1. Chapter 18
Let There Be Light

2. O 2 → 2O
hν

O2 → O 3 ( ozone ) UV
absorption
CCl 2F2 → CClF2 + ClO + Cl
hν
Cl + O 3   ClO + O 2 
→

O 3 → O + O 2
hν
 ⇒ 2O 3 → 3O 2

O + ClO  → Cl + O 2 

3. Why south pole ?
See p.374

4. 18.1 Properties of light -1
• .  1
E = hν = hc   = hc~
ν
λ
• Electromagnetic Spectrum (Fig 18.2)
• Absorption vs Emission of light

5. change of change of spin
nuclear change of e distribution change of change of NMR EPR
configuration X-ray configuration orientation
γ-ray uv.vis IR microwave

6. 18.2 Absorption of light -1
1) Spectrophotometer

7. 18.2 Absorption of light -2
1) .
P
Transmittance T =
P0
Po P
Absorbance A = -logT
Beer' s Law : A = εbc
    mol 
 ( cm )  = M
 cm ⋅ mol    
b

8. • A = εbc

9. 18.2 Absorption of light -3
(ex) How effective is sunscreen ?
at the peak absorbance near 300 nm ?
A ~ 0.35
T = 10-A
= 10-0.35
= 0.45
= 45%
⇒ 55%
UV-B is absorbed.

10. 18.3 Practical Matters -1
• Cuvets:
A glass made of SiO2 : Vis. UV.
Plastics & ordinary glass: Vis
NaCl(s) KCl(s) : IR

11. 18.3 Practical Matters -2
1) Good Operating Techniques :
• Cuvets handle: systematic errors/
random errors
• Most accurate at A~0.4–0.9
• Greatest sensitivity: λmax
• Baseline correction

12. 18.3 Practical Matters -3
Too little light : :
high A, P is small
Too much light :
low A, P ~ P0

13. 18.4 Using Beer’s Law -1
• A Colorimetric Reagent to Detect Phosphate

14. 18.4 Using Beer’s Law -2
Ex.1 : Bezene: find molar absorptivity (ε)
at p.387
250 mL
25.8 mg
+ hexane
b = 1.000 cm
?max = 256 nm e= ?

15. 18.4 Using Beer’s Law -2
Ex.2 : Nitrite in an aquarium
(using a standard curve)

543 nm

16. 18.4 Using Beer’s Law -3
NH3 animals & plant
[O]

17. 18.4 Using Beer’s Law -4
1) Standard Nitrite ex. at p389

18. 18.4 Using Beer’s Law -5
from least square (4.4)
A = 0.1769 [ppm] + 0.0015