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Exploring Chemical Analysis Fourth Edition

歐亞書局. Exploring Chemical Analysis Fourth Edition. 18. Let There Be Light. P.393. Let there be light and there was light. Spectrophotometry -the use of electromagnetic radiation to measure chemical concentrations. P.393. 18-1 Properties of Light.

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Exploring Chemical Analysis Fourth Edition

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  1. 歐亞書局 ExploringChemicalAnalysisFourth Edition 18 Let There Be Light P.393

  2. Let there be light and there was light.

  3. Spectrophotometry-the use of electromagnetic radiation to measure chemical concentrations P.393

  4. 18-1 Properties of Light

  5. Light can be described both as waves and as particles. • Light waves consist of perpendicular, oscillating electric and magnetic fields. • Wavelength, λ. • Frequency, ν, is the number of oscillations per second. • One oscillation per second is also called 1 hertz (Hz). •  Relation between frequency and wavelength: λν=c P.393

  6. Figure 18-1 Figure 18-1 Plane-polarized electromagnetic radiation. Ordinary, unpolarized light has electric and magnetic field components in all planes. Polarized light processes in one plane.

  7. Example:Relating Wavelength and Frequency • What is the wavelength of radiation in your microwave oven, whose frequency is 2.45 GHz? SOLUTION: C = n l P.394

  8. Light can also be thought of as particles called photons. The energy, E (measured in joules, J), of a photon is proportional to itsfrequency： h: Planck’s constant ( = 6.626×10-34 J‧s). : (=1/λ) is called the wavenumber 波數. 能量 ~~ 正比於波數 頻率 ~~ 反比於波長 P.394

  9.  High energy low energy  Figure 18-2 Figure 18-2 Electromagnetic spectrum, showing representative molecular processes that occur when radiation in each region is absorbed. The visible spectrum spans the wavelength range 380 to 780 nanometers (1 nm= 10－9 m). P.395

  10. vibration  low energy high energy  Figure 24-3 in Skoog’s analytical Chemistry, 8th edition

  11. 能量 ~~ 正比於波數 頻率 ~~ 反比於波長

  12. Example:Photon Energies By how many joules is the energy of a molecule increased when it absorbs (a) visible light with a wavelength of 500 nm (b) infrared radiation with a wavenumber of 1 251 cm-1? SOLUTION: (a) (b)

  13. Figure 18-3 The lowest energy state of a molecule is called the ground state. When a molecule absorbs a photon  its energy increases  the molecule is promoted to an excited state. 吸收能量 釋放能量 • Absorption of light increases the energy of a molecule. • Emission of light decreases its energy. P.395

  14. 18-2 Absorption of Light

  15. Figure 18-4 A spectrophotometer measures transmission of light. Figure 18-4 Schematic representation of a single-beam spectrophotometric experiment. In Figure 18-4, light passes through a monochromator (單色器,單光器), a device that selects a narrow band of wavelengths. Light with a very narrow range of wavelength is said to be monochromatic. 只有一個波長之光線的,單色光的 P.396

  16. Transmittance 穿透率 Transmittance, T, is the fraction of incident light that passes through a sample. P0≧ P 0 ≦ T < 1 P.396

  17. Absorbance 吸光率 The most useful quantity for chemical analysis is absorbance, A: P0≧ P When no light is absorbed, P = P0 and A = 0.  The sample does not absorb the light. P.397

  18. Figure 24-7 in Skoog’s analytical Chemistry, 8th edition

  19. Example:Absorbance and Transmittance • What absorbance corresponds to 99% transmittance? • What absorbance correspondsto 0.10% transmittance? SOLUTION: Transmittance 穿透率 ↗ Absorbance 吸光率↘ P.397

  20. Peak absorbance at 555 nm is proportional to concentration from 0.6μM to 3 mM. Absorbance is proportional to the concentration of light-absorbing molecules in the sample. Figure 18-5 shows that the absorbance of KMnO4 is proportional to concentration over four orders of magnitude (fro 0.6μM to 3 mM).

