Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods575A quantitative analysis based on the absorption of infrared radiation,although important, is less frequently encountered than those for UV/Visabsorption. One reason is the greater tendency for instrumental deviationsfrom Beer’s law when using infrared radiation. Because an infrared absorptionband is relatively narrow, any deviation due to the lack of monochromaticradiation is more pronounced. In addition, infrared sources are lessintense than UV/Vis sources, making stray radiation more of a problem.Differences in pathlength for samples and standards when using thin liquidfilms or KBr pellets are a problem, although an internal standard canbe used to correct for any difference in pathlength. Finally, establishing a100% T (A = 0) baseline is often difficult because the optical properties ofNaCl sample cells may change significantly with wavelength due to contaminationand degradation. We can minimize this problem by measuringabsorbance relative to a baseline established for the absorption band. Figure10.34 shows how this is accomplished.Another approach is to use a cell with afixed pathlength, such as that shown inFigure 10.32b.En v i r o nm e n t a l Ap p l i c a t i o n sThe analysis of waters and wastewaters often relies on the absorption of ultravioletand visible radiation. Many of these methods are outlined in Table10.6. Several of these methods are described here in more detail.Although the quantitative analysis of metals in waters and wastewatersis accomplished primarily by atomic absorption or atomic emissionspectroscopy, many metals also can be analyzed following the formationof a colored metal–ligand complex. One advantage to these spectroscopicmethods is that they are easily adapted to the analysis of samples in the fieldusing a filter photometer. One ligand that is used in the analysis of severalmetals is diphenylthiocarbazone, also known as dithizone. Dithizone isnot soluble in water, but when a solution of dithizone in CHCl 3 is shaken100Atomic absorption is the subject of Section10D and atomic emission is the subjectof Section 10G.The structure of dithizoneis shown to the right. SeeChapter 7 for a discussion ofextracting metal ions usingdithizone.SHNNNHN80% transmittance604020P 0P TA = –log P TP 00800 850 900 950 1000wavenumberFigure 10.34 Method for determining absorbancefrom an IR spectrum.