COMPARISON OF [CU2-EXCHANGE AND CATHODIC STRIPPING VOLTAMMETRY AND BYION-SELECTIVE ELECTRODE(] MEASUREMENTS IN LAKE WATER DETERMINED BY LIGAND)

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 determin e free cupric ion concentration ([Cu2+]) and organic complexation in l ake water samples. The techniques used were potentiometric measurement with a cupric ion-selective electrode (Cu-ISE) and ligand exchange me thods 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 li gands by adsorptive differential pulse cathodic stripping voltammetry (DPCSV). Good agreement was found among relationships between [Cu2+] a nd the total concentration of natural copper species ([Cu-nat]) determ ined via titration with the three ligand exchange/DPCSV methods. Relat ionships between [Cu2+] and [Cu-nat] measured by these methods also ag reed with those determined with a cupric ion electrode within the over lapping range of detection windows for each method ([Cu-nat] = 40-100 nM). At lower Cu concentrations, the Cu electrode gave erroneously hig h [Cu2+] readings, apparently due to a failure of the electrode membra ne to equilibrate with the sample. The agreement among the different m ethods provides support for the validity of each individual method of measuring [Cu2+] and natural Cu complexation. By combining Cu titratio n data measured by DPCSV and Cu-ISE we obtained a more complete descri ption of the Cu-complexing characteristics of water samples from two S wiss lakes than could be obtained with any single method. Equilibrium modeling of this composite data indicated the presence of at least thr ee ligand classes. These ranged in concentration from 38 +/- 19 nM far the strongest binding ligand class, which had conditional binding con stants in the range of 10(15) at pH 7.8-8.0, to 8 +/- 2 mu M for the w eakest class of ligands, which had 8 conditional constant of 10(8.6) M -1.