The absorption spectra of Cd–PAR and Hg–PAR complexes are shown in (Fig. 3). The total absorption spectrum of a mixture of Cd and Hg complexes is given. Due to the fact that absorption spectra of Cd and Hg complexes overlap, the determination of Cd and Hg in their mixtures by zero-order spectrophotometry is frequently difficult. The procedure for the simultaneous analysis, which involves the measuring of the total absorbance of the mixture at the two wavelengths of maximum absorption followed by simultaneous equations, was investigated. However, results of poor accuracy and reproducibility are obtained. This disadvantage has been overcome by use of derivative spectrophotometry.