Description of experiment
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Needed compounds:
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sodium sulfite : Na2SO3
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
potassium ferricyanide : K3 [Fe(CN)6]
sulphuric acid : H2SO4
sodium hydroxide : NaOH
vanadium pentoxide : V2O5
Class:
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elem=V,Fe
precipitation
redox
Summary:
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Vanadium (V) compounds can coexist with ferricyanide in solution. Reduction
of either one of them results in formation of a precipitate.
Description:
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Create a yellow acidic solution of vanadium (V) as follows:
- Add some V2O5 to a solution of NaOH and heat, until solid is dissolved
and liquid becomes clear and colorless.
- Add an excess amount of H2SO4 (15% by weight).
Sequence 1:
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Add a solution of potassium ferricyanide to the yellow liquid: No visible
changes can be observed. The liquid remains yellow (the solution of potassium
ferricyanide itself also is yellow).
Add a solution of Na2SO3 to the yellow vanadium (V)/ferricyanide solution:
At first glance no visible changes occur, but after a few seconds the liquid
turns turbid and a dark green precipitate is formed. The liquid appears to
solidify, the precipitate is formed throughout the whole liquid and forms
one large structure, which however can be broken down easily.
Add more (an excess amount) solution of Na2SO3: The dark green precipitate
turns much lighter and becomes yellow/green/brown.
Sequence 2:
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Add a solution of potassium ferrocyanide to the yellow liquid: Formation of
a dark green/blue precipitate.
Add a solution of NaOH: The precipitate dissolves again and the liquid becomes
clear. The color of the liquid is intense yellow. When some phenolphtalein is
added, the liquid turns red, so the intense yellow liquid was alkaline.
Remark:
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Vanadium (V) and ferrocyanide probably yield the same precipitate as
vanadium (IV) and ferricyanide.