Synthesis and Stability of Nitrilotrimethylenephosphonic Acid Mixed Ligand Complexes of Zn(II) and Cd(II) in Solution

The acid-dissociation constants of Nitrilotrimethylenephosphonic acid, C3H12O9N (NTPA) and its binary complexing equilibria as well as ternary chelates tendency with Zn (II) and Cd (II) ions involving some other chelating agents such as glycine (Gly) or -alanine (Ala) (N-O type donors), 5-sulphosalicylic acid (5SSA), oxalic acid (Oxa) or catechol (Cat) (O-O type donors), 2, 2 bipyridyl (Bipy) or 1, 10-phenanthroline (Phen) (N-N type donors) and iminodiacetic acid (imda) N-N-O type donor) have been determined potentiometrically in aqueous medium containing a supporting electrolyte (0.10 M KNO3) at different temperatures. All the ternary formation systems follow stepwise equilibria in which NTPA behaves as a primary ligand in presence of Gly, Ala or Cat and as a secondary ligand in presence of the remaining few other chelating agents, i. e. , 5SSA, Oxa, Bipy, Phen or imda. The stability constants (log K1, log K2, log 2, log KT and log KT), the percentage of relative stabilization values ( [ %RS]) and the distribution of total metal ions in different chelated species as a function of pH have been evaluated and discussed.