Colorimetric method –instrument and application
Introduction: - Colorimeter is device which is used to measure transmittance and absorbance of light passing through a liquid sample. It is a light sensitive device.
Principle: - Colorimeter works on the principle of Beer-Lambert’s Law.
Beer-Lambert’s Law states that when visible light from a natural or artificial source falls on a colored homogenous medium contained in a glass container, the color of the medium is the complementary color of the light that has been absorbed by the medium. During this interaction between the medium and the radiation, a portion of light is absorbed by the medium, a portion is reflected and the rest is transmitted.
The intensity of the incident light I0 is the sum
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another glass cell of similar dimensions containing the colorless solvent). Hence, for practical purposes the equation becomes
I0=Ia+It
P. Bouguer (1729) and J. H. Lambert (1760) investigated the effect of the thickness of the absorbing medium on the intensity of light. Lambert’s law states that when a monochromatic light passes through a transparent medium, the rate of decrease in the intensity with the thickness of the medium, the rate of decrease in the intensity with the thickness of the medium is proportional to the intensity of the incident light. The differential form of the law is given as
-dI0/dt=kI0
Where I0 is the intensity of the incident light, t is the thickness of the medium (also called the optical path length), and k is the proportionality factor. The integrated form of the above equation is given
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Real deviation:-This deviation from the law occurs at high concentration of the absorbing species. The absorptivity ε of the solution changes with concentration depending on the refractive index η of the solution and since refractive index varies considerably at concentration higher than 10-3 M, absorptivity also varies. Beer-Lambert law is based on the assumption of an incident beam of monochromatic radiation and even the best monochromator system provides only polychromatic beam of radiation spread over a few wavelengths.
2. Chemical deviation: - This deviation occurs due to presence of more than one absorbing species in the sample. The measured absorbance in such events is actually the sum of the absorbances of the individual species each having its own absorptivity.
3. Instrumental deviation: - This deviation arises depending upon the bandwidth of the instrument. The bandwidth of the instrument depends upon the wavelength resolving capacity of the system consisting of the monochromator and silts and varies from instrument to