Student Handout

Spritney Beers Blows it Bigtime

The big news is that celebrity singer, Spritney Beers has been arrested for driving under the influence (DUI). Spritney blew a “0.08%” on the Breathalyzer™ when she was stopped by the police after driving over the center line. This value reflects the blood alcohol concentration (BAC) and, in California, it is the value that legally designates intoxication. Despite considerable evidence that the Breathalyzer™ test gives an accurate approximation of the BAC, there is still much discussion about the validity of this test. While in jail, Spritney used her one phone call to contact her attorney.

A court date has been set for three weeks from today. Paparazzi photographed her as she left the courthouse in a heated discussion with her attorney. Reporters at the scene quoted him saying “I am very suspect of the validity of the Breathalyzer™ test given to my client. I only have to establish reasonable doubt in this case.”

Back at school, you are discussing the case with a classmate when your science teacher interrupts and decides that the topic is worthy of a class debate. To prepare for the class debate, you decide to learn how a Breathalyzer™ test is used to determine the BAC so that you can debate whether it is accurate or not.

 
Spritney Beers’ level of alcohol intoxication was assessed by measuring the concentration of ethanol she expelled from her lungs. This technique works because a water-based molecule like ethanol that is absorbed from the gut into the bloodstream will reach the lungs, where it is exhaled as a vapor in the air.

1. Diagram the path that alcohol will take from the gut (stomach & small intestine) to the lungs via the circulatory system. Which membranes will ethanol have to cross? Although some alcohol is exhaled from the lungs, the rest stays in the blood. Where does the ethanol in the blood go once it leaves the lungs?

Ethanol is dissolved in the blood and is distributed to organs around the body. It is a volatile molecule and can be vaporized quite easily. In the lung, ethanol is converted from a liquid to a gas, so it can be exhaled in the air.

2. Identify and describe the chemical and physical properties of ethanol that contribute to its volatility. It will help to draw the chemical structure of ethanol.

3. Where in the lung is ethanol vaporized? What role does the lung play in the vaporization of ethanol?

The Breathalyzer™ can approximate a person’s blood alcohol concentration (BAC) because the concentration of alcohol vapor in the lungs is directly related to its concentration in the blood. Alcohol vaporizes in the air sacs (alveoli) of the lungs and achieves an equilibrium with the concentration of alcohol that is still in the blood.

4. What is meant by equilibrium? Are ethanol molecules still moving across the membrane between the capillary and the alveolus?

5. What would happen to the equilibrium if some of the ethanol leaves the alveolar sac by exhalation?

When a person exhales into a breath analyzer such as the Breathalyzer™ tube, the exhaled alcohol reacts with compounds in the Breathalyzer™ chamber to produce a change in color from orange to green. The chemical reaction indicated by the color change involves oxidation and reduction. Silver nitrate catalyzes the reaction

6. What is oxidized in the Breathalyzer™? What is reduced? What role does the catalyst play in the chemical reaction?

The degree of the color change indicates how much alcohol is present in the expired air and the instrument calculates an actual concentration. However, for the Breathalyzer™ to calculate how much alcohol is present in the expired air sample it must take into account the volume of blood from which the alcohol originated. There is a standard way of describing this relationship; it is called the blood-to-breath ratio or partition ratio. The average blood-to-breath ratio is 2100:1 and this is the value used for legal purposes. The ratio assumes that an equilibrium exists between the blood and the alveolar air.

7. Explain what this ratio means in terms of the concentration of alcohol in the blood and the breath. Does the ratio change as one exhales?

Actually, the ratio can vary between 1500:1 and 3000:1 depending upon a number of factors including a person’s age, gender, and genetic makeup. In Spritny Beers’ case, the reported BAC was 0.08%, based on the 2100:1 ratio. In fact, she may be anywhere in this range from 1500:1 to 3000:1.

8. Calculate the underestimation and overestimation of Spritney’s BAC assuming she had a blood-to-breath ratio of 1500:1 and 3000:1. Would Spritney still be considered legally intoxicated based on your answers?

As a point of discussion, consider the advantages and disadvantages of using the Breathalyzer™ test and decide for yourself whether you would prosecute based on the Breathalyzer™ evidence in this case. Is there other evidence to consider?