Description of experiment
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Needed compounds:
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sodium persulfate : Na2S2O8
silver nitrate : AgNO3
nitric acid : HNO3
potassium permanganate : KMnO4
Class:
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elem=Mn,O,Ag
redox
Summary:
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Persulfate is capable of oxidizing Mn2+ in acid environments, but silver (I)
is needed as a catalyst. Oxidation, however, is not easy and just a small
part is oxidized to permanganate, a large part is oxidized no further than
MnO2.
Description:
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Dissolve some KMnO4 in dilute nitric acid (2 mol/l) and add a slight excess
amount of sodium sulfite: Formation of a colorless liquid with a slight
odour of sulphur dioxide.
Add a solution of AgNO3: Formation of a white precipitate (milky), which
quickly dissolves again. The liquid, however, remains slighty turbid.
Add an excess amount of sodium persulfate and heat to appr. 60 C: At first,
nothing appears to happen, but suddenly the liquid turns turbid and dark
brown and becomes completely opaque (due to formation of MnO2). From the
first sign of coloration till complete opaqueness the elapsed time is less
than 10 seconds.
Keep on heating for a few more minutes and then let liquid stand on a quiet
place for a few hours: At the bottom of the test tube, a thick brown layer
of the precipitate is formed. The liquid above the dark precipitate has a
bright purple color, the color of dilute permanganate.