Spectroscopy
ULTRA-VIOLET SPECTRA OF CAFFEINE IN SOFT DRINKS
(DEVELOPED BY SUSAN FIEDLER OF J.S. BATTLE HIGH SCHOOL, AND MARY LOU HEARN OF NOTTOWAY HIGH
SCHOOL, AND GARY LONG OF VIRGINIA TECH)
Introduction:
Caffeine is a naturally occurring stimulant found in coffee that is also used as an additive in many soft drinks and is
the active ingredient in No-Doze. A typical 8 oz. cup of coffee has about 110 mg of caffeine. Soft drinks can range
from 0 to 100 mg in a 12 oz can. A number of students avoid a cup of coffee in the morning but substitute a highcaffeine soft drink instead.
How can we determine the amount of caffeine in a solution? One of the tools we can use is the UV-Vis
Spectrophotometer. While there are a number of ingredients in soft drinks, by comparing the spectra of caffeinefree and regular diet Mountain Dew, we determined that the absorbance range for caffeine is around the 250 nm
range. This instrument will measure the absorbance of light in that range, and give us a comparative reading on the
amount of caffeine in the solution.
Purpose: To determine the amount of caffeine in selected sodas.
Safety: Wear safety goggles at all times on the MCL.
Procedure: For Caffeinated Mountain Dew:
1. Open up the OOI UV-Vis program on the desktop.
2. Put the black plastic cuvette in the port and press Dark, then press Store.
3. Use the large bore plastic pipette to measure 2 mL of distilled water into the quartz cuvette. Be sure to wipe off
the exterior, and avoid putting your fingerprints on the sides of the cuvette. Insert the cuvette into the port and
press Reference; then press Store. Click on Scan; your data should show a flat line at zero. (Look in the upper
right hand corner for the zero.)
4. Using the microtip pipette, add 1 drop of diet Mountain Dew with caffeine into the cuvette and stir carefully
with the glass stirring rod. Click Scan. Using the mouse cursor grab the vertical red crosshair on the screen
and move it back and forth. Then position the vertical crosshair at the most intense area of the peak between
250 – 300 nm. You must move the cross hair off and back on to the correct wavelength to get a new reading.
Take a reading of the peak by noting in the upper right hand corner of your screen the wavelength (in
nanometers, nm), and the absorbance value to the right in the box. Record your data in the data table.
5. Without removing the cuvette from the holder, using the microtip pipette add 1 drop of the caffeine standard
and stir. Click Scan. Again, move the red crosshair back and forth to get a fresh reading. Replace the crosshair
on the wavelength you recorded and write the new absorbance in your data table.
6. Repeat number 5 for 4 more drops of caffeine standard. CAUTION!! The drops of caffeine in the data table are
cumulative. Only one drop of caffeine standard for each scan.
Procedure for DeCaffeinated Mountain Dew:
7. Remove the cuvette, empty into the waste container, and rinse thoroughly with distilled water. Repeat numbers
2 and 3. Then add 1 drop of caffeine free diet Mountain Dew. Observe the graph. Is there a peak? Record the
absorbance.
8. Repeat numbers 5 and 6 using the caffeine standard, adding one drop of standard for a total of five drops.
9. Rinse cuvette thoroughly and repeat steps 2-8 for Regular Diet Coke and then decaffeinated Diet coke.
Page 1 of 3
UVVis of Soft Drinks
Spectroscopy
Data:
Wavelength: ____________ nm
Caffeinated Diet Mountain Dew
0 drops caffeine standard (1 drop soda)
Add 1 drop caffeine standard
Add 1 drop caffeine standard for a total of 2 drops
3 drops caffeine standard
4 drops caffeine standard
5 drops caffeine standard
Absorbance
Decaffeinated Diet Mountain Dew
0 drops caffeine standard (1 drop soda)
1 drop caffeine standard
2 drops caffeine standard
3 drops caffeine standard
4 drops caffeine standard
5 drops caffeine standard
Absorbance
Caffeinated Diet Coke
0 drops caffeine standard (1 drop Coke)
1 drop caffeine standard
2 drops caffeine standard
3 drops caffeine standard
4 drops caffeine standard
5 drops caffeine standard
Absorbance
Decaffeinated Diet Coke
0 drops caffeine standard (1 drop Coke)
1 drop caffeine standard
2 drops caffeine standard
3 drops caffeine standard
4 drops caffeine standard
5 drops caffeine standard
Absorbance
Page 2 of 3
UVVis of Soft Drinks
Spectroscopy
Analysis:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
On the computer, go to Excel.
Label cell A1 “Drops” and B1 “Absorbance”.
Fill in column A with 0 through 5 for the number of drops of standard used.. Fill in column B with the
corresponding absorbance from your data table.
Highlight the numbers by sweeping across them with the mouse.
Click on the chart icon at the top; the blue, yellow and red “bar” graph image on the toolbar. A box will appear
with graph type choices. This is step one of four in the Chart Wizard.
Select “XY scatter”. Click “Next.”
For step 2 of 4 just click “next”.
In step 3, fill in the title and x and y axes labels. Click “next”.
Leave graph as object in the sheet. This will put your graph and your data on the same page. Click “Finish”.
Drag the graph to left justified.
On the menu bar, click on the word “Chart”. Under “Type”, choose “linear”. (This is the default choice.)
Click on “Options” and check the boxes to “display equation” and “display R2 value”. Click “Okay”.
Drag the equation off the graph and onto the body of the chart.
You can add your names to the chart using the Text Box. (An icon with the letter A in the upper right had
corner.)
Print copies for each member of your team.
Calculations:
For each brand of soda (1caffeinated and 1 decaffeinated) we will do a process called “standard subtraction” by:
1. Using the equation from the graph, solve for x when y = 0. (Your answers will be negative.)
2. Subtract the x value of the decaffeinated soda from the x value of the caffeinated soda. (Your answer is now
positive.) This value represents the number of drops of standard caffeine that was present in 1 drop of soda.
3. Calculate ppm (parts per million) by: multiplying the answer from number 2 times 5 µg per drop and dividing
this product by 0.038 mL. Show all work on a separate piece of paper.
Caffeinated Mountain Dew:
Caffeinated Diet Coke:
4.
___________ ppm
___________ ppm
Compare this to the accepted value provided by your teacher. Percent error is found by finding the absolute
value (the accepted value minus your experimental value) and dividing the answer by your accepted value and
multiplying by 100.
% error Diet Mountain Dew __________
% error Diet Coke
_________
Conclusions:
1. Why is absorbance for decaffeinated drinks not zero?
2. How do you account for the difference in absorbance between cola drinks and non-cola drinks?
3. Why do you think we used diet drinks instead of regular drinks?
4. What was in the standard?
5. What might be sources of error for this experiment?
Page 3 of 3
UVVis of Soft Drinks