Understanding the Properties of Light and Spectrophotometry

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 -

14. 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?

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

16. 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.

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

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

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

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

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

22. Calibration Curve for Nitrite Analysis(blank subtracted)

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

24. The Spectrophotometer – Sec 19-1

25. Double-Beam Spectrophotometer

26. Hitachi UV-Vis – U2000

27. Sample Cuvettes

28. Light Sources

29. Monochromator

30. Light Detectors

31. 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

32. 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]