Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sulphuric acid : H2SO4
sodium persulfate : Na2S2O8
sodium hydroxide : NaOH
nitric acid : HNO3
nickel nitrate : Ni(NO3)2 . 6H2O
EDTA tetra sodium : (NaOOC CH2)2 N CH2 CH2 N (CH2 COONa)2
Class:
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elem=Ni
redox
coordination
Summary:
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Nickel forms colored coordination complexes with EDTA.
Description:
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Add an excess amount of EDTA tetra sodium to a solution of nickel nitrate,
which is slightly acidic by means of the presence of some nitric acid (this
nickel solution was obtained by dissolving the dutch nickel coin 'kwartje'
in nitric acid and boiling away most of the acid and water): The EDTA
dissolves, the liquid turns intense blue. The dutch 'kwartje' contains
trace amounts of manganese, but this does not cause the color of the
liquid (this was shown, by performing a similar experiment with EDTA
and manganese (II)).
Sequence 1:
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Add some NaOH: The liquid becomes turbid. The color of this liquid becomes
cyan, probably caused by formation of Ni(OH)2, suspended in a liquid, which
still contains some of the Ni-EDTA complex.
Add some solid Na2S2O8: The solid turns black, this confirms the presence
of Ni(OH)2 in the turbid liquid.
Add an excess amount of sulfuric acid (appr. 15% by weight): Liquid turns
pale green, the color of the simple aqueous nickel (II) ions. No purple
color of permanganate is obtained (this could be caused by the trace amounts
of manganese in the nickel nitrate solution).
Sequence 2:
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Add an excess amount of sulfuric acid to the intense blue liquid: The liquid
becomes pale green, ordinary color of dilute aqueous Ni2+.
Sequence 3:
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Add an excess aamount of a concentrate solution of NaOH to the intense blue
liquid: The resulting liquid becomes turbid and gets a green/cyan color.
A green precipitate is formed, which floats around in a green/cyan liquid.
Remark:
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By carefully mixing the resulting liquids from sequences 2 and 3, the deep
blue liquid can be obtained again, but a little too much acid results in
formation of the pale green aqueous nickel (II) and a little too much base
results in formation of pale green Ni(OH)2 in a not very strongly colored
cyan liquid.
The acid and the base must fairly precisely be neutralized in order to get
the deep blue coordination complex of EDTA and nickel (II).