Determination of percentage ethanol in aqueous solution through

Reduction-oxidation titration

Purpose:

-To determine the volume % concentration of ethanol in aqueous solution through an oxidation/reduction reaction

Background Information:

Determining the percentage of ethanol, whether it be in a sample of wine, beer, purified ethanol solution for alternative fuel, or commercial ethanol solution for scientific sale, is an important industrial process. Chemists and engineers in quality control departments for a number of companies utilize the set of reactions below to monitor purity, process and reactivity in their products. With ethanol production for fuel use drastically increasing across the United States, new experimental techniques for producing the largest yield of ethanol are tested daily. Questions about the energy efficiency of ethanol production have been raised, however, as purification and distillation techniques still require immense amounts of heat and energy.

This method uses a redox titration to find the concentration of ethanol in an aqueous solution. The ethanol is oxidized to ethanoic acid by reacting it with an excess of potassium dichromate in acid:

2 Cr2O72−+ 16 H++ 3 C2H5OH → 4 Cr3++ 11 H2O + 3 CH3COOH

The amount of unreacted dichromate is then determined by adding potassium iodide solution which is also oxidized by the potassium dichromate forming iodine:

Cr2O72−+ 14 H++ 6 I−→ 2 Cr3++ 3 I2+ 7 H2O

Soluble starch solution is added to the final solution, as elemental iodine forms a blue-black colored complex with starch. The iodine is then titrated with a standard solution of sodium thiosulfate. The titration endpoint can be visualized when the color due to the complex clears, as all of the elemental iodine is reacted with thiosulfate. The titration results are used to calculate the ethanol content of the original solution: according to the following equation:

2 S2O32−+ I2→ S4O62−+ 2 I−

Materials:

.01 M K2Cr2O7 in 5M H2SO41.0% starch solution

.03M Na2S2O3 solution1.2M KI solution

2-10 mL graduated cylinderburet and buret clamp

2-250 mL Erlenmeyer flask10.0 mL Ethanol sample

Procedure:

  1. Obtain a 10.0 mL ethanol sample from your instructor with a volumetric pipette.
  2. Dilute your 10.0 mL ethanol sample using a 1:10 dilution scheme in a 250 mL Erlenmeyer flask (10.0 mL ethanol in 90.0 mL of distilled water, for a total of 100.0 mL).
  3. Transfer 1.0 mL of the ethanol sample into a separate 250 mL Erlenmeyer flaskwith a volumetric pipette. Add 20.0 mL of the acid dichromate solution to this flask with a volumetric pipette. Swirl carefully in order to completely oxidize the sample. Allow this solution to sit for 10 minutes to complete oxidation.
  4. Add 100.0 mL of water to your oxidized sample. Then, add 1.0 mL of KI solution to react the remaining dichromate in solution with a volumetric pipette.
  5. Allow this solution to sit for 5 minutes to complete the reaction.
  6. Add an additional 1.0 mL of KI solution to your flask with a volumetric pipette, to ensure complete reaction. Your solution should be dark brown.
  7. Rinse a buret carefully with .03M Na2S2O3 solution. Then, fill the buret with .03M Na2S2O3 solution. Open the buret to allow solution to be dispensed, to a point where you can read the initial volume of the solution. Close the buret.
  8. Begin titrating your solution slowly. As the brown color of the solution fades to yellow, add 1 mL of starch solution. Continue to slowly titrate until the blue color begins to fade. Add the thiosulfate drop by drop until the blue color completely disappears. This is your endpoint.

Data and Calculations:

  1. Place all of the following information in a displayed data table, and include the calculations in a calculations section:
  2. Calculate the number of moles of thiosulfate used during titration, the number of moles of elemental iodine reacted, the number of moles of excess dichromate reacted, the total number of moles of dichromate added to your initial solution, and the number of moles of dichromate that initially oxidized the ethanol.
  3. From this data, you can calculate the number of moles of ethanol oxidized, the number of grams of ethanol oxidized, and the number of mL of ethanol oxidized.
  4. Your initial alcohol sample was diluted several times during the course of the experiment. Work backwards, and figure out how many mL of ethanol were in the original sample. Assuming the remainder of the sample is water, calculate the % by volume of alcohol in your original sample.

Conclusion:

  1. Given the actual % volume of your sample, calculate a percentage of error.
  2. In any sample of aqueous ethanol, be it beer, wine, or commercial fuel, there are other impurities, such as sugars, acids, and other organics that can potentially be oxidized as well. Discuss how this affected the accuracy of your results.
  3. Would it have been possible to conduct the same reduction/oxidation reactions using an alternate salt – instead of KI, utilizing KCl or KBr? Explain your answer.
  4. Discuss whether or not this oxidation/reduction technique could be performed on commercial gasoline to test the % ethanol added to our fuel.