Answer to Question #115540 in Physical Chemistry for DEEPAK SINGH

Question

What are the appropriate reasons for the deviation from the beer's law?

Answer 1

Answer:

The Beer's Law is a limiting expression, and is valid only for ideal solutions. The deviations from the Beer's law can be divided in:

1) originating from the sample,

2) originating from the instrumentation.

The first group of the reasons comprises:

a) interactions between the absorbing species (e.g. formation of dimers),

b) change of the refractive index of the media due to the high concentration of the solute,

c) light induced chemical reactions in the sample,

d) fluorescence (emission of the absorbed light),

e) light scattering (non-ideality of the solution).

The second group of reasons is mainly caused by:

a) logarithmic nature of the absorbance:

$A = \text{log}\frac{I_0}{I_t}$ ,

where $I_0$ and $I_t$ are the incident and the transmitted light intensities, respectively. This equation clearly shows that when the absorbance equals 2, the transmitted light is 100 times less intense than the incident light. Such a small intensity is hardly measurable, which causes a big instrumental error of the measurement,

b) non-monochromaticity of light.

The non-linearity of the absorbance/concentration dependence has been also recently addressed regarding the distance between the absorbing particles[1].

[1] Phys. Chem. Chem. Phys., 2019,21, 9793-9801