Quantitative approximation for the selectivity of analytical spectrophotometric procedures with systems which do not obey Beer's law.

A selectivity index is proposed for defining the selectivity of a spectrophotometric procedure that is subject to interference by species which do not obey Beer's law in the system. The interactions between analyte and interferents which affect the absorbance of an analytical system are studied by means of a simple mathematical model. Theoretical expressions are derived which represent the selectivity as a function of the analyte or interfering species concentration. The treatment is illustrated by a study of the Zr(IV)-chloranilic acid system in presence of thorium as interferent.