Download intro to spectroscopy - Mount Holyoke College

CHEMISTRY 101
GENERAL CHEMISTRY I
FALL 2000
INTRODUCTION TO ATOMIC AND MOLECULAR SPECTROSCOPY
Readings from text: Zumdahl, Chapter 12 (sections 12.1-12.4).
Substances appear "colored" if they absorb or emit visible light; the light absorptive properties of a
substance depend on its molecular structure. "Spectroscopy" is the study of the interaction of light
(electromagnetic radiation) and matter. Information about this interaction in turn has led to increased
understanding about the structure of matter, from ions in stellar atmospheres to complex biomolecules
here on earth. Moreover, as we'll examine in this lab, the color of a substance can be related to the light it
emits or that it absorbs.
Some substances emit light, as is the case with a star. The colors of light emitted by a star give the
"signatures" of the atoms in that star when viewed through a spectroscope. Other common light emitting
objects include jellyfish (which contain a protein, "green fluorescent protein" or GFP, which emits light
when activated by the jellyfish's nervous system) and glowing safety light sticks (in which a chemical
reaction, usually induced by breaking a glass tube in the stick, produces light). More commonly,
however, materials around us which appear "colored" are absorbing particular colors form the visible
spectrum; we see colors of light which are not absorbed but rather reflected back (a material which
reflects all colors of light appears white to the eye). The first part of this experiment will be to establish
the relationship between the color of a substance and its spectrum, in which the light it absorbs or emits
is resolved into its components. In the second part of the experiment, the extent to which light is
absorbed can be related to the concentration of the absorbing species present in a sample.
PROCEDURE
A. Emission Spectra: Gases and Solids
A spectroscope is a device in which light from an emitting source is resolved into its component
parts for analysis. In a simple spectroscope the detection device is the human eye. A prism breaks up the
visible light from the source into its component wavelengths. Make a note of what you see when the
light source is a gas such as neon or helium. The gas is confined in a glass tube and energy in the form of
an electric current causes the gas to emit light. (This is the principle on which neon signs operate.)
Compare the emission of light by your gas sample with the emission spectrum of a solid sample such as
the tungsten filament of an incandescent light bulb. Given the information that emission of light of a
single color indicates that the substance has given off light energy by going from a higher to a lower
energy level, what can you say about the distribution of energy levels in gases compared with those in
solids?
Another use of the spectroscopes that has a long history is the analysis of the colors emitted by
metals in a bunsen burner flame. The color is produced by dipping a spatula in a solution of some metal
salt and putting the spatula in the flame. The color can be determined by simple
observation. If you work with a partner, you can observe the flame through the spectroscope to analyze
the component colors of the emitted light. Try this with solutions of salts of several alkali and alkalineearth metals, such as LiCl, NaCl, KCl, CaCl2, SrCl2 and BaCl2. This method has been used for
qualitative detection of these metals and was used in the last century to determine the presence of the
elements that make up the sun and other stars.
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B. Absorption Spectra: Relation to Color
Use the spectroscope and the continuous light source for this part of the experiment. You will also
need some substance with an easily observed visible color in water solution. A good choice is potassium
permanganate (KMnO4). Not only does it have an easily visible purple color, it will also let you check
on your technique. Permanganate ion is a strong oxidizing agent that will react with most substances it
contacts, including skin. The brown stain that results is the reaction product, manganese dioxide
(MnO2), an insoluble brown substance. Fortunately, toxicity is not a problem; in fact, potassium
permanganate solutions were once used to treat poison ivy and other skin infections. Place a sample of
the potassium permanganate solution between the light source and the spectroscope. It is usually a good
idea to start with water and to add concentrated potassium permanganate dropwise to observe any
changes in the spectrum. You should be able to correlate these changes with the purple color of the
potassium permanganate.
With your visual observations in mind, supplement your eye with an electronic detector. With a
recording spectrophotometer you can obtain an absorption spectrum of a potassium permanganate
solution in which an electronic device for measuring light, rather than your eye, obtains the spectrum.
