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Question
Describe the oxidising action of potassium dichromate and write the ionic equation for its reaction with iron (II) solution.
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Solution
Potassium dichromate acts as a strong oxidising agent. It is used as a primary standard in volumetric analysis. The oxidation action of dichromate ion in an acidic medium can be represented as follows:
\[\ce{Cr2O^{2-}_7 + 14H^+ + 6e^- -> 2Cr^{3+} + 7H2O}\] (E– = 1.33 V)
When ferrous salts (Fe2+) are treated with acidified K2Cr2O7, they are oxidised to ferric (Fe3+) salts.
\[\ce{Cr2O^2-_7 + 14H^+ + 6e- -> 2Cr^3+ + 7H2O}\]
\[\ce{6Fe^2+ -> 6Fe^3+ + 6e-}\]
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\[\ce{Cr2O^2-_7 + 14H^+ + 6Fe^2+ -> 2Cr^3+ + 7H2O + 6Fe^3+}\]
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