# Absorption Spectroscopy Essay

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• Published : June 14, 2011

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Absorption Spectroscopy
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Chem 152
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Abstract
A Beer’s Law analysis was used to determine the molar absorptivity of a Red-40 food dye. Using a spectrometer, the best wavelength to use for the analysis of this dye was determined to be 501nm. A series of dilutions were performed and the concentrations were calculated to find ‘E,’ the molar absorptivity, which was determined to be 18035 M-1 cm-1.

Introduction
The experimental behavior of the absorption spectroscopy lab is to be able to determine the molar absorptivity of a food dye; in this case, Red-40. The determination of the best wavelength to use is found by measuring the highest peak that had an absorbance between 1 and 1.5. The dye concentration will be accomplished by preparing a series of serial dilutions and measuring the absorbance at the best wavelength previously determined to conduct a Beer’s Law analysis. Absorption spectroscopy is often used to calculate the amount of compound present in a solution sample by measuring the amount of light each wavelength it absorbs. Since the amount of light absorbed is directly proportional to the number of molecules in a solution, spectroscopy allows for the determination of a dye concentration and thus the molar absorptivity in the Red-40 dye. Beer’s Law is given by A=Ebc, where ‘A’ is the measured absorbance, ‘c’ is the concentration of the absorbing species, ‘b’ is the width of the cuvette, and ‘E’ is the molar absorptivity constant. This equation describes the linear relationship between the absorbance and the concentration, making it possible to calculate ‘E’ by graphing the absorbance versus concentration and finding the slope of the linear line. That being said, it is expected that the absorbance of the dye will decrease as the concentration decreases.

Results
Red-40 dilutions
Solutions (dye-water)| Wavelength (nm)| Volume (ml)| Absorbance| Concentration...