Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods665placing A mix /A SX on the y-axis and A SY /A SX on the x-axis. For example,at a wavelength of 266 nm the value A mix /A SX of is 0.766/0.042, or 18.2,and the value of A SY /A SX is 0.410/0.042, or 9.76. Completing the calculationsfor all wavelengths and plotting the data gives the result shown inFigure 10.67. Fitting a straight-line to the data gives a regression modelofAAmixSX= 0. 8147 + 1.7839×Using the y-intercept, the concentration of MnO 4 – isCC= [ MnO ]= 0.8147×−X4−4−SX10 . 10 MMnO4AASYSXA mix /A SX201510500 2 4 6 8 10A SY /A SXFigure 10.67 Multiwavelength linearregression analysis for the data in PracticeExercise 10.7.or 8.15 10 –5 M MnO 4 – , and using the slope, the concentration ofCr 2 O 7 2– isCCYSY[ Cr O ]2−= 2 7× MCrO=−42−10 . 102 71.78391.0or 1.7810 –4 M Cr 2 O 7 2– .Click here to return to the chapter.Practice Exercise 10.8Figure 10.68 shows a continuous variations plot for the data in this exercise.Although the individual data points show substantial curvature—enoughcurvature that there is little point in trying to draw linear branches forexcess metal and excess ligand—the maximum absorbance clearly occursat an X L of approximately 0.5. The complex’s stoichiometry, therefore, isFe(SCN) 2+ .Click here to return to the chapter.Practice Exercise 10.9The value of K a is−60. 225−0.000K a= (. 100× 10 ) ×= 495 . × 10 −70. 680 −0.225Click here to return to the chapter.Practice Exercise 10.10To determine K a we use equation 10.21, plotting log[(A – A HIn )/(A In – A)]versus pH, as shown in Figure 10.69. Fitting a straight-line to the datagives a regression model ofabsorbance0.80.60.40.20.00.0 0.2 0.4 0.6 0.8 1.0X LFigure 10.68 Continuous variationsplot for the data in Practice Exercise10.8.log A–A HInAIn–A1.00.50.0-0.5-1.03.0 3.5 4.0 4.5 5.0pHFigure 10.69 Determining the pKa ofbromothymol blue using the data inPractice Exercise 10.10.