32 Breathalyzer

							    32 The Breathalyzer™ Test for Alcohol
Purpose
    Explore the chemical oxidation of ethanol by acidic dichromate as used in
    Breathalyzer™ tests for alcohol.

Background
    Modern breath analyzers rely on infrared spectroscopy and/or fuel cell technology
    (electrochemical oxidation) to measure alcohol content in exhaled air. This was not
    always the case. The first commercial instrument for estimating a person’s blood
    alcohol content (B.A.C.) by analyzing a breath sample was introduced In 1954 by Dr.
    Robert Borkenstein. Named the Breathalyzer, its operation was based on redox
    chemistry and absorption photometry. In this lab the chemical reaction used in the
    original breathalyzer will be explored.

    Ethanol (ethyl alcohol) can be oxidized to acetic acid by the action of dichromate as
    shown in the following equation:
                        4 Cr3++ 3CH3COOH + 11 H2O
        3C2H5OH + 2Cr2O72-                                       Eq 1
    Because the reaction is fairly slow, silver nitrate is added as a catalyst to reduce
    reaction time.

    As ethanol is oxidized, the yellow orange dichromate ion is reduced to chromium ion
    giving the solution a greenish color. Beer’s law can be applied to find the
    concentration of dichromate in a solution using a colorimeter to measure the light
    absorbance of the solution. The concentration of dichromate remaining in a solution
    after reacting a known quantity of dichromate with an unknown quantity of ethanol can
    be used to calculate the quantity of ethanol in the unknown.

Materials


             Equipment
PASPORT Xplorer GLX
Beaker, 400 mL (2)


PASPORT Colorimeter
Beaker, 100 mL


PASPORT Extension Cable
Beaker, 50 mL


Volumetric flask, 100 mL
Graduated pipette, 5 mL


Erlenmeyer flasks, 125 mL (7)
Graduated pipette, 10 mL


Hot plate (2)
Plastic beral pipette
              Consumables
Potassium dichromate solution
15% Sulfuric acid


Silver nitrate solution




Safety Precautions
Wear chemical splash goggles, chemical-resistant gloves and apron
    •     Avoid contact with eyes skin or clothing
    •     Notify the instructor immediately of any spills or contact with the solutions

Procedure
Obtain about 40 mL of standard potassium dichromate stock solution in a 100 mL Beaker. Record the exact
concentration of dichromate in your data table.
    1)    Obtain about 15 mL of an ethanol solution of unknown concentration in a 50 mL
          beaker. Record the identification number of the unknown solution in your data
          table.

    2)    Label 7 125 mL Erlenmeyer flasks with the following: 1 mL, 2 mL, 3 mL, 4 mL, 5
          mL, unknown A, and unknown B.

    3)    Prepare 2 hot water baths in 400 mL beakers. Maintain the temperature at 75°C
          to 80°C.

    4)    Clean a 100 mL volumetric flask with soap and water, then rinse several times
          with tap water followed by several rinses with distilled water.

    5)    Using a 10 mL graduated pipette, transfer 5.0 mL of the potassium dichromate
          solution to the 100 mL volumetric flask. Fill the flask about half-full with 15%
          H2SO4 solution. Mix the solution by swirling. Using a plastic pipette, add about 1
          mL of silver nitrate solution to the flask and mix again. If any white precipitate is
          visible, stopper the flask and shake the solution until the precipitate dissolves.

    6)    Using a 5 mL graduated pipette, transfer 1.0 mL of ethanol solution to the
          volumetric flask and swirl to mix. Fill the flask with 15% H2SO4 to the calibration
          mark. Stopper the flask and mix the solution by inverting the flask several times.
          Transfer this solution to the flask marked “unknown A”. Place the flask containing
          the unknown into a water bath being careful to avoid getting water from the bath
          in the flask.
7) Rinse the volumetric flask thoroughly with distilled water and repeat the procedure
   using 2.0 mL of the same ethanol solution. Transfer the resulting solution to the
   flask marked “unknown B” and place in the second water bath. Leave the unknown
   solutions in the water baths for 45 minutes, then remove the flasks from the water
   baths and allow them to cool to room temperature.

8) While the unknown solutions are in the water baths, prepare a set of standard
   solutions:

     •     Using a 10 mL graduated pipette, transfer 1.0 mL of dichromate solution to a
         rinsed 100 mL volumetric flask. Fill the flask about half full with 15% H2SO4
         solution and mix well being sure that any white precipitate is dissolved.

     •     Fill the volumetric flask to the calibration mark with additional 15% H 2SO4
         solution. Mix well and transfer the solution from the volumetric flask to the flask
         labeled “1 mL”.

     •    Rinse the volumetric flask thoroughly with distilled water and prepare a new
         solution following the same steps but adding 2.0 mL of dichromate solution.

     •    Follow the same procedure to prepare solutions containing 3.0 mL, 4.0 mL and
         5.0 mL of dichromate solution.

     •     Based on the concentration of the stock dichromate solution, calculate the
         concentrations of the five standard solutions and record the values in your data
         table.

Xplorer GLX Setup
1)   Connect the Extension Cable to
     Port #1 on the Xplorer GLX.

2)   Connect the Colorimeter to the
     PASPORT Extension Cable.

3)   Turn on the Xplorer GLX.

4)   Set the sampling mode to Manual Sampling:

     •    From the Home screen press  to open the Sensor screen.

     •    Press , ,  to select Manual Sampling.

