University of Pittsburgh at Bradford
Science In Motion
Chemistry Lab 003
DETERMINATION OF WAVELENGTH OF MAXIMUM ABSORBANCE
Introduction:
Colored solutions are colored because they absorb certain wavelengths of light while allowing other
wavelengths of light to pass through. As observers, we see the wavelengths of light that are not absorbed. By
measuring the amount of light absorbed, we can find the concentration of solutions. Before doing this type of
spectral analysis, the wavelength at which absorbance is greatest needs to be determined. This is the
wavelength to be used for the analysis.
The purpose of this experiment is to demonstrate the procedure to find the wavelength of maximum
absorbance for any colored solution. The process involves recording the absorbance over the range of 350 nm
to 650 nm, usually in intervals of 25 nm. The data can be graphed to visualize the highest absorbance or the
data pairs can be examined to determine the wavelength.
Water with food color added works very well to teach this procedure. Alternatively, solutions of colored
ions may be used.
Purpose:
To find the wavelength of maximum absorbance of a colored solution using a spectrophotometer.
Equipment / Materials:
Spectrophotometer
2 cuvets
dropper bottle containing colored water
wash bottle containing distilled water
Safety:


Always wear goggles and an apron in the lab.
Gloves may be preferred to keep coloring from staining hands.
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Procedure:
1. Select a dropper bottle containing one of the four available colored solutions.
2. Hold cuvet by the rough sides. Rinse the cuvet with a small amount of the colored solution and then fill the
cuvet 3/4 full.
3. Fill the second cuvet with distilled water to be used as a blank.
4. Press A/T/C Button on the Spectronic 20 Genesys. Select absorbance.
5. Press the nm arrow up or down and select 350 nm.
6. Insert the blank into the cell holder and close the door. Position the cell so that the light passes through clear
walls.
7. Press 0 ABS/100% T to set the blank to 0 A.
8. Remove the blank and insert sample into the cell holder. The sample measurement appears on the LCD
display. Record the absorbance on the data sheet.
9. Reset the wavelength to 375 nm and repeat steps 6, 7, and 8.
10. Repeat steps 5, 6, 7 and 8, recording absorbances at every 25 nm using this technique until you reach 650 nm.
11. Locate the 50 nm region in which the absorbance is highest and record the absorbance every 10 nm in this
region repeating steps 5, 6, 7 and 8.
12. Determine the wavelength of maximum absorbance by creating a graph of the data - placing wavelength on the
x-axis and absorption on the y-axis.
Questions:
1.
What is the wavelength of maximum absorbance for each colored solution tested?
2.
What color of light corresponds to the wavelength of maximum absorbance for each colored solution
tested?
Name_______________________________
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Period______________________________
Date________________________________
DETERMINATION OF WAVELENGTH OF MAXIMUM ABSORBANCE
Data Table:
Color of solution _______________________________
wavelength
(nm)
absorbance
wavelength
absorbance
(nm)
350
375
400
425
450
475
500
525
550
575
600
625
650
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