When a beam of light falls upon a solution, a position of incident light is reflected, Ir, a portion of light is absorbed Ia, and the remainder is transmitted It. If Io denotes the intensity of incident light. Then,
for aqueous solution, Ir is small compared to Ia and It hence may be neglected without appreciable error.
Thus in passing through the solution, the light loses some of its intensity. The intensity increased with the increase in the path length and concentration of coloured species in the solution.
Absorption by a medium also depends on the wavelength of incident light. The wavelength of which the absorption is maximum is know as lambda max.
Lamberts Beer law is a combine form of the lambert’s law and Beer’s law. Lambert law states that,
“when a beam of a monochromatic radiation pass through absorbing medium the rate of decrease of intensity with thickness of the medium is directly proportional to the intensity of medium.
upon integrating between the limit I=Io at b=0
similarly, beer’s law states that, reduction of light intensity with concentration of absorbing species in the medium which states that when a beam of monochromatic radiation passes through a medium containing absorbing species, the rate of decrease in intensity with concentration of absorbing species is directly proportional to the intensity of radiation.
combining equation no 1 and 2 we get the integrated single relationship which is basis for all chlorometric analysis and is known as Beer-Lamberts law, which states that, the absorbance of solution is proportional to the product of concentration of the absorbing species and thickness of solution. From equation 1 and 2
the quantity log Io/I =A (absorbance) i.e A=abc
where a=molar extinction coefficient
c=concentration of solution (molL-1)
thus, A~c i.e. absorbance is directly proportional to the concentration of the solution. To verify the law, plotting of absorbance against concentration of solution should be a straight line through origin. Lambda max can be defined as the wavelength at which maximum absorbance of the solution occurs. Lambda max can be determined by considering the solution of intermediate concentration and varying the wavelength continuously. At continuous wavelength maximum absorbance occured called Lambda max.
Chromium(VI) with Diphenylcarbazide in acidic condition gives violet (reddish purple) colour proportion to the Chromium(VI) ion present. The DPC when reacts with dichromate, it is oxidising to Diphenylcarbazide and Cr+6 is reduced to Cr3+. This chromium ions formed in situ) reacts with diphenyl carbazene and forms the colour complex. The colour intensity is measured and absorbance is used for the calculation of concentration of Cr (VI).
Cr+6 +DPC+ H+-> violet colour complex