Limitations to Beer’s Law

According to Beer’s law, a calibration curve of absorbance versus the concentration of analyte in a series of standard solutions should be a straight line with an intercept of 0 and a slope of ab or  b. In many cases, however, calibration curves are found to be nonlinear (Figure 10.22).

Calibration curves showing positive and negative deviations from Beer’s law.

Deviations from linearity are divided into three categories: fundamental, chemical, and instrumental.

Fundamental Limitations to Beers Law

Beer’s law is a limiting law that is valid only for low concentrations of analyte. There are two contributions to this fundamental limitation to Beer’s law. At higher concentrations the individual particles of analyte no longer behave independently of one another. The resulting interaction between particles of analyte may change the value of . A second contribution is that the absorptivity, a, and molar absorptivity, , depend on the sample’s refractive index. Since the refractive index varies with the analyte’s concentration, the values of a and will change. For sufficiently low concentrations of analyte, the refractive index remains essentially constant, and the calibration curve is linear.