1
Breathalyzer Reaction
Introduction
A breathalyzer (breath analyzer) is a device for estimating blood alcohol content (BAC) from a
breath sample. The invention of the breathalyzer provided law enforcement with a non-invasive
test providing immediate results to determine an individual's BAC at the time of testing. It does
not, however, determine an individual's level of intoxication, as this varies by a subject's
individual alcohol tolerance. The BAC test result itself can vary between individuals consuming
identical amounts of alcohol, due to gender, weight, and genetic pre-disposition.
Breath analyzers do not directly measure blood alcohol content or concentration, which requires
the analysis of a blood sample. Instead, they estimate BAC indirectly by measuring the amount
of alcohol in one's breath.
There are several types of breathalyzer devices. Regardless of the type, each device has a
mouthpiece, a tube through which the suspect blows air, and a sample chamber where the air
goes. To measure alcohol in the breath, a suspect breathes into the device. The breath sample
is bubbled into a vial which contains a mixture of chemicals that react with alcohol. The reaction
of the alcohol results in a color change of the solution in the vial; the degree of color change is
directly related to the level of alcohol in the expelled air. To determine the amount of alcohol in
that air, the color of the reacted mixture is compared to the color of an unreacted mixture in
another vial. The breathalyzer device then gives a readout that indicates the degree of color
change and, hence, the level of alcohol in the suspect’s breath.
Ethanol, the alcohol present in alcoholic beverages, has the chemical formula C2H5OH. There
are many other chemicals that are classified as alcohols, but they have different names and
chemical formulas. For example, rubbing alcohol is called isopropanol and has the chemical
formula C3H7OH.
Continued on next page…
2
The Chemical Reaction Used by the Breathalyzer
Ethanol reacts with dichromate ion in acidic solution according to the chemical equation shown
below. In this reaction, dichromate ion is converted to chromium ion. In solution the dichromate
ion has an orange color, but the chromium ion is blue-green. This reaction is easily observed
because the orange color disappears as dichromate ion is used up, and the blue-green color
appears as chromium ion forms.
2-
2 Cr2O7
+
dichromate ion
orange color
3 C2H5OH
ethanol
colorless
+
+
3+
16 H

4 Cr
+
acid
chromium ion
colorless blue-green color
3 C2H3O2H
acetic acid
colorless
+
11 H2O
water
colorless
If there is enough ethanol present to react with all of the dichromate ion and use it up in the
reaction, the orange color will disappear entirely and the solution will become blue-green. On
the other hand, there may be only a little ethanol present relative to the amount of dichromate
ion present—in other words, there is not enough ethanol to react with and use up all of the
dichromate ion. In this case, some orange-colored dichromate ion remains after the reaction,
along with some blue-green chromium ion that has formed due to the reaction with the small
amount of ethanol present. This mixture of orange and blue-green ions will produce a solution
that is a golden yellow color. The exact shade of golden yellow will depend on the relative
amounts of orange dichromate and blue-green chromium ions in the mixture. For instance,
more dichromate will give the solution an orange-yellow color; more chromium ion will give the
solution a greenish-yellow color.
In the breathalyzer test, the breath sample is bubbled into a vial which contains a solution of
dichromate ion. Any ethanol in the breath sample reacts with the dichromate ion according to
the reaction above, and the solution changes to a shade of golden yellow as a result of the
reaction. The reacted mixture in this vial is compared to another vial which contains dichromate
ion but no ethanol (so it will be bright orange). The difference in color between the two vials
gives an indication of the amount of ethanol in the suspect’s breath.
In this experiment, you will carry out the breathalyzer reaction in test tubes. Rather than
determining the amount of ethanol in a breath sample, you will determine the amount of ethanol
in a wine sample.
3
Breathalyzer Reaction
Prelab
Name________________
1. The concentration of alcohol in one’s ___________________ is directly proportional to the
concentration of alcohol in one’s blood.
2. The device used to measure the alcohol content of a person’s breath is the
_____________________.
3. There are many different compounds that are classified as alcohols. For example, the
alcohol found in rubbing alcohol is a different chemical compound than the alcohol present in
alcoholic beverages. Ethanol is the name of the alcohol in alcoholic beverages. What is the
chemical formula for ethanol?
