Using the Beer–Lambert law

Three quantities determine the prediction

In the Beer–Lambert model, absorbance is proportional to concentration c and optical path length l. The coefficient ε describes the response for the substance, wavelength and conditions. We keep ε user supplied; the example value is illustrative.

Work through a synthetic example

Use an illustrative ε = 20,000 L·mol⁻¹·cm⁻¹, c = 25 µM and l = 10 mm. First convert: 25 µM = 0.000025 mol/L and 10 mm = 1 cm. Then A = 20,000 × 1 × 0.000025 = 0.5.

Reversing the calculation gives c = 0.5 / (20,000 × 1) = 0.000025 mol/L = 25 µM. Without ε and l, the same absorbance does not specify a unique concentration.

Know the model you are applying

The law describes a homogeneous medium with a sufficiently narrow wavelength band. Scattering, stray light and changes in sample chemistry can make a real response differ from this model. Solvent, pH and temperature can affect absorption behaviour.

The calculator checks arithmetic and input ranges. It does not choose an experimental coefficient, validate a method or calculate measurement uncertainty. Doubling concentration or path length doubles the predicted absorbance only while the selected model remains appropriate.

Sources & further reading

• IUPAC Gold Book — Beer–Lambert law
• Agilent — The Basics of UV-Vis Spectroscopy

Original educational material. Content review records a source and consistency check; it is not specialist peer review or laboratory method validation.