Beer Lambert Law and Determination of Concentration of a Colored Solution

Beer Lambert Law

Beer Lamberts Law
A = log\frac{Io}{I}
A =\varepsilon lc

‘A’ represents absorbance, which we will be measuring. ‘\varepsilon ’ is molar absorptivity. ‘I0’ is the intensity of light at zero absorbance or the blank. ‘I’ represents the absorbance of the colored solution or intensity of light passing through the sample solution. If I < I0 some light has been absorbed by the solution. ‘l’ represents the length of the light path through the solution and ‘c’ represents the concentration of the solution.
A = log\frac{Io}{I}



Determination of concentration of an unknown solution of CuSO4.5H2O

The objective of this experiment is to determine the unknown concentration of a colored solution like Copper(II)sulfate penta hydrate.

In order to do that five (standard) solutions of known concentration are prepared by serial dilution.

A spectronic-20 spectrophotometer or a colorimeter from vernier may be used to determine the absorbance of the five standard solutions that were prepared.

The five standard solutions are filled in the cuvettes and absorbance measured, after the colorimeter is calibrated with a blank solution.

Concentration Vs Absorbance

A plot of absorption versus concentration is plotted using Microsoft Excel.  A direction relation exists between absorbance and concentration. Which is Beer-Lambert law.

ConcVsAbs Graph


From this graph and the value of absorbance for the unknown concentration of CuSO4.5H2O, you can determine the concentration.

This is a very sensitive experiment hence extra caution should be taken while preparing the standard solutions and diluting them.

For more information on using the colorimeter and the labquest go to

 Colorimeter / Labquest