Principle of Photometry
- Measurement of intensity of light
- Technique used in photometry:
1. Colorimetry
2. Spectrophotometry
Beer’s law: When a ray of monochromatic light passes through an absorbing
medium, its intensity decreases exponentially, as the concentration
increases.
I α I0 x c
or, I = I0 x e– k1 c
Lambert’s law: When a ray of monochromatic light passes through an absorbing
medium,the intensity decreases exponentially, as the length of
absorbing medium increases.
I α I0 x l
or, I = I0 x e- k2 l
Colorimeter of filter used and colour of solution are complementary to
each other
Colour of filter Wave length(nm) Colour of solution
Violet 380-420 Yellow
Blue 420-470 Red
Green 470- 520 Red
Yellow 520- 580 Violet
Red 580-660 Green
Combining both the laws:
I = I0 x e–k3cl
or, I/ I0 = e– k3cl
or, log (I/ I0 ) = – k3cl
or, log (I0/I) = k3cl
or, log10 (I0/I) = k3cl/2.303
or, log10 (I0/I) = Kcl
or, log10 (I0/I) = absorbance/O.D
I0/I = Transmittance,
A = Kcl Where, k= molar absorption coefficient
or, A α c
Deriviation of Beer’s & Lambert’s law
Verification of Beer’s & Lambert’s law
Colorimeter
Application of Beer’ & Lambert’s law
Beer-Lambert law has various applications, some of which are discussed here:
1. Drug Analysis: Beer-Lambert Law is used to analyze drugs and biological and osimetric
materials. It can be used to determine the molecular concentration of a
tablet or bilirubin in blood plasma samples. In electromagnetic spectroscopy, we scan the
tablet with electromagnetic radiation (UV-rays) to ascertain its qualitative (drug presence)
and quantitative (concentration) characteristics.
2. Spectroscopy: In spectroscopy, the concentration of a material in a solution is determined
by examining the light’s absorption at a particular wavelength. By examining
the absorption of light at a particular wavelength, we can use this method to estimate the
concentration of a material in a solution in a variety of disciplines, including environmental
science, biochemistry, and pharmacology.
3. Chemical Analysis:Using chemical analysis, one can ascertain a substance’s concentration
in a solution. By observing the absorption of light at a particular
wavelength, analytical methods like UV-Visible Spectroscopy, chromatography, and capillary
electrophoresis use the Beer-Lambert Law to calculate the concentration of an ingredient in
a solution. The same procedure can be applied to find out how much bilirubin is absorbing in
blood plasma samples.
4. Quality Assurance: To ascertain the concentration of specific components in diverse items,
such as food, beverages, and medication, beer-lambert law is applicable.
5. Graphene mother liquor (GML): A materials composition design approach for GML is devised
using the Lambert-Beer law. This technique improves the
dispersion and composite modification effects of GML, and GML made using the same type of
dispersant has a similar percentage curve, which serves as a benchmark for subsequent
research.
6. Applications in Physics and Chemistry: Chemistry uses Beer’s Law to gauge polymer
deterioration, research oxidation, and determine
the concentration of chemical solutions. The rule also explains the attenuation of radiation as
it passes through the Earth’s atmosphere. The rule, which is typically used to describe light,
also helps scientists comprehend how particle beams like neutrons attenuate. The Bhatnagar-
Gross-Krook (BKG) operator is utilized in the Boltzmann equation for computational fluid
dynamics. The Beer-Lambert Law is a theoretical physics solution to the BKG operator.
Limitation of Colorimetry:
To obtain linearity between absorbance and concentration, the following
condition are fulfilled:
a. Light must be of narrow wavelength, preferably monochromatic
b. The wavelength used should be at maximum absorbance of the solution
c. There must be no ionisation association, dissociation or solvation of solute with
concentration of urine
d. Linearity exists only between min. max. threshold conc. Of given sample
For a colorimeter, maxm absorbance =
OD = -log (transmission %) / 100
= log 100- log (transmission %)
= 2- log(transmission %)
