Speciation studies of ternary complexes of Co(II), Ni(II), and Cu(II) with 5-Sulfosalicylic acid and 5-Hydroxysalicylic acid in Urea-water mixtures

Speciation of complexes of Co(II), Ni(II) and Cu(II) with 5-Sulfosalicylic acid as primary ligand (L) and 5-Hydroxysalicylic acid as secondary ligand (X) has been studied in various concentrations (0.0–42.47% w/v) of Urea-water mixtures at ionic strength of 0.16 mol L⁻¹ and at a temperature of 303.0 K. All the titrations were carried out at different relative concentrations (M: L: X = 1.0: 2.5: 2.5; 1.0: 2.5: 5.0; 1.0: 5.0: 2.5) of metal (M) to 5-Sulfosalicylic acid (L) and 5-Hydroxysalicylic acid (X) with NaOH as titrant. The species detected during the refinement were MLXH, ML2XH and MLX2H2 for Co(II), Ni(II) and Cu(II). The extra stability of ternary complexes compared to their binary complexes was understood to be due to electrostatic interactions of the side chains of ligands, charge neutralisation, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of Urea-water mixtures and plausible equilibria for the formation of species were also presented.