Chemistry 161
Lab 6: Spectroscopy – Introduction and Procedure
Introduction:
Part 1: Atomic emission and absorption spectroscopy was one of the experiments that lead
to the discovery of energy levels or “shells” and the notion of the “quantized” nature of light
(photons). The colors of light emitted or absorbed by atoms, ions and molecules correspond
to the differences in energy between the levels. This phenomenon is responsible for the
deep shades of dyes, the brilliant colors of fireworks or even the shiny appearance of metal
surfaces.
Atoms, ions and molecules have two types of states; a ground state and multiple excited
states. In the ground state, the electrons are in the lowest energy levels possible (atoms
naturally are in the ground state). This means the electrons have the lowest possible values
for "n" the principal quantum number. When electrons are promoted to higher energy levels,
this is known as an excited state. Promotion of electrons to higher energy levels requires the
addition of energy through heat, electrical discharge or absorption of specific wavelengths of
light. The excited state is unstable, and the electrons will return to the ground state resulting
in the emission of energy in the form of light.
The energy of a photon of light is related to the wavelength and frequency of that light. Due
to the law of conservation of energy, the wavelengths of light absorbed by a material when
the electrons are promoted from the ground state to an excited state, will relate to the energy
difference between the levels (∆E). Similarly, the wavelengths light emitted when electrons
fall from excited states to the ground state will correspond to difference in energy between
the levels. The difference in energy between the levels varies from chemical to chemical and
consequently the wavelengths of light absorbed or emitted will vary from chemical to
chemical. These wavelengths of light a like a fingerprint for a particular type of atom.
Part 2: Absorption or emission of ultraviolet or visible light by a molecule depends on electron
transitions between molecular orbital energy levels, just as absorption or emission of
electromagnetic radiation by an atom is determined by electron transitions between different
atomic energy levels. Molecular spectra follow rules analogous to the rules for atomic
spectra: energy is absorbed only when the amount of energy provided matches the difference
in energy, E, of 2 energy levels. When an electron goes from a lower to a higher energy
state, a photon of definite wavelength and frequency is emitted. Every atom or molecule has
a characteristic electronic spectrum depending on its characteristic E’s. The peaks for
solids, liquid and aqueous materials tend to be much more broad due to the interactions with
surrounding molecules.
Part 3: As seen in Part 2, different solutions have different spectral properties. In this portion
of the experiment those properties will be utilized to determine the concentration of an
unknown sample. In looking at the absorbance of a solution it can be noted that the
absorbance of a solution is directly proportional to the concentration of the solution and the
length of solution the light has to pass through. This relationship is known as Beer’s Law and
can be expressed as follows: A = εLc where A is absorbance, ε is the molar absorptivity (in
L/mol*cm), L is the path length (in cm), and c is the concentration (in mol/L).
A calibration curve can be generated by measuring the absorbance of a series of standard
solutions (of known concentration) and plotting Absorbance (A) versus concentration, with A
on the y-axis and concentration on the x-axis. The linear regression for this plot will yield a
familiar equation of the line y = mx + b. In this case, we can relate this to the equation above
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and think of it as A = εl * c + b. Then by measuring the absorbance of an unknown solution,
we can plug it into our equation and solve for the concentration of our unknown.
Pre-lab Question: (To be completed in your Lab Notebook)
1. Write the equation that describes the relationship between frequency and wavelength.
Write the equation that describes the relationship between frequency and energy.
2. In your own words, describe the purpose of the calibration curve generated in Part 3 of
this experiment.
3. Write a short paragraph explaining why atomic emission spectra have narrower peaks
than molecular spectra.
Procedure:
Part 1
1. Using the TV Specs, observe and record atomic emission spectra from He lamp.
- Sketch the atomic emission spectrum of He. Include color and wavelength.
2. Repeat step 1 with the Hg lamp.
- Sketch the atomic emission spectrum of Hg. Include color and wavelength.
Part 2
3. Use a USB cable to connect a Vernier Spectrometer to your computer.
4. Start the Logger Pro program on your computer.
5. Calibrate the spectrometer.
a. Prepare a blank by filling an empty cuvette ¾ full with distilled water.
b. Open the Experiment menu and select Calibrate → (Spectrometer:1). The
following message appears in the Calibrate dialog box: “Waiting … seconds for
the device to warm up.” Wait until the message changes to: “Warmup
complete.”
c. Place the blank in the cuvette holder of the spectrometer. Align the cuvette so
that the clear sides are facing the light source of the spectrometer. Click “Finish
Calibration”, and then click
.
