Quantitative Chemical Analysis Seventh Edition Quantitative Chemical Analysis Seventh Edition Chapter 18 Fundamentals of Spectrophotometry Copyright © 2007 by W. H. Freeman and Company Daniel C. Harris

Light has: Intensity Color (wavelength) Polarization

Intensity

Transmittance Absorbance

Transmittance Absorbance Beer’s Law

ε = 313 M -1 cm -1 b = 2 cm T = 3.16% = A = -logT = 1.50 c = 1.5/(313 M -1 ) (2 cm -1 ) = M

dP(x)/dx = -κ P(x)  P(x) = exp(-κ x)

Beer’s law

Molar extinction Coefficient ~250 L.mol -1 cm -1 Cross section ~ cm 2

Absorption of a mixture

Isosbestic point

(wavelength) x (frequency) = speed [m/s] λν = c [10 8 m/s]

436 nm  what frequency?

Visible spectrum

Your Eyes See the Color that is NOT Absobed

Hexaquotitanium (III) Solutions Appear Violet Due to Absorption of Yellow and Green Light

Subtraction of Light by Absorption Leads to Color you See

What makes a molecule absorb light?

Electron motion (fs)  resonant frequency ~ 300 nm UV-VIS ~ s  ν ≈ s -1

What makes a molecule absorb light? Electron motion UV-VIS excites electrons!

E = h ν

Electron diffraction pattern from a beryllium atom

What makes a molecule absorb light? Nuclear motion ~ 3000 nm infrared

What makes a molecule absorb light? Nuclear motion ~ 300,000 nm microwave

What makes a molecule absorb light? Oscillating DIPOLE moment

What makes a molecule absorb light?

green house gas

What makes a molecule absorb light? [cm -1 ] = 1/λ = ν/c =E/hc

What makes a molecule absorb light? 3200 cm −1 broad, strong O-H stretch (alcohols) 3000 cm −1 broad, medium O-H stretch (carboxylic acids) 1200 cm −1 strong, O-H bending 2800 cm −1 strong, C-H stretch 1400 cm −1 variable, C-H bending 1700 cm −1 strong, C=O stretch 1200 cm −1 strong, C-O stretch

What makes a molecule absorb light?

E = h ν