Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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nickel sulfate : NiSO4 . 6H2O
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
sodium persulfate : Na2S2O8
oxone : K2SO4.KHSO4.2KHSO5
Class:
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elem=O,S,Ni
redox
Summary:
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Both oxone, active ion is HSO5(-), and peroxodisulfate, active ion is S2O8(2-),
produce a black precipitate when added to nickel(II) ions at high pH. Hydrogen
peroxide, on the other hand, only produces green nickel(II) hydroxide, and if
the black precipitate is present, it is destroyed by hydrogen peroxide, with
formation of oxygen and green nickel(II) hydroxide.
Description:
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Do this experiment with oxone:
- Dissolve some nickel sulfate (or the nitrate, but not the chloride) in water.
- In a separate test tube, dissolve quite some NaOH or KOH in water.
- In a third test tube, dissolve some oxone (be sure to have excess hydroxide,
relative to the oxone).
- Mix the solution of NaOH or KOH and the solution of oxone. The resulting
liquid must be strongly alkaline. You see very fine bubbles of gas (oxygen) in
the solution.
- Add the solution of the nickel(II) salt.
The result is a black precipitate (really black, like carbon black) and a lot
of bubbles of gas (oxygen). The precipitate remains black, also when production
of oxygen stops.
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Do the same kinf of experiment with sodium peroxodisulfate:
- Dissolve some nickel sulfate in water.
- In a separate test tube, dissolve quite some NaOH or KOH in water.
- In a third test tube, dissolve some sodium peroxodisulfate (be sure to have
excess hydroxide).
- Mix the solution of NaOH or KOH and the solution of sodium peroxodisulfate.
The resulting liquid must be strongly alkaline.
- Add the solution of the nickel(II) salt.
The result again is a black precipitate, but there is not such a strong
production of oxygen as in the experiment with oxone.
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Do a similar experiment with 10% H2O2:
- Dissolve some NaOH or KOH in water and add a few drops of 10% H2O2.
- In a separate test tube dissolve some nickel sulfate or nickel nitrate.
- Mix the two solutions
The result is a pale green precipitate of Ni(OH)2. No special reaction occurs.
It does not matter whether a very small amount of H2O2 is used or an excess
amount of H2O2 is used, relative to the nickel. The result is the same in both
tests.
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Finally, add a few drops of 10% H2O2 to the test tubes with the black
precipitate. This results in immediate destruction of the black precipitates,
both from the oxone and from the peroxodisulfate. Bubbles of oxygen are
produced and the precipitate becomes pale green.
The black compound in both cases most likely is the same compound, it must be
some higher oxidation state hydrous nickel oxide. This can be produced with
oxone and with peroxodisulfate. Hydrogen peroxide cannot produce this compound,
actually this black compound is destroyed by hydrogen peroxide.