# Instrumental techniques in chemical analysis

Apr 27 • General • 1248 Views • No Comments on Instrumental techniques in chemical analysis

QUESTION

Q1.What is Lambert-Beer law?

Q2. What does  Beer’ law and Lambert’ law states?

Q3.State Lambert- Beer law.

Q4. How is absorbance measured?

Q5. Write the mathematical form of Lambert-Beer  law.

Q6. Give an example of Lambert-Beer law.

Q7. What are the limitation of Lambert-Beer law?

Q8. Write the co-efficient Factor of Lambert-Beer law.

Instrumental techniqes in chemical analysis

Lambert’s and Beer’s law:

• In optics, Lambert-Beer law is also known as Beer’s law or the Beer-Lambert law, or the Beer-Lambert – Bouguer law.
• Beer-Lambert law is the linear relationship between absorbance and concentration of an absorber of an electromagnetic radiation.
• In Beer-Lambert’s law, Beer’s law states that absorbance is proportional to the concentration of sample and Lambert’s law states that absorbance is directly proportional to the thickness of the sample.
• Lambert-Beer law states that optical absorbance of a chromophore in a transparent solvent varies linearly with both the sample cell pathlength and chromophore  concentration. This law is accurate enough for a range of chromophore, solvents and concentrations and is widely used in quantitative spectroscopy.
• Absorbance is measured in a spectrophotometer. It is denoted as Aλ, which is calculated from yhe ratio of light energy passing through the sample (10) to the energy that is incident on the sample(I): Aλ = -log (I/I0)
• Lambert-Beer law is a mathematical means of expressing how light is absorbed by matter.
• The mathematical form of Lambert-Beer law is:

log(lo/l)=a b c

where, lo is the intensity of light entering the solution.

I is the intensity of light existing the solution.

b is the thickness of the solution path.

c is the concentration of the solution.

• This Law  also states that there is a logarithmic dependence between the transmission,T, of light through a substance and the product of the absorption coefficient of the substance,,and the distance the light travels through the material,l.
• When the concentration of the absorbing material becomes too high,this law fails.
• An example of Lambert-Beer law is green laser light in a solution of Rhodamine 6B.The beam intensity becomes weaker as it passes through the solution.

Co-efficient Factor of Lambert-Beer law

A=ebc

Where,  A is absorbtion.

e is molar absorbtivity with units of L mol-1 cm-1

b is the path length of the sample.

C is the concentration of the compound in solution

Expressed in mol L-1.

Limitations of Beer-Lambert law:

The Linearity of Beer-Lambert law is limited by chemical and instrumental factors. Causes of non-linearity include;

1. Scattering in light due to particulates in the sample

2. fluorescence   or phosphorescence in the sample.

3. Changes   in refractive index at high analyte concentration.

4. shifts  in chemical equilibria as a function of concentration.