1 / 45

Summary: (Last lecture)

Absorption spectroscopy. Summary: (Last lecture). definition electromagnetic spectroscopy matter absorption spectroscopy fundamental terms (transmittance, absorbance absorptivity, molar absorptivity). Absorption spectroscopy. Molar absorptivity ( e ). A = e bc.

kelsie-sharp
Download Presentation

Summary: (Last lecture)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Absorption spectroscopy Summary: (Last lecture) • definition • electromagnetic spectroscopy • matter • absorption spectroscopy • fundamental terms • (transmittance, absorbance • absorptivity, molar absorptivity)

  2. Absorption spectroscopy Molar absorptivity (e) A = ebc • = the molar absorptivity, L mol-1 cm-1 (the characteristic of a substance that tells how much light is absorbed at particular wavelength) b= the pathlength of cell, cm C = the concentration of absorbing species, M

  3. Absorption spectroscopy Quantitative aspects of absorption measurements Beer’s Law A = ebc (The heart of spectrophotometry) *Application of Beer’s Law to mixture

  4. Absorption spectroscopy A solution containing more than one kind of absorbing substances: Atotal = A1 + A2 + … + An = e1bc1 + e2bc2 + … + enbcn Conditions: no interaction among the various species

  5. Absorption spectroscopy Limitations to the Applicability of Beer’s Law A a c • monochromatic radiation • dilute solutions ( 0.01 M) only Why?

  6. Absorption spectroscopy Limitations to the Applicability of Beer’s Law At high concentration (> 0.01M): • in concentrated solution, solutes • molecules influence one another • as a result of their proximity. • When solute molecules get close • to one another, their properties • (including molar absorptivity) • change somewhat.

  7. Absorption spectroscopy Limitations to the Applicability of Beer’s Law At high concentration (> 0.01M): • The solute becomes the solvent. • Properties of a molecule are not • exactly the same in different solvent.

  8. Absorption spectroscopy Deviations of Beer’s Law DEVIATIONS 1. Chemical Deviations 2. Instrumental Deviations • Polychromatic Radiation • Stray Light

  9. Chemical deviations • arise when an analyte dissociates, • associates, or reacts with a solvent • to produce having a different • absorption spectrum from the analyte. Ex: acid/base indicators HIn = H+ + In- colour 1 colour 2 Deviations of Beer’s Law

  10. e430 e570 HIn 6.30 x 102 7.12 x 103 In 2.06 x 104 9.61 x 102 Ex: The molar absorptivity of the weak acid HIn (Ka=1.42 x 10-5) and its conjugate base In- at 430 and 570 nm were determined by measurements of strongly acidic and strongly basic solutions of the indicator (where essentially all of the indicator was in HIn and In- forms respectively). The results were

  11. (1) Derive absorbance data for unbuffered solutions having total indicator concentrations ranging from 2 x 10-5 to 16 x 10-5 M Soln. Calculate the [HIn] and [In-] in a solution in which the [indicator] is 2.00 x 10-5 M Here HIn = H+ + In-

  12. From the equation for the dissociation process; Substitution of these relationships into (1) for Ka:

  13. From Beer’s Law:

  14. At 570 nm:

  15. Note: The direction of curvature is opposite at the two wavelengths.

  16. Instrumental deviations • polychromatic radiation Consider a beam consisting of just two wavelengths and  at ,

  17. at , When an absorbance measurement is made with radiation composed of both wavelengths, the measurement A, Am:

  18. when

  19. In experiment, deviations from Beer’s Law resulting from the use of a polychromatic radiation is not appreciable.

  20. Instrumental deviations • stray light Causes: scattering and reflections from various internal surface Characteristic: • differs greatly in wavelength from that • of the principal radiation • may not have passed through the sample

  21. Instrumental deviations • stray light Ps is the power of nonabsorbed stray radiation

  22. note At high concentration and at longer path lengths, stray radiation can also cause deviations from the linear relationship between ABS and path length. M.R. Share, Anal. Chem. 1984, 56, 339A

  23. Instrumental deviations:stray light Summary: The instrumental deviations result in absorbance that are smaller than theoretical. OR The instrumental deviations always lead to negative absorbance error.