The instrument also makes a record of the spectrum. The use of the recording spectrophotometer will be
demonstrated. Once again you will be able to correlate the absorption spectrum with the purple color of
potassium permanganate. Note that there is a wavelength at which potassium permanganate absorbs
visible light most strongly. Obtain spectra of the following solutions: nickel chloride (NiCl 2), copper (II)
sulfate (CuSO4) and iron (III) nitrate (Fe(NO3)3) and correlate color with the wavelength at which they
absorb most strongly.
C. Study of the Relation of Transmittance and Absorbance to the Concentration of the Absorbing
Species
So far, we have been focussing mainly on the wavelength of light absorbed (in other words, on
the "x axis" of the spectra collected in part B. There is also a great deal of information about a material
contained in the y-axis of these plots, which describes the amount of light absorbed at a particular
wavelength. In this section, we will explore the relationship between the amount of light absorbed and
the concentration of a substance, i.e., the amount of the substance present in a mixture. Instinctively, the
basic relationship between concentration and light absorbed is pretty obvious: the more of a substance
present, the more light it should absorb. For example, if we look at with a dilute solution of potassium
permanganate (i.e., a small amount of potassium permanganate mixed with water), it should absorb less
than a concentrated solution (a larger amount of potassium permanganate mixed with water).
You will be provided with a set of solutions which vary in potassium permanganate concentration.
Take note in your records: how do the appearances of the mixtures differ from each other?
We will be discussing concentration at length later in this course. For now, it is important to note
that concentration is measured in units of molarity (M), where larger values indicate a more concentrated
solution. Make a table in your notebook which lists the molarity of each of the solutions and leaves room
for the absorbance and transmittancemeasurement of each solution:
Sample #
Concentration
Transmittance
Absorbance
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An instrument such as a Spectronic 21 is best suited for making measurements at a selected
wavelength; the instructor or TA will demonstrate its operation.. Set the wavelength control of the
Spectronic 21 for the wavelength of maximum absorbance of the compound on which you are making
measurements. Leave the wavelength at this setting throughout the measurements. You are now about
to compare the extent to which your samples either transmit or absorb light compared to the solvent
(water) as a standard. With a sample of water in the light beam you can adjust the readout to indicate
zero absorbance (100% transmittance). As the name indicates, the percent transmittance refers to the
percentage of light that is transmitted by a sample compared to the amount transmitted by the solvent.
Absorbance is a defined quantity and is related to percent transmittance by the following relation.
Absorbance = log10
100
% transmittance
Obtain readings of both percent transmittance and absorbance for your solutions and record these in the
table in your notebook.
Make a plot of percent transmittance against concentration and another plot of absorbance against
concentration. One plots values of the y coordinate against values of the x coordinate, so you will have
two plots with values of concentration on the x axis. In one of these the values of percent transmittance
will lie on the y axis, and in the other the y axis will be absorbance. One of these plots will be a straight
line. The slope of the straight line is a quantity known as the molar absorptivity; it has a different value
for each substance and is an important property that can be used to characterize compounds.
D. Safety and Clean-up
Most of the substances used in this experiment are compounds of transition metals, many of which
are essential trace elements but toxic at high concentrations. The way for you to deal with used solutions
is to put them in marked bottles for subsequent disposal.
REPORT
Next week in lab, you should hand in an informal report which contains the following information.
The report should be neat and organized, but it can be either handwritten or word processed.
Section A: Answer all questions raised in the Procedures section, and record your observations with the
different lamps and solids.
Section B: Describe your observations with all aspect of the experiment. Include copies of your spectra
of all solutions studied, and correlate the spectra with the observed color (explain the relationship).
Section C: Hand in your table (concentration, absorbance, transmittance) and both plots (absorbance vs.
concentration; transmittance vs. concentration). Draw conclusions about the relationship (1) between
percent transmittance and molar concentration and (2) between absorbance and molar concentration. For
the relationship that produced a straight line, give a mathematical equation for the relationship by
expressing the equation for a straight line in terms of the variables in your experiment
Acknowledgments
This experiment was modified from the experiment "What Color is Your T-shirt?" prepared by
Prof. Mary Campbell. Development of this laboratory was made possible by a grant to Sweet Briar
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College by the Jessie Ball duPont Fund and grants to the College-University Resource Institute, Inc.
(CURI) by the National Science Foundation and the Camille and Henry Dreyfus Foundation.