  21. b Beer’s Law • A : absorbance • ε:molar absorptivity 莫耳吸光係數 • b: the pathlength that the light travels through a substance • c : concentration P.398

  22. Color Plate 13 shows that color intensity increase as the concentration of the absorbing molecule increases. Absorbance is a measure of the color. Color Plate 13 COLOR PLATE 13 Fe(phenanthrolines)32+ Standards for Spectrophotometric Analysis (Section 18-2) Volumetric flasks containing Fe(phenanthroline)32+ solutions with iron concentrations ranging from 1 mg/L (left) to 10 mg/L (right). P.398

  23. Example:Using Beer’s Law The peak absorbance of 3.16×10-3 M KMnO4 at 555 nm in a 1.000-cm-pathlength cell in Figure 18-5 is 6.54. • Find the molar absorptivity and percenttransmittance of this solution. • What would be the absorbance if the pathlength were 0.100 cm? • What would be the absorbance in a 1.000-cm cell if the concentration were decreased by a factor of 4? P.398

  24. The peak absorbance of 3.16×10-3 M KMnO4 at 555 nm in a 1.000-cm-pathlength cell in Figure 18-5 is 6.54. (a) Find the molar absorptivity and percenttransmittance of this solution. SOLUTION: Percent transmittance is 100T=2.88×10-5 %. When the absorbance is 6.54, transmittance is very tiny. P.399

  25. (b) What would be the absorbance if the pathlength were 0.100 cm? A = (2.07×10-3 M-1 cm-1) (0.1cm) (3.16×10-3 M) = 0.654 If we decrease pathlength by a factor of 10, we decrease absorbance by a factor of 10 to 6.54/10=0.654. P.400

  26. (c) What would be the absorbance in a 1.000-cm cell if the concentration were decreased by a factor of 4? A = (2.07×10-3 M-1 cm-1) (10cm) (3.16×10-3 M/4) = 1.64 If we decrease concentration by a factor of 4, we decrease absorbance by a factor of 4 to 6.54/4 to 1.64.

  27. Example: Finding Concentration from Absorbance Gaseous ozone has a molar absorptivity of 2700 M-1cm-1 at the absorption peak near 260 nm in the spectrum at the beginning of this chapter.  Find the concentration of ozone (mol/L) in air if a sample has an absorbance of 0.23 in a 10.0-cm cell. Air has negligible absorbance at 260 nm. SOLUTION: P.400

  28. 280-320 nm 320-400 nm Absorption Spectra An absorption spectrum is a graph showing how A (or ε) varies with wavelength (or frequency or wavenumber). Figure 18-6 Absorption spectrum of typical sunscreen lotion shows absorbance versus wavelength in the ultraviolet region. P.400

  29. SPF = 1/T SPF↗ T ↘ A ↗ See Problem 18-12

  30. Example:How Effective Is Sunscreen? What fraction of ultraviolet radiation is transmitted through the sunscreen in Figure 18-6 at the peak absorbance near 300 nm? • From the spectrum, the absorbance A is about 0.35 at 300 nm.  The transmittance is T=10－A = 10－0.35 = 0.45 = 45%. Just over half the ultraviolet radiation (55%) is absorbed by sunscreen and does not reach your skin. Absorption spectrum of typical sunscreen lotion SOLUTION: P.401

  31. White light contains all colors of the rainbows. A substance that absorbs visible light appear colored, when white light is transmitted through it or reflected from it. The substance absorbs certain wavelengths of white light, and our eyes detect wavelengths that are not absorbed. P.402

  32. 18-3 Practical Matters

  33. The instrument represented in Figure 18-4 is called a single-beam spectrophotometer because it has only one beam of light.

  34. Figure 18-7 Sample is usually contained in a cell called a cuvet, which has flat, fused-silica faces. P.402

  35. Common cuvets for ultraviolet and visible measurements.

  36. Transmittance ranges for various optical materials. Figure 25-2, Skoog • Simple glasses are fine in the visible region. • Fused silica or quartz is necessary in the UV region (< 380 nm). • Halide salts (KBr, NaCl, AgCI) are often used in the IR region but have the discadvantages of being expensive and somewhat water soluble.

