Spectrophotometric Analysis of Copper
Spectrophotometric Analysis of Copper
Spectrophotometric Analysis of Copper
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EXPERIMENTAL PROCEDURE
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.
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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 = 400 to = 700 nm (where 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.
In this portion of the experiment, two students may work as partners to prepare the calibration curve (using solutions 14 below). However, each student must do his or her own unknown solution (solutions 5 and 6).
Make sure you read the ABSORBANCE scale and not the transmittance scale on the instrument dial.
1. Preparing Solutions:
Clean and dry the six 18 x 150 mm test tubes from your desk and label them 1 through 6. Four of these tubes will be used for solutions of known copper concentration and the other two will contain two different concentrations of
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the unknown copper compound. For each desk side (three people) you should have a set of two burets, one containing standard copper solution and one containing 1 M HNO3. Use them to prepare the following solutions:
Test tube
Remarks
1 2 3 4 5 6
Reference standard for standardizing light control Be sure to mix thoroughly Be sure to mix thoroughly
Add about 0.3 g of unknown (accurately weighed). Be sure to mix thoroughly Add about 0.2 g of unknown (accurately weighted). Be sure to mix thoroughly
Be sure that the tubes are clean. It is probably a good idea to rinse out each tube with a small amount of the solution before filling it with the solution. Be especially careful that there are no smudges or scratches near the bottom of the tube.
Page E-14 Note that solution 1 is pure nitric acid and so [Cu2+] = 0.0 M. Solution 4 was not diluted and so has [Cu2+] equal to that concentration marked on the bottle of standard copper solution.)
a) Calculate the actual concentration of Cu2+ ion in your samples (2 and 3). Because you are simply diluting a copper-containing solution of known concentration, you can use the equation (Conc. of original solution)(Volume of standard Cu2+) = (Conc. of dilute solution)(total solution volume) b) Use one of the sheets of graph paper provided in the laboratory (or a computer program) to construct a calibration plot of concentration (vertical or y-axis) vs. absorbance (horizontal or x-axis) for the four known samples (samples 1-4). Be sure that your scales are chosen so that almost all of the graph paper is used. c) Determine the copper concentrations in samples 5 and 6 from the calibration plot. (Find the concentration that matches the measured absorbance of each sample.) Record this information. d) Based on the concentrations calculated above, determine the mass of copper (in grams) in each of the two unknown samples and use this information to determine the mass of copper in your original solid samples. e) Now calculate the weight percent of copper in each of the separate samples and also the average of these two results. Record all of this information on your Report Form.
You may use MS Excel or the programs on the General Chemistry CD-ROM to prepare your plots.
INTERNET RESOURCES
1. About Spectronic 20 instruments http://www.dartmouth.edu/~chemlab/techniques/spectrometer.html