DIMER SPECIES IN AQUEOUS-SOLUTIONS OF 2,6-TOLUENEDIAMINE-N,N,N',N'-TETRAACETIC ACID WITH COPPER(II), NICKEL(II), COBALT(II) AND ZINC(II)

The complex species formed in aqueous solution between 2.6-toluenediamine-N,N,N',N'-tetraacetic acid (2,6-TDTA) and Cu(II), Ni(II), Co(II), Zn(II), Cd(II) and Mn(II), respectively, were studied at 25-degrees-C and ionic strength 0.1 M in KCl. The application of the least-squares computer program SUPERQUAD to the experimental potentiometric data indicates that, upon varying the ligand: metal relationships and concentrations, the following complex species are formed (ligand, H4L): Cu(II) (CuH2L, [CuHL]-, [CuL]2-, [Cu2H2L]2+, Cu2L, [Cu2HL2]3- and [Cu2L2]4-); Ni(II) (NiH2L, [NiHL]-, [NiL]2-, [Ni2HL]+, [Ni2HL2]3- and [Ni2L2]4-); Co(II) ([CoHL)-, [CoL]2-, [Co2H2L]2+, [Co2HL]+, [Co2HL2]3- and [Co2L2]4-); Zn(II) (ZnH2L, [ZnHL]-, [ZnL]2-, [Zn2H2L]2+, [Zn2HL2]3- and [Zn2L2]4-); Cd(II) (CdH2L, [CdHL]-, [CdL]2- and [CdHL2]5-; and Mn(II) (MnH2L, [MnHL]- and [MnL]2-). The dimer species [M2L2]4- has been identified for the first time in potentiometric studies in aqueous solution. Compared with m-PDTA acid, the order of coordinating ability 2,6-TDTA much less than m-PDTA (m-PDTA = meta-phenylenediamine-N,N,N',N'-tetraacetic acid) is attributed to the repulsive steric effects of the methyl group.