  37. Silicate glass is completely adequate for use in the visible region • In the UV region, fused silica or quartz must be substituted. • In the IR region, glass, quartz, and fused silica all absorb at wavelengths longer than about 2.5 μm. • Optical elements for IR spectrometry are typically made from halide salts or, in some cases, polymeric materials. Figure 25-2, Skoog

  38. In a single-beam spectrophotometer, we do not measure the power of incident beam P0, directly. Rather, the radiant power passing through a reference cuvet containing pure solvent is measured and defined as P0. Skoog, chapter 24 P0和Pe(或P)之間的差異未必是被樣品吸收 而是以其他方式如散射(scattering)或反射(reflection)

  39. P0: radiation power passing through cuvet filled with solvent • P: radiation power passing through cuvet filled sample (solvent + solute) • Transmittance = P/P0 In a single-beam spectrophotometer (1) Measure P0 (reference: pure solvent) (2) Measure P (sample: solvent + solute)

  40. Double-beam spectrophotometer • P0: radiation power passing through cuvet filled with solvent • P: radiation power passing through cuvet filled sample (solvent + solute) • Transmittance = P/P0 With a double-beam system, you can measure P and P0 simultaneously!

  41. Good operating techniques • Cuvets should be held with a tissue to avoid putting fingerprints on the cuvet faces and must be kept scrupulously clean. Do not touch the clear face of a cuvet. Cuvet cleaning reagent

  42. 18-4 Using Beer’s Law

  43. Spectrophotometric analysis with visible radiation is called colorimetric analysis. For a compound to be analyzed by spectrophotometry, it must absorb electromagnetic radiation, and this absorption should be distinguishable from that of other species in the sample. Biochemists assay proteins in the ultraviolet region at 280 nm because the aromatic amino acids tyrosine, phenylalanine, and tryptophan have maximum absorbance near 280 nm. tryptophan tyrosine phenylalanine

  44. SOLUTION: Example:Measuring Benzene in Hexane • A solution prepared by dissolving 25.8 mg of benzene (C6H6, FM 78.11) in hexane and diluting to 250.0 mL has an absorption peak at 256 nm, with an absorbance of 0.266 in a 1.000-cm cell. Hexane does not absorb at 256 nm.  Find the molar absorptivity of benzene at this wavelength. Molarabsorptivity =ε = A/bc = 0.266/(1.000m)(1.321×10-3 M) = 201.3 M-1cm-1. P.404

  45. (b) A sample of hexane contaminated with benzene has an absorbance of 0.070 at 256 nm in a cell with a 5.000-cm pathlength.  Find the concentration of benzene. SOLUTION: P.405

  46. BOX 18-2 Designing a colorimetric reagent to detect phosphate Free pyrocatechol violet indicator is yellow Pyrocatechol violet indicator bound to metal is blue Near neutral pH, phosphate binds tightly to the two Zn2+ ions  The pyrocatechol is displaced and is free A designed ligand that binds to two Zn2+ ions  The distance between the two Zn2+ ions is just right to bind to the metal ion indicator pyrocatechol The change in adsorption provides a quantitative measure the amount of phosphate.

  47. UV/Vis spectra obtained by additions of [Zn2(H-bpmp)]3+ solution (final concentrations: 0, 10, 20, 30, 40, and 50 mm) to the pH 7.0 aqueous buffer (HEPES, 10 mm) containing pyrocatechol violet (50 mm); b) UV/Vis spectra obtained by additions of HPO42- solution (final concentrations: 0, 25, 50, 100, 150, 200, 250, and 500 mm) to the pH 7.0 aqueous buffer (HEPES, 10 mm) containing [Zn2(H-bpmp)(pyrocatechol violet)]t (50 mm). UV/Vis spectra of the [Zn2(H-bpmp)]3+ -pyrocatechol violet mixture (50 mm in a pH 7.0 aqueous solution) in the presence of various anions (250 mm).

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