Effects of the detection window on the determination of organic copper speciation in estuarine waters

The complexation of copper by natural organic ligands in sea water was measured by cathodic stripping voltammetry with ligand (catechol and quinolin-8-ol) competition. Two methods to determine copper complexation in estuarine waters were compared, one based on a complete titration of the complexing capacity of the sample and the other on measurement of the labile and total dissolved metal concentrations only. Values for log α_CuL (ihe α-coefficient for complexation of Cu²⁺ by natural organic ligands) ranging from 3.2 to 7.7 and from 3.3 to 7.8 could be detected by varying long α_CuAL (the α-coefficient for complexation of Cu²⁺ by the added competing ligand) from 3.4 to 8.9 in samples from the Tamar estuary and from the Channel. The two methods gave comparable results and showed that the type of sites detected depends on the detection window of the technique. This effect is due to the sea and estuarine water samples containing a series of complexing ligands forming complexes of greatly varying strength, thus causing a rnage of complex stabilities to be measured as a function of the detection window of each technique. A comparison showed that lower values for α_CuL are obtained by anodic stripping voltammetry as a result of that technique having a lower detection window. A detailed study of the Tamar estuary revealed a decrease in log α_CuL from 10.8 to 8.3 with increasing salinity, demonstrating that major cations compete with copper for the complexing sites. The free Cu²⁺ concentrations were very low throughout the estuary (16.2 < pCu²⁺ < 18.2) even though the total measurements to establish potential toxic effects of copper in natural waters.     

Integrated Microanalytical System for Simultaneous Voltammetric Measurements of Free Metal Ion Concentrations in Natural Waters

A complexing gel integrated microelectrode (CGIME) for direct measurements of free metal ion concentrations in natural waters has been developed. It is prepared by the successive deposition of microlayers of a chelating resin, an antifouling agarose gel and Hg on a 100‐interconnected Ir‐based microelectrode array. The trace metals of interest are in a first step accumulated on the chelating resin in proportion to their free ion concentration in solution, then released in acidic solution and detected simultaneously by using square wave anodic stripping voltammetry (SWASV). The reliability of this sensor for the simultaneous measurement of copper, lead and cadmium has been studied by a series of replicate laboratory tests. The proportionality between the voltammetric peak current intensity and the free metal ion concentrations in solution has been demonstrated by using malonate as a model ligand. Finally, the CGIME sensor was applied to the Cu and Pb free concentration measurement in sea water samples and the results compared to the free metal ion concentrations measured using hollow fiber based permeation liquid membrane (HF‐PLM) coupled to inductively coupled plasma mass spectrophotometer (ICP‐MS). Comparable concentration values were found for both metals with both techniques allowing to validate the CGIME measurements in complex media.     

Evaluation of micromolar compleximetric titrations for the determination of the complexing capacity of natural waters

The titration of micromolar levels of complexing agents with metal ion titrants, and voltammetric methods to locate the equivalence point, has been evaluated experimentally and theoretically. Both anodic stripping voltammetry and differential pulse polarography give systematically low results if labile metal ions are used as titrants. Low-temperature (0 °C) dual-cell anodic stripping voltammetry greatly minimizes the effects of metal complex lability but the mercury film electrodes deteriorate rapidly because of temperature cycling. A micromolar compleximetric titration with a voltammetric end-point is not a practical method for determining the complexing capacity of natural waters.     

Analytical methods for determination of free metal ion concentration, labile species fraction and metal complexation capacity of environmental waters: A review

Different experimental approaches have been suggested in the last few decades to determine metal species in complex matrices of unknown composition as environmental waters. The methods are mainly focused on the determination of single species or groups of species.The more recent developments in trace elements speciation are reviewed focusing on methods for labile and free metal determination.Electrochemical procedures with low detection limit as anodic stripping voltammetry (ASV) and the competing ligand exchange with adsorption cathodic stripping voltammetry (CLE-AdCSV) have been widely employed in metal distribution studies in natural waters. Other electrochemical methods such as stripping chronopotentiometry and AGNES seem to be promising to evaluate the free metal concentration at the low levels of environmental samples. Separation techniques based on ion exchange (IE) and complexing resins (CR), and micro separation methods as the Donnan membrane technique (DMT), diffusive gradients in thin-film gels (DGT) and the permeation liquid membrane (PLM), are among the non-electrochemical methods largely used in this field and reviewed in the text. Under appropriate conditions such techniques make possible the evaluation of free metal ion concentration.     

