Question

4.

a. State Beer’s Law. What is meant by the statement that Beer’s Law is a limiting law?

b. Describe two apparent deviations from Beer’s Law and indicate how the deviation specifically affects the calibration curve.

c. Phosphate ions do not absorb radiation in the visible region of the electromagnetic spectrum. Nonetheless, Phosphate ions can be detected quantitatively utilizing absorption spectrophotometry in the visible region. Explain.

d. Why can’t an ultraviolet spectrophotometer be used to obtain infrared spectra? Be specific.

e. State three factors that contribute to photometric error.

f. Why is fluorescence spectroscopy more sensitive than absorbance spectroscopy?

g. Why must there be a vertical and horizontal adjustment of the hollow cathode lamp position with respect to the flame for an analysis using atomic absorption spectroscopy?

h. Why is a narrow slit width important for qualitative analysis, but not as critical for quantitative analysis?

i. What is the advantage of using a double beam instrument over a single beam instrument?

j. The molar absorptivity of p-nitrophenol at 407 nm is shown in the table two pH’s. What does the information tell you about p-nitrophenol?

                       

                        pH                   molar absorptivity

                        4.0                             330

                        9.0                         18330

k. The temperature of the plasma used in ICP is about twice as high as the temperature of a flame, yet there are few ionization problems associated with the use of an ICP. Explain.

l. Use an energy diagram to illustrate the change(s) in energy that an electron undergoes when fluorescence is observed for a molecule. Explain the energy diagram.

m. An acetylene/air flame (2450 K) can be used to analyze a sample containing Pd by atomic absorption spectroscopy whereas the analysis of V requires an acetylene/nitrous oxide flame (3000K). Explain.

Answer 1

(A) According to Beer, the absorbance A (for a monochromatic radiation beam) of a solution containing an absorbing compound, is proportional to its concentration C, and is given by:

A= εbC

Where ε is the molar absorptivity of compound at the particular wavelength, and b is the optical path length.

Limitting Law: Beer law is capable of describing absorption behavior of solutions containing relatively low amounts of solutes (<10mM) dissolved in it. When the concentration of the analyte in the solution is high (>10mM), the analyte begins to behave differently due to interactions with the solvent and other solute molecules and at times even due to hydrogen bonding interactions.

(B) Beer's law is subjected to certain real and apparent deviations.

Real deviations are mostly encountered in concentrated solutions of the absorbing compounds. These deviations are due to interactions between the absorbing species and to alterations of the refractive index of the medium.

Apparent deviations are due to chemical reasons arising when the absorbing compound, dissociates, associates, or reacts with a solvent to produce a product having a different absorption spectrum.

In principle, it is possible to determine the concentration of an unknown solution of by solving the Beer equation for concentration

C= A/εb

Values of  ε are tabulated for many common molecules, but the trouble is that ε varies as a function of wavelength, temperature, solvent, pH, and other chemical conditions, so if the conditions of the sample don't match those with which the ε was measured, the calculated concentration won't be correct. Also, the absorbances measured on your instrument may not vary linearly with concentration, due to the deviations discussed above, in which case no single value of ε would give accurate results. As a result, it is much more common in practice to prepare a series of standard solutions of known concentration, whose chemical conditions as close as possible to those of the sample, measure their absorbances on your instrument, and plot a calibration curve with concentration of the standards on the x-axis vs measured absorbance on the y-axis. Once the calibration curve is established, unknown solutions can be measured and their absorbances converted into concentration using the calibration curve.

(c) By the use of Colourimetric technique one can easily determine the phosphate content of natural and wastewater sources. Phosphate is considered to be one the most important nutrients in natural water. The phosphate found in natural waters mainly exists as the orthophosphate species, PO₄^3-, however, the polyphosphates P₂O₇^4- and P₃O₁₀^5- are frequently encountered. These polyphosphate species may be hydrolysed to produce the orthophosphate, however, the species which dominates will depend on the pH prevailing in the particular environment. Phosphate will readily react with ammonium molybdate in the presence of suitable reducing agents to form a blue coloured complex, the intensity of which is directly proportional to the concentration of phosphate in the solution. The phosphate content of an unknown water sample can be obtained by first plotting the absorbances of a series of standard solutions against the corresponding concentrations, thus giving a calibration curve. The concentration of phosphate in the unknown sample can then be determined from the graph.

(D) Ultraviolet (UV) and IR spectroscopy both interrogate molecules via electromagnetic radiation. But they differ is wavelength (and thus energy) and the types of molecular/atomic interactions they probe. IR is a lower energy radiation and is generally close in energy to many vibrational transitions of molecules while, UV is much higher energy, and closer to the electronic transitions of molecular orbitals (ie: affects electronic structure/electrons in orbitals). Additionally, UV absorption by molecules correspond to differences in the (covalent) bonding of atoms. Compounds with metal ions or with (conjugated) double bonds often show UV or visible absorption in a single broad band that can help identify the compound but rarely deliver enough information to identify it unambiguously by itself.