2.2: Absorbance Spectra
- Page ID
- 38630
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RELATED READING: Pages 92-94
OBJECTIVES
Upon completion of this exercise, appropriate discussion, and related readings, the student will be able to:
- Measure absorbance values for a solution at a variety of wavelengths.
- Construct a graph plotting wavelengths vs. absorbance values.
- Identify the wavelength of maximum and minimum absorbance for each solution used.
- Explain the relationship between solution color, spectrophotometer wavelength, and absorbance.
GLOSSARY
Absorbance maxima and minima: those wavelengths at which compounds in solution exhibit minimal or maximal absorbance of light. These minima and maxima are fixed for a stated set of conditions (pH, solvent, and temperature), but may vary if these conditions vary.
Rectilinear (linear) graph paper: most comonly used graph paper, in which the X (horizontal) and Y (vertical) axes are divided into equally spaced segments.
MATERIALS
- Spectrophotometer
- Sample Solutions
- Cuvettes
PROCEDURE
- Fill one cuvette approximately 1/2 full with distilled water (water blank).
- Fill additional cuvettes approximately 1/2 full with each of the five colored solutions (sample).
- Select a spectrophotometer and adjust the wavelength to 375 nm.
- Adjust the zero control.
- Insert the water blank and adjust for zero absorbance (100% transmittance).
- Remove the blank and insert the sample cuvette containing the red solution.
- Read the absorbance and record it on the data sheet.
- Read and record the absorbance of each solution at this wavelength.
- Repeat steps 3 through 7 for each wavelength listed on the data sheet.
- Empty the sample cuvettes and rinse them with water.
- Adjust the spectrophotometer to 420 nm and blank it with distilled water.
- Insert a strip of white paper into an empty cuvette and position the cuvette in the sample well.
- Look down into the sample well and observe the color of the light that is visible on the paper strip. Record your observation on the data sheet.
- Repeat steps 11, 12, and 13 at 540 nm and 610 nm.
- Using the graph paper provided at the end of this exercise, construct a graph plotting wavelength on the X axis and absorbance on the Y axis. Plot the absorbance values for each different colored solution on the same graph., using a different colored ink or symbol for each curve.
| DATA SHEET, EXERCISE #2 |
NAME: ___________ DATE: ___________ |
TEST SolutionS
| Wavelength | RED | BLUE | GREEN | YELLOW | VIOLET |
|---|---|---|---|---|---|
| 375 | |||||
| 400 | |||||
| 425 | |||||
| 450 | |||||
| 475 | |||||
| 500 | |||||
| 525 | |||||
| 550 | |||||
| 575 | |||||
| 600 | |||||
| 625 | |||||
| 650 |
| Solution Color | Absorbance Maximum | Absorbance Minimum |
|---|---|---|
| Red | ||
| Blue | ||
| Green | ||
| Yellow | ||
| Violet |
| Color Observation | |
|---|---|
| Wavelength | Color Observed |
| 420 nm | |
| 540 nm | |
| 610 nm | |
Discussion Questions
- Why were two absorbance peaks seen with the violet and green solutions?
- Are there instrument factors that will cause variance from the curves you plotted?
- Would you expect to see identical curves produced by different spectrophotometers?
