Beer-Lambert law is the combined form of Beer’s and lambert’s laws. This law relates the intensity of light absorbed with the thickness of the absorbing medium and the concentration of the solution.

Definition

According to Beer lambert law, ” when a beam of monochromatic light is passed through a solution of an absorbing substance then the rate of decrease in intensity of radiation with the thickness of absorbing solution is directly proportional to intensity as well as to the concentration of the solution.”

Derivation

Mathematically, Beer lambert law can be expressed as:

A=εCb

ε =the molar extinction coefficient of the absorbing medium.

C= concentration of solution

Where, A = Absorbance

Hence, the molar extinction coefficient(ε) can be defined as absorbance for a concentration of 1 mol dm-3 and a path length of 1 cm.

When C= 1 mol dm-3 and b=1 cm then, A=ε.

The term log(Io/It) is called absorbance(A)

As we know,

log(Io/It) = εCb

A=log(Io/It)A=log(1/T)A=-log( T )

The ratio (It/Io) is called Transmittance(T). Now, relate absorbance(A) and Transmittance(T).

Hence, the absorbance is equal to the negative logarithm of transmittance.

Verification of Beer-lambert law

This law can be verified by plotting the absorbance(A) against the molar concentration (C) of the solution. A straight line passing through the origin and having a slope εb is obtained.

Application of Beer -lambert law

1. The concentration of the unknown solution can be determined by comparison with a solution of known concentration using a spectrophotometer, the principle is based on Beer-Lambert Law.
2. The purity of a substance can be checked by the measurement of the absorbance of a compound by using a spectrophotometer.

Limitations of Beer-lambert law

1. Beer-Lambert law is only valid on monochromatic light
2. This law is applicable under a low concentration range where interactions between molecules are not considered.
3. This law is also invalid when radiations of very high intensities are used.