Beer's Law_Spectrophotometry

# Absorbance a if no light is absorbed the absorbance

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absorbance ( A ). If no light is absorbed, the absorbance is zero (100% transmittance). Each unit in absorbance corresponds with an order of magnitude in the fraction of light transmitted. For A = 1, 10% of the light is transmitted ( T = 0.10) and 90% is absorbed by the sample. For A = 2, 1% of the light is transmitted and 99% is absorbed. For A = 3, 0.1% of the light is transmitted and 99.9% is absorbed.

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Absorbance Spectrum The extent to which a sample absorbs light depends strongly upon the wavelength of light. For this reason, spectrophotometry is performed using monochromatic light. Monochromatic light is light in which all photons have the same wavelength. In analyzing a new sample, a chemist first determines the sample's absorbance spectrum. The absorbance spectrum shows how the absorbance of light depends upon the wavelength of the light. The spectrum itself is a plot of absorbance vs wavelength and is characterized by the wavelength ( λ max ) at which the absorbance is the greatest. The value of λ max is important for several reasons. This wavelength is characteristic of each compound and provides information on the electronic structure of the analyte. In order to obtain the highest sensitivity and to minimize deviations from Beer's Law (see subsequent pages on this topic), analytical measurements are made using light with a wavelength of λ max . 1. How does the cell path length affect the intensity of light reaching the detector and why is this behavior observed? 2. How does the transmittance vary with the cell path length? Is this plot linear? 3. How does the absorbance vary with the cell path length? Is this plot linear? 4. How does the concentration affect the intensity of light reaching the detector and why is this behavior observed? 5. How does the transmittance vary with the concentration? Is this plot linear? 6. How does the absorbance vary with the concentration? Is this plot linear? Introduction to Beer’s Law: Colorimetry of Food Dyes Introduction: A simple and accurate method used to determine the concentration of colored solutions is spectrophotometry. The use of a spectrometer and the principles of Beer’s Law, allow one to plot a graph
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