Bouguer-Lambert-Beer law

So:

 The absorbance of a solution is proportional to concentration of light-absorbing substance and a thickness of a layer

Or

 The relationship between a sample’s absorbance and the concentration of the absorbing species

where: A – optical density (absorbance), ε – the molar absorptivity, C – concentration (molarity)

Additivity of optical densities

Beer’s law can be extended to samples containing several absorbing components provided that there are no interactions between the components. Individual absorbances, A_i, are additive. For a two-component mixture of X and Y, the total absorbance, A_tot, is

So

A = l(e₁С₁ + e₂С₂ + …e_kС_k)

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.