Prepared by
Nam Sun Wang
Department of Chemical & Biomolecular Engineering
University of Maryland
College Park, MD 20742-2111

Table of Contents


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 popular because potassium dichromate is easily available in high purity and the solution is indefinitely stable in air. The theoretical reaction stoichiometry is shown below:
     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.

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+++ + 7H2O
To 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.

List of Reagents and Instruments

A. Equipment

B. Reagents


Prepare the dichromate reagent solution and the secondary s-Diphenylcarbazide solution.
  1. Pipet 1 ml of ethanol solution to be analyzed into of glucose sample in a lightly capped test tube. (To avoid the loss of liquid due to evaporation, cover the test tube with a piece of paraffin film if a plain test tube is used.)
  2. Heat the mixture at 90ºC for 5-15 minutes to develop the red-brown color.
  3. Add 1 ml of a 40% potassium sodium tartrate (Rochelle salt) solution to stabilize the color.
  4. After cooling to room temperature in a cold water bath, record the absorbance with a spectrophotometer at 575 nm.
      3ml                             1ml                         O.D.
    reagent  --->----+            Rochelle soln --->----++-------> at 575nm
                   | |  |                             | || |
                   |-+--|                             |-++-|
                   |    |      heat                   |    |
                   |    |    -------->                |    |
                   |    |                             |    |
                   |    |                             |    |
                   |____|                             |____|
              3ml sample soln


  1. Phenol, up to 2g/l, intensifies the color density. It changes the slope of the calibration curve of absorbance versus glucose concentration but does not affect the linearity. The above procedure yields an absorbance of 1 for 1 g/l of glucose in the original sample in the absence of phenol in the reagent, as opposed to an absorbance of 2.5 for 1 g/l of glucose in 2 g/l of phenol. This property can be exploited to achieve the maximum sensitivity for dilute samples.


  1. How much time was needed for the complete color development? Justify your answer with a plot of color intensity as a function of time.
  2. Obtain an absorption spectrum over wavelengths in the visible range (i,e. 400-700 nm). Justify the use of 575 nm chosen in the Procedure.
  3. Find one other method commonly used to determine ethanol concentration in aqueous solutions. What are the claimed advantages and disadvantages of this method.


  1. Williams, M. B. and Reese, Darwin, Colorimetric determination of ethyl alcohol, Anal. Chem., 22, 1556, 1950.

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Ethanol Assay by Dichromate Colorimetric Method
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Nam Sun Wang
Department of Chemical & Biomolecular Engineering
University of Maryland
College Park, MD 20742-2111
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e-mail: nsw@umd.edu