Description of experiment
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Needed compounds:
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sodium metabisulfite : Na2S2O5
hydrochloric acid : HCl
sodium hydroxide : NaOH
ammonium persulfate : (NH4)2S2O8
sulphuric acid : H2SO4
sodium sulfite : Na2SO3
potassium permanganate : KMnO4
Class:
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elem=Mn,O
redox
Summary:
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Persulfate is not capable of oxidizing manganese to the (VII) state
in acidic environments.
Description:
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Dissolve some solid KMnO4 in dilute sulphuric acid and add a slight excess
amount of sodium sulfite: The liquid becomes colorless and clear, a pungent
odour of SO2 is observed. The liquid contains Mn2+.
Add an excess amount of ammonium persulfate: The odour of SO2 disappears,
the ammonium persulfate dissolves, no further changes. The liquid remains
colorless.
Heat to appr. 60 C for a few minutes: No visible changes.
Add an excess amount of a solution of NaOH (such that all acid is neutralized
and the liquid becomes alkaline): A dark brown precipitate appears immediately
on contact between the two liquids. The persulfate oxidizes manganese(II) to
manganese(IV) immediately.
Add more ammonium persulfate, dissolve and heat to appr. 60 C for a few
more minutes: No changes observed. After a few minutes of standing the
precipitate goes down and the liquid above the precipitate becomes pale
brown and a little bit milky. No visible traces of green [MnO4]2- or
purple [MnO4]-.
Add an excess amount of a 10% solution of HCl: No visible changes, the
liquid remains pale brown and milky. The dark brown precipitate does
not dissolve.
Add an excess amount of solid Na2S2O5 and shake: The liquid almost
instantly turns colorless and clear.
Conclusion: Persulfate is not capable of oxidizing Mn2+ in acid environments,
in alkaline environment it cannot oxidize manganese further than MnO2 (the
(IV) state).