Titration of soil-derived fulvic acid by copper(II) and measurement of free copper(II) by anodic stripping voltammetry and copper(II) selective electrode

Abstract

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...

A voltammetric method for determining free metal activities in aqueous solutions. Part 2: anodic stripping voltammetry of Cd, Cu, Pb and Zn in synthetic and soil solutions

“…Some previous attempts have been made to measure free metal activities of Cu 2+ by ASV, but the interpretation of the results was not straightforward (Bhat et al, 1981;Zur and Ariel, 1977). Bhat et al (1981), for instance, presented an equation for the kinetic correction for systems showing chemical irreversibility; however, the difference found between the ISE and the ASV method when measuring free copper activity in solution was such that the best agreement between the two techniques was reflected in ISE/ASV ratios ranging from 0.41 to 0.83. This is probably due to the lack of consideration of the peak potential shift as shown in this study.…”

Complexation of Copper by Sewage Sludge-derived Dissolved Organic Matter: Effects on Soil Sorption Behaviour and Plant Uptake

“…However, the extent of complexation is also affected by solution variables such as pH, ionic strength and the presence of inorganic anion pairs, which may alter the competition for reactive sites on organics and/or adjust their effective charge. Although some work has considered the metal, and in particular Cu, complexing capacity of DOM (Bhat, Saar, Smart, & Weber, 1981;Dudley, McNeal, Baham, Corey, & Cheng, 1987;Sposito, Holtzclaw, & LeVesque-Madore, 1979), there is a lack of information concerning the effects of soil solution variables on the process, particularly in sludged soils. Indeed, this may be of especial importance in sludgeamended soil where the sludge addition itself has the potential to cause marked changes in soil solution chemistry (e.g.…”

Exchange of comments on evaluation of the copper anodic stripping voltammetry complexometric titration for complexing capacities and conditional stability constants

“…Sir: The correspondence of Bhat and Weber (1) is mainly a reiteration of discussion presented both in our original paper (2) and in a recent exchange of comments (3,4); i.e., for copper ASV titrations of several pure organic compounds we were unable to obtain correct ß' values because of kinetic dissociation or direct reduction of complexed copper. We are well aware that if neither phenomenon were occurring, undetectable to barely detectable currents would have been observed in the case of Cu-EDTA.…”