Determination of Wavelength of Maximum Absorbance
Equipment / Materials:
a piece of index card cut to fit inside a test tube
spectrophotometer
test tubes (cuvettes)
food coloring (red, blue, yellow: 1 drop/100ml)
Safety: Always wear goggles and an apron in the lab.
Procedure:
Part One- Observing the visible spectrum
1. Place a piece of index card in a test tube.
2. Set the wavelength knob to 525 nm.
3. Place the test tube in the spectrophotometer so the index card is perpendicular to the front of the
Spec 20.
4. If you can see a small spot of green light on the index card, proceed to step 5. If not, turn the
transmittance (Right Front) adjustment knob clockwise until green light is visible.
5. Set the wavelength knob to 350 nm.
6. Look down into the sample compartment and record on the data sheet the Observed color of the
light striking the paper on the data table. If no color is visible, simply write “no color.”
7. Now set the wavelength knob to 375 nm and record the color. Repeat this every 25 nm until you
reach 650 nm.
Part Two – Determining the wavelength of maximum absorbance
1. Fill one test tube ¾ full with the blue food coloring solution, one with red food coloring solution,
and one with yellow food coloring solution.
2. Fill a second test tube with distilled water to be used as a blank.
3. Set the spectrophotometer to 350 nm.
4. With the sample compartment empty and the lid closed, adjust the left front knob until it reads 0%
transmittance on the left side of the meter.
5. Place the blank into the unit and close the lid. Adjust the right front knob until it reads zero
absorbance on the right side of the meter.
6. Remove the blank and place the cuvette containing the blue colored solution into the
spectrophotometer. Close the lid. Record the absorbance on the data sheet. Then record the
absorbance of the red solution and then the yellow solution.
7. Reset the wavelength to 375 nm and repeat steps 4, 5, and 6. Record absorbances every 25 nm
until you reach 650 nm.
Data Table:
Wavelength ()
350
375
400
425
450
475
500
525
550
575
600
625
650
Observed color
Absorbance
Absorbance
Absorbance
blue
red
yellow
Name of lab partners____________________________________________
Lab Worksheet for Spec 20 lab
Graphs
You will draw three graphs, one each for the blue, red, and yellow food coloring. The x-axis will be
Wavelength in nanometers, and the y-axis will be Absorbance. Absorbance does not have a unit. Then
graph your points from the data table. Staple your graphs to this paper.
Questions:
1. In this lab we only looked at visible light. What would be an approximate wavelength of infrared
light?
2. What would be an approximate wavelength of ultraviolet light?
3. What color of light corresponds to the wavelength of maximum absorbance for each solution
tested? Is this what you would expect? Explain in detail.
4. If we tested a violet colored dye, what would you expect the spectrum to look like?