UV-Visible Spectroscopy

Unit-1
UV-VISIBLE SPECTROSCOPY NOTES VIKAS KUMAR
1
UV Visible Spectroscopy
Spectroscopy:- Spectroscopy is the branch of science dealing the
study of intraction of electromagnetic radiation with matter
Principle Of Spectroscopy :- The principle is based on the
measurement of spectrum of a sample containing atoms.
 Spectrum is a graph of intensity of absorbed or emitted radiation by
sample verses frequency (V) or wavelength (λ).
 Spectrometer is an instrument design to measure the Spectrum of
a compound.
Absorption Spectroscopy :- An analytical technique which concerns
with the measurement of absorption of electromagnetic radiation. Eg
UV (185-400nm) / Visible (400-800nm) spectroscopy. IR Spectroscopy
(0.76-15µm).
Emission Spectroscopy :-An analytical technique which emission of a
particle or radiation is dispersed according to some property of the
emission & the amount of dispersion is measured eg. Mass
spectroscopy.
Electromegnetic Radiation:-
Electromegnetic radiation consist of discrete packets of energy which
called as photons.

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A photons consists of an oscillating electric field (E) & An oscillating
magnetic field (M) which are perpendicular to each other.
Properties of electromagnetic radiation:-
1) Wavelength:- it is distance b/w two successive maxima on an
electromagnetic wave. Unit are m,cm,mm,nm and micrometer.
2) Frequency:- Number of wavelength units pass time is called as
frequency. It is denoted by “V” and units are cycle per sec. Hertz.
3) Wavelength:-
𝒗 =
𝟏
𝒘𝒂𝒗𝒆𝒍𝒆𝒏𝒈𝒕𝒉
As the number of waves per cm in vaccum.

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4) Vilocity:- it is the product of wavelength and frequency and is
equal to the velocity of the wave in the medium.
𝑣 = 𝑛 ∗λ
 The relationship b/w wavelwngth & frequency can be written
as
C=Vλ
 As photons is subjected to energy.
So 𝐄 = 𝐡𝐯 =
𝐡𝐜
𝛌
Electronic Transition:-
1) σ → σ* transition:
 The energy required is large.
 For exp. Methane (which has only C-H bonds and can only
undergo σ → σ* transition Transition) shows an absorbance
maximum at 125nm.
 Absorption maxima due to σ →σ transition are not seen in
typical UV-Vis Spectra (200-700nm) but in UV- region (125-
135nm).
2) n→σ* Transition:-
 These transition Usually need less energy then n→σ* transition.
 They can be initiated by light whose wavelength is in the range
150-250nm.
 The number of organic functional groups with n →σ* peaks in the
UV region in small.
3) π →π* Transition:-
 Π electron in a bonding orbital is exaited to corresponding
antibonding orbital π* and obserbed in conjugated compounds

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 Eg. Alkenes generally absorb in the regin 170 to 205 nm.
4) n→π* transition:-
 n→π9 transition require minimum energy and show absorption at
longer wavelength around 300nm.
Terms Used In UV/Visible Spectroscopy
Chromophore:- The part of a molecule responsible for
imparting color are called as chromospheres.
The functional group comtaining multiple bonds capable
of absorbing radiations above 200nm due to n→π* &

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π→π* transitions eg. NO2 , N=O, C=N, C≡N, C=C,
C=S etc.
Auxochrome:- The functional group with non bonding
electrons that dose not absorb radiation in near UV
region but when attached to a chromophore alters the
wavelength & intensity of absorption.
Eg. Benzene λmax=255nm.
Phenol λmax=270nm.
Aniline λmax=280nm.
UV Visible Spectroscopy
Theory involved :
When a beam of light falls on a solution or homogenes
media a surface of the media a portion is absorbed with
in the medium and remaining is transmitted through the
medium

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Thus if I0 is the intensity of radiation falling on the
media.
Ir is the amount of radiations reflucted
Ia is the amount of radiation absorbed
It is the amount of radiation transmitted
Then I0=Ir+Ia+It
Law involved
1. Beer’s law
2. Lambert’s law
3. Beer- Lambert’s law
1. Beer’s law : When a beam of monochromatic
light I paased through a honogenous absorbing
medium , the rate with decrease of intensity of
radiation with increase in the concn (c) of absorbing
species is directly proportional to the intensity (I) of
incident light radiation.

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-dI/dc=KI
-dI/I=Kdc
On integration of above equation
-In I=KC+B--------------------eq(1)
When concn =0 then there is no absorbance
Here I=I0
Therefore substituting in equation (1)
-In I=k *0+b
-In I=b
Substituting the value of b in equation (1)
-InI=Kc-In I0
In I0-InI=kc
In I0/InI=Kc (Since logA-logB=logAB)
I0/I=ekc (remaining natural logarithm)
I/I0 =e-kc (Inverse both side)
I=I0 e-kc---------------------------(2)

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Lambeart’s law :- when a beam of monochromatic light
is passed through a homogenous absorbing medium the
ratio of decrease of intensity of radiation with thickness
of absorbing medium is directly proportional to the
intensity of the incident light (radiation)
dI/dt=KI
I=intensity of incident light of wavelength λ&
t= Thickness of medium.
Since I=I0e-kt------------------------(3)
Now combine the eq (2) and eq(3) we get
I=I0e-kct
Converting natural logarithm to base 10
I=I0 10-kct
Inverse on both side
I0I=10kct
Taking log on both side
logI0/I=Kct--------------------------(4)

