The document discusses the principles of UV-Visible spectroscopy. It explains that UV-Visible spectroscopy is based on Beer-Lambert's law, which states that absorbance is directly proportional to the concentration of the absorbing substance in a solution. It also describes Lambert's law, which indicates that the intensity of incident radiation decreases exponentially with increasing thickness of the absorbing medium. Finally, the document combines both Beer's and Lambert's law into the Beer-Lambert equation that relates the intensity of radiation transmitted through a solution to the properties of the absorbing substance.

1. Principle involved in
Principle involved in
Principle involved in
UV -Visible
UV -Visible
UV -Visible
Spectroscopy
Spectroscopy
Spectroscopy
Dr.M.Sumithra M.Pharm,Ph.D
Associate Professor
Department of Pharmaceutical Chemistry and Analysis
School of Pharmaceutical Sciences
VISTAS,Chennai.
UNIT :1
UNIT :1
UNIT :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 Law
Beer’s Law
Beer’s Law
According to Beer’s Law, when a beam of
According to Beer’s Law, when a beam of
According to Beer’s Law, when a beam of
monochromatic radiation passes through an
monochromatic radiation passes through an
monochromatic radiation passes through an
absorbing medium, the intensity of incident radiation
absorbing medium, the intensity of incident radiation
absorbing medium, the intensity of incident radiation
decreases exponentially with an increase in the
decreases exponentially with an increase in the
decreases 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 proportional
In other words, absorbance is directly proportional
In 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 Law
Lambert’s Law
Lambert’s Law
According to lambert’s law the rate of
According to lambert’s law the rate of
According to lambert’s law the rate of
decrease in the intensity of the
decrease in the intensity of the
decrease in the intensity of the
incident radiation (I) with the
incident radiation (I) with the
incident radiation (I) with the
thickness of the medium (t) is directly
thickness of the medium (t) is directly
thickness of the medium (t) is directly
proportional to the intensity of the
proportional to the intensity of the
proportional to the intensity of the
incident light.
incident light.
incident light.

7. when the concentration of
when the concentration of
when the concentration of
absorbing substance is zero,
absorbing substance is zero,
absorbing substance is zero,
Hence
Hence
Hence
*The intensity of incident light at
*The intensity of incident light at
*The intensity of incident light at
zero and the concentration is
zero and the concentration is
zero and the concentration is
equal to the transmitted light (It).
equal to the transmitted light (It).
equal to the transmitted light (It).
I = It
I = It
I = It When C=0
When C=0
When C=0
Substitute the values of I and C
Substitute the values of I and C
Substitute 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. References
References
i)Gurdeep.R Chatwal.K, and Anand., Instrumental
i)Gurdeep.R Chatwal.K, and Anand., Instrumental
method of Chemical Analysis, Himalaya Publishing
method 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 Pharmaceutical
ii)Beckett A.H,Stenlake,J.B,Practical Pharmaceutical
Chemistry,4 th edn.CBS Publishers and
Chemistry,4 th edn.CBS Publishers and
Distributors,NewDelhi.2000
Distributors,NewDelhi.2000

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