Anh văn chuyên ngành hóa 3


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Anh văn chuyên ngành hóa 3

  2. 2. Introduction.  Before the beginning of the 20th century most quantitative chemical analyses used titrimetry as the analytical method analysts achieved highly accurate result.  But limited Other methods developed during this period extended quantitative analysis to include trace level analytes Colorimetry.  One example of an early colorimetric analysis is Nessler’s method. SPECTROSCOPY Group 4 – DHHC6B
  3. 3. Nessler’s method.  The Nessler’s for ammonia.  It was first proposed in 1856.  Nessler’s found that adding an alkaline solution of HgI2 and KI to a dilute solution of ammonia produced a yellow to reddish brown colloid with the color determined by the concentration of ammonia. A comparison of the sample’s color to that for a series of standards was used to determine the concentration of ammonia. SPECTROSCOPY Group 4 – DHHC6B
  4. 4. Introduction  At the end 19th century, spectroscopy was limited to:  The absorption.  Emission.  Scattering of UV/VIS.  Infrared electromagnetic radiation.  During the 20th , spectroscopy has been extended to include other form of electromagnetic radiation (photon spectroscopy) • X-rays • Microwaves • Radio waves • Energetic particles such as: electrons and ions SPECTROSCOPY Group 4 – DHHC6B
  5. 5. Introductions. SPECTROSCOPY Group 4 – DHHC6B
  6. 6. Introduction  Spectroscopy is used to qualitatively or quantitatively study the atoms or molecules, or to study physical processes.  The interaction of radiation with matter can cause redirection of the radiation and/or transitions between the energy levels of the atoms or molecule. SPECTROSCOPY Group 4 – DHHC6B
  7. 7. Introduction  A transition from a lower level to a higher level absorption ( transfer energy)  A transition from a higher level to a lower level  emission (transfer energy)  Redirection of light due to its interaction with matter scattering (may or may not occur with transfer of energy) SPECTROSCOPY Group 4 – DHHC6B
  8. 8. Absorption SPECTROSCOPY Group 4 – DHHC6B Type of excitation depend on the wavelength of the light UV/Visible promoted electrons to higher orbital Infared excited vibrations Atoms or molecules absorb light  a higher energy level Microwaves excited rolations Measuring the concentration of absorbing species in a sample is accomplished by Beer-Lambert Law
  9. 9. Absorption Useful for indentifying of compounds Depend on its energy level structure A function of wavelength The absorption of light SPECTROSCOPY Group 4 – DHHC6B
  10. 10. Emission SPECTROSCOPY Group 4 – DHHC6B 1.How is the emitting radiation? 2. Atomic – emission spectroscopy and Atomic – fluorescence spectroscopy 3. How is the flourescence of molecules and the phosphorescence of molecules?
  11. 11. Emission SPECTROSCOPY Group 4 – DHHC6B  Atoms or molecules at higher energy level low levels by emitting radiation (emission or luminescence)  Atoms by a high-temperature energy source, this light emission atomic or optical emission.  Atoms excited with light atomic fluorescence. decay Excited called called
  12. 12. Emission Group 4 – DHHC6B  For molecules it is called : • Fluorescence if the transition is between states of the same spin. • Phosphorescence if the transition occur between states of different spin. The emission intensity of an emitting substance is linearly proportional to analytes concentration at low concentration, and is useful for quantitating emitting species. SPECTROSCOPY
  13. 13. UV/VIS and infrared spectrophotometry 1 Colorimetric: visible light was absorbed by sample, was the earliest application of molecular absorption spectroscopy 2 Concentration of analyte was determined by: • Using Nessler tubes. • Using an instrument called a colorimeter. 3 IR was discovered in 1800, their uses in optical molecular absorption spectroscopy Group 4 – DHHC6B SPECTROSCOPY UV radiation was discovered in 1801, was limited by the lack convenient for detecting the radiation. 4
  14. 14. Introduction UV/VIS spectrophotometer  The UV/VIS spectrophotometer uses two light sources:  A deuterium (D2) lamp for ultraviolet light.  A tungsten (W) lamp for visible light. SPECTROSCOPY Group 4 – DHHC6B
  15. 15. Introduction UV/VIS spectrophotometer  Principles of machine operation: SPECTROSCOPY Group 4 – DHHC6B
