pH-metric determination has been used for the simultaneous equilibrium of cobalt (II) with picolinic acid, L-serine and L-threonine. Picolinic acid has been used as a primary ligand where as Lserine and L-threonine as secondary ligands in ternary complexes. Ionic strength was maintained i. e. 1.0 m NaClO4 and complex formation was seen on the basis of the equilibrium curve in aqua–organic media. pH-metry is most widely used method for calculation of stability constants. The results are computed using software program SCOGS.