Spectroscopy, the study of how light and matter interact, is one of the most widely used and powerful analytical tools of modern-day chemists. In this investigation you will measure the absorption of light by your aspirin and copper(II) aspirinate products using infrared and ultraviolet-visible spectrometers. Comparing the spectra of your products with those of the starting materials will help you determine whether or not you did, in fact, synthesize aspirin and copper(II) aspirinate.
Background
The two types of spectroscopy to be used are infrared (IR) and ultraviolet-visible (uv-vis). IR spectroscopy measures energy changes involving vibrations of groups of atoms, usually a bond or set of bonds, within a molecule. The vibrations of different types of bonds and groups of atoms are excited by specific and characteristic wavelengths of infrared light. You will use an IR spectrometer to measure the ability of your samples to transmit IR light at various wavelengths. A graph of percent transmission vs. wavenumber will be produced, where wavenumber is just the reciprocal of wavelength, in cm.
Ultraviolet-visible spectroscopy depends on the excitation of a molecule's electrons by uv or visible light. Although this is a much more sensitive technique than IR spectroscopy, and thus better suited for analytical purposes, uv-vis spectra are rarely characteristic of the sample. In other words, by looking at the IR spectrum of an unknown molecule a chemist can often work out the molecule's structure. That's rarely possible with a uv-vis spectrum. In this experiment you will record the uv-visible spectrum of copper(II) acetate and your copper(II) aspirinate. Here you are looking for a difference in the two spectra, to confirm that you really did carry out a reaction. Unlike the case with IR spectroscopy, your uv-vis spectra will be recorded as absorbance vs. wavelength. Absorbance (A) is defined as A = - log(%T/100) where %T is percent transmission.
The photos below show the IR (left) and uv-vis (right) spectrometers you will use.
Experimental: Infrared Spectroscopy
CAUTION: Do not use water for any cleaning in this part of the experiment. Water is a very efficient absorber of IR light and will ruin both your experiment and some of the equipment.
To begin, start the software for the IR spectrometer. Take a reference spectrum of a pair of clean, circular salt (NaCl) plates. Handle them only by the edges. Next, grind about 0.01 g of your solid sample to a fine powder using a mortar and pestle. Add about 10 drops of Nujol, a mineral oil, to the mortar. The Nujol will be in a small plastic dropper bottle, as shown in the picture below. Use the pestle to stir the mixture until it has a smooth, uniform appearance.
 | Tools for IR spectroscopy: acetone, Nujol, spatula, mortar and pestle, and container with NaCl plates |
After the mixing is complete, use a Pasteur pipet to place two drops of the aspirin-Nujol mixture on the face of one of the salt plates. Put the second salt plate over the liquid, thereby creating a "sandwich" of sorts.
Put the salt plates and sample into the beam of the IR spectrometer, record the spectrum, and save it on a floppy disk. Take the disk to the computer room to print it with the IR software there. You can also analyze your spectrum in the computer room.
Repeat the above with your copper aspirinate product.
When you are finished, use acetone to clean the salt plates and the mortar and pestle.
Experimental: Ultraviolet-Visible Spectroscopy
CAUTION: You will be using plastic cuvettes in this investigation. Try to avoid touching the clear sides through which light will pass.
Put about 0.01 g of copper(II) acetate in a 1-cm cuvette and add water. You may have to shake the cuvette and sonicate to get the solid to dissolve. After the solution takes on a blue color, place the cuvette into the Cary 100 uv-vis spectrometer in the front cell holder. In the back cell holder, place a cuvette filled with water, as a reference. Record the sample's spectrum. Go ahead and print the spectrum before going further. Also, write down the wavelength of greatest absorbance (the peak position).
Now for the hard part. Dissolve some of your copper(II) aspirinate in a 1:1 mixture of 95% ethanol and water. Shaking and sonication will be needed to get any blue color at all. Again, put a few milliliters of the solution into a 1-cm cuvette and record the spectrum. Print the spectrum and note the peak position.
If time permits, record the uv-vis spectrum of your aspirin product dissolved in 95% ethanol.
Data Analysis and Calculations
Your first task is to show that the IR spectra of your two products are different from the spectra of your reactants. Check with your instructor for reference spectra of the reactants, either in printed or digital (floppy disk) form. To begin, compare the spectrum of your aspirin product with the IR spectra of Nujol and salicylic acid. Nujol has IR bands that will obscure your spectra around 2900 (intense band), 1460, 1377, and 723 cm-1. Salicylic acid and aspirin (acetylsalicylic acid) have similar spectra, but the latter, your product, has diagnostic peaks at 1752, 1688, 1187, and 917 cm-1 that you should look for. -- The best way to check for these peaks in your spectrum is to use the IR software to superimpose your aspirin spectrum on a spectrum of salicylic acid.
For your copper(II) aspirinate, you need to show that its IR spectrum differs from that of copper(II) acetate. Diagnostic peaks for copper(II) aspirinate are near 1756, 1725, 1692, 1187, and 917 cm-1. Again, the best way to check your spectrum is to superimpose it on a spectrum of copper(II) acetate.
Reference spectra are here in the jpg and pdf formats.
Complete your work by comparing the uv-vis spectra of copper(II) acetate and your copper(II) aspirinate product. Are they different or the same? What does this say about your product?
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