Description of experiment
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Needed compounds:
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potassium dichromate : K2Cr2O7
potassium chromate : K2CrO4
formic acid : HCOOH
Class:
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elem=C,Cr
redox
Summary:
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Dichromate and chromate both quickly are oxidized by pure formic acid,
resulting in a fairly violent and exothermic reaction, in which a dark
purple/blue/grey compound is formed.
Description:
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Add a few 100's of ml of pure formic acid (>98%) to a small spatula of
potassium dichromate: The solid at once turns dark brown and then a violent
reaction sets in, in which a lot of gas is produced. All potassium dichromate
dissolves and a syrup-like liquid remains, which is very dark purple. On
dilution with water, a dark purple/blue/grey solution is formed.
Add a few 100's of ml of pure formic acid to a small spatula of potassium
chromate: Also here, the solid at once turns dark. First orange and then dark
brown. For the rest, the experimental results are the same as in the experiment
above.
Remark: With formic acid, there is no real difference between chromate and
dichromate. This is in contrast with thionylchloride, which dissolves potassium
dichromate, giving a deep red solution, while potassium chromate does not
dissolve in this.