Stripping voltammetry metal complexes

The methods of investigation of metal species in natural waters must possess by well dividing ability and high sensitivity and selectivity to determination of several metal forms. The catalytic including chemiluminescent (CL) techniques and anodic stripping voltammetry (ASV) are the most useful to determination of trace metals and their forms. The methods considered ai e characterized by a low detection limits. Moreover, they allow detection of the most toxic form of metals, that is, metal free ions and labile complexes. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

Analytical techniques such as adsorptive stripping voltammetry rely on complex formation to improve detection limits of metals such as V(IV) and V(V) [51]. An example is in the use of cupferron and KBr03 as additives to acidic aqueous solutions, which result in the adsorption of the V(V)-cupferron complex to the electrode surface that catalyzes the reduction of the Br03 ion in solution. This technique has good selectivity over Cu, Pb, Cd, Fe, and Ti, but the response is dependent on pH. 

Analytical Applications In addition to the above-mentioned analytical aspects of the processes at Hg electrodes, in this section, we briefly review the papers focused on the subject of the affinity of various compounds to the mercury electrode surface, which allowed one to elaborate stripping techniques for the analysis of inorganic ions. Complexes of some metal ions with surface-active ligands were adsorptively accumulated at the mercury surface. After accumulation, the ions were determined, usually applying cathodic stripping voltammetry (CSV). Representative examples of such an analytical approach are summarized as follows. 

Separations may also be made on the basis of complex stability In such cases, a complexant is added to the sample and metal complexes are fractionated by a particular technique and their concentration is monitored (Apte and Batley, 1995 Miller and Bruland, 1997), typically by fluorescence (Aster et al., 1997), ultraviolet-visible (UV-Vis) spectrophotometry (Senesi, 1992 Bjorklund and Morrison, 1997), atomic absorption following solvent extraction (Itabashi et al., 1997), cathodic stripping voltammetry (CSV) (van den Berg et al., 1990 Yokoi et al., 1995 Maxwell and Smyth, 1996 Achterberg et al., 1997) or anodic stripping voltammetry (ASV)... 

Morrison, G.M., Florence, T.M. and Stauber,J.L. (1990) The effects of complexing agents and surfactants on the deposition and stripping processes in differential pulse anodic stripping voltammetry of metals at the hanging mercury drop electrode. Electroanalysis (NY), 2, 9-14. 

Twenty years ago the main applications of electrochemistry were trace-metal analysis (polarography and anodic stripping voltammetry) and selective-ion assay (pH, pNa, pK via potentiometry). A secondary focus was the use of voltammetry to characterize transition-metal coordination complexes (metal-ligand stoichiometry, stability constants, and oxidation-reduction thermodynamics). With the commercial development of (1) low-cost, reliable poten-tiostats (2) pure, inert glassy-carbon electrodes and (3) ultrapure, dry aptotic solvents, molecular characterization via electrochemical methodologies has become accessible to nonspecialists (analogous to carbon-13 NMR and GC/MS). 

Anodic and cathodic stripping voltammetry electrochemical techniques, combined with metal titration analyses, for obtaining ligand concentrations and conditional stability constants, can be used to determine how many ligands are involved in metal complexation as well as their relative differences in strength. 

The four variations of this technique are to be found in Table 14.2. The schemes of operation are shown in Fig. 14.6. Important applications for trace metals are the use of anodic stripping voltammetry (ASV) to determine trace quantities of copper, cadmium, lead and zinc, and adsorptive stripping voltammetry (AdSV) of trace quantities of nickel and cobalt—pre-concentration by adsorption accumulation of the oxime complexes followed by reduction to the metal is employed, as reoxidation of these metals in ASV is kinetically slow and does not lead to well-defined stripping peaks. 

Anodic stripping voltammetry (ASV) can be very usefiil in studies of metal complexation in natural aquatic systems. The technique can be used, under favorable conditions, to determine metal concentrations as low as 10 molL. Further, and again under appropriate conditions, stabihty constants may be determined in a manner analogous to that used in polarography. The principle of the technique involves deposition of a metal ion in reduced form on a static mercury electrode followed by reoxidation through reversal of the polarity. The reoxidation current is related to the metal concentration in the analyte. In the presence of a... 

Shuman M. S. and Woodward G. P. (1973) Chemical constants of metal complexes from a complexometric titration followed with anodic stripping voltammetry. Anal. Chem. 45, 2032-2035. 

It is used in combination with square wave anodic stripping voltammetry (SWASV) using a PalmSens portable instrument (Palm Instrument BV, Houten, The Netherlands) for the measurement of metals such as Cu (II), Cd (II) and Pb (II) (labile metallic complexes and free metals) in water. These disposable sensors require no calibration for use in the screening mode, so, many samples may be tested for the presence or the absence of metals in water. The quantification can also be performed using the standard addition method in less than 15 min. 

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