Spectroscopy Analysis

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Chemistry 111 Lab: Spectrophotometry

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This experiment has two purposes: • To explore the technique of spectrophotometry, the use of light to determine some property of a substance. • To determine the weight percent of copper in an unknown compound using spectrophotometry. In the first part of the laboratory, you will explore a spectrophotometer, the instrument used in the laboratory. Then you will the conditions most suitable to use in the experiment. Finally, you will determine the concentration of copper in a solution, and from this information determine the amount of copper in an unknown sample. In order to determine the concentration of copper in the unknown solutions we shall first construct a standard plot of the concentration of copper vs. the absorbance of solutions having known copper concentrations. According to Beer's Law, absorbance is directly proportional to concentration and so the resulting plot should be a straight line. This graph will be used to determine the concentrations of solutions containing known amounts of the unknown copper compound and from this we can determine the weight percent of copper.

Part A. Visible Spectrum
1. Take a piece of chalk that is about 2 cm long and rub it on the blackboard to produce a 45˚ edge or bevel on one end of the chalk. 2. Place the beveled piece of chalk into a test tube with the bevel pointed up, and put the test tube into the sample compartment of the Spec 20. 3. With the sample port open, look directly down at the piece of chalk and turn the sample tube until a small spot or slit of light is reflected off the bevel of the chalk. Adjust the light control knob until the light that is reflected off the bevel of the chalk is at maximum brightness. NOTE: This portion of the experiment may be done with a partner.

Figure A piece of beveled chalk in a small test tube allows you to see inside the sample container of the spectrophotometry and to observe the colors as a function of wavelength.

Revised: June 2005

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Chemistry 111 Lab: Spectrophotometry

4. Turn the wavelength selector knob and notice how the color of the light reflected off the chalk changes color. By turning this knob, you can scan the entire visible spectrum. Note that, although it is possible to find the absorbance of each of these solutions at any visible wavelength (~400 nm → ~700 nm), the Spec 20 gives most accurate results between 420 and 640 nm.

5. As you vary the wavelength, carefully note which wavelength ranges between λ = 400 to λ = 700 nm (where λ is the symbol for wavelength) correspond to which observed colors of the visible spectrum and record these wavelength ranges and colors in Table A in the Data Section of this write-up. 6. On Graph A provided in the Data Section, draw lines that correspond to the observed wavelength range for each primary color of the spectrum (red, orange, yellow, green, blue and violet) you recorded in Table A.

Part B. Light Absorption of Colored Solutions
1. Make sure the test tubes you are using are clean. Fill one with 0.24 M CuSO4, one with 0.1 M CoSO4 and one with distilled H2O. Wipe off the outside of each test tube with a clean, dry cloth. Avoid fingerprints on the test tubes as you proceed with the experiment. 2. Starting at a wavelength setting of 420 nm, measure the absorbance (A) of both solutions at 420 nm. Record these values in Table B in the Data Section. 3. After zeroing the instrument at each different wavelength, measure the absorbance readings for both solutions at 20 nm increments from λ = 420 to at least 720 nm. (For the copper containing solution, you will have to extend your measurements beyond 640 nm.) Record absorbance readings for each of these solutions vs. wavelength in Table B in the Data Section. 4. Plot the absorbance on Graph B in the Data Section. Make sure that you carefully record the different readings for the...
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