Analytical Chem istry - DePauw University

588 Analytical Chemistry 2.0IR spectra traditionally are displayed usingpercent transmittance, %T, along they-axis ( for example, see Figure 10.16).Because absorbance—not percent transmittance—isa linear function of concentration,spectral searching and spectralsubtraction, is easier to do when displayingabsorbance on the y-axis.0.80.4absorbance1.20.0A: mixture4000 3500 3000 2500 2000 1500 1000 500wavenumber (cm -1 )0.80.4absorbance1.20.0B: cocaine hydrochloride4000 3500 3000 2500 2000 1500 1000 500wavenumber (cm -1 )0.80.4absorbance1.20.0C: first subtraction (A–B)4000 3500 3000 2500 2000 1500 1000 500wavenumber (cm -1 )Figure 10.37 Identifying the componentsof a mixture by spectral searchingand subtracting. (a) IR spectrum of themixture; (b) Reference IR spectrum ofcocaine . HCl; (c) Result of subtractingthe spectrum of cocaine . HCl from themixture’s spectrum; (d) Reference IRspectrum of mannitol; and (e) The residualspectrum after removing mannitol’scontribution to the mixture’sspectrum.0.80.4absorbance1.20.00.80.4absorbance1.20.0D: mannitol4000 3500 3000 2500 2000 1500 1000 500wavenumber (cm -1 )E: second subtraction (C–D)4000 3500 3000 2500 2000 1500 1000 500wavenumber (cm -1 )stoichiometry of metal–ligand complexes and determining equilibriumconstants. Both of these examples are examined in this section.St o i c h i o m e t r y o f a Me t a l-Li g a n d Co m p l e xWe can determine the stoichiometry of a metal–ligand complexation reactionM+ yL MLyusing one of three methods: the method of continuous variations, themole-ratio method, and the slope-ratio method. Of these approaches, the