The Beer–Lambert law is a fundamental relationship in IB Chemistry Topic 11 (Measurement & Data Processing). It explains how the amount of light absorbed by a solution depends on its concentration. This law forms the basis of colorimetry and spectrophotometry—techniques used to determine concentrations of unknown solutions with high accuracy. Once you understand how absorbance, concentration, and path length are linked, quantitative analysis becomes much easier.
What Is the Beer–Lambert Law?
The Beer–Lambert law states that absorbance is directly proportional to the concentration of a solution and the path length of the light through it.
The equation is:
A = ε c l
Where:
- A = absorbance (no units)
- ε = molar absorptivity (L mol⁻¹ cm⁻¹)
- c = concentration of solution (mol L⁻¹)
- l = path length of cuvette (cm)
In simple terms:
More concentrated solutions absorb more light.
What Absorbance Means
When light passes through a colored solution, some wavelengths are absorbed.
Absorbance measures how much light is absorbed rather than transmitted.
Important points:
- Absorbance increases with concentration.
- A = 0 → no light absorbed (solution is clear).
- Higher A values → darker solution.
Absorbance is logarithmic, not linear, but Beer–Lambert makes the relationship effectively linear for practical lab use.
Why the Beer–Lambert Law Works
Two principles combine to make the law valid:
