Lecture 19

Spectrophotometry-III

Light Source

Sample

Monochromator(filter,

wavelength selector)

Detector

Spectrometer

Data Processing

Single beam

Double beam

A tungsten lamp

3000 K

Deuterium Lamp

hHHHEH '''202

Deuterium: higher intensity

UV sources

Limits to Beer’s Law

strayP

strayPT

0

1

Incident light

P0

Emergent light

P1

Stray light

Solvents for UV/vis Spectrophotometry

Solvent Lower Wavelength Limit

Water 180 nm

Ethanol 220 nm

Hexane and 200 nmCyclohexane

Acetone 320 nm

AClT logBeer’s law Bugert, Lambert and Beer

A

concentration

Straight line!

Wavelength, nm

Aor

B

Other components of the mixture

Measure @this wavelength!

Limits to Beer’s Law

Polychromatic light (several wavelength)

Wavelength, nm

Aor

B

Other components of the mixture

Intensity is too low!

Wavelength, nm

Aor

B

Other components of the mixture

Measurement is difficult

Wavelength, nm

Aor

C

Other components of the mixture

Measure @this wavelength!

B + reagent CCompound is chemically derivitized (transformed) prior to themeasurement of absorbance

C2H5N

S-

SC2H5Cu

S

S S

SN

C2H5

C2H5

NC2H5

C2H52 +

Cu2+

NH

NN

S

N

Hg

N

S

NN

HN

N

H

N N

SH

NHg2+

NO2

NO2

HNNH2

+O

NO2

NO2

HNN