2Cr2O7-- + 3C2H5OH + 16H+ -----> 4Cr+++ + 3CH3COOH + 11H2ODichromate (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.
Other methods of determination primarily based on the above reaction are commonly used. In these methods, any one of the reactants or products participating in Reaction (1) can be analyzed through another separate reaction. For example, the excess dichromate remaining in the solution can be further reduced by titration with other oxidizing reagents such as ferrous ammonium sulfate:
Cr2O7-- + 6Fe++ + 14H+ -----> 2Cr+++ + 6Fe+++ + 7H2OTo enhance the visualizatio of titration endpoint, organic indicators such as sodium diphenylamine sulfonate and 1,10-o-phenanthroline are added. Another similar method is based on iodometric titration.
Proper concentration of sulfuric acid in the surronding solution will direct the oxidation of ethanol toward acetic acid instead of acetaldehyde.
Or the excess dichromate can be analyzed.
3ml 1ml O.D. reagent --->----+ Rochelle soln --->----++-------> at 575nm | | | | || | |-+--| |-++-| | | heat | | | | --------> | | | | | | | | | | |____| |____| 3ml sample soln