Beer–Lambert Law

What is this formula?

The Beer–Lambert Law relates the absorbance of light by a sample to the concentration of the absorbing species, the optical path length, and the molar absorptivity coefficient.

When to use it

Use this formula in spectrophotometric analysis to determine the concentration of a substance from absorbance measurements.

Example

A solution has:

ε = 15000 L·mol⁻¹·cm⁻¹

l = 1.0 cm

c = 2.0×10⁻⁵ mol/L

Formula:

A = εlc

Substitution:

A = 15000 × 1.0 × 2.0×10⁻⁵

Result:

A = 0.30

The measured absorbance is 0.30.

Applications

- UV-Visible spectrophotometry

- Quantitative chemical analysis

- Pharmaceutical assays

- Environmental monitoring

- Clinical chemistry

- Biochemical analysis

Note

The Beer–Lambert Law is an ideal model that is most accurate for dilute solutions, monochromatic radiation, and homogeneous samples. Deviations may occur at high concentrations, due to chemical interactions, scattering, or instrumental limitations.