1. ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 18 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 18 ELECTROMAGNETIC RADIATION

3. - Also known as radiant heat or radiant energy - One of the ways by which energy travels through space - Consists of perpendicular electric and magnetic fields Examples heat energy in microwaves light from the sun X-ray radio waves

4. Three Characteristics of Waves Wavelength (λ) - Distance for a wave to go through a complete cycle (distance between two consecutive peaks or troughs in a wave) Frequency (ν) - The number of waves (cycles) per second that pass a given point in space Speed (c) - All waves travel at the speed of light in vacuum (3.00 x 10 8 m/s) ELECTROMAGNETIC RADIATION

5. one second λ1λ1 λ3λ3 λ2λ2 ν 1 = 4 cycles/second ν 2 = 8 cycles/second ν 3 = 16 cycles/second amplitude peak trough ELECTROMAGNETIC RADIATION node

6. Gamma rays X rays Ultr- violet InfraredMicrowaves Radio frequency FM Shortwave AM Visible Visible Light: VIBGYOR Violet, Indigo, Blue, Green, Yellow, Orange, Red 400 – 750 nm - White light is a blend of all visible wavelengths - Can be separated using a prism Wavelength (m) Frequency (s -1 ) 10 -11 10 3 10 20 10 4 ELECTROMAGNETIC RADIATION

7. - Inverse relationship between wavelength and frequency λ α 1/ν c = λ ν λ = wavelength (m) ν = frequency (cycles/second = 1/s = s -1 = hertz = Hz) c = speed of light (3.00 x 10 8 m/s) ELECTROMAGNETIC RADIATION

8. An FM radio station broadcasts at 90.1 MHz. Calculate the wavelength of the corresponding radio waves c = λ ν λ = ? ν = 90.1 MHz = 90.1 x 10 6 Hz = 9.01 x 10 7 Hz c = 3.00 x 10 8 m/s λ = c/ ν = [3.00 x 10 8 m/s]/[9.01 x 10 7 Hz] = 3.33 m ELECTROMAGNETIC RADIATION

9. Albert Einstein proposed that - Electromagnetic radiation is quantized - Electromagnetic radiation can be viewed as a stream of ‘tiny particles’ called photons h = Planck’s constant (6.626 x 10 -34 joule-second, J-s) ν = frequency of the radiation λ = wavelength of the radiation = 1/ λ = wavenumber (m -1 ) THE ENERGY OF PHOTONS

10. THE ATOMIC SPECTRUM Transmission - Electromagnetic radiation (EM) passes through matter without interaction Absorption - An atom (or ion or molecule) absorbs EM and moves to a higher energy state (excited) Emission - An atom (or ion or molecule) releases energy and moves to a lower energy state

11. THE ATOMIC SPECTRUM Energy AbsorptionEmission Excited state Ground state

12. Gamma rays X rays Ultr- violet InfraredMicrowaves Radio frequency FM Shortwave AM Visible 10 -11 10 3 10 20 10 4 ELECTROMAGNETIC RADIATION Bond breaking and ionization Electronic excitation vibration rotation Molecular Processes Occurring in Each Region

13. ABSORPTION OF LIGHT Spectrophotometry - The use of EM to measure chemical concentrations Spectrophotometer - Used to measure light transmission Radiant Power (P) - Energy per second per unit area of a beam of light - Decreases when light transmits through a sample (due to absorption of light by the sample)

14. ABSORPTION OF LIGHT Transmittance (T) - The fraction of incident light that passes through a sample PoPo P 0 < T < 1 P o = radiant power of light striking a sample P = radiant power of light emerging from sample

15. ABSORPTION OF LIGHT Transmittance (T) - No light absorbed: P = P o and T = 1 - All light absorbed: P = 0 and T = 0 Percent Transmitance (%T) 0% < %T < 100%

16. ABSORPTION OF LIGHT Absorbance (A) - No light absorbed: P = P o and A = 0 - 1% light absorbed implies 99% light transmitted - Higher absorbance implies less light transmitted

17. ABSORPTION OF LIGHT Beers Law A = εbc A = absorbance (dimensionless) ε = molar absorptivity (M -1 cm -1 ) b = pathlength (cm) c = concentration (M)

18. ABSORPTION OF LIGHT Beers Law - Absorbance is proportional to the concentration of light absorbing molecules in the sample - Absorbance is proportional to the pathlength of the sample through which light travels - More intense color implies greater absorbance

19. ABSORPTION OF LIGHT Absorption Spectrum of 0.10 mM Ru(bpy) 3 2+ λ max = 452 nm

20. ABSORPTION OF LIGHT λ max = 540 nm Absorption Spectrum of 3.0 mM Cr 3+ complex

21. ABSORPTION OF LIGHT Maximum Response (λ max ) - Wavelength at which the highest absorbance is observed for a given concentration - Gives the greatest sensitivity

22. ABSORPTION OF LIGHT Calibration Curve

23. ABSORPTION OF LIGHT Complementary Colors - White light contains seven colors of the rainbow (ROYGBIV) - Sample absorbs certain wavelengths of light and reflects or transmits some - The eye detects wavelengths not absorbed

24. ABSORPTION OF LIGHT Complementary Colors λ max 380-420 420-440 440-470 470-500 500-520 520-550 550-580 580-620 620-680 680-780 Color Observed Green-yellow Yellow Orange Red Purple-red Violet Violet-blue Blue Blue-green Green Color Absorbed Violet Violet-blue Blue Blue-green Green Yellow-green Yellow Orange Red

25. ABSORPTION OF LIGHT Complementary Colors

26. ABSORPTION OF LIGHT Complementary Colors Ru(bpy) 3 2+ λ max = 450 nm Color observed with the eye: orange Color absorbed: blue Cr 3+ -EDTA complex λ max = 540 nm Color observed with the eye: violet Color absorbed: yellow-green

27. ABSORPTION OF LIGHT Cuvet - Cell used for spectrophotometry Fused silica Cells (SiO 2 ) - Transmits visible and UV radiation Plastic and Glass Cells - Only good for visible wavelengths NaCl and KBr Crystals - IR wavelengths

28. ABSORPTION OF LIGHT Single-Beam Spectrophotometer - Only one beam of light - First measure reference or blank (only solvent) as P o PoPo P Light source monochromator (selects λ) samplecomputer detector b

29. ABSORPTION OF LIGHT Double-Beam Spectrophotometer - Houses both sample cuvet and reference cuvet - Incident beam alternates between sample and reference with the aid of mirrors (rotating beam chopper) PoPo P Light source monochromator (selects λ) samplecomputer detector reference b