  16. 16. Single-Beam UV/VIS Spectrophotometer  Single-Beam spectrophotometer are often sufficient for making quantitative absorption measurements in the UV/VIS spectral region.  The concentration of analyte in solution can be determined by: - Measuring the absorbance at a single wavelength. - Applying the Beer-Lambert Law. SPECTROSCOPY Group 4 – DHHC6B
  17. 17. Single-Beam UV/VIS Spectrophotometer A light- emitting diode (LED) Instrumentation A photodiode dectector A sample container. The simplest instruments use a single-wavelength light source. SPECTROSCOPY Group 4 – DHHC6B
  18. 18. Single-Beam UV/VIS Spectrophotometer SPECTROSCOPY Group 4 – DHHC6B
  19. 19. Dual-Beam UV/VIS Spectrophotometer  In UV absorption spectroscopy, obtaining a spectrum requires manually measuring the transmittance of the sample and solvent at each wavelenght.  The double-beam design greatly simplifies this process by measuring the transmittance of the sample and solvent simultaneously. SPECTROSCOPY Group 4 – DHHC6B
  20. 20. Dual-Beam UV/VIS Spectrophotometer  Instrumentation. reference sample detector detector ratio Mono- chromator LAMP SPECTROSCOPY Group 4 – DHHC6B
  21. 21. Applications.  Absorption measurements based upon ultraviolet or visible radiation find widespread application for the qualitative and quantitative determination of molecular species SPECTROSCOPY Group 4 – DHHC6B
  22. 22. Applications. Quantitative analysis by absorption measurements Applications to absorbing species Applications to nonabsorbing species Application of absorption measurement to qualitative SPECTROSCOPY Group 4 – DHHC6B
  23. 23.  UV/VIS spectrophotometry have somewhat limited application for qualitative analysis. Unambiguous identification is impossible.  Confirmation of the presence of an aromatic amine or a phenolic structure may be obtained by comparing the effects of pH on the spectra of solutions containing the sample with those. Application of absorption measurement to qualitative analysis SPECTROSCOPY Group 4 – DHHC6B
  24. 24. Quantitative analysis by absorption measurements  Absorption spectroscopy is one of the most useful and widely used tools available to the chemist for quantitative analysis.  Important characteristics of spectrophotometric and photometric methods include: SPECTROSCOPY Group 4 – DHHC6B
  25. 25. Quantitative analysis by absorption measurements • Wide applicability to both organic and inorganic systems1 • Typical sensitivities of 10^-4 to 10^-5 M2 • Moderate to high selectivity3 4 5 • Ease and convenience of data acquisition • Good accuracy SPECTROSCOPY Group 4 – DHHC6B
  26. 26. Applications to absorbing species.  Spectrophotometric analysis for any organic compound containing one or more of these groups is potentially feasible.  A number of inorganic species also absorb and are thus susceptible to direct determination; we have already mentioned the various transition metals. In addition, a number of other species also show characteristic absorption.  Examples include nitrite, nitrate, and chromate ions; osmium and ruthenium tetroxides; molecular iodine; and ozone SPECTROSCOPY Group 4 – DHHC6B
  27. 27. Applications to nonabsorbing species  Numerous reagents react selectively with nonabsorbing species to yield products that absorb strongly in the ultraviolet or visible regions.  The successful application of such:  Reagents to quantitative analysis usually requires that the color  Forming reaction be forced to near completion SPECTROSCOPY Group 4 – DHHC6B
  28. 28. Applications to nonabsorbing species • Forming reagents are also frequently employed for the determination of absorbing species such as transition- metal ions • The molar absorptivity of the product will frequently be orders of manitude greater than that of the uncombined psecies Note SPECTROSCOPY Group 4 – DHHC6B
  29. 29. Applications to nonasorbing species.  A host of complexing agents find appilication in the determination of inorganic species.  Typical inorganic reagents include:  Of even more importance are organic chelating agents that form stable, colored complexes with cations. The thiocyanate ion for Fe, Co, Mo The anion of H2O2 for Ti, Va, Cr Iodide ion for Bi, Pb, Te SPECTROSCOPY Group 4 – DHHC6B
  30. 30. Procedure Cleaning and handing of cells Selection of wavelength Standard addition method Variables that influence absorbance Determination of the relationship between absorbance and concentration SPECTROSCOPY Group 4 – DHHC6B
