Question

Give me two reasons for deviations from Beers’ law that would cause a nonlinear relationship

between absorbance and concentration. Explain why the deviations from linearity occur.?

Answer 1

According to Beer's law, absorbance is proportional to concentration. It is a linear plot describing the relationship between absorbance and concentration. Deviations do occur however to cause non-linearity. This can be attributed to a range of chemical and instrumental errors.

1. Beer's law loses linearity at the high and low concentrations ends of the relationship. It successfully describes the behavior of dilute solutions. At high concentrations, There may be interaction between analyte and other atomic species in the sample and thus absorption characteristics of the analyte are affected. High concentration may result in a shift in the absorption wavelength of the analyte.

The absorption spectrum of the sample changes with the change in pH of the solvent. The analyte molecules may associate, dissociate and interact with the solvent, and form a product with different absorption characteristics. For example, Phenol red, which undergoes a resonance transformation when moving from acidic form(yellow) to the basic form(red). Resonance affects the electron distribution and thus changes the absorption spectrum.

2. Instrumental factors also cause deviations from linearity.

a. Beer's law is strictly followed when a monochromatic source of radiation is used. It is common to use a polychromatic source with a filter or a grating (monochromator) to create a monochromatic beam from the source. However, if the band of wavelengths is selected such that the molar absorptivity of the analyte at these wavelengths changes a lot, the absorbance of the analyte will not follow Beer's law.

b. The another problem is of stray radiation or scattered radiation from the instrument. This radiation is due to reflection and scattering by the surfaces of lenses, mirrors, gratings and filters. This radiation is outside the wavelength band selected. If the analyte absorbs at this wavelength of stray radiation, a deviation is observed.

c. If the cells used for the analyte sample and the blank solutions have different path-lengths, then there would be a deviation.