Method

     2Cr2O7-- + 3C2H5OH + 16H+ -----> 4Cr+++ + 3CH3COOH + 11H2O

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

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

• Test tubes
• Pipets
• Spectrophotometer

B. Reagents

• Potassium Dichromate Reagent Solution
  • Potassium dichromate: 1 g
  • Add concentrated (6N) sulfuric acid to: 100 ml
  • Shake to dissolve; heat slightly if necessary
• Saturated s-Diphenylcarbazide solution in 95% ethanol
  • s-Diphenylcarbazide: ? g to saturate
  • 95% ethanol: ? ml
  • Collect supernatant

Procedures

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
   

Notes

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.

Questions

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.

References

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