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Study Notes: Deviations from Beer’s Law

It is often assumed that Beer’s Law is always a linear plot describing the relationship between absorbance and concentration. Deviations do occur however that cause non-linearity. This can be attributed to a range of chemical and instrumental factors, some of which are briefly considered below.

Beer’s Law successfully describes the behaviour of dilute solutions only. At high concentrations (ie greater than 10-2 M) there is interaction between absorbing particles such that the absorption characteristics of the analyte are affected.

Also at high concentrations the refractive index of a solution can be altered causing departures from Beer’s Law.

Instrumental factors including stray light, noise and effects due to polychromatic radiation also cause spectrophotometers to suffer from non-linearity. The selection of an analytical wavelength other than lambdamax can also be a contributing factor.

The net outcome of all of these influences is that Beer’s Law loses linearity at the high and low concentration ends of the relationship. As a consequence the best results (minimum overall error) are obtained when absorbance is in the range 0.2 to 0.8. For absorbances below 0.1 and above 1.0 considerable errors may be expected (but this will be dependent upon the performance characteristics of the individual spectrophotometer).

A calibration curve (concentration versus absorbance) where at high concentration/absorbence the curve becomes non-linear.

It is always important for the analyst to check or know that the concentration range being worked on is indeed linear and not just simply apply Beer’s Law assuming linearity when in fact substantial errors are being introduced. The preparation of a calibration curve with standards bracketing the concentration of the samples to be analysed will highlight non-linearity. Where there is non-linearity the calibration curve should be used for quantitation rather than the Beer’s Law equation.

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