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Applications of Ultraviolet/VisibleMolecular Absorption Spectrometry

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Absorption of light is a two step process:

Absorption M + h M*

RelaxationM* M + heat

The heat evolved (very minute) does not affect the system temperature.

There are other modes of relaxation that cause deviations in Beers Law.

A) PhotodecompositionB) Fluorescence

C) Phosphorescence

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A. Absorbing species containing s , p , and n electrons

(organic compounds).

Bonding Antibonding

Formaldehyde

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Multiple chromophores have little effect on max but increase .

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Shifts l max to longer l and e is greater than expected due to the additiveeffect of conjugated double bonds.

Conjugation causes delocalization of p electrons stabilizing p *, thereforeshifting absorbance to longer wavelength (lower energy).

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Aromatic compound have three sets of bands arising from p p * transitions.

Benzene:184 nm ( e ~ 60,000)E2 band at 204 nm ( e = 7900)B band at 256 nm ( e = 200)

Each band has fine structure fromvibrational transitions that are reducedor disappear in polar solvents.

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-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

400 450 500 550 600 650

Wavelangth (nm )

A b s o r

b a n c e

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C

N

C 2 H 5

CH 2

SO 3- +

N CH 2

SO 3 Na

SO 3 NaC 2 H 5

N N

OCH 3

NaO 3 S

H 3 C

HO

SO 3 Na

Allura Red ACl

max = 506 nme = 11,600 L/mol cm

Brilliant Blue FCFl

max = 630 nme = 85,000 L/mol cm

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Usually have a pair of non-bonding electrons that interact with the p electrons of the chromophore.

Common auxochromes are hydroxides and amines.

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B. Absorption by d and f electrons 1. 4 f and 5 f metals absorb because of f electron excitation

Lanthanide and actinide serieselements

Little effect from solvent orligands because f electrons arenot the outer most electrons andare shielded form solvent andligand effects.

Spectra have sharp, well definedabsorption peaks.

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B. Absorption by d and f electrons 2. Most period 4 & 5 transition metals have at least one species (cation)

that absorbs radiation in the visible region.

Absorption is due to the excitation of the outer shell d electrons. The d orbitals are partially filled and the d electrons are highly

influenced by external forces since they are in the outermost shell. Transition metals have broad band absorption spectra and l max is

affected by solvent and ligands.

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C. Charge Transfer Absorption

Involve charge transfer complex,usually a complex anion (e - donor) anda metal (e - acceptor).

Absorption of radiation causes thetransfer of an e - from the donor orbitalto the acceptor orbital.

Results in an excited state where aredox reaction has occurred.

In most cases the metal ion in thecomplex serves as the e - acceptor.

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Generally a very poor method for

qualitative analysis because l max isusually influenced by the solvent.

Polar solvents obliterate fine structure

from vibrational transitions in thespectrum due to collision broadening.

Gas phase spectra can be useful

qualitative tools since the fine structureof the spectrum is preserved.

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One of the most common methods for quantitative analysis of compoundsin aqueous solution.

UV/Vis quantitative methods are characterized by:

1) Applicability to many organic and inorganic compounds.

2) Good sensitivity 10 -4 10 -5 M, 10 -7 with some modifications.

3) Moderate to good selectivity.

4) Good accuracy.

5) Ease of data collection.

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Usually UV/Vis is used to quantify the relationship between analyteconcentration and absorbance.

1. Selection of wavelength ( l ) for quantitation.

Usually the maximum absorption peak ( l max ) is selected

because:a) sensitivity is greatest (greatest DA/Dc)

b) absorption curve is flat (small DA/Dl ) at the peak soBeers Law will most likely be valid.

A. Procedural Details

2. Variables that influence absorption spectra. .

a) Solvent

b) pH

c) Temperature

d) Sample matrix (i.e. electrolyte concentration and interferingspecies)

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Absorbing species can be either analyte, titrant, or product

B. Photometric Titrations

Example 1: analyte non-absorbingtitrant absorbing

product non-absorbing

Example 2: analyte absorbingtitrant absorbing

product non-absorbing

Example 3: analyte absorbingtitrant non-absorbing

product non-absorbing