Use of a modified, high-sensitivity, anodic stripping voltammetry method for determination of zinc speciation in the North Atlantic Ocean

Zinc speciation is considered to be an important determinant of the biological availability of zinc. Yet in oceanic surface waters, characterization of zinc speciation is difficult due to the low concentrations of this essential micronutrient. In this study, an anodic stripping voltammetry method previously developed for the total determination of cadmium and lead was successfully adapted to the measurement of zinc speciation. The method differs from previous zinc speciation anodic stripping voltammetry methods in that a fresh mercury film is plated with each sample aliquot. The fresh film anodic stripping voltammetry method was compared to competitive ligand exchange cathodic stripping voltammetry in a profile from the North Atlantic Ocean. Results using the fresh film anodic stripping voltammetry method were similar to those determined using the cathodic stripping voltammetry method, though ligand concentrations determined by fresh film anodic stripping voltammetry were generally slightly higher than those determined by cathodic stripping voltammetry. There did not seem to be a systematic difference between methods for the estimates of conditional stability constants. The ligand concentration in the North Atlantic profile ranged from 0.9 to 1.5 nmol L⁻¹ as determined by fresh film anodic stripping voltammetry and 0.6 to 1.3 nmol L⁻¹ as determined by cathodic stripping voltammetry. The conditional stability constants determined by fresh film anodic stripping voltammetry were 10^9.8-10^10.5 and by cathodic stripping voltammetry were 10^9.8-10^11.3.     

A Novel Scheme for the Classification of Heavy Metal Species in Natural Waters

Abstract

A scheme is proposed which permits the quantitative assay of seven different heavy metal species in natural waters. The concentration of each species is calculated from measurements using anodic stripping voltammetry of labile and total metal in samples which were (a) untreated, (b) u.v. irradiated, (c) passed through a chelating resin column, and (d) u.v. irradiated then passed through a chelating resin column. The scheme was applied to the analysis of Cd, Pb and Cu in seawater.     

Voltammetric methods for speciation analysis of trace metals in natural waters

Trace metals play an important role in the regulation of primary productivity and phytoplankton community composition. Metal species directly affects the biogeochemical cycling processes, transport, fate, bioavailability and toxicity of trace metals. Therefore, developing powerful methods for metal speciation analysis is very useful for research in a range of fields, including chemical and environmental analysis. Voltammetric methods, such as anodic stripping voltammetry (ASV) and competing ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV), have been widely adopted for speciation analysis of metals in different natural aquatic systems. This paper provides an overview of the theory of voltammetric methods and their application for metal speciation analysis in natural waters, with a particular focus on current voltammetric methods for the discrimination of labile/inert fractions, redox species and covalently bound species. Speciation analysis of typical trace metals in natural waters including Fe, Cu, Zn, Cd, and Pb are presented and discussed in detail, with future perspectives for metal speciation analysis using voltammetric methods also discussed. This review can elaborate the particular knowledge of theory, merits, application and future challenge of voltammetric methods for speciation analysis of trace metals in natural waters.     

In situ electrodeposition for the determination of lead and cadmium in sea water

Electrodeposition techniques for the direct determination of lead and cadmium in sea water at the natural pH and in the presence of dissolved oxygen are examined. Anodic stripping voltammetry, at either the hanging mercury drop electrode or glassy carbon thin film electrode, is suitable for the determination of labile lead and cadmium. The presence of dissolved oxygen increases the height of the lead wave with a shift to more negative potentials. A more versatile technique is in situ deposition of labile metals on a mercury-coated graphite tube electrode. The mercury film, deposited in the laboratory, is stable on the dried tubes which are used later for field electrodeposition. The deposited metals are determined by electrothermal atomic absorption spectrometry.     

