Use of the Copper(II)-Selective Electrode for the Determination of the Stability Constants of Copper(II) Complexes

In the determination of stability constants of metal complexes, usually the measurements of the activities of free metal ions at very low levels are required. The electrode response of an Orion Model 94–29 copper(II) ion selective electrode at very low activity levels was studied in the various metal buffers composed of copper(II) ions and an excess of ligand. Usually the linear calibration range of the copper(II) ion selective electrode was extended to very low activity levels in the presence of a strong complexing agent. The stability constants of copper(II) complexes with acetate, ammonia, ethylenediamine, glycine, iminodiacetic acid, 1,10-phenanthroline, etc. were determined successfully. In the cases of EDTA and NTA, however, the lower limit of the linear calibration range was abnormally high and the stability constant of copper–EDTA or –NTA chelate could not be obtained. Some experimental results on the potentiometric titration of copper(II) ion with EDTA were also presented.