Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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nickel sulfate : NiSO4 . 6H2O
sodium persulfate : Na2S2O8
sodium hydroxide : NaOH
sodium sulfite : Na2SO3
hydrochloric acid : HCl
sulphuric acid : H2SO4
nitric acid : HNO3
potassium permanganate : KMnO4
Class:
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elem=Ni,Mn
redox
Summary:
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This experiment describes a qualitative method, useful for showing the
presence of minute quantities of manganese (II), which cannot be detected
by oxidation with H2O2 in alkaline environments anymore. Chloride ions
may not be present besides the manganese to be detected.
Description:
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First a dilute solotion, containing Mn2+ is prepared. In this experiment
this was done as follows:
Dissolve a crystal of KMnO4 in dilute H2SO4 (a few mass percents) and
add a slight excess amount of sodium sulfite.
One drop of the solution, containing Mn2+ is taken apart and added to some
distilled water.
The following steps are performed for detecting Mn2+ in the water.
Dissolve some nickel sulfate and a fairly large amount of sodium persulfate
in water and add a solution of sodium hydroxide: A black precipitate is
formed.
Add the water, containing the manganese (II): No visible changes.
Now the liquid is shaken and divided in three parts, each part containing
the black precipitate.
Add an excess amount of dilute HNO3 (appr. 2 mol/l) to part 1: The liquid
becomes clear and purple. Permanganate is formed and even in small quantities
this masks the pale color of the nickel (II) ions.
Add an excess amount of dilute H2SO4 (a few mass percent) to part 2: The same
result as with dilute HNO3.
Add an excess amount of dilute HCl (a few mass percent) to part 3: The liquid
becomes clear and pale green, smell of chlorine. Now the chloride is oxidized
instead of the Mn2+, resulting in just the pale green color of the Ni2+ ion.
This method uses the fact that the higher nickel oxide, formed from nickel
hydroxide in excess alkali by oxidation with persulfate, is capable of
oxidizing Mn2+ to permanganate in acidic environments.
When more concentrated solutions of Mn2+ are used, then a deep purple color
of permanganate is obtained. At too high concentrations of Mn2+ this method
does not work anymore, on addition of the acid, a brown precipitate of MnO2
is formed, instead of purple [MnO4]-.
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This method was used to show that in the dutch 'dubbeltje' and 'kwartje'
traces of manganese are present. This was done by dissolving such a coin
in nitric acid (appr. 40% by weight) completely. These coins consist for
almost 100% of nickel.
To a sample of the resulting green solution much sodium hydroxide is added,
such that a pale green precipitate is formed. After that a fairly large
amount of solid sodium persulfate is added, resulting in a black precipitate.
The alkaline liquid with precipitate then is acidified with an excess amount
of dilute nitric acid, resulting in a pale pink liquid, instead of a pale
green liqiud, which was expected. When a drop of H2O2 (3%) is added to the
pale pink liquid, then the color turns to pale green.