other liquid.
Add an excess amount of 2M sulphuric acid to the rose-purple liquid: The
liquid becomes green, with a bluish tinge.
Remark: The heating of the liquid with hydroxylamine, hydroxide and ferro-
cyanide apparently changes its composition. On addition of thiocyanate a
complex can be formed of nice rose-purple color. When there was no heating,
then the rose-purple color is not obtained, not even after many hours.
Remark2: The rose-purple color reverts to light yellow/brown after a long
time of standing (several days). When an acid is added to this light yellow/
brown liquid, then the liquid becomes much more intense yellow/green, and
there is evolution of a colorless gas.
The change from rose-purple to light yellow/brown may be due to picking up
of CO2 from the air, which causes the liquid to become less alkaline. More
research is needed in order to determine the real cause of the change of
color.