Research on Electroanalytical Chemical Methods for Trace Metal Speciation in Sea Water Samples

Since bioavalability of a trace metal in natural waters is affected by its chemical forms, the speciation of a trace metal becomes more and more important in environmental chemistry. Differential pulse anodic stripping voltammetry (DPASV) titration procedures are developed and applied to identify organic trace metal species of copper (Cu), lead (Pb) and zinc (Zn) in sea water and to determine their complexing capacity.     

Overoxidized polypyrrole doped with 4,5-dihydroxy-3-(p-sulfophenylazo)-2,7-naphthalene disulfonic acid as a selective and regenerable film for the stripping detection of copper(II).

A conducting polymer modified electrode based on the incorporation of 4,5-dihydroxy-3-(p-sulfophenylazo)-2,7-naphthalene disulfonic acid, SPADNS, as an anionic complexing ligand into polypyrrole film during electropolymerization was prepared. The electroanalysis of copper(II) in this modified electrode was achieved by medium exchange and differential pulse voltammetry. Copper ions were accumulated from ammonia buffer on the electrode surface by the formation of a chemical complex at open circuit. The resulting electrode with complexed Cu(2+) was then transferred to an acetate buffer and subjected to anodic stripping voltammetry. The analytical performance was evaluated and, finally, linear calibration graphs were obtained in the concentration range of 2 - 250 ng ml(-1) for Cu(II). The detection limit was found to be 1.1 ng ml(-1) and RSD was obtained at 3.1 and 1.9% for two different concentrations. Many coexisting metal ions had little or no effect on the determination of copper. The developed method was applied to Cu(II) determination in natural water and human hair samples. Also, the rapid and convenient regeneration of electrode allows the use of a single modified electrode in multiple analyses.     

Complexing capacity profiles of naturally occurring ligands in Tempranillo wines for Cu and Zn: an electroanalytical approach for cupric casse

Complexing capacity of naturally occurring ligands in Vitis vinifera (Tempranillo variety) wines has been studied with respect to two target metals (Cu and Zn) by differential pulse anodic stripping voltammetry (DPASV). Eight commercial wines of two certified brands of origin (CBO) and a young wine along its vinification process were monitored. Conditional stability constants and total complexing ligand(s) concentration(s) have been calculated for both metals. Discussion of the particular electrochemical responses for Cu and Zn for all samples is presented. A follow-up of the Cu stripping response allowed differentiating a commercial wine from one under processing related to the cupric casse phenomenon. Interaction of Cu with two molecular forms of cyanidin has been theoretically modeled at natural wine pH.     

Trace element speciation by anodic stripping voltammetry.

The effects of a range of complexing agents and a detergent on the stripping process in anodic stripping voltammetry (ASV) were compared by using both differential-pulse and d.c. techniques. The detergent and most of the ligands, added after deposition was complete, seriously affected the height of the differential-pulse waves, and fulvic acid even decreased the area of the d.c. stripping peak. In order to obtain a correlation between ASV-labile metal measurements and bioassays, it is important that complexing agents in the sample solution affect only the deposition stage, and not the stripping stage in ASV. In order to achieve this result, the double acidification technique is recommended, where a voltammogram is recorded with deposition at the natural pH of the sample, but with stripping carried out at pH less than 2, then a second voltammogram run on the same solution, but with both deposition and stripping effected at pH less than 2. Excellent agreement was obtained between ASV-labile metal measured by the double acidification method and algal assays for a variety of natural and synthetic waters.     

