Alcohol Content in an Unknown Beverage

Transcription

1 Alcohol Content in an Unknown Beverage Purpose The purpose of this experiment is to determine the alcohol content in an unknown beverage using spectrophotometry. You will learn how to use a spectrophotometer and how to make a calibration plot. Introduction There are instances when beverages are found at a scene of a crime and the investigator does not know the content of the beverage. Even if it is determined that the drink contains alcohol, exactly what is the percentage? In order to answer these questions, we will use a technique called spectrophotometry. An example of a test that uses spectrophotometry is a breathalyzer test. This instrument contains a small spectrophotometer which is used to separate light according to its individual colors. The breathalyzer test measures the alcohol level in the breath which can then be related to the blood alcohol concentration of someone suspected of drunk driving. This may seem odd to someone not familiar with chemistry, since we all know that alcohol is a clear colorless liquid. How can alcohol levels be determined using an instrument which is sensitive to color? In order to use this method, the alcohol present in the suspect s breath, blood or in a beverage reacts with an orange-yellow chemical called potassium dichromate (K 2 Cr 2 O 7 ). When the alcohol reacts with the potassium dichromate, a blue-green compound called chromium(iii) sulfate is produced. The reaction is as follows: 2K 2 Cr 2 O 7 + 8H 2 SO 4 + 3CH 3 CH 2 OH 2Cr 2 (SO 4 ) 3 + 2K 2 SO 4 + 3CH 3 COOH + 11 H 2 O orange-yellow alcohol blue-green The spectrophotometer is set to absorb the blue-green (560 nm) color of light that is produced in the reaction. Using the above reaction, it is safe to say that the more blue-green light that is absorbed by the spectrophotometer, the more alcohol present in the sample. In order to determine the precise amount of alcohol present, a calibration curve must be produced. A calibration curve shows the relationship between the absorbance of light and the concentration of a chemical in a sample. You will make four solutions where the concentration of alcohol is known. By plotting the absorbance readings on the y-axis of the graph and alcohol concentration values on the x-axis, you can use this information to determine the concentration of an unknown alcohol sample. An example of a calibration plot is given on the next page. Once you have produced this calibration plot, you will then use it to determine the percentage of alcohol in an unknown sample.

2 Alcohol concentration vs. Absorbance of Blue-Green Light A Alcohol concentration (%) Figure 1: Example of a calibration plot for known alcohol concentrations.

3 Procedure Write all data on the results page at the end of this laboratory experiment. Part I 1. Label four 150 ml or 250 ml beakers 1 to Using a 10 ml graduated cylinder, measure 5.0 ml of 0.25 M potassium dichromate (K 2 Cr 2 O 7 ) solution into each beaker. 3. Add 1 drop of 0.1 M silver nitrate (AgNO 3 ) to each beaker. Swirl the beaker immediately after addition. 4. Using a buret or automatic dispenser, add 5.0mL of 6 M sulfuric acid (H 2 SO 4 ) to each beaker. Swirl the beaker immediately after addition. Be careful, sulfuric acid can cause severe burns to your skin. 5. Add distilled water and alcohol to the beakers as indicated below: Beaker # Alcohol Concentration Alcohol Water 1 (BLANK) 0% none 20 drops 2 2.5% 5 drops of 10% alcohol solution 15 drops 3 5.0% 10 drops of 10% alcohol solution 10 drops % 20 drops of 10% alcohol solution none BE SURE TO SAVE YOUR BLANK SOLUTION FOR PART II!!!!!!! 6. Allow the chemical reaction to proceed for 5 minutes. Swirl the beakers several times during the five minutes. 7. Using the graduated cylinder, dilute the contents of each beaker by adding 39.0 ml of distilled water to each beaker. Stir each beaker with a clean stirring rod. 8. On the spectronic 20, set the wavelength number to 560 nm. 9. With the cover of the sample holder closed and no cuvette in place, turn the left knob (the zero control) until you read 0% transmittance. 10. Using your BLANK solution (beaker #1), fill the cuvette two-thirds full. Place the cuvette in the sample holder and set your instrument, using the right knob (light control) to 100% transmittance.

4 11. Remove and save your BLANK to adjust the transmittance before EVERY sample reading. 12. In another cuvette, fill it two-thirds full with the contents from beaker #2 and read the absorbance. After the reading, pour the contents of the cuvette into a waste container. (Your instructor will inform about waste at the start of this experiment.) 13. Repeat step 12 for beakers #3 and # Obtain graph paper from your laboratory instructor and create a calibration curve. Take the absorbance values and plot the corresponding alcohol concentrations. Make sure you draw your line of best fit going through the 0/0 point of the x and y axes. Part II Unknown Scenario Police arrive on the scene of a serious car accident. A car has smashed head on into a tree just off the road. Both of the people in the car are unconscious and have appeared to be out on this desolate stretch of road for many hours. It appears to be a drunk driving accident since the weather has been perfect for driving. Police do not find any sources of alcohol in the vehicle. They find a pack of cigarettes and what appears to be a water bottle filled with a clear liquid. Later the blood alcohol levels of the people in the car turn out to be normal. The police insist it had to be a drunk driving accident since there were no skid marks on the road before the point of impact. The police give you a sample of the clear liquid from the water bottle and ask you to analyze it. 1. Write the number of the unknown sample on your data sheet and label your beaker with the unknown number. 2. Repeat steps 2 through 4 from Part I. 3. To your unknown beaker add 20 drops of the unknown sample and no distilled water. 4. Repeat steps 6 through 12 as in Part I. 5. Use the absorbance value you obtain for your sample and using your calibration plot from Part I, determine the alcohol concentration in your unknown sample.

5 DATA/RESULTS Part I Beaker # Alcohol Concentration Absorbance Value 1 0% % % % Part II Unknown Number Absorbance Value Alcohol Concentration Staple your calibration curve to this page. Questions 1. Was the clear liquid an alcoholic beverage? 2. If your sample had turned out to not be an alcoholic beverage, what would be your conclusions for what happened at the time of the accident? ANSWER QUESTION #2 REGARDLESS OF WHETHER YOUR SAMPLE WAS AN ALCOHOLIC BEVERAGE OR NOT.