Question

Lab 9: Iodometric Determination of Ethanol

1. Why must wine samples be stored in the refrigerator before analysis?

2. Other procedures suggest leaving the wine sample in the flask, in a warm place, overnight before titrating the unreacted dichromate solution. Why would this procedure have the flask incubating in the over at 70 degrees for 2.5 hours instead?

3. How would the change in procedure affect results?

Answer 1

1. The wine samples must be stored in the refrigerator before analysis since a low temperature reduces the possibility of oxidation of ethanol, which is the target compound for the analysis.
2. The method utilizes a redox titration to find the concentration of ethanol in an aqueous solution. The ethanol is oxidized to ethanoic acid by an excess of dichromate in acid. Because wine contains the other oxidizable substances that could interfere with the titration, the dichromate solution is placed in a flask, and the wine sample is suspended in a small container above it. The water and the ethanol slowly evaporate, and as the ethanol comes in contact with the dichromate it first dissolves, and is then oxidized. More ethanol evaporates until eventually all the ethanol from the wine has left the sample, and reacted with the dichromate. Since this transfer is time-consuming (and, a high temperature facilitates the reaction), it is recommended to leave the flask with the suspended sample in a warm place overnight, or incubate it in the oven at 70 degrees for 2.5 hours.
3. The change in the procedure may dramatically affect the results. Since both the time and the temperature control the extent of oxidation of ethanol, if all the ethanol is not oxidized, the estimation of ethanol will be wrong.