Solutions for Spectrophotometry Exercises

Solutions for Spectrophotometry Exercises 1. a) Doubling the frequency doubles the energy. b) Doubling the wavelength halves the energy. 2. Microwave energies correspond to molecular rotation energies. Infrared wavelengths can produce vibrational excitation. Visible light can promote electrons to excited states (in colored compounds). Ultraviolet radiation can electronically excite and ionize compounds and break chemical bonds. 3. Transmittance (T) is the fraction of incident light that passes through a sample. T = P/Po, where Po is the incident radiant power and P is the transmitted power. Absorbance (A) is logarithmically related to transmitance, A = -logT. When all light is transmitted, absorbance is zero. When no light is transmitted, absorbance is infinite. Molar absorptivity (ε) is a measure of how likely a particular wavelength will be absorbed. Absorbance is proportional to concentration, A = εlc. Transmittance, absorbance and molar absorptivity all depend on the wavelength at which they are measured. 4. An absorption spectrum is a graph of absorbance versus wavelength. 5. Absorption of wavelngths in the 400-550 nm range means that only the red end (yellow-orange-red) of the visible light spectrum is not absorbed so the sample appears to be red-orange. 6. Solutions for Spectrophotometry Exercises 7. Since absorbance is directly proportional to concentration, the absorbance will double as the concentration doubles. From Prob. 6, it was noted that if T = 0.450, then A = 0.347. Thus, for the 0.0200 M solution A = 0.694 and 8. Beer’s law only applies to very dilute solutions and to the use of monochromatic radiation. 9. The concentration of the diluted standard is Assuming Beer’s law behavior, Using data for the diluted standard, 10. Basic components of a single-beam spectrophotometer light source monochromator sample holder detector Solutions for Spectrophotometry Exercises 11. Continuous radiation from the light source enters the monochromator through a slit and the radiation reflects off a diffraction grating where it is dispersed (spread out). After focusing, only a small band of radiation emerges from the exit slit of the monochromator. By varying the orientation of the diffraction grating with respect to the incoming radiation, a different wavelength can be selected for passage through the monochromator. 12. In a single-beam instrument only a narrow band of wavelengths passes through the sample and its intensity is detected. In a diode array spectrophotometer all wavelengths emitted from the light source pass through the sample and are dispersed by a diffraction grating onto an array of detectors (photodiodes). Thus the intensity of the transmitted light at all wavelengths is measured simultaneously. 13. Radiation reaching the PMT first strikes a surface called the photocathode where electrons are ejected by the incident photons. These ejected electrons are drawn to another surface (dynode) maintained at a slightly more positive potential, where each incident electron ejects 2 or more electrons. This process is repeated with a total of 8-10 dynode surfaces and results in a significant number of electrons being collected as the electrical signal. 14. In order to obtain the sample absorbance, the intensity of light that passes through a blank solution is first measured (Pblank). Then the intensity of light that passes through the sample (Psample) is measured. The instrument then computes the sample absorbance from -log (Psample/Pblank).