Polarography to anodic stripping voltammetry

Direct-current polarography dates back some 60 years to the discovery by Heyrovsky that highly reproducible i-E curves could be obtained using a dropping mercury electrode (DME) (under natural convection) as the working electrode. Thus, in a polarographic analysis, an i-E curve is recorded using a slow linear potential scan and a DME with a droptime of 2-10 s the i-E curve will, ideally, have the S-shape shown in Fig. 12.1(a). The half-wave potential E can be used for qualitative analysis since it is a measure of how readily the electroactive species is reduced or oxidized and therefore is a reflection of its 

A mercury valve creates a slight vacuum, pulling the previous sample from the cell. 

A fresh sample is transferred to the cell by a stream of nitrogen gas exhorting venturi action over the sample tubing orifice. 

The sample is nebulized into small droplets to allow efficient and rapid removal of dissolved oxygen. 

The sample droplets collect at the bottom of the nebulization chamber and the deaerated solution is transferred to the cell (sensing electrodes in the cell ensure that an appropriate volume is transferred). 

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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). 

When the complexation reaction is studied by titration with a metal ion, the concentration of the free metal ion is monitored typically by an electrochemical method, such as potentiometry with an ion-selective electrode, differential pulse polarography, or anodic stripping voltammetry. Fluorescence spectroscopy is being increasingly used to study the reaction of humic material with paramagnetic metal... 

From the nature of the process described above it has been referred to as stripping polarography , but the term anodic stripping voltammetry is preferred. It is also possible to reverse the polarity of the two electrodes of the cell, thus leading to the technique of cathodic stripping voltammetry. 

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. 

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... 

Anodic stripping voltammetry has been used to determine total arsenic spedes . Pulse polarographic methods have been applied to aqueous and non-aqueous solutions of methyl- and dimethylarsenic adds at concentration levels down to 0.1 /rg/ml . These arsenicals are electroactive in aqueous buffers and in non-aqueous media in which the acidic supporting electrolyte, guanidinium porchlorate, is employed. A direct method of analysis, based on differential pulse polarography, is reported. Detection limits of roughly 0.1/xg/ml (for MMAA) and 0.3p[Pg.190]

Part IV is devoted to electrochemical methods. After an introduction to electrochemistry in Chapter 18, Chapter 19 describes the many uses of electrode potentials. Oxidation/reduction titrations are the subject of Chapter 20, while Chapter 21 presents the use of potentiometric methods to obtain concentrations of molecular and ionic species. Chapter 22 considers the bulk electrolytic methods of electrogravimetry and coulometry, while Chapter 23 discusses voltammetric methods including linear sweep and cyclic voltammetry, anodic stripping voltammetry, and polarography. 

Terminology related to electroanalytical chemistry are chronoamperometry, voltammetry, coulometry, amperometric titrimetry, coulometric titrimetry, conductivity, con-ductimetry and high frequency titrimetry, electrometric titrimetry, electrogravimetry, electrodeposition, anodic stripping voltammetry (ASV), cathodic stripping voltammetry (CSV), polarography, differentia] pulse polarography (DPP), ion-selective electrode (ISE), ion-specific electrode (ISE), molecular selective electrode, potentiometry, potentio-metric titrimetry, and chronopotentiometric titrimetry. 

Methods such as anodic stripping voltammetry, polarography, and spectroscopy not only can differentiate between "free" and "complexed" metal ion species but in some instances can be used to identify the nature and determine the concentrations of individual complex species. 

Anodic stripping voltammetry, potentiometric stripping voltametry, and differential pulse polarography are used for the simultaneous determination of up to 10 analytes at extremely low concentrations (detection limits <0.01 pgl ). Electroanalytical techniques are applicable for 30 elements. Stripping analysis allows differentiation between chemical forms but is subject to interference from adsorption... 

What are the advantages of mercury electrodes for electrochemical measurements What are the advantages of the DME versus a Pt microelectrode for polarography What are the disadvantages of the DME Describe the method of anodic stripping voltammetry. What analytes can it be used to determine Why is stripping voltammetry more sensitive than other voltammetric methods ... 

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