9/9/09 1 Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Ultraviolet-Visible Absorption Spectrometry 1- Fundamental principles of molecular absorption 2- Beer’s Law 3- Factors that affect Beer’s Law -Sections 6A, 6B, 6C, 13A, 13B, 13D Suggested exercises 6-5, 6-14, 6-15, 6-18, , 6-19, 13-4, 13-6, 13-22, 13-12 (Cover calculations) Solve before quiz on Monday Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Electromagnetic Spectrum Figure 6-3, Section 6B Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Spectroscopy Methods Section 6B9/9/09 2 Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Absorption of Radiation Figure 6-23 and 6-24, Section 6C Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Absorption Spectra Figure 14-2, Section 14B Key Features: Molar absorptivity, ε (cm-1 M-1) Maximum absorption wavelength, λMax (nm)"Absorption range, , λ Range (nm)"Transmittance, T = P/Po"% Transmittance, %T = 100% x P/Po"Absorbance = -log10 T = log (Po/P)"Lecture 02 – Sep 11 Chem 4101 – Fall 2009 UV-VIS Photometer or Spectrophotometer Figure 13-13(a), Section 13D9/9/09 3 Lecture 02 – Sep 11 Chem 4101 – Fall 2009 What happens at the cuvette? Figure 13-1, Section 13A Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Use a narrow wavelength selector Figure 13-4, Section 13B Suggestions: Measure absorbance at the maximum. Use a wavelength selector with no more than 1/10 the width of the absorption band."Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Beer’s Law Log (Po/P) = εbC = A Figure 13-4, Section 13B Assumptions: Diluted solutions (< 0.01M) Equilibrium"Stable molecules or species"Monochromatic radiation"9/9/09 4 Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Chemical deviations from Beer’s law (Indicators) Figure 13-3, Section 14B; Example 13-1 At 430 nm, In-1 is the main absorber At 570 nm, HIn is the main abosrber In mixtures at equilibrium, use equation for multicomponent system: Atotal = A1 + A2 +… = ε1bC1 + ε2bC2 Lecture 02 – Sep 11 Chem 4101 – Fall 2009 Stray light effects on beer’s law Figure 13-6, Section 13B A’ = log [(Po + Ps)/ (P +
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