AGNES at vibrated gold microwire electrode for the direct quantification of free copper concentrations

The free metal ion concentration and the dynamic features of the metal species are recognized as key to predict metal bioavailability and toxicity to aquatic organisms. Quantification of the former is, however, still challenging. In this paper, it is shown for the first time that the concentration of free copper (Cu²⁺) can be quantified by applying AGNES (Absence of Gradients and Nernstian equilibrium stripping) at a solid gold electrode. It was found that: i) the amount of deposited Cu follows a Nernstian relationship with the applied deposition potential, and ii) the stripping signal is linearly related with the free metal ion concentration. The performance of AGNES at the vibrating gold microwire electrode (VGME) was assessed for two labile systems: Cu-malonic acid and Cu-iminodiacetic acid at ionic strength 0.01 M and a range of pH values from 4.0 to 6.0. The free Cu concentrations and conditional stability constants obtained by AGNES were in good agreement with stripping scanned voltammetry and thermodynamic theoretical predictions obtained by Visual MinteQ. This work highlights the suitability of gold electrodes for the quantification of free metal ion concentrations by AGNES. It also strongly suggests that other solid electrodes may be well appropriate for such task. This new application of AGNES is a first step towards a range of applications for a number of metals in speciation, toxicological and environmental studies for the direct determination of the key parameter that is the free metal ion concentration.     

Voltammetry of copper species in estuarine waters induced adsorption of copper on the hanging mercury drop electrode in complexing ligand/surfactant/chloride media

The adsorption of copper species on a positively charged hanging mercury drop electrode in complexing ligand/surfactant/chloride solution is discussed. Techniques used were differential pulse voltammetry of the copper in the adsorbed film, and potential-step reduction of adsorbed copper followed by different pulse anodic stripping voltammetry of Cu(Hg). The CuCl^?₂ species is shown to be the most important copper moiety adsorbed on the electrode and the adsorption is enhanced by organic films. This can be a critical pathway in the reduction of copper(II) in estuarine waters. The induced adsorption of copper in organic layers has biogeochemical implications associated with the nature of organic films and their influence on the Cu(II)/Cu(I) redox couple. There are also analytical applications, e.g., the compositional assay of organic monolayers by utilising Cu(II) and Cu(I) adsorption as electoractive probes and the determination of solution copper-organic binding.     

Trace metal speciation by adsorptive stripping voltammetry of metal chelates of solochrome violet RS

Adsorptive stripping voltammetry has become one of the most sensitive methods for trace metal determinations. The growing application of the method to natural water systems prompted an investigation into the fraction of the metal concentration that contributes to the adsorptive stripping response. Recent procedures for trace measurement of iron, titanium and gallium, based on chelation with solochrome violet RS (SVRS) are coupled to systematic ligand competition experiments. Tannic acid, EDTA, NTA, glycine, cysteine, carbonate and chloride ions are used as model natural ligands. It is shown that adsorptive stripping voltammetry measures the free ion and metal displaced from complexes by the “analytical” ligand. The exact fraction of the metal measured thus depends on the thermodynamic stability of the metal-SVRS chelate (compared to that of natural complexes), and on the relative concentrations of the competing ligands. The method offers possible distinction between metal complexes, based on their thermodynamic stabilities. The use ofa large excess of the “analytical” ligand can lead to measurement of the total metal content. Implications of these results relative to the use of this procedure for studying the speciation of trace elements in natural waters are discussed.     

Determination of selenium in sea water by adsorptive cathodic stripping voltammetry

A procedure is presented for determining Se(IV) and total dissolved Se in sea water using cathodic stripping voltammetry in the presence of added copper. Experiments using cyclic voltammetry indicate that the preconcentration step consists in adsorption of a Cu(I)₂Se complex species on the hanging mercury drop electrode. The optimized analytical conditions include a copper concentration of 40 μM and a solution pH of 1.6. Differential pulse modulation is used. Interference caused by organic surface-active substances present in natural waters in eliminated by UV photolysis of the sample. Cadmium interferes with the determination of Se only when present at a concentration 100 times higher than normal. UV photolysis at pH ≈ 8 is used to convert Se(VI), which is the electroactive species. The response is linear for Se concentrations between 0 and 200 nM. The limit of detection is 0.01 nM Se when a deposition time of 15 min is used.     

