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Lab section 1EF

Aim: The purpose of this experiment was to obLab Experiment 7: Identifying Unknown Compounds by Infrared Spectroscopytain and interpret IR samples after preparing solid sample of Potassium Bromide and liquid samples of p-Anisaldehyde .

Table of Physical Properties and Hazards


Melting Point (°C)





-harmful to aquatic life

-harmful to environment


Potassium Bromide






Benzoic Acid


-potential for environmental hazard

-skin and eye irritant

-respiratory irritant 




-highly flammable

-eye irritation



IR, known as Infrared Radiation, is the radiation in the energy range between the visible and microwave regions of an electromagnetic spectrum. An IR spectrum is a plot of percentage of IR radiation that passes through a sample versus the frequency of radiation. The radiation that passes through the sample is measured as percent transmission and the frequency of the radiation  is measured in wavelengths per centimeter, which is also known as the wavenumber. When the frequency of the infrared light applied o a compound is exactly the same as the natural vibrational frequency of the interatomic bond, the molecule absorbs the light and the amplitude of the bond vibration increases and gives rise to what are known as peaks. IR spectroscopy is concerned with the study of absorption of infrared radiation, which causes vibrational transition in the molecule. IR spectra is used to elucidate structures to determine their functional group. Spectroscopy is a method of essentially “seeing the unseeable”, where electromagnetic radiation is used to obtain information about atoms and molecules that are too small to see.The main functional groups that are seen in an IR spectrum are carbonyls and alcohols. The 4000 to 1500 cm^-1 portion of the graph is useful for identifying various functional groups. Bonds in the 1500 to 60cm^-1  portion, also known as the fingerprint region is the result of many types of vibrations that characterize the molecule as a whole.

Spectrometer machines such as an FT-IR spectrophotometer, which was used in this experiment, helps to split the electromagnetic radiation on two beams. One beam travels over a longer path inside the spectrometer than the other beam. A combination of the two beams produces an interferogram, in which Fourier transformation, a computerized mathematical manipulation of data from the interferogram and converts that info into an IR spectrum. In the FT-I spectrophotometer, the radiation from the IR spectrum passes through the sample simultaneously, which saves time. The frequency of the radiation absorbed is related to the rigidity or strength of the bond and the total mass and of the bonding atoms. To be specific,the vibrational energy is higher for stronger bonds and lighter atoms. The two types of vibration we see in IR are stretching and bending vibrations. Bending vibrations require less energy than stretching motion, thus, absorbs at lower frequencies, and therefore, has smaller wavenumbers.

Procedure & Observation

In this experiment, a liquid and  solid sample compound were made and then analyzed in an infrared spectrometer. In preparing the liquid sample, half a drop of p-Anisaldehyde was placed on top of a salt plate. Another salt plate was then placed on top on the salt plate with the sample compound. The salt plates were then placed inside the salt plate holder. A metal cover was used to cap the salt plate holder and was then inserted inside the FT-IR Spectrophotometer. The FT-IR Spectrophotometer served to get an accurate IR spectrum of the sample compound. On the computer connected to the FTIR machine, the “hit sample” button was pressed and the IR spectrum of that sample was displayed on the computer screen. The sensitivity was set to 50, the IR spectrum was printed, and the salt plate with the sample was removed from the machine and the cleansed with Acetone. To prepare the solid sample, a micro spatula was used to scoop a small of amount of potassium Bromide powder, which was mixed with benzoic acid. Approximately 90% of Potassium Bromide and 10% of Benzoic Acid was used for this mixture. The solid mixture was mixed and grinded to fine powder. It is important to keep everything dry and to avoid moisture from the hands to or water to get on the salt plates or fine powder, because it can permanently ruin the salt plates and the outcome of the experiment. The cell windows are easily fogged by exposure to moisture and require frequent polishing with buffer powder which returns them to their original condition. The fine powder was placed inside a pellet and a bolt is screwed in to compress the powdered mixture. The pellet is then unscrewed and observed for transparency. In the case, we observed little to no transparency in our solid sample. The pellet was then placed on the sample holder and then placed inside a an FT-IR spectrophotometer where the sample ran for no longer than 15 minutes. The computer displayed an IR spectrum data once we collected the sample information.

Results - see attached


We were able to identify the liquid and solid samples when analyzing the print out of the data. As mentioned earlier, our solid sample had little to no transparency which was due to an abundance of Potassium Bromide. As a result, our IR Spectrum for our solid sample showed for more absorbance than transmittance.




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