Chapter 14

Applications of Ultraviolet-Visible Molecular Absorption Spectrometry

Constructive and destructive interference of electron waves.

Electronic Transitions Between Molecular Orbitals

A center of light absorption in a molecule.

*

*

Angular Momentum of the Electron is Conserved

n*

n*

Angular Momentum of the Electron is Not Conserved

Angular Momentum of the Electron is Conserved

“forbidden”

“allowed”

possible transitions? (nm)

* (allowed)

*(allowed)

n*(allowed)

n*(forbidden)105

alkanes

alkenes, alkynes,

aromatics carbonylscmpd’s with heteroatoms

104

103

102

10

150 300

1,2,4,5-tetrazine

1. Solvent Shifts – is a peak a * or an n* transition?

2. Effect of ring substituents on the absorbance spectrum of cpmd’s with aromatic rings

3. Effect of pH on absorbance spectra, e.g. indicators HIn H+ + In-

* "red shift“ in a

polar solvent

* energy level is polar and stabilized by a polar solvent

n*“blue shift“ in a polar solvent

electron donating (stabilizes ring, lowers energy, red shift):

methyl, phenol, R2N -, RO-

electron withdrawing (destabilizes ring, raises energy, blue shift): :

nitro, carboxylic, ester, ketones, aldehydes

phenolphthalein

crystal violet

phenol red

1. Mixture Analysis

2. Sample “matrix” – Standard Addition methods

3. Derivative Spectroscopy – enhancing weak spectral details

4. Enzyme Kinetics

1. Mixture Analysis

A = AM + AN

e.g. determination of caffeine + sodium benzoate in soft drinks (Ch312)

2. Can be used with any instrumental technique; useful if there is an irreproducible sample “matrix” = all the impurities in the sample, e.g. microorganisms, dissolved metals and organics, particulate matter, humics, etc.

(Ch 1, p. 13-17)

Vspike →

Sig

nal →

add increasing volumes of known concentration (Vspike) to fixed volume of sample

Vx, Cx

"spike" = added analyte of known concentration and volume

Vx = volume unknown

Cx = concentration unknown

Vs = volume "spike"

Cs = concentration "spike"

Vt = total volume

Vs, Csdilute toVt

A = Ax + As = bCdx + bCds

Plot A vs. Vs

m = b = Cx =

t

xxdxtdxxx V

VCC so VCVC

t

ssdstdsss V

VCC so VCVC

t

ss

t

xx

V

VCεb

V

VCεbA

Example 1-1

10 mL aliquots of a natural water sample were pipetted into 50.00 mL volumetric flasks. Exactly 0.00, 5.00, 10.00, 15.00 and 20.00 mL of a standard solution containing 11.1 ppm of Fe3+ was added to each, followed by an excess of SCN- ion to give the red complex Fe(SCN)2+. After dilution to volume, the instrument response S for each of the 5 solutions, measured with a colorimeter, was found to be 0.240, 0.437, 0.621, 0.809, and 1.009 respectively. What was the concentration of Fe3+ in the water sample?

3. Derivative Spectroscopy - used to enhance weak spectral features

4. Enzyme Kinetics – “Michaelis-Menten” Mechanism

E + S ES P + Ek1

k-1

k2