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Applications of UV-Vis Molecular Absorption Spectrometry Chap 14

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Applications of UV-Vis Molecular Absorption Spectrometry Chap 14

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  1. The APXS is designed to study the alpha particles (from radioactive decay) and X-rays emitted by rocks and soils in order to determine their elemental chemistry. Knowing the elemental composition of Martian rocks will provide the scientists with information about the formation of the planet's crust, as well as any weathering that has taken place. Clues as to the nature or weathering on the surface of Mars, will, again raise the issue of whether or not there was surface water.

  2. Applications of UV-Vis Molecular Absorption Spectrometry Chap 14

  3. Application to Qualitative Analysis • Spectral data may be plotted with ordinate as: • absorbance (A) • percent transmittance (%T) • log absorbance (log10 A) • Abscissa normally: • wavelength (nm) • wavenumber (cm-1) • frequency (GHz) ordinate abscissa

  4. Table 14.1 Absorption Characteristics of Some Common Chromophores

  5. Table 14-4 Absorption Characteristics of Aromatics

  6. Fig 6-24 (a) Partial energy level diagram for UV-Vis absorption in an organic molecule

  7. Fig 14-1 UV-Vis Absorption Spectra for 1,2,4,5 tetrazine

  8. Fig 14-6 Effect of solvent on absorption spectrum of acetaldehyde

  9. Transparency: Solvents for the UV-Vis Table 14-3 Cutoff wavelengths

  10. Solvents: Effect on absorbing system • Polar solvents tend to “blur” vibrational • features more than nonpolar solvents • Polar solvents more likely to shift λmax • n→π*blue shifted • solvation of lone pair lowers energy of n • ∼ 30 nmwhen hydrogen-bonding possible • π→π*red shifted • ∼ 5 nm due to attractive polarization

  11. Solvent effects imply that: UV-Vis is not reliable for qualitative but excellent for quantitative analysis

  12. Quantitative Analysis Develop Calibration Curve with Standards Challenge: Preparing standards in same matrix as analyte

  13. Standard Addition Method Fig. 14-8 Determination of NO2ˉ in water by UV-Vis

  14. Analysis of a Binary Mixture • Case (a) : Significant overlap • Best treated by using a • spreadsheet procedure • with data obtained at • many λ’s. • Case (b) : Little overlap • Measure Abs of X at λʹ • Measure Abs of Y at λʺ • Solve simultaneous • equations for [X] and [Y] Analogous to Fig. 14-9

  15. Analysis of a Binary Mixture of Ti and V

  16. Analysis of a Binary Mixture of Cr2O72- and MnO4-

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