The proton-ligand dissociation constant of 5-(4′-alkylphenylazo)-3-phenylamino-2-
thioxothiazolidin-4-one (HL n)(n= 1, R=–CH 3; n= 2,–H and n= 3,–Cl) and metal–ligand 
stability constants of its complexes with metal ions (Mn 2+, Co 2+, Ni 2+ and Cu 2+) have 
been determined potentiometrically in 0.1 mol· dm− 3 KCl and 40%(by volume) DMF–water 
mixture and at 298, 308 and 318 K. The stability constants of the formed complexes increase 
with the order Mn 2+< Co 2+< Ni 2+< Cu 2+.