Question

1. What does the Beer-Lambert Law allow you to do?

2. Under what conditions is the Beer-Lambert Law valid?

3. Under what conditions is the Beer-Lambert Law not valid?

Answer 1

1.Beer-Lambert Law allows you to measure concentration of a sample by providing a linear

relationship between absorbance and concentration of an absorber of electromagnetic radiation. The general Beer-Lambert law is usually written as:

A=lc where A is the measured absorbance, is a wavelength-dependent absorptivity coefficient, l is the path length, and c is the analyte concentration.

2.Beer-Lambert Law is valid if the basic assumptions are fullfilled which require following conditions :

• The molecules of absorbing sample must act independently of each other without any electrostatic interactions
• The absorbing medium must be homogeneous in the interaction volume.
• The absorbing medium must not scatter the radiation—no turbidity.
• The incident radiation must consist of parallel rays, each traversing the same length in the absorbing medium.
• The incident radiation should preferably be monochromatic.
• The incident flux must not influence the atoms or molecules.

3.Beer-Lambert Law is not valid under the following conditions:

• Deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity.
• Absorbing mediumis not homogenous.
• Absorbing medium scatters light.
• Incident radiation does not contain parallel rays.
• Non-monochromatic incident radiation.
• Incident radiation influences atoms or molecules.