* Orange Color: The orange color of potassium dichromate (K₂Cr₂O₇) in acidic solution is due to the presence of the dichromate ion (Cr₂O₇²⁻). This ion has a +6 oxidation state for chromium.
* Green Color: When the dichromate ion is reduced, it forms the chromic ion (Cr³⁺), which is green in color.
The reduction reaction:
The dichromate ion (Cr₂O₇²⁻) is reduced to the chromic ion (Cr³⁺) by a reducing agent. Common reducing agents include:
* Ethanol (C₂H₅OH): In the presence of acid, ethanol is oxidized to acetaldehyde, while the dichromate ion is reduced to the chromic ion.
* Sulphur dioxide (SO₂): Sulphur dioxide is oxidized to sulphate ions (SO₄²⁻) while reducing the dichromate ion.
The balanced chemical equation for the reaction with ethanol:
2 K₂Cr₂O₇ + 8 H₂SO₄ + 3 C₂H₅OH → 2 Cr₂(SO₄)₃ + 2 K₂SO₄ + 11 H₂O + 3 CH₃CHO
Key takeaway: The change in color from orange to green in acidified potassium dichromate signifies the reduction of chromium from a +6 oxidation state to a +3 oxidation state.
