Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium bicarbonate : NaHCO3
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
copper sulfate penta hydrate : CuSO4 . 5H2O
ammonia : NH3
nitric acid : HNO3
nickel : Ni
Class:
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elem=Ni,Fe,Cu
coordination
redox
Summary:
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Analysis of the composition of the dutch coin 'dubbeltje' shows that it
does not contain any copper or just a very small quantity of it.
Description:
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Add a solution of moderately concentrated nitric acid (appr. 50% by weight)
to a nickel coin 'dubbeltje', which was cleaned well before doing the
experiment: At first the reaction is very slow. When heated to appr. 60 C
the reaction suddenly becomes very vigorous and a thick dark red/brown gas
appears above the liquid. After ceasing of the reaction, a large part of
the coin is dissolved. The resulting liquid still is very acidic, which can
easily be demonstrated by adding some NaHCO3 (vigorous bubbling/foaming, due
to formation of CO2). The resulting liquid after reaction is bright green,
the color of Ni2+(aq).
Add a solution of K4[Fe(CN)6].3H2O to a part of the bright green liquid: A
pale green precipitate appears.
Add an excess amount of NH3 (15% by weight) to the liquid, containing the
pale green precipitate: The precipitate turns pale purplish grey.
Add an excess amount of NH3 (15% by weight) to a part of the bright green
liquid: The liquid turns purple/blue and remains clear. When some water is
added, the liquid turns bright purple and still remains clear. This purple
color is due to the formation of the coordination complex [Ni(NH3)6]2+.
Add a very small amount (a few sub-mm particles) of solid copper sulfate to
a part of the bright green solution: The copper sulfate dissolves. No visible
change of the liquid occurs.
Add a solution of K4[Fe(CN)6].3H2O to the bright green solution, with the
small amount of copper sulfate added: A dirty-looking gray precipitate
appears, instead of the pale green precipitate. From this can be concluded,
that if a dutch 'dubbeltje' contains copper, then this will only be in
trace amounts.