4. The color of dichromate ion in solution is ___________________.
The color of chromium ion in solution is _____________________.
5. Which chemical should you handle with care in this experiment (see next page)?
What is the first thing you should do if you get some on your skin?
4
Breathalyzer Reaction
Procedure
Caution!
The chromic acid solution (which contains the dichromate ion) used in this
experiment is very corrosive and a strong oxidizing agent. Be very careful in handling
and pouring this solution. Wear your safety glasses at all times. If you happen to spill
some of this solution on yourself, immediately wash the affected area with plenty of cold
water, then report the accident to the instructor. If you spill some of the solution on the
lab bench, inform the instructor so that he/she can clean up the spill.
A. Preparation of Color Standards for Comparison to the Wine Sample
1. Note: It is important that volumes be measured accurately and precisely! For a given
step, use the type of pipet indicated. If you are unsure which type of pipet to use, or how
to use it, ask the instructor. Use the 10 mL pipets provided to place the following amounts of
distilled water and 0.1% ethanol solution into 11 large test tubes. Measure the volumes very
carefully, because the accuracy of your measurements will determine how well this experiment
turns out! When you are finished, each tube will contain a total of 10 mLs of liquid.
Test tube #
1
mLs ethanol solution: 0
mLs distilled water: 10
2
1
9
3
2
8
4
3
7
5
4
6
6
5
5
7
6
4
8
7
3
9
8
2
10
9
1
11
10
0
Mix the contents of each large test tube well with a glass stirring rod. If you stir the test tubes
beginning with tube #1 and working up to tube #11 in order, there is no need to clean the stirring
rod between test tubes.
2. Label 11 small test tubes with the numbers 1 to 11 using a wax pencil (do not try to use tape
as labels). Using a 1 mL volumetric pipet, take 1 mL of liquid out of each of the large test tubes
and place it into the small test tube with the corresponding number. If you begin with tube #1
and work in order up to tube #11, there is no need to clean the pipet between test tubes. After
this, you will have a set of 11 small test tubes, each containing 1 mL of liquid.
3. Obtain a new Pasteur pipet and use it to add 20 drops of H2SO4 (sulfuric acid) to each of the
small test tubes. Then obtain another new Pasteur pipet and use it to add 1 drop of chromic
acid solution (containing the dichromate ion) to each test tube. When finished, save the two
pipets because you will need them later. Mix the contents of the small test tubes well with a
glass stirring rod by beginning with tube 1 and working up to tube 11 in order (then there is no
need to rinse the stirring rod between tubes).
4. Set up a hot water bath by filling a large beaker about 1/4 full with distilled water and placing
it on a hot plate. Heat the water to ~80 oC. If it becomes too hot (~90 oC or higher), add a little
cold water to cool it down. Try to find a setting on the hot plate that maintains the temperature
near 80 oC. Then place the 11 small test tubes into the water bath and heat them for 5 minutes.
5
5. Remove the test tubes from the water bath and observe the colors of the solutions. Record
your observations on the report sheet. Keep the water bath set up because you will need it later.
B. Determination of the Ethanol Content of the Wine Sample
The 11 small test tubes contain known amounts of ethanol since you carefully measured the
amount of the 0.1% ethanol solution that went into each tube; the table below shows the %
ethanol in each test tube. The 11 test tubes provide a set of color standards so that you can see
the color produced as increasing known amounts of ethanol react with the dichromate ion. The
more ethanol in a test tube, the fewer orange dichromate ions remain after the reaction, and the
more blue-green chromium ions produced (see the chemical reaction in the introduction).
Test tube #
% ethanol by
mass-volume
1
0.00
2
3
0.01 0.02
4
5
0.03 0.04
6
0.05
7
8
9
10
11
0.06 0.07 0.08 0.09 0.10
Note: % ethanol by mass-volume means: mass of ethanol (in grams) per 100 mL of liquid.
Example: 0.1 gram of ethanol in 100 mL of liquid:
(0.1 g / 100 mL) x 100% = 0.1%
Next you will mix chromic acid with a wine sample and observe the color that results when the
ethanol in the wine reacts with the dichromate ion. The amount of ethanol in the wine is
unknown. In order to determine the percentage of ethanol in the wine, you will compare the
color of the wine sample to the 11 test tubes containing known amounts of ethanol. For
instance, if the wine sample has the same shade of color as test tube #4, you can conclude that
the wine sample contains 0.03% ethanol by mass-volume (see table above).
