Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium persulfate : Na2S2O8
sodium hydroxide : NaOH
EDTA tetra sodium : (NaOOC CH2)2 N CH2 CH2 N (CH2 COONa)2
nickel sulfate : NiSO4 . 6H2O
Class:
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elem=Ni
redox
coordination
Summary:
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Nickel (II) builds a stable complex with EDTA. Once this complex is formed,
it is not precipitated with NaOH and persulfate does not oxidize nickel (II).
Description:
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Sequence 1:
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Add a solution of EDTA tetra sodium to a solution of nickel sulfate: The
liquid becomes blue/grey and remains clear.
Add some solution of NaOH: The liquid remains clear. Its color becomes slightly
less intense and it becomes a little more greenish (these changes may be due
to dilution of the colored complex).
Add some solid Na2S2O8: The solid dissolves, no black precipitate is formed,
the liquid remains clear.
Sequence 2:
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Add a solution of NaOH to a dilute solution of nickel sulfate: A pale green
precipitate is formed, the liquid becomes turbid.
Add an excess amount of a solution of Na2S2O8: The liquid becomes dark brown/
black. A few seconds are needed for the change from pale green to brown/black.
Add a large excess amount of a solution of EDTA tetra sodium: The black
precipitate does not dissolve.