6. Conduct a full spectrum analysis of a colored solution.
a. Fill a cuvette ¾ full with the colored solution and place it in the spectrometer.
Align the cuvette so that the clear sides are facing the light source of the
spectrometer.
b. Click
. A full spectrum graph of the colored solution will be displayed.
c. Allow the spectrometer to collect date for a few seconds and then click
d. Examine the graph, noting the peak or peaks of very high absorbance or other
distinguishing features. Compare the color of the solution with the wavelength
associated with that color. Use the ‘rainbow’ chart drawn above.
e. To Save your graph, select Store Latest Run from the Experiment menu.
7. Repeat Step 6 with the remaining colored solutions. Remember to keep a copy of
each graph. To export your data open the file menu and select Export As → CSV
(Excel, InspireData, etc.)… You will want to store these runs onto a USB drive or
email them to yourselves. You will want to store these runs onto a USB drive or email
them to yourselves.
Part 3
8. Obtain ~40 mL of 0.20 M CoCl2 solution and distilled water in separate beakers.
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9. Label five clean, dry, test tubes 1–5. Use pipets to prepare four standard solutions
according to the chart on the next page. Thoroughly mix each solution with a stirring
rod. Clean and dry the stirring rod between uses.
Test Tube
number
1
2
3
4
5
0.20 M CoCl2
(mL)
10.00
8.00
6.00
4.00
2.00
Distilled H2O
(mL)
0
2.00
4.00
6.00
8.00
Concentration
(M)
0.20
0.16
0.12
0.080
0.040
10. In Test Tube 6, obtain ~ 5 mL of an unknown sample.
11. Calibrate the spectrometer.
a. Prepare a blank by filling an empty cuvette ¾ full with distilled water.
b. Open the Experiment menu and select Calibrate → (Spectrometer:1). The
following message appears in the Calibrate dialog box: “Waiting … seconds for
the device to warm up.” After the allotted time, the message changes to:
“Warmup complete.”
c. Place the blank in the spectrometer; make sure to align the cuvette so that the
clear sides are facing the light source of the spectrometer. Click “Finish
Calibration”, and then click
.
12. Determine the  max for CoCl2 (aq) and set up the data collection mode.
a. Rinse and fill a cuvette ¾ full with the 0.20 M CoCl2 solution and place it in
the spectrometer.
b. Click
. A full spectrum graph of the CoCl2 solution will be displayed. Note
that one area of the graph contains a peak absorbance ( max). Click
to
complete the analysis.
c. To save your graph of absorbance vs. wavelength, select Store Latest Run from
the Experiment menu.
d. Click the Configure Spectrometer icon, , on the toolbar. A dialog box will
appear.
e. Select Abs vs. Concentration under Set Collection Mode. The  max will be
automatically selected. Click
to proceed.
13. Collect absorbance-concentration data for the five standard solutions.
a. Leave the cuvette in the spectrometer. Click
. When the absorbance
reading stabilizes, click
. Enter “0.20” (the solution concentration). Click
.
b. Using the solution in Test Tube 2, rinse and fill the cuvette ¾ full. Wipe the
cuvette and place it in the spectrometer. When the absorbance reading
stabilizes, click
. Enter “0.16” and click
.
c. Repeat these steps for the remaining test tubes of the standard CoCl2 solution
and when you have finished testing the standard solutions, click
.
d. Place the unknown solution into the spectrometer. Record the absorbance
(found on the bottom left of the screen) in your lab notebook.
14. To export your data open the file menu and select Export As → CSV (Excel,
InspireData, etc.)… Alternatively, you can cut and paste your data from the columns
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on the left side of the sceen into Excel. You will want to store these runs onto a USB
drive or email them to yourselves.
15. Bring your lab notebook to the instructor for inspection. You may be assigned a small
cleaning task. The instructor will also check to see if your laptop was properly
returned to the cart.
Analysis
A1) Create a graph showing the data and y=mx+b equation for C1-C5 solutions.
A2) Calculate/find the molar concentration of your unknown CoCl2 solutions. Clearly show directly on
the graph how it was calculated/found.
There are no postlab questions for this lab.
Summary:
What would you do differently if you repeated this experiment?
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