Question

Beer-Lambert law along with its limitations

Answer 1

Question

Beer-Lambert law along with its limitations

Answer

The Beer-Lambert law is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as:

$$A = a (\lambda) \cdot b \cdot c,$$

where $A$ is the measured absorbance, $a(\lambda)$ is a wavelength-dependent absorptivity coefficient, $b$ is the path length, and $c$ is the analyte concentration.

When working in concentration units of molarity, the Beer-Lambert law is written as:

$$A = \varepsilon \cdot b \cdot c,$$

where $\varepsilon$ is the wavelength-dependent molar absorptivity coefficient with units of $\mathrm{M}^{-1}\mathrm{cm}^{-1}$.

Limitations of the Beer-Lambert law

The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes of nonlinearity include:

- deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity

- scattering of light due to particulates in the sample

- fluorescence or phosphorescence of the sample

- changes in refractive index at high analyte concentration

- shifts in chemical equilibria as a function of concentration

- non-monochromatic radiation, deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band

- stray light