UV visible spectroscopy
Electronic Transition
( σ → σ* )
Eg:- saturated alkanes
(n → s* )
Eg:- Oxygen , Nitrogen & Sulphur
(n → p*)
Eg:- Saturated aliphatic ketones
(p → p*)
Eg:- Conjugated Dienes / carbonyl compound
Chromophore
Auxochrome
An auxochrome is a group of atoms
attached to a chromophore which
modifies the ability of that chromophore
to absorb light
Also called as colour enhancing group
Eg:- NH2 , OH , NHR, OR
Spectral Shift
Bathochromic shift ( Red shift)
Hyperchromic effect
Hypochromic effect
Chromophore was considered a system
responsible for imparting colour to the compound..
I. Independent chromophore:-
single chromophore is
sufficient to import color to the
compound e.g. Azo group
II. Dependent chromophore: -
When more than one chromophore
is required to produce color.
e.g. acetone having one
ketone group is colorless
.
INSTRUMENTATION of UV/VISIBLE :-
Sources (UV and visible)
Tungsten / Halogen
Hollow Cathode Discharge Lamp
Deuterium Arc Lamp
Hydrogen discharge lamp
Mercury discharge lamp
MONOCHROMATORS-
1-Grating Monochromator:-
• Reflection grating
• Echellette Reflection Grating
2- Prism monochromators
CUVETTE (sample container)
Quartz or fused silica cuvettes are required
DETECTORS
The photomultiplier tube
The linear photodiode array
Charge-Coupled Devices (CCDs)
UV-Vis Spectrophotometers
- Single beam
2.Double beam
Beer-Lambert’s law
A = abc
A= Absorbance
a= Absorptivity
b= path length
c= Concentration of solution
Limitation of Beer-Lamberts
law
fluoresecence or
phosphorescence
scattering of light
stray light
non-monochromatic radiation
The linearity is limited by chemical
and
instrumental factors.
Hypsochromic Shift
( Blue shift)