Description of experiment
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Needed compounds:
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sodium persulfate : Na2S2O8
ammonium persulfate : (NH4)2S2O8
sodium hydroxide : NaOH
nickel sulfate : NiSO4 . 6H2O
Class:
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elem=Ni,O
precipitation
redox
Summary:
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A precipitate of nickel hydroxide is oxidized by persulfate to a black
compound (probably NiO2).
Description:
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Add an excess amount of NaOH solution to a solution of nickel sulfate:
Formation of a pale green precipitate of nickel hydroxide. This precipitate
has no solid structure, but many small flakes of it float through the liquid.
Slowly the precipitate goes to the bottom of the glass container. The liquid
above the precipitate becomes clear and colorless.
Heat to appr. 65 C: No visible changes.
Add some solid (NH4)2S2O8: The liquid immediately turns completely black
by formation of a black precipitate. A slight evolution of a gas can be
observed.
Add an excess amount of HCl (10% by weight): The precipitate dissolves,
a gas is evolved in fairly large quantities (O2, N2??). A suffocating
odour of chlorine is present, although it is not expected that the gas
bubbles consist of gaseous chlorine (in that case the smell of chlorine
would be untolerable in the near vicinity of the glass container). After
all of the precipitate has dissolved, the liquid becomes pale brown/green
and slightly milky, just as the initial solution of nickel sulfate.
The same set of experiments is performed with sodium persulfate and exactly
the same results are obtained. For Na2S2O8, the addiotion of HCl to the
black precipitate also results in formation of gas bubbles and a suffocating
odour of chlorine. Due to this result, it is expected that these gas bubbles
consist of oxygen.