Exp. 2:  An Introduction to Spectroscopy: Determining the
Absorption Spectra of Chromium(III) Nitrate
The purpose of this experiment is to introduce the basic concepts important to the study of spectroscopy, specifically the operation of aSpec 20 Spectrophotometer, the meaning of absorption as related to light, the interpretation of a spectral curve.

Your lab book introduction should therefore, include:

  • the theory behind the operation of any spectrophotometer
  • information about the operation of the Spec 20, with diagrams of its internal schematics and a sketch showing the location and purpose of the different knobs on its front side
  • information about the wave nature of light as it relates to spectroscopy, and visible light in particular
  • the care and use of cuvettes

There are several good websites where you can find the information you need. A good starting point is www.chem.vt.edu/chem-ed/ac-meths.html. This website is a result of the efforts of Dr. Brian Tissue, one of the chemistry professors at Virginia Tech.

Procedure:

Part A. Introducing the Spec 20

Plug in and turn on the Spec 20 and allow at least twenty minutes for it to warm up. Turn the left front knob (the zeroing knob) so that the %Transmittance is 0%.

While you are waiting for the Spec 20 to warm up, you can start making up the solution of Cr(NO3)3 that you are going to use in Part C. Pipet 10.00 mL of the 0.0500 M stock solution into a 25 mL volumetric flask and dilute to the line. Cover the flask tightly with a small square of Parafilm and invert it about 15 times to mix well. Set it aside until you get to Part C.

Now put distilled water into a clean cuvette so that the top is just below the white circle on the cuvette. Wipe the outside with a Khem-wipe and make sure there are no air bubbles in the water. Turn the light control knob (also known as the brightness knob) which is the right-front knob counterclockwise as far as it will turn, to lower the amount of light that will pass through the water in the cuvette and hit the phototube. Place the cuvette in the sample holder lining up the index lines directly.

Turn the wavelength selection knob so that the wavelength is 510 nm. Next rotate the brightness knob clockwise until the %T is about 90 %. Rotate the wavelength knob from about 350 nm to 750 nm and note the response of the phototube (indicated by % T) as the wavelength changes.

Now determine the wavelength of light to which the Spec 20 is most responsive. It should be in the neighborhood of 510 nm. Adjust the % T so that it is 100% for this wavelength. Then without touching either the zeroing or brightness knobs, turn the wavelength knob to 350 nm and record the % T. Take readings at each of the following wavelengths: 375, 400, 425, 450, 475, 500, 512, 525, 550, 600, 612, and 625 nm.

Part B. The Visible Spectrum

Take a piece of chalk cut at a slant and place it in a test tube the same size as the cuvettes and put it into the sample holder with the slanted side facing the light source of the Spec 20.

Observe and record the color of the beam every 50 nm from 650 nm until 350 nm. You may adjust the brightness knob for those wavelengths where the light appears dim or too bright.

Next, set the wavelength to 600 nm. Note the color(s) you see. How many different wavelengths of light make up the band of light you see?

Now adjust the wavelength to 550 nm. Rotate the brightness knob and see how the light intensity changes with the rotation. Be sure you note the variation in light intensity across the band that you see.

Part C. Absorption Spectrum of Cr(NO3)3 Solution

Now you are going to find the absorption spectrum of the 0.0200 M Cr(III) solution you prepared earlier.

First, set the wavelength to 375 nm. Make sure the Spec 20 is showing 0 %T with nothing in the sample holder. Then place the cuvette containing the distilled water (from now on called the blank) from Part A into the sample holder and set the %T to be 100%. Remove the water-filled cuvette and insert the second cuvette that contains an equal amount of the 0.0200 M Cr(III) solution. Record the %T. Repeat the previous steps with the blank and the Cr(III) solution at each of the following wavelengths (as long as the Spec 20 is showing 0%T with no cuvette in the sample holder you needn’t worry about redoing that step.): 400, 405, 415, 425, 440, 455, 470, 490, 500, 520, 530, 540, 550, 570, 575, 580, 600, 625. Remember, it’s put in the blank, set %T to 100%, remove the blank, put in the sample, record the %T. Every time you change the wavelength, you must reset the %T using the blank.

Carefully mark the cuvette used for the blank with a small "B" written near the top and the cuvette used for the sample with a small "S". You will use these same two cuvettes for the blank and different Cr(III) solutions and the same Spec 20 for Experiment 3.

Results and Discussion:

Parts A and B:

Graph the data from Part A and label it as Relative Overall Response of the Spec 20. Graph the %T vs the wavelength (l). Do this graph by hand using a full page of your lab book. The y-axis should be at least as high as the x-axis is wide.

Also, on the same graph, plot a curve using the following data and label it as Relative Response of the Phototube. Plot the phototube response vs the l.

At the top edge of the graph shade in the colors representing the wavelengths of light on the x-axis as you recorded them in Part B.

Now you want to calculate a number that we will call the Relative Intensity of the Spec 20’s Lamp. For each wavelength studied, divide the Relative Overall Response by the Relative Response of the Phototube. Multiply this value by 100/3 and you will get the Relative Intensity of the Spec 20’s lamp. The actual value of this ratio is not important. What is important is how the value changes as you go from one wavelength to the next wavelength. Plot these values as a third line on the graph from above.

 
Wavelength (l)Relative Response ( %)
350
375
400
425
450
475
500
512
525
550
575
600
612
625
 		90
98
100
98
91
81
68
61
53
37
21
10
7
5

Part C.

Make a second graph by plotting the absorption spectrum (%T vs l) for the 0.0200M Cr(III) solution.

Describe the first two curves you plotted on Graph 1. Notice that they do not coincide. Compare the wavelengths where the phototube response is high with those wavelengths that represent a high overall response of the Spec 20. What can you say about the intensities of the light that the tungsten filament lamp emits at the different wavelengths and how sensitive the phototube is at those same wavelengths?

Questions:

  1. To what color of light is the Spec 20 most responsivse? What color of light isemitted most strongly by the tungsten lamp?

  2. In what way do the various knobs on the Spec 20 affect the light beam which is passed through the sample holder?

  3. The effective bandwidth of the Spec 20 is only 20 nm and is constant over the entire wavelength region. What is meant by bandwidth?

  4. When light is diffracted from a grating, the sine of the angle of diffraction, q , is directly proportional to the wavelength. That is, sin q is proportional to l. Which color of light is diffracted at a greater angle, red or yellow light?

  5. If both red and yellow light pass through the second slit of the Spec 20 at the same time, the intensity of which color light will be affected most by the V-shape of the light-control aperture, and why? (The Spec 20’s diffraction grating is set up so that it give out a horizontal dispersion of light.)

  6. When a solution is red in color, does the solution absorb red lilght strongly or transmit red light strongly?

  7. Look at the Cr(III) absorption spectrum and decide which wavelength(s) would be best to use for analyzing chromium(III) nitrate solutions in the range of 0.02 – 0.04 M. Why would this/these wavelength(s) possibly be undesirable if the concentration was much outside of this range, either smaller or larger?

  8. It was assumed that the water and the glass in the cuvette absorbed none of the light in the regions of the spectrum used. Is this a safe assumption? Why or why not?

Gwen Sibert
Roanoke Valley Governor's School
gsibert@rvgs.k12.va.us
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