Chem. Anal. (Warsaw), 54, 783 (2009) * Corresponding author. E-mail: abkhedr2001@yahoo.com, Fax: +20 40 3350804 Keywords: Cobalt(II) determination; Triazole azo dyes; Pharmaceutical products Spectroscopic Investigation of Cobalt(II) Complexes with Some Triazole Azo Dyes for Microdetermination of Cobalt(II) in Pharmaceutical Products by Abdalla M. Khedr Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt P.O. Box: 31527, Tanta, Egypt By coupling diazonium salt of 3-amino-1,2,4-triazole with resorcinol, p-hydroxytoluene, p-hydroxyethylbenzoate, p-hydroxybenzophenone, p-hydroxybenzaldehyde and p-bromo- phenol, six triazole azo dyes HL 1 –HL 6 were obtained. They represented good complexing properties towards Co(II) ions, resulting in colored complexes. In this paper, the use of the synthesized triazole azodyes as reagents in spectrophotometric determination of Co(II) was presented. Absorption maxima, optimum pH values, molar extinction coefficients, sto- ichiometric ratios, apparent stability constants for the complexes, and coresponding free energy of formation were determined. HL 1 was found to be the best chromogenic reagent due to its high selectivity and sensitivity towards Co(II) ion. HL 1 was successfully applied with high precession and good accuracy to the spectrophotometric determination of Co(II) ion in injection solutions, tablets, capsules, and liquid formulations. Bonding type in the formed complexes was studied. Otrzymano szeœæ triazolowych barwników Azo HL 1 –HL 6 . Otrzymano je przez przy³¹czenie rezorcyny, p-hydroksytoluenu, benzoesanu p-hydroksyetylu, p-hydroksybenzaldehydu i p-bromofenolu do diazoniowej soli 3-amino-1,2,4-triazolu. Otrzymane ligandy dobrze kompleksowa³y jony Co(II), a otrzymane kompleksy by³y kolorowe i zosta³y u¿yte w tej pracy do oznaczania Co(II). Wyznaczono maksima absorpcji, optymalne pH, wspó³czynniki ekstynkcji, stechiometriê reakcji kompleksowania i jej sta³¹ trwa³oœci oraz odpowiednie wartoœci entalpii swobodnej. HL 1 okaza³ siê najlepszym ligandem pod wzglêdem selektyw- noœci i czu³oœci. Zosta³ on zastosowany z du¿¹ precyzj¹ i dok³adnoœci¹ do spektrofotome- trycznego oznaczania Co(II) w zastrzykach, tabletkach, kapsu³kach i ciek³ych preparatach. Dyskutowano równie¿ typ wi¹zania w zwi¹zkach kompleksowych.