Purpose: To develop an absorption spectrum and calibration curve on a spectrophotometer. To determine whether Beer's law applies to the solution being analyzed.
Materials:
Procedure:
1. Turn on the spectrophotometer and allow 30 minutes for the machine to warm up. Follow the attached instructions for information on operating the Spec 20.
2. Obtain an approximately 150-175 mL sample of 0.1 M cobalt chloride solution.
3. Determine the absorption spectrum of the solution, taking readings at 480, 490, 500, 510, 520, 530, and 540 nm. Take additional readings at regular (5 nm) intervals in the region of maximum absorbance.
4. a) Be certain to set 100% absorbance with no cuvette in the cell holder.
b) Set 0% absorbance when the blank, distilled water, is in the cell holder (after wiping the cuvette).
c) Wipe the cuvette containing the sample and record the absorbance.
5. Plot the absorbance spectrum with the wavelength on the x-axis and the absorbance on the y-axis, and determine the wavelength of maximum absorbance.
6. Use the wavelength selected from your graph to prepare a calibration curve.
7. Prepare five dilutions of the stock solution that give approximately equal space absorbance values between 0.1 and 0.5. Use concentrations of 0.04, 0.05, 0.06, 0.07, and 0.08 M. Use volumetric glassware to make the dilutions. (You may want to prepare a table to keep track of quantities of stock solution needed to attain specific concentrations.)
8. Graph a calibration curve with varying concentrations on the x-axis, and absorbance on the y-axis.
Questions:
1. (5 points) Does cobalt chloride follow Beer's Law at the wavelength selected?
2. (10 points) Why must you be certain to wipe the cuvette dry before each reading? Why do you have to do this even when there might be only one drop of distilled water on the surface, and this obviously has no absorbance of its own?
3. (5 points) Why is the Spec 20 a good instrument for measuring the concentration of cobalt chloride, but not sodium chloride?
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