Principle involved in uv visible spectroscopy

Short introduction about principle involved UV
Visible Spectroscopy

1. Principle involved inPrinciple involved inPrinciple involved in
UV -VisibleUV -VisibleUV -Visible
SpectroscopySpectroscopySpectroscopy
Dr.M.Sumithra M.Pharm,Ph.D
Associate Professor
Department of Pharmaceutical Chemistry and Analysis
School of Pharmaceutical Sciences
VISTAS,Chennai.
UNIT :1UNIT :1UNIT :1
Dr.M.Sumithra M.Pharm,Ph.D
Associate Professor
Department of Pharmaceutical Chemistry and Analysis
School of Pharmaceutical Sciences
VISTAS,Chennai.

2. The principle of UV-Visible spectroscopy is based
on the fundamental law of absorption called
Beer-Lambert’s law.
The law governs the absorption of
radiation by an absorbing medium.

3. When a beam of monochromatic light is allowed to pass
through a transparent cell containing a homogeneous medium
of an absorbing substance, a portion of an incident light is
reflected at the surface of the medium, a portion is absorbed
and the remainder is transmitted through the medium.

4. I = Ir + Ia + It
Where,
I = Intensity of the incident radiation
Ir = Intensity of the reflected radiation
Ia = Intensity of the absorbed radiation
It = Intensity of the transmitted radiation
UV, Visible & IR Spectrophotometry
The amount of radiation reflected at the surface of the
medium is very small (about 4%) hence it can be
neglected.
Therefore, equation (1) becomes:
I = Ia + It

5. Beer’s LawBeer’s LawBeer’s Law
According to Beer’s Law, when a beam ofAccording to Beer’s Law, when a beam ofAccording to Beer’s Law, when a beam of
monochromatic radiation passes through anmonochromatic radiation passes through anmonochromatic radiation passes through an
absorbing medium, the intensity of incident radiationabsorbing medium, the intensity of incident radiationabsorbing medium, the intensity of incident radiation
decreases exponentially with an increase in thedecreases exponentially with an increase in thedecreases exponentially with an increase in the
concentration of the absorbing medium.concentration of the absorbing medium.concentration of the absorbing medium.
In other words, absorbance is directly proportionalIn other words, absorbance is directly proportionalIn other words, absorbance is directly proportional
to the concentration of the absorbing substance.to the concentration of the absorbing substance.to the concentration of the absorbing substance.

6. Lambert’s LawLambert’s LawLambert’s Law
According to lambert’s law the rate ofAccording to lambert’s law the rate ofAccording to lambert’s law the rate of
decrease in the intensity of thedecrease in the intensity of thedecrease in the intensity of the
incident radiation (I) with theincident radiation (I) with theincident radiation (I) with the
thickness of the medium (t) is directlythickness of the medium (t) is directlythickness of the medium (t) is directly
proportional to the intensity of theproportional to the intensity of theproportional to the intensity of the
incident light.incident light.incident light.

7. when the concentration ofwhen the concentration ofwhen the concentration of
absorbing substance is zero,absorbing substance is zero,absorbing substance is zero,
HenceHenceHence
*The intensity of incident light at*The intensity of incident light at*The intensity of incident light at
zero and the concentration iszero and the concentration iszero and the concentration is
equal to the transmitted light (It).equal to the transmitted light (It).equal to the transmitted light (It).

I = ItI = ItI = It When C=0When C=0When C=0

Substitute the values of I and CSubstitute the values of I and CSubstitute the values of I and C

8. - Log It =K(0) + b (OR) -Log It = b
Substitute the b value in equation
-Log I =K(C) –Log It
Rearrange the equation
Log It -Log I =KC
Apply the natural log
[log It/I=KC]
[It/I=e^KC ]
Inverting Both sides
[I/It=e^(-KC) ]
I =It e^(-KC)
Similarly by Lambert’s Law
I =It e^(-Kt)
Combining both Beer’s and Lambert’s law
I =It e^(-KCt)
Beer's and Lambert's Law Equation

It =I e^KCt

9. ReferencesReferences
i)Gurdeep.R Chatwal.K, and Anand., Instrumentali)Gurdeep.R Chatwal.K, and Anand., Instrumental
method of Chemical Analysis, Himalaya Publishingmethod of Chemical Analysis, Himalaya Publishing
house, 5 th edition ,New Delhi.2000.house, 5 th edition ,New Delhi.2000.
ii)Beckett A.H,Stenlake,J.B,Practical Pharmaceuticalii)Beckett A.H,Stenlake,J.B,Practical Pharmaceutical
Chemistry,4 th edn.CBS Publishers andChemistry,4 th edn.CBS Publishers and
Distributors,NewDelhi.2000Distributors,NewDelhi.2000

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