The complexation equilibria of Cd(II) with PAR were studied in solutions containing 50% (v/v) ethanol–water medium within the pH range 3.0–10.0. The solution spectra were recorded in equimolar solutions and in solutions containing an excess of one component. The absorption spectrum of the Cd complex shows an absorbance maximum at 510 nm, where the reagent PAR does not absorb.
The optimum concentration of PAR for maximum colour development was determined by carrying out a set of experiments using a 7.86 μg ml−1 Cd2+ concentration. The reagent concentration was varied over a wide range from 1×10−5 M to about 5×10−3 M. The maximum absorbance was attained using 2×10−4 M PAR.