Description of experiment
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Needed compounds:
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potassium chlorate : KClO3
potassium nitrate : KNO3
manganese (II) sulfate : MnSO4 . H2O
phosphoric acid : H3PO4
Class:
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elem=P,Mn
coordination
redox
Summary:
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In concentrated phosphoric acid, manganese (II) ions are oxidized easily to
manganese (III) ions, which are stabilized by formation of a phosphato-complex
with a remarkably bright purple color. This reaction is remarkable in the sense
that oxidation stops at manganese (III).
Description:
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Add some manganese sulfate to 85% H3PO4 and heat: The solid dissolves and a
colorless liquid is obtained.
Experiment 1
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Add some solid KNO3 to the hot solution of manganese sulfate in conc. H3PO4 and
heat, until the liquid slightly boils: The KNO3 dissolves and slowly, the color
of the liquid changes from colorless to beautifully bright purple. If heating
is continued for several minutes, then the liquid becomes quite dark purple.
Experiment 2
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Add some solid KClO3 to the hot solution of manganese sulfate in conc. H3PO4:
As soon as the solid touches the hot liquid, it oxidizes the manganese (II) to
a brown solid. Probably this is MnO2. Beware, the reaction of the KClO3 can be
violent. A crackling noise is heard, when the KClO3 reacts. On further heating,
the brown solid dissolves and the liquid becomes bright purple. A faint smell
of Cl2 can be observed.
Remark: The deep purple compound is a complex of manganese (III), it is
[Mn(PO4)2](3-). When a lot of water is added, then the liquid becomes turbid
and brown. This complex only is stable in very concentrated acids.