Comparison of [Cu2+] measurements in lake water determined by ligand exchange and cathodic stripping voltammetry and by ion-selective electrode

While there has been a proliferation of techniques for measuring free metal ion concentrations and complexation in natural waters, there has been little direct intercomparison among methods. Here we report the results of an intercomparison among different methods used to determine free cupric ion concentration ([Cu2+]) and organic complexation in lake water samples. The techniques used were potentiometric measurement with a cupric ion-selective electrode (Cu-ISE) and ligand exchange methods involving the addition of three well-characterized ligands with different binding affinities for copper (catechol, 8-hydroxyquinoline, and tropolone) followed by measurement of the Cu chelates of these ligands by adsorptive differential pulse cathodic stripping voltammetry (DPCSV). Good agreement was found among relationships between [Cu2+] and the total concentration of natural copper species ([Cu-nat]) determined via titration with the three ligand exchange/DPCSV methods. Relationships between [Cu2+] and [Cu-nat] measured by these methods also agreed with those determined with a cupric ion electrode within the overlapping range of detection windows for each method ([Cu-nat] = 40-100 nM). At lower Cu concentrations, the Cu electrode gave erroneously high [Cu2+] readings, apparently due to a failure of the electrode membrane to equilibrate with the sample. The agreement among the different methods provides support for the validity of each individual method of measuring [Cu2+] and natural Cu complexation. By combining Cu titration data measured by DPCSV and Cu-ISE we obtained a more complete description of the Cu-complexing characteristics of water samples from two Swiss lakes than could be obtained with any single method. Equilibrium modeling of this composite data indicated the presence of at least three ligand classes. These ranged in concentration from 38 +/- 19 nM far the strongest binding ligand class, which had conditional binding constants in the range of 10(15) at pH 7.8-8.0, to 8 +/- 2 mu M for the weakest class of ligands, which had 8 conditional constant of 10(8.6) M-1.