Appropriate parameters were selected to record the first-derivative spectra using the zero-crossing method to select suitable wavelengths for obtaining linear calibration graphs. The reproducibility of zero-crossing wavelengths of derivative spectra was checked by recording the first-derivative spectra of Cd–PAR and Hg–PAR systems (pH 5.5) at different concentrations of the analytes (Fig. 4). The zero-crossing wavelengths of Cd and Hg in the first-derivative spectra are obtained at 510 and 522 nm, respectively, (Fig. 4(a), (b)), whereas the first-derivative spectra of two series containing increasing amounts of Cd or Hg ions are shown in (Fig. 5(a), (b)). Cd or Hg can be determined using the absolute value of the total derivative spectrum at a wavelength corresponding to the zero-crossing point of the other component. The heights h1 and h2 in the first-derivative spectra of mixtures (Fig. 5(a), (b)) corresponding to values taken at 510 nm (zero-crossing point of Cd complex) and at 522 nm (zero-crossing point of Hg complex) are proportional to Cd and Hg concentrations, respectively.