An aliquot of a sample solution, containing up to 9.2 μg ml−1 of Cd, and up to 7.4 μg ml−1 of Hg, was transferred into a 25 ml calibrated flask, then, 10 ml PAR (1×10−4 mol l−1) was added, the pH was adjusted to 5.5 and diluted to the full volume while keeping a final 50% (v/v) ethanol content. The derivative spectrum was recorded in the range from 400–600 nm against a reagent blank. The absolute value of the first-derivative was measured at 510 nm (zero-crossing point of Cd complex) and 522 nm (zero-crossing point of Hg complex) for the determination of Cd and Hg, respectively.