Download Applying UV-vis Spectrophotometry - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Applying UV-vis Spectrophotometry Qualitative (as discussed) 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) docsity.com 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. docsity.com
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2) Study Equilibria (acid-base, complex formation, redox, etc.) M + nL MLn Fe3+ + SCN- FeSCN2+ [L]/[M] A Products absorb Reactants don’t absorb 1 2 Essentially all M is reacted to ML and excess L added Hold [M] constant and increase [L] docsity.com Can calculate εML from A at excess [L] Get n from extrapolated curve Can calculate equilibrium constant (K) from [MLn] and mass balancesK = ----------- [M] [L]n e.g. CM = [M] + [MLn] 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 docsity.com Round cells, e.g. Spectronic 20 cells should be positioned the same way for each measurement because of imperfections in glass (square cells too) 3) Source drift – in single beam instruments. Must: a) Allow source sufficient time to warm up b) Check 0 and 100 % T settings frequently docsity.com 4) Stray Radiation – strikes detector without going through sample IoAtrue = log ------I Io + Is where Is is intensityAmeasured = log -----------I + Is of stray radiation Stray Light Specification – important characteristic of monochromators docsity.com Stray Light Specification e.g. Cary 14 Is = < 10-6 Io at all λ’s Spectronic 505 Is = < 10-3 Io Stray light more serious at high concentrations (i.e. high A) makes it difficult to measure high A (i.e. A = 3 or 4) Some instruments are capable of measuring large absorbance Spec 505 100 + 0.1 Ameasured = log ------------- = 1.96for Atrue = 2 1 + 0.1 Io 100= log --- = log ----- = 2I 1 Is = 10 -3 x 100 = 0.1 docsity.com
