Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
hydroxyl amine sulfate : (NH3OH)2 SO4
sodium hydroxide : NaOH
sodium chloride : NaCl
sodium sulfite : Na2SO3
copper sulfate penta hydrate : CuSO4 . 5H2O
silver nitrate : AgNO3
potassium iodide : KI
nitric acid : HNO3
mercury (II) nitrate : Hg(NO3)2
Class:
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elem=Hg,I,Ag,Cu
precipitation
redox
Summary:
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This is a very nice experiment, involving beautifully coloured compounds,
but it is a hazardous experiment as well, due to the use of mercury (II)
compounds.
Mercury (II) builds a complex with excess iodide, [HgI4]2-, which gives a
beautiful bright yellow precipitate with Ag+ and a beautiful bright brick-
red precipitate with Cu+.
Description:
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Add a solution of KI to a solution of mercury (II) nitrate (this solution was
prepared by dissolving metallic mercury in fairly concentrated nitric acid
and hence still is very acidic, estimated to be appr. 3 M HNO3. The acid
does not affect this experiment): Formation of a bright orange/red precipitate
of HgI2.
Add more solution of KI: The precipitate dissolves again and the liquid
becomes almost colorless (a very pale yellow color remains, but this may be
due to oxidation of trace amounts of iodide by oxygen from the air, due to
the strongly acidic environment).
This almost colorless liquid is called (A) and contains [HgI4]2-.
Sequence 1: Yellow precipitate
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Add a solution of AgNO3 to part of (A): Formation of an orange and a yellow
precipitate. When the liquid is shaken, all of the precipitate becomes bright
yellow. The orange precipitate is formed at those places, where the
concentration of (A) is low, compared to the concentration of the solution
of AgNO3. The precipitate is Ag2HgI4.
Heat the yellow precipitate: The precipitate becomes bright orange. When it
cools down, it becomes yellow again. This can be repeated.
Sequence 2: Brick-red precipitate
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Add an excess amount of a solution of Na2SO3 to a solution of copper sulfate:
Formation of a brown/green precipitate.
Add an excess amount of solid NaCl: The solid dissolves and the brown/green
precipitate dissolves. The liquid becomes almost clear and almost colorless
(just a very light yellow colour remains).
Add this almost colorless liquid to part of (A): Formation of a bright brick-
red precipitate. The precipitate is Cu2HgI4.
Heat the brick-red precipitate: The precipitate becomes dark purple. When it
cools down, it becomes brick-red again. This can be repeated. After 24 hours,
part of the brick-red precipitate has become light dirty brown.
Rinse both precipitates with water and add an excess amount of a solution
of sodium hydroxide to each of the precipitates:
- The yellow precipitate remains yellow.
- The brick-red precipitate becomes grey/black and the liquid above it
becomes turbid/blue. When some HCl (10%) is added to the grey/black
precipitate, then it does not change visibly.
Heat the yellow precipitate in NaOH-solution: The precipitate becomes brown/
yellow. When it cools down, it remains brown yellow.
Add some hydroxyl amine sulfate to the brown/yellow precipitate in NaOH-
solution: The precipitate turns grey/black. A little amount of a gas is
evolved.