  24. Analysis of Mixtures of Absorbing Substances : : • two components behave independently of one • another.

  25. Example 1 The molar absorptivities of compounds X and Y were measured with pure samples of each. e(M-1 cm-1) X Y  (nm) 272 16440 3870 327 3990 6420 A mixture of compounds X and Y in a 1.000 cm cell has an absorbance of 0.957 at 272 and 0.559 at 327 nm. Find the concentrations of X and Y in the mixture.

  26. Example 2 The figure shows the spectra of 1.00x10-4 M MnO4-, 1.00x10-4 M Cr2O72-, and unknown mixture mixture of both. Absorbances at several wavelengths are given in the table. Find the concentration of each species in the mixture Wavelength MnO4- Cr2O72- Mixture (nm) standard standard • 0.042 0.410 0.766 288 0.082 0.283 0.571 320 0.168 0.158 0.422 350 0.125 0.318 0.672 360 0.056 0.181 0.366

  27. Quiz 2: สารละลายของสารอินทรีย์ตัวหนึ่งเตรียมขึ้นจากสารละลาย 0.287 mgในเอธานอล 10 mLพบว่าหากใช้เซลที่มี ความหนา 1.0cm จะให้ค่าการดูดกลืน 1.25 ที่ 305 nm จงคำนวณmolar absorptivity กำหนดให้น้ำหนักโมเลกุล ของสารเท่ากับ 500

  28. Summary: key terms Beer’s Law the relationship between a sample’s absorbance and the concentration of the absorbing species Stray Light any light reaching the detector that does not follow the optic path from the source to the detector

  29. Transmittance the ratio of the radiant power passing through a sample to that from the radiation’s source Absorbance The attenuation of photons as they pass through asample (A) Absorbance spectrum a graph of a sample’s absorbance of electromagneticradiation versus wavelength (frequency or wavenumber)

  30. photon a particle of light carrying an amount of energy equal to hv Next topic: Instruments for absorption measurements

  31. l selector sample optical source detector Instrument components: UV-VIS hn1 hn2 signal processor

  32. Instrument components: UV-VIS Sources: A sources must: • generate a beam of radiation with • sufficient power for easy detection • and measurement • provide output power that is both • stable and intense Types of spectroscopic sources: 1. continuous sources 2. lines sources

  33. Instrument components: UV-VIS continuous sourceslines sources H2 and D2 lamp Tungsten filament lamps Xe arc lamp hollow cathode lamp Hg vapor lamp laser

  34. Instrument components: UV-VIS Tungsten filament lamp: Vis/near IR source • 320-2500 nm

  35. Evaporation: W(s) W(g) W(g) + I2(g) WI2(g) Redeposition: WI2(g) + W(s) W(s) + I2(g) Instrument components: UV-VIS Quartz Tungsten Halogen (QTH) lamp • 200-3000 nm • high temperature (3500 K)

  36. Instrument components: UV-VIS H2 and D2 lamp • provide continuous spectrum in the UV • region (180-375 nm) by electrical • excitation of deuterium or hydrogen • at low pressure mechanism H2 + Eelectrical H2*  H(KE1) + H(KE2) + hv ‘bond dissociation energy’

  37. Instrument components: UV-VIS sample containers

  38. Instrument components: UV-VIS sample containers Note: • a liquid sample is usually contained • in a cell called a cuvet that has a • flat • material • fused silica • glass  only Vis • quartz

  39. Instrument components: UV-VIS wavelength selectors Types 1. Filters 1.1 interference filters 1.2 absorption filters 2. Monochromators

  40. Instrument components: UV-VIS Filters “a wavelength selector that uses either absorption, or constructive and destructive interference to control range of selected wavelengths” • the simplest method for isolating a • narrow band of radiation

  41. Instrument components: UV-VIS Absorption filters • work by selectively absorbing radiation • from a narrow region Interference filters • use constructive and destructive • interference to isolate a narrow range • of wavelengths

  42. Instrument components: UV-VIS Absorption filters • use coloured glass • provide effective bandwidths, range • 30-250 nm the width of the band of radiation passing through a wavelength selector measured at half the band’s height

  43. Instrument components: UV-VIS

  44. Instrument components: UV-VIS Relationship between Absorption and Observed Colour wavelength region removed by absorption (nm) complementary colour of the residual light, as seen by eye colour observed 400-450 violet yellow-green 450-480 blue yellow 480-490 green-blue orange 490-500 blue-green red 500-560 green purple 560-580 yellow-green violet 580-600 yellow blue 600-650 orange green-blue 650-750 red blue-green

More Related