Introduction
Infrared Spectroscopy is the study of matter and its interaction with electromagnetic radiation and it can be used in order to determine the functional group within a sample (2). When infrared light hits a certain molecule it can cause pulling and bending between the bonds present, which also determined the amount of absorbance there is. The intensity of an absorption band depends on the molecule and the dipole moment between the bond and the amount of bonds present (2). The bigger the dipole moment between a bond, the stretch at a given percent transmittance will be bigger as well. IR Spectroscopy is recorded in wavenumber or inverse centimeters of the wavelength and demonstrates the percent transmittance of infrared that is transmitted into the molecule. The range on the data goes from 400 cm−1 to 4000 cm−1. The fingerprint region is located from 400 cm−1 to 1500 cm−1 (2). Certain regions corresponding to certain functional groups. An OH bond is found around 3300 cm−1, while a C-O bond is found around 1044 cm−1. Using IR, the functional groups present within a sample can be found; however, IR alone cannot determine the structure of a molecule.
The purpose of this experiment was to determine whether or not an IR instrument can determine the concentration of alcohol within an aqueous solution. The amount of infrared absorbed is proportional to the concentration of the substance (1). As ethanol concentration increases, the “alcohol” peak would increase with the “water” peak decreasing. To observe the concentration of ethanol within water, the C-O bond was observed to make the distinction since both water and ethanol have an OH peak but only ethanol has a C-O peak. Using the IR absorbance from the known solutions and comparing the calibration curve, the unknown solutions containing different concentrations of ethanol were estimated.
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Experimental
Solutions with different concentrations of ethanol were created starting from 0% to 100%. Zero percent being pure water and 100 percent being pure ethanol. Two solutions per student were prepared in order to gather IR spectrum readings. One solution of 4 mL of ethanol and 6 mL of water with a second solution containing 5mL of ethanol and 5 mL of water were created. A few drops of the solution were placed onto the clean sample plate/crystal. Using the computer, images of the spectrum were recorded and the peaks were analyzed. On the second week, an alcoholic beverage was then read by the IR spectrum instrument to determine the unknown ethanol concentration. Altos Reposado Tequila was the sample used within the experiment in order to compare the ethanol concentration using IR spectrum.
Discussion
Since both water and ethanol have an OH group, observing the OH stretch at 3300 cm−1 would not be accurate. In order to compare and distinguish the concentration of ethanol from water, the C-O bond was observed. As concentration of ethanol increased within the solution, absorbance increased as well. As concentration of ethanol increased, the percent transmittance decreased. This would mean that as ethanol concentration increases, the C-O stretch would be stronger and percent transmittance would increase. At zero percent EtOH, there would be a strong OH bond created by the H2O but no C-O bond that would be present within a high ethanol concentration. The variation between the different concentrations can be observed in Table 1 and 2. The further the concentrations were apart, the more distance the absorbance and transmittance was. However, the closer the concentrations of ethanol, the harder it was to distinguish the percentage of transmittance but a good range is still known.
The C-O stretch was observed on the IR of Altos Reposado Tequila at 1044 cm−1. The IR demonstrated a percent transmittance of 73.107% and an absorbance of 0.136. Using the y=mx+b or slope equation from the graph where y is the absorbance and x is the concentration of ethanol, and plugging in the absorbance gave the concentration of EtOH within the alcoholic beverage which was 34.09. This would mean that the range of absorbance would be around 0.194 or 0.205. The absorbance calculated can be backed up with the absorbance seen within the class data (table 1 and 2). However, the known percentage of alcohol within Altos Reposado Tequila is 40%. The calculated value differed from the known value of 40%, representing 34.09%.
Error and variability within the experiment could be due to cleaning the plate or crystal on the IR spectroscopy instrument with ethanol, which could result in excess ethanol if it was not completely cleaned off. This may result in a higher transmittance at the 1044 cm−1 wavenumber. Using more data from known concentrations would be a more efficient way to define a more accurate linear trend line and determine the unknown samples ethanol concentration. Although there are a few errors, it is confirmed that as concentration of ethanol increases, the absorbance increases and vice versa.
References
- Padias, A. (2016). Making the Connections 2: A How-To Guide for Organic Chemistry Lab Techniques (2nd ed.). Hayden-McNeil.
- Wade, Leroy G. “Organic Chemistry” Glenview, Il; Pearson, 2017. Print
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