B.Sc. Second year Undergraduate degree course (CBCS Pattern)
Semester Third
Physical Chemistry (CHE-312)
Chapter – Colorimetry
Lamberts Law
This law can be stated as follows when a beam
of light as allowed to pass through a transparent medium, the rate of decrease
of intensity with thickness of medium is directly proportional to intensity of
light.
Mathematically,
the lambart’s law may be stated as follows.
Where,
I
= intensity of incident light of wave length (λ)
t
= thickness of medium
k
= Proportionality constant
Integrating equation & putting I= I0 & t=0
ln I0/It =kt
I=I0.e-kt
It= I010-kt (changing
equation to natural log)
Where,
k = 1/ 2.303
Beer's
Law
When a monochromatic beam of radiation passes through an absorbing medium, the intensity of the transmitted radiation decreases exponentially with the concentration of the absorbing substance. The law is expressed as
It = Io10 – k’C……………..(2)
Where
C
is the molar concentration of the absorbing substance and k’ is another
constant
Lambert-Beer's Law
When a beam of monochromatic radiation is
passed through a transparent absorbing medium, the decrease in the intensity of
radiation is directly proportional to the concentration of the absorbing
substance and the thickness of the absorbing medium.
-dI /I = kC dx
Where
I is the intensity of radiation, C is the molar concentration of the absorbing
species, x is the thickness of the absorbing medium and k is the
proportionality constant. If Io is the intensity of incident radiation and I is
the intensity of transmitted radiation, after passing through a path length (thickness)
of l cm in the solution, and upon integrating the above equation, between the limits
I = Io when x= 0 and I= I at x= l, we get,
ln I/Io = -kCl
2.303 log I/Io = -kCl
log I/Io = -kCl/2.303
log I/Io = Є Cl (Є= -k/2.303)
ε is the molar absorptivity or molar
extinction coefficient, and log I/Io = A which is known as the absorbance of
the material
A = Є
Cl……………..(3)
Thus absorbance A, also known as optical
density, is directly proportional to (i) the concentration C of the absorbing
species and (ii) the path length l and has no units. Eq. (3) is the mathematical
expression for Lambert’s Beer law. ε is defined as the absorbance of the
solution of unit molar concentration (1M) placed in a cell of path length one
cm. If C is expressed in mol dm-3, then the unit for ε is dm3
mol-1cm-1.
Molar absorptivity
Molar absorptivity, also known as molar
extinction coefficient or molar absorptivity coefficient, is a fundamental
parameter used in spectroscopy to describe how strongly a substance absorbs
light at a particular wavelength. It is denoted by the symbol ε (epsilon) and
is expressed in units of L/(mol∙cm) or sometimes cm-2/(mol).
Molar absorptivity is a measure of how effectively a substance absorbs light at a specific wavelength, and it takes into account the concentration of the absorbing species. It's commonly used in the Beer-Lambert law, which relates the absorbance (A) of a sample to the concentration (c) of the absorbing species, the path length (l) of the sample, and the molar absorptivity (ε):
A = ЄCl
Where:
A is the absorbance of the sample (unitless).
Є is the molar absorptivity (L/(mol∙cm) or cm2/(mol)).
C is the concentration of the absorbing
species (mol/L).
l is the path length of the sample (cm).
Molar absorptivity is often determined experimentally by measuring the absorbance of solutions with known concentrations of the absorbing species at a specific wavelength. These measurements allow researchers to establish a relationship between absorbance and concentration and calculate the molar absorptivity coefficient for that substance at that particular wavelength.
Colorimetry: Interaction of electromagnetic radiation with matter
Colorimetry:Lamberts Law,Beer's Law,Lambert-Beer's Law, Molar absorptivity
Colorimetry: Limitations of Beer –Lambert’s law,Deviation from Beers Law,Reasons for Deviation from Beer's Law.
Colorimeter: Principle, Construction and components, working, Applications
Colorimetry Multiple Choice questions
