Spectroscopy
The branch of science which describes the interactions of electromagnetic radiation with matter is known as spectroscopy.
Principle of Spectroscopy
During the absorption some of the molecules falling in the path of the incident beam, collide with the photons of radiant energy. However only those photons get absorbed whose energy exactly equal to the difference of energy, ΔE between the ground and the excited states of the molecules. ΔE = hv = hc/ λ
Spectrophotometer
“The instrument used to measure the amount of electromagnetic radiations absorbed by a compound is called spectrophotometer.” It consists of light source of radiation with the prism that can select the desired wavelengths which can pass through the sample of a compound being investigated. The radiation that is absorbed by the sample is detected and is recorded on a chart against the wavelength or wave number. Absorption peaks are plotted as minima in infrared, and usually as maxima in ultraviolet spectroscopy.
Beer-Lambert Law
The absorption of a light by a substance at a particular wavelength is proportional to molar concentration in a fixed path length. Mathematical it expressed as;
A = εcl
A is called absorption of a solution, c is the concentration of absorbing compound in moles /dm3, ε is called molar absorptivity, l is the length of the sample tube in cm.[1]
Verification of Beer-Lambert Law Through a lab Experiment
Prepare the solutions of any salt of given concentration;
0.01M, 0.03M, 0.05M, 0.07M, 0.09M,
Then check absorption of solutions against λmax with help of spectrophotometer.
We examine the Cobalt(II) sulfate solution of given concentrations
Observations
λmax of Cobalt(II) sulfate solutions = 512nm
| Concentration | Absorption |
| 0.01M | 0.046 |
| 0.03M | 0.145 |
| 0.05M | 0.202 |
| 0.07M | 0.381 |
| 0.09M | 0.474 |
Graph
Graph; Showing the Abs Vs Conc.
According to Beer-Lambert Law
ε = A/cl
If l = 0.1cm
| Concentration | Absorption | ε = A/cl (mol-1cm-1) |
| 0.01M | 0.046 | 46 |
| 0.03M | 0.145 | 48.3 |
| 0.05M | 0.202 | 40.4 |
| 0.07M | 0.381 | 54.4 |
| 0.09M | 0.474 | 52.7 |
Discussion
According to Beer-Lambert Law molar absorptivity ε is the constant value for a substance. But our results deviate with Beer-Lambert Law.This is may be due to the presence impurities or some personal errors during solution preparations.
References
[1]. A Text Book of Organic Chemistry by Ghulam Rasool Chaudhary, First Addition 2009, Azeem Publishers Faisalabad
