In the laboratory, when an aqueous solution of a primary alcohol such as ethanol is mixed with potassium dichromate (VI) and sulfuric acid, and the mixture heated under reflux, the alcohol is fully oxidized to a carboxylic acid.
The oxidation reaction of the primary alcohol (e.g. ethanol) to a carboxylic acid may be represented simply by an equation in which the symbol [O] represents the oxygen supplied by the oxidizing agent:
Alternatively, we may write half-equations and a complete equation to represent the redox nature of the reaction:
When secondary alcohols are oxidized, ketones are formed.
This reaction is very similar to the one in which aldehydes are produced, but the placement of the hydroxyl group results in the production of a ketone rather than an aldehyde and, ultimately, a carboxylic acid.
When these oxidation reactions are performed in a laboratory investigation, the change in colour of the oxidizing agent indicates that the reaction has proceeded.
Potassium dichromate, K 2 Cr 2 O 7 , changes colour from orange (Cr 2 O 72– ) to green (Cr 3+ ) during this reaction. If potassium manganate(VII), KMnO 4 , is used instead, it changes colour from purple to colourless.