1. Use a clean 1 mL volumetric pipet to place 1 mL of the diluted wine into another small test
tube. (Note: the wine has been diluted 100-fold with water by the stockroom manager.) Use
the Pasteur pipet you saved from step 3 above to add 20 drops of H2SO4 (sulfuric acid) to this
test tube. Then use the other Pasteur pipet you saved to add 1 drop of chromic acid solution to
the test tube. Stir the contents of the tube with a glass stirring rod. Place the tube in the hot
water bath at 80 oC for 5 minutes.
2. Compare the wine sample test tube to the 11 standards and determine which one it best
matches. Record this on the report sheet. Then calculate the concentration of ethanol in the
wine by following the steps on the report sheet.
3. Discard the contents of all small test tubes (which contain chromium, a heavy metal) in the
waste bottle provided by the instructor. Discard the contents of the large test tubes (which
contain only water and ethanol) down the drain.
6
Breathalyzer Reaction
Report Sheet
Name __________________
A. Colors of Ethanol Standards
Describe the pattern of color you observe in moving from tube #1 (no ethanol) to
tube #11 (highest % of ethanol).
(Here is an example to guide you: Tube #1 is colorless, and tubes #2 - 11 are blue— the blue
color is very pale in tube #2 but becomes increasingly darker in progressing from tube #2 to tube
#11.)
B. Ethanol Content of the Wine Sample
1. The diluted wine sample best matches the color of test tube # ________.
2. % ethanol by mass-volume in diluted wine sample:
(Based on color comparison; see table on p. 5.
% ethanol by mass-volume means the number of
grams of ethanol in 100 mL of wine. See p. 5 for
an example.)
_________ %
3. % ethanol by mass-volume in undiluted wine:
(#2 X 100 since the wine was diluted 100-fold
by the stockroom manager)
_________ %
4. % ethanol by volume in undiluted wine:
(#3 ÷ 0.789)
(% ethanol by volume means the number of
mLs of ethanol in 100 mL of wine.)
__________ %
5. Proof of the undiluted wine:
(#4 X 2)
(“Proof” means twice the % alcohol by volume.
Therefore, 100% ethanol would be 200 proof.)
__________ %
7
Breathalyzer Reaction
Postlab
NAME__________________
1. In the last calculation on the report sheet you determined the proof of the wine. What does
“proof” mean with respect to alcoholic beverages?
A beverage that is 180 proof contains _______ % alcohol by volume.
Does the proof you calculated agree with the usual proof range for wine of 16-30? ________
2. In all the alcohol solutions you prepared today, there was an excess of dichromate (in other
words, there was more dichromate than ethanol). Therefore, there was dichromate “left over”
after the reaction.
Assume you have two alcohol solutions, described below, and you add dichromate to each of
them.
Solution 1 has twice as much ethanol as dichromate.
Solution 2 has three times as much ethanol as dichromate.
What color do you expect solution 1 to turn after heating it in a water bath? _____________
What color do you expect solution 2 to turn after heating it in a water bath? _____________
If the labels came off so that you didn’t know which tube contained solution 1 and which
contained solution 2, would you be able to tell them apart by their colors?
YES or NO
Explain your answer.
3. Imagine that you have a wine sample containing 14% ethanol by volume. After adding
excess dichromate and heating the sample, it turns a yellowish color.
Suppose that you prepare a sample of a different wine which also contains 14% ethanol by
volume. However, this wine contains another compound (besides the ethanol) that is able to
react with the dichromate ion and convert it to Cr3+. After heating this sample, what do you
expect to observe? Check the correct answer:
_____ It will be more orange than the first sample, appearing to have less
ethanol than the first sample (less than 14%).
_____ It will be the same yellowish color as the first sample, appearing to have
the same amount of ethanol as the first sample (exactly 14%)
_____ It will be more greenish-yellow than the first sample, appearing to have more
ethanol than the first sample (more than 14%).
EXPLAIN below why you chose the answer you did.