Trace amounts determination of lead,zinc and copper by adsorptive stripping voltammetry in the presence of dopamine

A selective and sensitive method for simultaneous determination of lead, zinc and copper by adsorptive differential pulse cathodic stripping voltammetry is presented. The method is based on adsorptive accumulation of the complexes of Pb(II), Zn(II), and Cu(II) ions with dopamine onto hanging mercury drop electrode (HMDE), followed by reduction of adsorbed species by differential pulse cathodic stripping voltammetry. The effect of experimental parameters such as pH, dopamine concentration, accumulation time and potential and scan rate were examined. Under the optimized conditions, linear calibration curves were established for the concentration of Pb, Zn, and Cu in the ranges of 5–150, 5–250, and 1–150 ng/mL, respectively. Detection limits of 0.06, 0.25, and 0.04 ng/mL for Pb, Zn, and Cu were obtained. An application of the proposed method is reported for the determination of these elements in some real samples such as natural waters and alloys.     

Speciation of trace metals in natural waters: the influence of an adsorbed layer of natural organic matter (NOM) on voltammetric behaviour of copper

The influence of an adsorbed layer of the natural organic matter (NOM) on voltammetric behaviour of copper on a mercury drop electrode in natural water samples was studied. The adsorption of NOM strongly affects the differential pulse anodic stripping voltammogram (DPASV) of copper, leading to its distortion. Phase sensitive ac voltammetry confirmed that desorption of adsorbed NOM occurs in general at accumulation potentials more negative than −1.4 V. Accordingly, an application of negative potential (−1.6 V) for a very short time at the end of the accumulation time (1% of total accumulation time) to remove the adsorbed NOM was introduced in the measuring procedure. Using this protocol, a well-resolved peak without interferences was obtained. It was shown that stripping chronopotentiogram of copper (SCP) in the depletive mode is influenced by the adsorbed layer in the same manner as DPASV. The influence of the adsorbed NOM on pseudopolarographic measurements of copper and on determination of copper complexing capacity (CuCC) was demonstrated. A shift of the peak potential and the change of the half-peak width on the accumulation potential (for pseudopolarography) and on copper concentration in solution (for CuCC) were observed. By applying a desorption step these effects vanished, yielding different final results.     

Electrochemical methods for speciation of trace elements in marine waters. Dynamic aspects

The contribution of electrochemical methods to the knowledge of dynamic speciation of toxic trace elements in marine waters is critically reviewed. Due to the importance of dynamic considerations in the interpretation of the electrochemical signal, the principles and recent developments of kinetic features in the interconversion of metal complex species will be presented. As dynamic electrochemical methods, only stripping techniques (anodic stripping voltammetry and stripping chronopotentiometry) will be used because they are the most important for the determination of trace elements. Competitive ligand exchange-adsorptive cathodic stripping voltammetry, which should be considered an equilibrium technique rather than a dynamic method, will be also discussed because the complexing parameters may be affected by some kinetic limitations if equilibrium before analysis is not attained and/or the flux of the adsorbed complex is influenced by the lability of the natural complexes in the water sample. For a correct data interpretation and system characterization the comparison of results obtained from different techniques seems essential in the articulation of a serious discussion of their meaning.     

Parameter evaluation for the determination of selenium by cathodic stripping voltammetry at the hanging mercury drop electrode

The behaviour of selenium(IV) in cathodic stripping voltammetry is evaluated systematically. The effects of copper concentration, pH, deposition potential and complexing agents on the stripping peak are examined and criteria are given for the choice of suitable quantitative parameters. The detection limit was found to be 20 ng l^?1 and the background contamination level was 35 ng l^?1. Zinc and lead do not affect the determination of selenium if EDTA is added to the solution whereas cadmium interferes badly; the corresponding mechanisms are discussed.