Gang Wang Instrumental_Week5&6_CH13_15-2010

Gang Wang Instrumental_Week5&6_CH13_15-2010 -...

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Applying UV-vis Spectrophotometry Qualitative I) The UV-vis absorption spectrum is related to molecular structure, therefore it provides qualitative information as to structure both from ε and λ . In order to take advantage of this it is desirable to have a scanning instrument to measure the entire spectrum (either a double beam, direct reading instrument or a single beam unit with computer interface. By itself an electronic spectrum is not sufficient to identify a compound (while an IR spectrum is)
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Quantitative II) The UV-vis region of the spectrum is particularly well suited to quantitative analysis for several reasons (both chemical & spectroscopic reasons): 1) Large values for ε make it possible to measure low concentrations 2) Many compounds (both organic & inorganic) absorb in the UV-vis region, but most common solvents do not. This is a limiting factor in IR. 3) Absorption bands are broad making it easy to achieve conditions where there are no instrumental deviations from Beer’s Law 4) Good instrumentation is available, i.e. good sources, detectors & monochromators III) UV-vis can be used for studying chemical processes that involve conversion of a non-absorber to an absorber or vice versa
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1) Spectrophotometric Titrations Example where titrant absorbs Sample + Titrant Æ Product MnO 4 - + Fe 2+ Æ MnO 2 + Fe 3+ Volume of Titrant Added A Endpoint Reaction incomplete at the endpoint absorbs
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2) Study Equilibria (acid-base, complex formation, redox, etc.) M + nL ÅÆ ML n Fe 3+ + SCN - ÅÆ FeSCN 2+ [L]/[M] A Products absorb Reactants don’t absorb 12 Essentially all M is reacted to ML and excess L added Hold [M] constant and increase [L]
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Can calculate ε ML from A at excess [L] Get n from extrapolated curve Can calculate equilibrium constant (K) from [ML n ] and mass balances K = ----------- [M] [L] n e.g. C M = [M] + [ML n ] Other calculation methods can be used involving graphical or computer curve fitting procedures, e.g. Job’s Method or the Method of Continuous Variation A Mole ratio 3) Study Kinetics X + Y Æ Z Measure A vs time after mixing For quantitative applications, we usually want to measure at one particular λ .
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Gang Wang Instrumental_Week5&6_CH13_15-2010 -...

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