Protonation constant of monoaza-12-crown-4 ether and stability constants with selected metal ions in aqueous solution in the presence of an excess of sodium ion: a potentiometric and differential pulse polarographic study at fixed ligand to metal ratio and varied pH

The ligand monoaza-12-crown-4 ether (A12C4) was studied in aqueous solution at 298 K and an ionic strength of 0.5 mol dm⁻³ in the presence of an excess of sodium ion (0.5 mol dm⁻³ NaNO₃). The protonation constant of A12C4, determined by glass electrode potentiometry (GEP) in the same background electrolyte, was found to be log K=9.36±0.03. Polarographic experimental and calculated complex formation curves (ECFC and CCFC) for labile metal–ligand systems, studied at a fixed total ligand (L_T) to total metal (M_T) concentration ratio and varied pH, were used for the modelling of the metal species formed and the refinement of their stability constants. The metal–ligand model and formation constants are optimised by solving mass-balance equations written for the assumed model and by fitting the CCFC to the ECFC. The CCFC can be generated for any metal–ligand model, including polynuclear metal species, for any L_T:M_T ratio, and for more than one ligand competing in the complex formation reaction. Three lead complexes with the ligand A12C4, viz. PbL²⁺, PbL(OH)⁺ and PbL(OH)₂, were found and their overall stability constants from differential pulse polarography (DPP), as log β, were estimated to be 3.75±0.03, 9.30±0.05 and 12.70±0.05, respectively. Two copper complexes CuL²⁺ and CuL(OH)₂ are reported and their stability constants (from DPP) were estimated to be 6.00±0.05 and 21.77±0.1, respectively. Two cadmium complexes CdL²⁺ and CdL(OH)⁺ are reported. The stability constant for CdL²⁺ was estimated from DPP and GEP as 2.80±0.05 and 2.68±0.03 (the latter value was obtained from a few potentiometric experimental points), respectively, and the stability constant for CdL(OH)⁺ from DPP was estimated to be 7.88±0.05. GEP could not be used for the stability constants determination of other metal complexes studied because of precipitation occurring prior the completion of a complex formation reaction.