Spectroscopy and Beers Law.docx

Spectroscopy and Beers Law.docx - Spectroscopy and Beers...

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Spectroscopy and Beer’s Law Introduction: The advancements in chemistry beginning in the 1930’s, was a result of the development of instrumentation that interacted with matter on a small scale. One of the important interactions is between electromagnetic radiation and matter. Spectroscopy is the name of the analytical technique concerned with the measurement of the interaction of radiant energy with matter. Specifically, the interaction that you will be using is the absorption of light by the electrons in a particular atom and the instrument you will use is called a spectrophotometer. Background: Why is a solution green (or any other color, for that matter)? The reason for the color is due to the selective absorption of light in the visible region. In your textbook, you have seen that the energy levels in an atom are quantized, that is they require specific amounts of energy to cause an electron to jump from one level to another. When this happens in an atom, it is said to absorb the energy at that specific wavelength. The actual color that is seen, though, is usually the result of the wavelengths that are not absorbed. These are the wavelengths that are rejected by the atom. Thus, a solution is green not because it absorbs green light, but because that is the light that it does not absorb. If you look at the visible spectrum found in your book, you can see that this compound must absorb light in the red region. Below is an example of the spectrum of chlorophyll, which is green. Picture 1: Spectrum of Chlorophyll This substance absorbs almost no light around 480 – 560nm, which are the green wavelengths and absorbs strongly in the 650 – 680nm, which are the red wavelengths. In this lab, you will have the opportunity to test several different colored solutions to see the relationship between what each absorbs the strongest light. One common use of a spectrophotometer is to determine the concentration of an ion in solution. As the concentration of the colored species increases, the absorption of light also increases. The relationship between the absorbance readings and the concentration of an ion is found by the formula known as Beer’s Law. Beer’s Law: A = bC Where: A = absorbance = molar absorptivity, a constant specific to a substance b = path length traveled by the light C = concentration of the substance in molarity The relationship between A and C is proportional or linear. Thus, the precise values of and b are not needed if the absorbance at various known concentrations can be measured. Then, a graph is
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constructed with the absorbance on the y-axis and the concentration on the x-axis.
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