Ethanol estimation by potassium dichromate method
Most of the chemical oxidation methods are based on the complete oxidation of ethanol by dichromate in the presence of sulfuric acid with the formation of acetic acid. This reaction is highly preferred because potassium dichromate is easily available in high purity and the solution is indefinitely stable in air. The reaction that occurs between alcohol and potassium dichromate is:
2Cr2O7– + 3C2H5OH + 16H+ 4Cr+++ + 3CH3COOH + 11H2O
Dichromate (Cr2O7–, Cr(VI)) is yellowish in color and the reduced chromic product (Cr+++, Cr(III)) is intensely green. Because the absorption spectra of dichromate and chromic ions overlap significantly, Beer’s law is not obeyed. Instead, the spectra of the solution of interest must be analyzed at multiple wavelengths to calculate the individual concentrations of dichromate and chromic ions in a mixture subject to the material balance that the total number of chromium atoms must be conserved.
Proper concentration of sulfuric acid in the surrounding solution will direct the oxidation of ethanol toward acetic acid instead of acetaldehyde.