Journal of Solution Chemistry, Vol. 33, No. 2, February 2004 ( C  2004) Potentiometric and Thermodynamic Studies of 5-(4 ′ -Sulfonylazidophenylazo)-3-phenyl-2- thioxothiazolidin-4-one and Its Metal Complexes Ahmed T. Mubarak 1 Received September 23, 2003; revised February 23, 2004 The proton dissociation constant of 5-(4 ′ -sulfonylazidophenylazo)-3-phenyl-2-thio- xothiazolidin-4-one (SPT) and the stability constants of its complexes with some metal ions were calculated potentiometrically in 0.1 M KCl and 40% (v/v) ethanol–water mixture. The order of stability was found to be Mn 2+ < Co 2+ < Ni 2+ < Cu 2+ < Zn 2+ . The effect of temperature on the dissociation of SPT and the stability of its complexes were studied. The corresponding thermodynamic functions were derived and discussed. The dissociation process is unspontaneous, endothermic, and entropically unfavorable. The formation of the metal complexes was found to be spontaneous, endothermic and entropically favorable. KEY WORDS: 5-(4 ′ -Sulfonylazidophenylazo)-3-phenyl-2-thioxothiazolidin-4-one; potentiometry; dissociation constant; stability constants; thermodynamic parameters. 1. INTRODUCTION Sulfonamides were the first effective chemotherapeutic agents employed for curing bacterial infections. (1) Azo compounds, based on rhodanine, play a central role as chelating agents for a large number of metal ions. (2) They form a stable six-membered ring by complexation with a metal ion and can also be used as analytical reagents. (3) It is also accepted that knowledge of the stability constants of such azorhodanines and their metal complexes may eventually help to throw some light on the deactivation of essential trace metals in biological systems. (4) Our intrest at present is focused on potentiometric analyses of azorhodanines and their complexes (5−8) to develop pendant ligands containing heteroatoms that might have the potential for selective coordination of metal ions. 1 Department of Chemistry, College of Science, King Khalid University, Abha 61413, Saudi Arabia; e-mail: ahmadtahir20@hotmail.com 149 0095-9782/04/0200-0149/0 C  2004 Plenum Publishing Corporation