Protonation constant of monoaza-12-crown-4 ether and stability constants with selected metal ions in aqueous solution in the presence of an excess ofsodium ion: a potentiometric and differential pulse polarographic study atfixed 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 exc ess 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 experimen tal and calculated complex formation curves (ECFC and CCFC) for labile meta l-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 met al species formed and the refinement of their stability constants. The meta l-ligand model and formation constants are optimised by solving mass-balanc e equations written for the assumed model and by fitting the CCFC to the EC FC. The CCFC can be generated for any metal-ligand model, including polynuc lear metal species, for any L-T:M-T ratio, and for more than one ligand com peting in the complex formation reaction. Three lead complexes with the lig and A12C4, viz. PbL2+, PbL(OH)(+) and PbL(OH)(2), were found and their over all stability constants from differential pulse polarography (DPP), as log beta, were estimated to be 3.75 +/- 0.03, 9.30 +/- 0.05 and 11.70 +/- 0.05, respectively. Two copper complexes CuL2+ and CuL(OH)(2) are reported and t heir 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 fe w potentiometric experimental points), respectively, and the stability cons tant for CdL(OH)(+) from DPP was estimated to be 7.88 +/- 0.05. GEP could n ot be used for the stability constants determination of other metal complex es studied because of precipitation occurring prior the completion of a com plex formation reaction. (C) 1998 Elsevier Science B.V. All rights reserved .