Analytical Chem istry - DePauw University

646 Analytical Chemistry 2.08. A second instrumental limitation to Beer’s law is stray radiation. Thefollowing data were obtained using a cell with a pathlength of 1.00 cmwhen stray light is insignificant (P stray = 0).[analyte] (mM) absorbance0.00 0.002.00 0.404.00 0.806.00 1.208.00 1.6010.00 2.00Calculate the absorbance of each solution when P stray is 5% of P 0 , andplot Beer’s law calibration curves for both sets of data. Explain any differencesbetween the two curves. (Hint: Assume that P 0 is 100).9. In the process of performing a spectrophotometric determination ofFe, an analyst prepares a calibration curve using a single-beam spectrophotometersimilar to that shown in Figure 10.26. After preparing thecalibration curve, the analyst drops and breaks the cuvette. The analystacquires a new cuvette, measures the absorbance of his sample, anddetermines the %w/w Fe in the sample. Will the change in cuvette leadto a determinate error in the analysis? Explain.10. The spectrophotometric methods for determining Mn in steel and fordetermining glucose use a chemical reaction to produce a colored specieswhose absorbance we can monitor. In the analysis of Mn in steel,colorless Mn2+ is oxidized to give the purple MnO4- ion. To analyzefor glucose, which is colorless, we react it with a yellow colored solutionof the Fe(CN)63-, forming the colorless Fe(CN)64- ion. The directionsfor the analysis of Mn do not specify precise reaction conditions, andsamples and standards may be treated separately. The conditions for theanalysis of glucose, however, require that the samples and standards betreated simultaneously at exactly the same temperature and for exactlythe same length of time. Explain why these two experimental proceduresare so different.11. One method for the analysis of Fe 3+ , which can be used with a varietyof sample matrices, is to form the highly colored Fe 3+ –thioglycolic acidcomplex. The complex absorbs strongly at 535 nm. Standardizing themethod is accomplished using external standards. A 10.00 ppm Fe 3+working standard is prepared by transferring a 10-mL aliquot of a 100.0ppm stock solution of Fe 3+ to a 100-mL volumetric flask and dilutingto volume. Calibration standards of 1.00, 2.00, 3.00, 4.00, and 5.00ppm are prepared by transferring appropriate amounts of the 10.0 ppm