This paper emphaslzes measurement of [Cu2+] during the Cu2+ tltretlon of soil-derlved fulvlc acid (SFA). We detected Cu2+ by anodic strlpplng voltammetry (ASV) and ion selective electrode (ISE) technlques under the same experimental Conditions. Correctlon of observed ASV stripping current ( I , )after Wing the data to a theoretlcal eqiiatlon allowed calculatlon of SFA complexing capacities for 10, 20, and 40 mg/L SFA. The use of In situ calibration curves in the ASV oxperlments allowed us to convert klnetlc current-corrected I , values into [Cu"], and to compare [Cuzc] measured by ASV and ISE experlments. 'Three major conclusions arlse from thls work. Researchers can use ASV to measure [Cu2+] In the presence of fulvic acid wlth appropriate correctlons. It would be advantageous to compare the results of at least two technlques when measuring free metal lion concentrations in fulvlc acid solutions. For estlmatlon of free metal ion by ISE in fulvic acid sailutlons, researchers should exercise cautlon while calibratlng the ISE.Recent attempts to understand the extent and nature of metal ion binding to isolated and in situ humic materials utilize dialysis (I), fluorescence spectrometry (2), ion selective electrode (ISE) potentiometry (2-9), and voltammetry (10-13). Although the most popular techniques are ISE and voltammetry, they are controversial. Sekerka and Lechner (14) noted that the Cu2+ ISE responds to humic materials in the absence of Cu2+, but Buffle et al. (15) using microprobe analysis observed no carbon adsorption on Cu2', PbZ+, or Cd2+ ISE. During voltamnietric experiments humic materials adsorb on mercury electrodes (II), and observed plating (reduction) currents should be corrected for kinetic current due to metal complex dissociation during deposition at the electrode (16-18). Both of these problems with voltammetry cause complications and necessitate assumptions in calculating free metal ion concentrations from observed currents.The metal-complexing ability of an aqueous solution of fulvic acid (FA) or of humic matter in a natural water sample is measuired by adding excess metal ion to the sample and distinguishing free metal ion and complexed metal ion concentrations. The major requirement of differentiating between free and total metal ion can be fulfilled by techniques such as ISE patentioinetry or anodic stripping voltammetry (ASV).The organic matter complexing capacit!? or total ligand concentration (C,) is typically obtained from the upper and lower portion of the titration curve of a free metal ion vs. total metal ion graph. CL values obtained by different experimental techniques do not always agree. For example, CL values €or different FA samples me lower for ASV (16) than ISE (15) results. The disparity might be caused by differences in propertieu of humic materials obtained from different sources, or in experimental conditions. For instance, ASV experiments usually use maximum metal ion concentrations of ca. 10" M compared t o CEI. M for ISE experiments. It is more appropriate to adopt more...

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Sir: Tuschall and Brezonik (1) in a recent paper criticize the titrametric anodic stripping voltammetry (ASV) method (2, 3) for determining total ligand concentration (CL) values and conditional stability constants (ß') for copper(II) complexes. The authors conclude that the method is unsuitable for titration of natural water samples or natural organic matter. In contrast, our recent paper (4) concludes that the ASV method is suitable for titration of soil fulvic acid with Cu2+.

By using the same current-time (I-t) curves, electrochemical kinetic parameters are determined by two methods, (a) using the ratio of current at a given potential to the diffusion-controlled limiting current and (b) curve fitting method, for the reduction of Cu(II)–CyDTA complex. The analysis by the method (a) shows that the rate determining step involves only one electron although the overall reduction of the complex involves two electrons suggesting thereby the stepwise reduction of the complex. The nature of I-t curves suggests the adsorption of intermediate species at the electrode surface. Under these circumstances more reliable kinetic parameters can be obtained by the method (a) compared to that of (b). Similar observations are found in the case of reduction of Cu(II)–EDTA complex.