Deviations from Beers Law

Beer s law generally holds good over a wide range of concentration if the structure of the coloured non-electrolyte in the dissolved state does not change with concentration. Small amount of electrolytes, which do not react chemically with the coloured components, do not usually affect the light absorption, large amounts of electrolytes may result in a shift of the maximum absorption and may also change the value of extinction coefficient. Discrepancies are normally observed when the coloured solute ionises, dissociates or associates in solution as because the nature of the species in solution will vary with the concentration. The law also fails if the 

Also discrepancies may occur when monochromatic light is not used. The plot of log j versus 

It cannot always be assumed that Beer s law will apply, that is, that a linear plot of absorbance versus concentration will occur. Deviations from Beer s law occur as the result of chemical and instrumental factors. Most deviations from Beer s law are really only apparent deviations because if the factors causing nonlinearity are accounted for, the true or corrected absorbance-versus-concentration curve will be linear. True deviations from Beer s law will occur when the concentration is so high that the index of refraction of the solution is changed from that of the blank. A similar situation would apply for mixtures of organic solvents with water, and so the blank solvent composition should closely match that of the sample. The solvent may also have an effect on the absorptivity of the analyte. 

Deviations from Beer s law result in nonlinear calibration curves, especially at higher concentrations. 

Chemical causes for nonlinearity occur when nonsymmetrical chemical equilibria exist. An example is a weak acid that absorbs at a particular wavelength but has an anion that does not  

Apparent deviations may also occur when the substance can exist as a dimer as well as a monomer. Again, the equilibrium depends on the concentration. An example is the absorbance by methylene blue, which exhibits a negative deviation at higher concentrations due to association of the methylene blue. 

The existence of an isosbestic point is not proof of the presence of only two components. There may be a third component with e = 0 at this particular wavelength. The absence of an isosbestic point, however, is definite proof of the presence of a third component, provided the possibility of deviation from Beer s law in the two-component system can be dismissed. For a two-component system, the isosbestic point is a unique wavelength for quantitative determination of the total amount of two absorbing species in mutual equilibrium. 

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