Analytical Chem istry - DePauw University

584 Analytical Chemistry 2.0Practice Exercise 10.6The absorbance spectra for Cr 3+ and Co 2+ overlap significantly. To determinethe concentration of these analytes in a mixture, its absorbance wasmeasured at 400 nm and at 505 nm, yielding values of 0.336 and 0.187,respectively. The individual molar absorptivities (M –1 cm –1 ) aree 400 e 505Cr 3+ 15.2 0.533Co 2+ 5.60 5.07Click here to review your answer to this exercise.For example, in Example 10.6 the molarabsorptivity for Fe 3+ at 550 nm is 119that for Cu 2+ , and the molar absorptivityfor Cu 2+ at 396 nm is 10.2 that forFe 3+ .To obtain results with good accuracy and precision the two wavelengthsshould be selected so that e X > e Y at one wavelength and e X < e Y at theother wavelength. It is easy to appreciate why this is true. Because the absorbanceat each wavelength is dominated by one analyte, any uncertaintyin the concentration of the other analyte has less of an impact. Figure10.35 shows that the choice of wavelengths for Practice Exercise 10.6 arereasonable. When the choice of wavelengths is not obvious, one methodfor locating the optimum wavelengths is to plot e X /e Y as function of wavelength,and determine the wavelengths where e X /e Y reaches maximum andminimum values. 8When the analyte’s spectra overlap severely, such that e X ≈ e Y at allwavelength, other computational methods may provide better accuracy andprecision. In a multiwavelength linear regression analysis, for example, amixture’s absorbance is compared to that for a set of standard solutions atseveral wavelengths. 9 If A SX and A SY are the absorbance values for standardsolutions of components X and Y at any wavelength, then8 Mehra, M. C.; Rioux, J. J. Chem. Educ. 1982, 59, 688–689.9 Blanco, M.; Iturriaga, H.; Maspoch, S.; Tarin, P. J. Chem. Educ. 1989, 66, 178–180.1.00.8mixtureabsorbance0.60.4Figure 10.35 Visible absorption spectra for 0.0250 MCr 3+ , 0.0750 M Co 2+ , and for a mixture of Cr 3+ andCo 2+ . The two wavelengths used for analyzing the mixtureof Cr 3+ and Co 2+ are shown by the dashed lines. Thedata for the two standard solutions are from reference 7.0.20.0Cr 3+Co 2+400 450 500 550 600wavelength