Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods6455. The accuracy of a spectrophotometer can be evaluated by preparing asolution of 60.06 ppm K 2 Cr 2 O 7 in 0.0050 M H 2 SO 4 , and measuringits absorbance at a wavelength of 350 nm in a cell with a pathlength of1.00 cm. The absorbance should be 0.640. What is the molar absorptivityof K 2 Cr 2 O 7 at this wavelength?6. A chemical deviation to Beer’s law may occur if the concentration of anabsorbing species is affected by the position of an equilibrium reaction.Consider a weak acid, HA, for which K a is 2 10 –5 . Construct Beer’slaw calibration curves of absorbance versus the total concentration ofweak acid (C total = [HA] + [A – ]), using values for C total of 1 10 –5 ,3 10 –5 , 5 10 –5 , 9 10 –5 , 11 10 –5 , and 13 10 –5 M for the followingsets of conditions:(a) e HA = e A – = 2000 M –1 cm –1 ; unbuffered solution.(b) e HA = 2000 M –1 cm –1 and e A – = 500 M –1 cm –1 ; unbuffered solution.(c) e HA = 2000 M –1 cm –1 and e A – = 500 M –1 cm –1 ; solution is bufferedto a pH of 4.5.Assume a constant pathlength of 1.00 cm for all samples.7. One instrumental limitation to Beer’s law is the effect of polychromaticradiation. Consider a line source that emits radiation at two wavelengths,l′ and l″. When treated separately, the absorbances at thesewavelengths, A′ and A″, areP ′A′ =−log T ′ = ε′bCP0P ′′A′′ =−log T ′′ = ε′′bCP0If both wavelengths are measured simultaneously the absorbance isP ′ + P ′′A =−log ( )T T( P′ + P′′)0 0(a) Show that if the molar absorptivity at l′ and l″ are the same (e′ =e″ = e), the absorbance is equivalent toA=εbC(b) Construct Beer’s law calibration curves over the concentrationrange of zero to 1 10 –4 M using e′ = 1000 and e″ = 1000, ande′ = 1000 and e″ = 100. Assume a value of 1.00 cm for the pathlength.Explain the difference between the two curves.