  31. 31. Procedural details The pH of the solution The temperature High electrolyte concentration Variables that influence absorbance The nature of the solvent The presence of interfering subtances SPECTROSCOPY Group 4 – DHHC6B
  32. 32. Procedural details Group 4 – DHHC6B  Spectrophotometric absorbance measurements are ordinarily made at a wavelength corresponding to an absorption peak, because the change in absorbance per unit of concentration is greatest at this point; the maximum sensitivity is thus realized.  In addition, the absorption curve is often flat in the region; under these circumstances, good adherence to Beer’s law can be expected. Finally, the measurements are less sensitive to uncertainties arising from failure to reproduce precisely the wavelength setting of the instrument. SPECTROSCOPY
  33. 33. Procedural details Group 4 – DHHC6B  After deciding upon the conditions for the analysis, it is necessary to prepare a calibration curve from a series of standard solutions. These standards should approximate the overall composition of the actual samples and should cover a reasonable concentration range of the analyte.  Seldom, if ever, is it safe to assume adherence to Beer’s law and use only a single standard to determine the molar absorptivity. The results of an analysis should never be based on a literature value for the molar absorptivity. SPECTROSCOPY
  34. 34. Procedural details Group 4 – DHHC6B  It is apparent that accurate spectrophotometric analysis requires the use of good – quality, matched cells. These should be regularly calibrated against one another to detect differences that can arise from scratches, etching, and wear.  Equally important is the use of proper cell cleaning and drying techniques.  Erickson and Suries recommend the following cleaning sequence for the outside windows of cell. SPECTROSCOPY
  35. 35. Procedural details Group 4 – DHHC6B  Prior to measurement, the cell surfaces are cleaned with a lens paper soaked in spectrograde methanol.  The paper is held with a hemostal; after wiping, the methanol is allowed to evaporate, leaving the cell surfaces free of contaminants.  The authors showed that this method was far superior to the usual procedure of wiping the cell surfaces with a dry lens paper, which apparently leaves lint and films on the surface. SPECTROSCOPY
  36. 36. Procedural details Group 4 – DHHC6B  Ideally, calibration standards should approximate the composition of the sample to be analyzed not only with respect to the analyte concentrations of the other species in the sample matrix, in order to minimize the effect of various components of the sample on the measured absorbance.  For example, the absorbance of many colored complexes of metal ions is decreased to a varying degree in the presence of sulfate and phosphate ions as a consequence of the tendency of these anions to form colorless complexes with metal ions. SPECTROSCOPY
  37. 37. Procedural details Group 4 – DHHC6B  The color – formation reaction is often less complete as a consequence, and lowered absorbances are the results. The matrix effect of sulfate and phosphate can often be counteracted by introducing into the standards amounts of the two species that approximate the amounts found in the samples.  When complex materials as solid, minerals, plant ash are being analyzed, preparation of standards that match the samples is often impossible. When this is the case, the standard addition method is often helpful in counteracting matrix effects. SPECTROSCOPY
  38. 38. Procedural details Group 4 – DHHC6B  The standard addition method can take several forms. The one most often chosen for photometric or spectrophotometric analyses, and the one that will be discussed here, involves adding one or more increments of standard solution to the sample aliquots of the same size. SPECTROSCOPY
  39. 39. Procedural details Group 4 – DHHC6B  Each solution is then diluted to a fixed volume before measuring its absorbance. It should be noted that when the amount of sample is limited, standard additions can be carried out by successive introductions of increments of the standard to a single measured aliquot of the unknown. Measurements are made on the original and after each addition. This procedure is often more convenient for voltammetric and potentiometric measurements and will be discussed in later sections of the text. SPECTROSCOPY
  40. 40. LOGO