Midterm Exam with Solutions - Instrumental Methods in Analytical Chemistry | CHEM 105, Exams of Chemistry

Download Midterm Exam with Solutions - Instrumental Methods in Analytical Chemistry | CHEM 105 and more Exams Chemistry in PDF only on Docsity! Chemistry 105 November 3, 2003 Midterm Exam - Solutions 1. (20 pts.) Consider atomic absorption (AA) and atomic emission (AE) spectroscopies. A. Draw the schematic diagrams of both AA and AE spectrometers and identify all the major components. 6 pts. The drawing should include: AA: light source, atom cell monochromator and detector AE: source/atom cell, monochromator, tetector B. Would the same monochromator used in AA be also well suited for the AE spectrometer? Explain your answer focusing on the role of the monochromator in each of these two techniques. 7 pts. No, in atomic absorption the spectral resolution is achieved by means of the narrow linewidth of the hollow cathode lamp. In AA, the monochromator is used to limit the amount of light emitted by the flame reaching the detector. In atomic emission, however, the resolution is achieved by the monochromator which must have several orders of magnitude better resolving power than the one used in AA. C. Flame is commonly used as an “atom cell” in AA but only occasionally in AE. What limits the usefulness of the flames as sources (atom cells) in AE? 7 point. The temperature of the flames is insufficiently high. The temperature determines, via Boltzmann distribution, the relative population of the atoms in the excited state and thus the sensitivity of AE determinations. ICP source are used instead since plasma offers higher temperatures. 2. (20 pts.) Describe the idea and the physical phenomena involved in the Smith- Hieftje background correction method. What type of “background” problems was this method designed to address? Smith-Hieftje is a background correction method in AA designed to correct for light absorption by “broad band” absorbers, the molecular species introduced into the flame together with the analyte. 2 The method relies on the changes of the spectral profile of light emitted by HCL at different lamp current. At a high lamp current, the emitted light is significantly broadened and self-reversal phenomenon eliminates a large portion of the light intensity at and around the maximum emission observed at low (normal) current level. Consequently, absorbance due to the background plus the analyte is measured at low current while mainly background absorbance is measured at high lam current. The difference approximates well the absorbance due to the analyte. 2 pts : It is an AAS technique. 4 pts : The effect is achieved by running the hollow cathode lamp at alternating high and low currents. 4 pts : The correct profile for the lamp emission at high and low current? 4 pts : Explanation of the self-reversal phenomenon. 6 pts : The method is used to correct for a broad-band absorbers? 3. (20 pts.) Consider a chronoamperometric (potential step) experiment carried out with a hanging mercury drop working electrode with a drop radius, r = 3 x 10 -2 cm. Assume that the solution contains 0.5 mM solution of Ox with E o = -0.40 V vs SCE and 0.1 M NaNO3. The potential step of -0.5 V is applied at t=0 from Einitial= -0.15 V vs SCE so that Efinal = -0.65 V. A. Is it possible to use linear diffusion equations to theoretically predict the current vs time plot expected in this experiment? If NO, explain why. If Yes, calculate under what specific set of conditions this would be acceptable. 10 pts. Yes it is possible to use linear diffusion equations to describe i(t) but only as relatively short times after the potential pulse is applied. In other words: 1/ 2 * * nFAD C nFADC rtπ >> B. Using one correctly labeled graph, sketch as quantitatively as possible the plots of the faradaic (iF) and charging (ic) currents expected in this experiment over a broad