Skip to main content

Beer-Lambert Law

The Beer-Lambert Law describes the relationship between the absorbance of light by a solution and the properties of the solution. It is a fundamental principle in spectroscopy and analytical chemistry for determining the concentration of substances.

Statement of the Law​

The Beer-Lambert Law can be expressed as:

A = ε × l × c

Where:

  • A = absorbance (unitless)
  • ε = molar absorptivity or extinction coefficient (L·mol⁻¹·cm⁻¹)
  • l = path length of the sample (cm)
  • c = concentration of the absorbing species (mol·L⁻¹)

Key Concepts​

  • Absorbance (A): A measure of how much light is absorbed by a solution.
  • Molar Absorptivity (ε): A constant that indicates how strongly a species absorbs light at a given wavelength.
  • Path Length (l): The distance light travels through the solution.
  • Concentration (c): The amount of solute per unit volume of solution.

Applications​

  • Determining concentrations of unknown solutions using spectrophotometry.
  • Studying reaction kinetics by monitoring changes in absorbance over time.
  • Investigating chemical equilibria and solution properties.

Important Considerations​

  • The Beer-Lambert Law is valid only for dilute solutions where absorbance is linearly proportional to concentration.
  • Deviations can occur due to high concentration, chemical interactions, or stray light.
  • Accurate measurements require selecting the correct wavelength where the substance absorbs light strongly.

Example​

If a solution has an absorbance of 0.75, path length of 1.0 cm, and molar absorptivity of 1.50 × 10⁴ L·mol⁻¹·cm⁻¹, the concentration can be calculated as:

c = A / (ε × l) = 0.75 / (1.50 × 10⁴ × 1) ≈ 5.0 × 10⁻⁵ mol·L⁻¹

Have Questions?​

If you have any questions, or are looking for more resources, you can join our Discord Server