     •     On the Data Properties screen, use  to scroll to the Number Of Digits
         field. Press  twice to set the number of digits to 2, then press  to accept.
   5) On the Sensors screen use the arrow keys to scroll through the list of
      measurements. Using the  key to toggle the Visible/Not Visible field, set Blue
      (468 nm) Absorbance to visible and all other measurements to Not visible.

   6) Fill a clean cuvette with 15% H2SO4 solution. Cap the cuvette and wipe the outside
      with a lint free tissue to remove any smudges. Place the cuvette in the colorimeter
      being sure to align the arrow on the cap of the cuvette with the screw on the
      colorimeter. Close the lid of the Colorimeter tightly and press the Calibrate button
      on the Colorimeter.

   7)   When the green calibration light on the Colorimeter goes out, Remove the “blank”
        cuvette from the colorimeter.

   8)   Empty the solution used during calibration from the cuvette. Using the 1.0 mL
        dichromate solution, rinse the cuvette twice with approximately 1.0 mL of the
        solution from the flask, and then fill the cuvette with about 6.0 mL of the 1.0 mL
        dichromate solution. Cap the cuvette. Gently tilt the cuvette back and forth to
        distribute the solute molecules.

   Note: Do not shake the cuvette or create air bubbles in the sample.

   9) Thoroughly wipe the outside of the cuvette with a tissue to remove smudges and
      place the cuvette in the Colorimeter. Close the Colorimeter lid.

   10) From the Home Screen, Open the Digits display ().
Record Data
   1)   Press Record to begin recording data. A flashing flag will appear in the upper
        right corner of the screen.

   2)   Press  to record the first value of absorbance. When prompted, type the
        concentration of the sample in the Colorimeter into the Keyboard Data field and
        press .

   3)   Open the Colorimeter lid and remove the cuvette. Discard the cuvette contents as
        directed by your instructor. Using the 2.0 mL dichromate solution rinse the cuvette
        twice with approximately 1.0 mL of solution from the flask and then fill the cuvette
        with 6.0 mL of the solution. Cap the cuvette.

   4)   Thoroughly wipe the outside of the cuvette with a tissue and place the cuvette in
        the Colorimeter. Close the Colorimeter lid.

   5)   When the new value of absorbance becomes stable, press  again to record
        the second value of absorbance.
6)   Repeat the three previous steps for each of the remaining dichromate
     solutions(3.0 mL, 4.0 mL and 5.0 mL).

7)   When you have finished measuring the absorbance of the known solutions of
     dichromate, press  again to end data collection.

8)   Open the Colorimeter, remove the cuvette, and discard the cuvette contents as
     directed by your instructor.

The next step is to find the absorbance of the solutions with the unknown
concentrations.

1)   From the Home screen, open the Graph display ().

2)   In the Tools menu (), select Linear Fit ().

3)   Open the Tools menu again and select the last option, Create Calculation from
     Linear Fit. The calculator screen will open with an equation already displayed.
     Press  to accept the calculation.

4)   Return to the Home screen and open the Digits display.

5)   Press  to highlight the active fields.

6)   Set the top data source to Blue (468 nm) Absorbance and the lower data source
     to Linear Fit Calculation.

7)   Rinse the cuvette twice with ~1.0 mL of “unknown A” solution then fill the cuvette
     with about 6.0 mL of the unknown solution. Cap the cuvette.

8)   Thoroughly wipe the outside of the cuvette with a tissue and place the cuvette into
     the Colorimeter. Close the Colorimeter lid securely.

9)   Press  to view the value of the unknown solution. When the value stabilizes,
     press  to record the value (Linear Fit Calculation = Concentration of the
     unknown solution).

10) Press  to end data recording. Record the values of Absorbance and
    Concentration of the unknown solution in your data table.

11) Repeat the previous 4 steps to record the absorbance and concentration of
    “Unknown B”.
Analyze
    Record calculations in your lab notebook as you complete your analysis.

    Data Table
Solution
ID
nu
mb
er

                                     Molarity
Un
kn
ow
 n
Et
ha
nol
sol
uti
on




Co Ab
 nc so
ent rb
rat an
ion ce




Po
tas
siu
 m
dic
 hr
om
ate
sto
ck
sol
uti
on




1.0
mL
Po
tas
siu
 m
dic
 hr
om
ate




2.0
mL
Po
tas
siu
 m
dic
 hr
om
ate




3.0
mL
Po
tas
siu
 m
dic
hr
om
ate




4.0
mL
Po
tas
siu
 m
dic
 hr
om
ate




5.0
mL
Po
tas
siu
 m
dic
 hr
om
ate




Un
kn
ow
 n
 A
(di
ch
ro
ma
te
ex
ce
s)




Un
kn
ow
 n
 B
(di
ch
ro
ma
te
ex
ce
s)




    Analysis Questions
The concentrations of dichromate in the unknown solutions is the excess dichromate remaining after the ethanol in the
solution was oxidized. Calculate the number of moles of unreacted dichromate in 100 mL of reaction solution for each
“unknown” solution. Calculate the number of moles of dichromate in 100 mL of reaction mixture before the ethanol
was added.
    1)   Calculate the number of moles of dichromate that were consumed by the reaction
         in each unknown mixture.

    2)   From Eq1, calculate the number of moles of ethanol which would have reacted
         with the dichromate in each reaction mixture. Using the molar mass of ethanol
         and the volume of ethanol used in each reaction mixture, calculate the ethanol
         content, in grams of ethanol per 100 mL of solution for the ethanol solution used.
         Find the mean of this final result for the two unknown solutions.
3)   What are possible sources of error in this activity?

4)   Would each of the errors that you listed contribute to a higher or lower calculated
     molarity of the unknown solution?