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Here transmittance (T)=I/I0 and Absorbance
(A)=logI/T
Hence, A=logI0/I-------------------------(5)
Using eq. (4) and(5)
A=Kct
Instead of K we can use E, the above equation will be
as followes
A=Ect
This is mathematical equation for beer’s Lambert’s
Law
A=Ect
Where :
A= Absorbance
E= Molecular extinction coefficient
c= Concn of sample
t= path length (normally 10mm or 1cm)

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E can be expressed as follows
𝐸 = 𝐸1%
1𝑐𝑚 ∗ 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟
𝑤𝑒𝑖𝑔ℎ𝑡
10
Instrumentation :-
1. Radiation sourse :- Both the tungsten and D2
lamp are present in the UV-VS.
2. Wavwlength Selector :-
A) Filter →Absorption and interference filter are
mainly used.
B) Monochromator → It gives the desired
wavelength in the entire UV or Visible region.
C) Slits → There are two slits ie →entrance slit
And exit slit.
3. Cell or Cuvettes :- For holding the sample
solution and the pure solvent.
4. Detector :- The most commonly used detectors
are photo emissive cell or phototubes and
photomultiplier tube
5. Recording System :- Recording is done by
recorder pen.
6. Power Supply

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Spectrophotometers are of two types
1. Single team spectrophotometers
2. Double beam Spectrophotometers
→ Single beam UV Spectrophotometer :

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→ Double Beam UV Spectrophotometer :
 A tungsten lamp (400-800nm)
 A D2 lamp (200-400nm)
APPLICATIONS FOR UV-VISIBLE SPECTROSCOPY
 Concentration Measurement
 Detection of Impurities
 Chemical Kinetics
 Detection of functional Group
 Molecular weight determination
→ Spectrophotometer Titratiion :
In a spectrophotometer titration the
equivalence point is determined with a
spectrophotometer. In this technique the titration vessel

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is kept directly in the light path of the instrument then
the absorbance of the solution is determined after
adding titration and a plot of absorbance as a function of
volume of titration is prepared.

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Single Component Analysis Methods
1. Direct Analysis : essentially all compounds containing conjugated
double bond or aromatic rings and many inorganic species absorb
light in the UV Visible region.
2. Indirect Analysis : This method involves analysis after addition
of some reasen. Chemical derivation may be adopted for any of
the several reasons.
1. If the analyte absorbs weekly in the UV-region
2. This technique can be used to improve the selectivity of the
assay in presence of other UV radiation absorbing substance.
3. Cost.
Methods of calculating concentration in
single compartment analysis
1. By using the relationship A=abc
2. By using the formula Cu=(Au/As)*Cs
3. By using the equation Y= mx+c
4. By using the Beer’s Curve
Multicomponents analysis methods
UV Spectrophotometric techniques are mainly used for multicomponent
analysis, thus minimizing the cumbersome task of seprating interferents
and allowing the determination of an increasing number of analytes
consequently redusing analysis time and cost.

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Multicomponent UC Spectrophotometric method are based on
recording and mathmetically processing absorption Spectra.
Advantages :
1. Avoid prior spectration technique and clean up steps that might be
required.
2. Spectral data are readily acquired with ease.
3. The process is fast accurate and simple.
4. Wide applicability to both organic and inorganic systems.
5. Typical detection limits of 10-4
to 10-5
m and moderate to high
selectivity.
Different UV Spectrophotometric multicomponent analysis
method include.
1. Difference Spectrophotometry.
2. Derivative Spectrophotomerty.
3. Absorbance ratio spectra method.
4. Derivative ratio spectra method.
5. Dual wavelength method.
Fluorimetry
It is the measurement of fuoressence intensity at a particular wavelength
with the help of a filter fluorimeter or a spectrofluorimeter.
PRINCIPAL : Molecular contain (σ electron) (π electron) and non
bonding n electron
The electron may be prevent in bonding molecular orbital also
called highest occupied molecular orbital (HOMO).

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It has least energy and more stable.
→ Single State : A state in which all the electron in a molecular are
paired (↓↑).
→ Double State : A state in which an paired electrons is present (↓or↑).
→State (triplet) : A state in which impaired electrons of same spin
present (↑).
→ Silgle exited state : A state which electron are unpaired but of
opposite spin like ↓↑ (Unpaired and opposite side).
→ Factor Effecting Fluoresence :
1. Concn.
2. Quantum Yield of fluorescence

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3. Intansity of incident light
4. Oxygen
5. pH
6. Temprature
7. Absorbance
Instrumentation :
Fluoracence in the instrument measuring fluorescence
intensity (F) is includes the Following Components.
1. Light sources
2. Monochromators/Filter.
3. Sample cells (Cuvetts).
4. Detectors
5. Polarisers
Source collimator Monochromotar(P1) Sample Detector Amplifire
Slit Monochromator(P2)
→ Instruments :→
1. Singe Beam (Filter) Fluorimeter
2. Double Beam (Filter) Fluorimeter
3. Spectrofluorimeter (Double Beam)

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→ Singl Beam Fluorimeter :
Single beam instruments are simple in construction
cheaper and easy to operate.
→ Double Beam Fluorimeter :

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→ Conclusion :
1. Fluorimetric method are not usefull in qualitative analysis
vand much use in quantitative analysis.
2. Fluorience is the most sensitive analytical techniques.
3. Detection studies will increase the development of flaurence
field.
→ Application :
1. Determination of ruthenium.
2. Determination of vanadium.
3. Determination of boron in steel.
4. Determination of aluminium in alloys.
5. Determination of chrominium and manganese.
6. Determination of uranium salts.
Thank You
Made by DRx Vikas

UV-Visible Spectroscopy

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