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Understanding the Properties of Light and Spectrophotometry

This article discusses the relation between frequency and wavelength of light, as well as the properties of light and its applications in spectrophotometry. It covers topics such as the electromagnetic spectrum, absorption of light, Beer's Law, and the use of spectrophotometers in analysis.

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Understanding the Properties of Light and Spectrophotometry

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  1. Ch 18 – Let There Be LightCh 19 – Spectrophotometry: Instruments and Applications

  2. Properties of Light- Sec 18-1

  3. Relation between frequency and wavelength •  = c • = wavelength (meters, cm, nm, etc) • = frequency (cycles per second, Hertz, s-1) • c = speed of light (2.997 x 108 m/s) • Unit analysis:

  4. Example (p. 376) - Relating Wavelength and Frequency What is the wavelength of radiation in your microwave oven, whose frequency is 2.45 GHz?

  5. Light can also be thought of as a particle or PHOTON • E = h E = energy (Joules) H = Planck’s constant (6.626 x 10-34 J·s) • = frequency (cycles per second, Hertz, s-1) • And combining with  = c -

  6. Electromagnetic Spectrum X-Rays: UV-Vis: Infrared: Microwave: Radio:

  7. Ground state: Excited state:

  8. Example (p. 377) – 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 or (b) infrared radiation with a wavenumber of 1,251 cm-1 ?

  9. Absorption of Light – Sec 18-2 Spectrophotometer: Radiant power: Monochromator:

  10. Transmittance and Absorbance

  11. Example (p. 379) – Absorbance & Transmittance What absorbance corresponds to 99% transmittance? To 0.10% transmittance?

  12. Beer’s Law: absorbance is proportional to the concentration of light-absorbing molecules in the sample A = bc Beer’s Law for a mixture -

  13. Example (p. 381) – Using Beer’s Law The peak absorbance of 3.16 x 10-3 M KMNO4 at 555 nm in a 1.000 cm pathlength cell in Fig 18-5 is 6.54. (a) Find the molar absorptivity and percent transmittance of this solution. (b) What would the absorbance be if the pathlength was 0.100 cm? (c) What would the absorbance be in a 1.000 cm cell if the concentration was decreased by a factor of 4?

  14. R O V B Y G Absorption Spectrum – CoCl2

  15. Example (p. 382) – Finding Concentration from the Absorbance Gaseous ozone has a molar absorptivity of 2700 M-1cm-1 at the absorption peak near 260 nm in the spectrum below. 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.

  16. Example (p. 383) – How Effective is Sunscreen? What fraction of ultraviolet radiation is transmitted through the sunscreen in the spectrum below at 300 nm?

  17. Using Beer’s Law – Sec 18-4 e.g. measuring NO2¯ in aquarium water

  18. Analysis based on the absorbance of the colored product of this reaction-

  19. Absorbance Spectrum of the Colored Product Absorbance max = __________ nm

  20. Construction of a Calibration Curve (Standard Curve) Table 18-2

  21. Calibration Curve for Nitrite Analysis(blank subtracted)

  22. Example (p. 389) – Using the Standard Curve From the data from Table 18-2, find the molarity of nitrite in the aquarium.

  23. The Spectrophotometer – Sec 19-1

  24. Double-Beam Spectrophotometer

  25. Hitachi UV-Vis – U2000

  26. Sample Cuvettes

  27. Light Sources

  28. Monochromator

  29. Light Detectors

  30. Spectrophotometric Analysis of a Mixture – Sec 19-2 A = total absorbance at wavelength 1 A = total absorbance at wavelength 2 X = molar absorptivity of molecule X at wavelength 1 X = molar absorptivity of molecule X at wavelength 2 Y = molar absorptivity of molecule Y at wavelength 1 Y = molar absorptivity of molecule Yat wavelength 2

  31. A = AX + AY because Beer’s Law is additive = X b [X] + Y b [Y] A″ = A″X + A″Y = ″X b [X] + ″Y b [Y]

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