Description of experiment
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Needed compounds:
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sodium persulfate : Na2S2O8
sodium hydroxide : NaOH
potassium bromate : KBrO3
vanadium pentoxide : V2O5
chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr,V
redox
Summary:
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Chromium (III) cannot be oxidized to chromium (VI) by vanadium (V) species
or bromates in strongly alkaline environments. Peroxodisulfate is capable
of achieving this.
Description:
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Dissolve some chrome alum in water and add this solution to an excess amount
of 1) potassium bromate, dissolved in a solution of sodium hydroxide
2) vanadium pentoxide, dissolved in a strong solution of sodium hydroxide:
Addition to both (1) and (2) results in formation of a green precipitate,
which dissolves on shaking and results in a clear green liquid.
Heat both liquids: The green liquids remain green. On cooling down, a green
precipitate is formed again.
Add some sodium peroxodisulfate to both green turbid liquids: In liquid (1),
there is no visible change, in liquid (2), the direct neighbourhood of the
solid sodium peroxodisulfate becomes bright yellow, but on shaking, both
liquids remain green. The green color, however, has shifted a little bit, it
shifted from greyish green to moss-green.
Heat both liquids and boil gently for a while: Within two minutes, both liquids
slowly change color from green to bright yellow. Liquid (1) becomes completely
clear, liquid (2) still has some flakes in it and is slightly turbid.