Description of experiment
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Needed compounds:
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nitric acid : HNO3
sodium hypophosphite : NaH2PO2
potassium dichromate : K2Cr2O7
Class:
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elem=P,Cr
redox
Summary:
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Hexavalent chromium is reduced to trivalent chromium by hypophosphite, but
there is no further reduction to chromium(II).
Description:
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Dissolve some potassium dichromate in dilute nitric acid (1 ... 2 M is
suitable): An orange solution is obtained.
Add an excess amount of solid sodium hypophosphite: This quickly dissolves and
the liquid remains orange, but it obtains a slight brownish hue.
Heat the liquid until it starts boiling gently: While the liquid is heated, the
orange color shifts to brown and on further heating it turns deep green.
Finally, the liquid has a deep green color, which on dilution looks bright
green. There is no further change of color on prolonged heating.