Mikrochim. Acta 129, 225-231 (1998) Mikrochimica Acta Springer-Verlag 1998 Printed in Austria Spectrophotometric and Derivative Spectrophotometric Determination of Palladium(II) Using Pyridopyridazine Dithione in the Presence of Non-Ionic Surfactant A. Youssef EI-Sayed* and Fathi A. Abu-Shanab Department of Chemistry, Faculty of Science, Al-Azhar University, Assiut, Egypt Abstract. Pyridopyridazine dithione (PPD) was synthesized as a new sensitive and selective reagent for spectrophotometric and derivative spectrophoto- metric determination of palladium(II). In aqueous and micellar medium, PPD forms 1:4 complexes hav- ing molar absorptivities of 4.68 x 104 and 5.74 x 1041mol-lcm -t at 570 and 615nm, respec- tively. Beer's law was obeyed up to 2.2 and 2.5 gg m1-1 with detection limits of 0.2 and 0.1 gg m1-1. The relative standard deviations for 1.23 ggm1-1 were 2.6 and 1.3%, in the absence and presence of Triton X-100. In fourth-derivative mode, the regression equa- tion, linear range, detection limit and RSD for 0.075 ~tgm1-1 were D4 : 4.3C + 1.5 x 10 -3, 0.013 - 0.23 ggm1-1, 3.7ngml -a and 0.78%, respec- tively. The ionization constants of the reagent and stability constants of the complexes were evaluated. The method is free from interference by most common metal ions and anions. The method was applied for determination of palladium in activated charcoal. Key words: palladium determination, pyridopyridazine dithione, surfactants, spectrophotometry, derivative spectrophotometry. Numerous spectrophotometric methods are available for the determination of palladium [1-17], but most have low sensitivity [2-7], are nonselective [6-9] and are based on solvent extraction [10-17]. Moreover, the determination of palladium in the presence of noble metals without preliminary extraction or use of masking agent is difficult, because the spectra of the * To whom correspondence should be addressed respective complexes with the chromogenic reagent tend to overlap [14-17]. Micellar media are one of the commonest ways of avoiding extraction procedures and improving the sensitivity and selectivity of analytical methods [2, 18]. Derivative spectrophoto- metry has opened up possibilities for increasing sensitivity [1, 19-21] and selectivity [22-24] of analytical methods without any increase in cost. Pyridazinones comprise a very interesting class of compounds because of their significant biological and pharmaceutical activities [25-27]. The reactions of pyridazinones with carbon disulfide in dioxane - DMSO mixture and sodium hydroxide, affording thiopyranopyridazine derivatives, have been reported [281. In this paper, pyridopyridazine dithione (PPD) derivative was synthesized by reaction of pyridazi- none with carbon disulfide in the presence of pyridine and potassium hydroxide. The pyridopyridazine derivative is utilized as a new sensitive and selective reagent for determination of palladium(II) in an aqueous micellar medium of Triton X-100 by ordinary and fourth-derivative spectrophotometry based on the formation of colored complexes with PPD in alkaline medium. The methods are used to determine palla- dium in activated charcoal. Experimental Apparatus A Perkin-Elmer Lambda 3B double-beam UV-visible spectro- photometer loaded with Per/cAn-Elmer PECSS software and interfaced to a Mitsuba 386/33 DX computer in conjunction with