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Experiment 1 - EXPERIMENT 1 Absorption Spectrophotometry...

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E XPERIMENT 1 Absorption Spectrophotometry 1 E XPERIMENT 1 Absorption Spectrophotometry 1.1 Purpose In experiment 1, we establish the amount of a reductant in a redox reaction using absorption spectrophotometry. 1.2 Background In physics and in physical chemistry, spectrophotometry is the quantitative study of electromagnetic spectra. In this experiment, we employ quantitative measurements of concentrations of reagents by monitoring the absorption of visible light. Absorption is the process by which the energy of a photon is taken up by another entity, for example by an atom or molecule whose valence electrons make transitions between two electronic energy levels. The frequencies ν of light absorbed by a molecule or polyatomic ion are related to the energy differences Δ E between allowed quantum states, Δ E = h ν . Here, we are concerned with the absorption of radiation in the visible region of the electromagnetic spectrum, which causes a valence electron in an atom, a molecule, or polyatomic ion to be promoted to an excited electronic state. The perceived color of a substance is closely related to its visible absorption spectrum. When a substance absorbs visible light, the color perceived is complementary to the color most strongly absorbed by the sample. In Table 8.1, pairs of complementary colors and the corresponding wavelength ranges are presented. As an illustrative example, we consider the permanganate ion MnO 4 - . Permanganate is a strong oxidizing agent, and we will measure the change of concentration of permanganate to deduce an unknown concentration of a reductant. Permanganate ion in an aqueous solution absorbs visible radiation around 550 nm with an absorption maximum at 525 nm. The visible absorption spectrum of permanganate is displayed in Figure 1.1. Permanganate ion absorbs light of a wavelength corresponding to a green color, and therefore appears as purple colored solution.
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E XPERIMENT 1 Absorption Spectrophotometry 2 Table 1.1 Pairs of absorbed and observed colors for selected wavelength spanning the range of the visible spectrum of light. Wavelength λ Color Absorbed Color Observed 380 – 430 nm Blue-Violet Yellow 430 - 500 nm Blue Orange 500 – 520 nm Blue-Green Red 520 – 565 nm Green Purple 565 - 590 nm Yellow Blue-Violet 590 - 625 nm Orange Blue 625 - 740 nm Red Blue-Green The axes in the plot in Figure 1.1 are labeled absorption A and wavelength λ . The wavelength of light provides a measure of its frequency ν , since the product of wavelength and frequency result in the speed of light c. The speed of light is an important physical constant, and the speed of all electromagnetic radiation, including visible light, in a vacuum is 3.00 × 10 8 m/s. Therefore, the frequency of light can be calculated as ν = c/ λ , and using the relationship Δ E = h ν , we get for the energy of electromagnetic radiation Δ E = (h × c)/ λ .
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