Q1(h). Molar absorptivity

Answer in brief

ε=A/(cl) is the molar absorptivity at a specified wavelength; commonly its unit is L mol⁻¹ cm⁻¹.

Key points

• Beer–Lambert law is A=log₁₀ (I₀/I)=ε cl. The coefficient expresses how strongly one molar concentration of the species absorbs per unit path length.
• With SI concentration in mol m⁻³ and path length in metres, the SI unit is m² mol⁻¹; 1 L mol⁻¹ cm⁻¹=0.1 m² mol⁻¹.
• It depends on wavelength, molecular structure and transition probability, solvent and chemical form.
• For a fixed species under Beer–Lambert conditions, ε does not depend on path length or concentration.