Voltammetry stripping

For a simple anodic stripping experiment the potential is held at a negative potential so that the metal is reduced onto the working electrode 

After a suitable accumulation time the potential is swept in a positive direction so as to oxidise all of the different accumulated metals. The voltage required for the stripping depends on the standard potential for the M/M couple and the corresponding electrode kinetics. The magnitude of the stripping peak is then a function of both the concentration of metal ions contained within the solution and the length of the accumulation time. Low detection hmits can then be achieved by extending the period of accumulation. 

If a solution contains a mixture of two or more metal ion species, and assuming that they are both deposited onto the electrode during the accumulation stage, then as the electrode is swept positively the metals will be oxidised at their respective potentials. Hence it is possible to identify different metals from the potential at which they are stripped (oxidised) in the voltammetric scan. Problems may arise when two metals oxidise at similar potentials, so that their voltammetric peaks overlap. If a solid electrode is used, then alloy formation may also present problems. Various methodologies have been developed to overcome this problem a prime example is the use of mercury film electrodes as opposed to using a hanging mercury drop electrode. 

A major disadvantage is the limited anodic range available, due to the relatively facile oxidation of mercury. A further issue is the known toxicity of mercury and its compounds. In some countries there are stringent controls on its use. As a result of this, there is increasing interest in finding alternative electrode materials. One such example is the use of bismuth (see, for example, J. Wang, Electroanalysis 17 (2005) 1341). 

Stripping process, for example, by sweep voltammetry or differential pulse 

It is the preconcentration period that enhances the sensitivity of this technique. In the preconcentration phase, precise potential control permits the selection of species whose decomposition potentials are exceeded. The products should form an insoluble solid deposit or an alloy with the substrate. At Hg electrodes, the electroreduced metal ions form an amalgam. Usually, the potential is set 100-200 mV in excess of the decomposition potential of the analyte of interest Moreover, electrolysis may be carried out at a sufficiently negative potential to reduce aU of the metal ions possible below hydrogen ion reduction at Hg, for example. Concurrent H+ ion reduction is not a problem, because the objective is to separate the reactants from the bulk electrolyte. In fact methods have been devised to determine the group I metals and (NH4) ion at Hg in neutral or alkaline solutions of the tetraalkylammonium salts. Exhaustive electrolysis is not mandatory and 2%-3% ranoval suffices. Additionally, the processes of interest need not be 100% faradaicaUy efficient provided that the preconcentration stage is reproducible for calibration purposes, which is usually ensured by standard addition. 

It is important to stir the solution or rotate the electrode during the preconcentration stage. The purpose of this is to increase the analyte mass transport to the electrode by convective means, thereby enhancing preconcentration. In general, in electroanalysis, one seeks to obtain proper conditions for diffusion alone to permit mathematical expression of the process rate (the current). 

In this and controlled flow or rotation cases, it is advantageous to purposely increase the quantity of material reaching the electrode surface. Preconcentration times are typically 3 min or longer. 

In the rest period or quiescent stage, the stirrer is switched off for perhaps 30 s but the electrolysis potential is held. This permits the concentration gradient of material within the Hg to become more uniform. The rest period is not obligatory for films produced on a solid substrate. 

As mentioned above, voltammetric methods are suitable for the determination of metal ions in biological materials in concentrations down to the sub-ppb range. For determinations at rather low concentrations (10 -10 M) mostly stripping methods are used. Stripping methods consist of two basic steps  

The pre-electrolysis, in which the species to be determined is electrochemically enriched on a suitable stationary working electrode with constant mixing of the solution during a precisely defined time (tpE pre-electrolysis time) and a suitable voltage (t/pg pre-elec- 

V = sweep rate of the continuously changed DC voltage (A(//Ar) c = concentration of the species 

For anodic stripping voltammetry (ASV) mostly HMDE and MFE are used as working electrodes. The enrichment step can occur by three different kinds of reactions  

The cation is reduced to the metal which forms an amalgam soluble in the mercury of the working electrode. 

Pertcchnetatc and Tc(lV) could be more sensitively analyzed in acidic media in the presence of thiocyanate by adsorption stripping voltammetry at the hanging mercury drop electrode using the differential pulse mode. Determinations down to 5-fO g Te per ml were feasible. An intense eurrent signal at -1.32 V vs SCE was observed if only technetium and thiocyanate were present in the solution. Larger quantities of salts, e.g. chlorides and sulphates, decreased the sensitivity of the method considerably. This, however, could easily be avoided if, after electrodeposition was completed, the primary electrolyte was replaced by a pure solution of dilute acid for the stripping voltammetric step [100]. 

The stripping voltammetric determination of pertcchnetatc by means of a glassy carbon electrode, chemically modified with a tctraphenylarsonium chloride loaded copolymer film, was reported. A detection limit of 10 M TCO4 was achieved after a 5 min enrichment time [101]. 

The first stage in stripping electroanalytical methods is the accumulation of the electroactive substance either on the surface or in the bulk of a liquid electrode (mercury electrodes). The second stage consists of electrode polarization, obtaining cathodic or anodic voltam-mograms (stripping voltammetry (SV)) that give information on the nature and concentration of electroactive analytes [7]. 

In this context, anodic stripping voltammetry (ASV) is a very sensitive electrochemical technique mainly used for the analysis of trace concentrations of metallic species in solution, although detection of some organic species has also been carried out. Detection limits 

Cathodic stripping voltammetry (CSV) is the inverse of ASV. Mercury electrodes can be used to detect and quantify species that form sparingly soluble mercury salts such as halide ions, sulfide, cyanide, thiols, and penicillins. CSV is not restricted to mercury electrodes. Any electrode material that, on oxidizing, forms sparingly soluble salts will work, such as silver electrodes for the determination of halide ions. Another possibility is that the ion of interest oxidizes to form a precipitate on the electrode surface. 

Potentiometric stripping analysis (PSA) on the other hand, is very similar to ASV in that the accumulation stage is the same. The difference is that in the stripping step, an oxidant in solution, or alternatively an anodic current, causes the oxidation of the reduced analyte. In mixtures, the stripping potential identifies the analyte. 

Adsorptive stripping voltammetry (AdSV) is based on the accumulated adsorption of a complex between the analyte and a complexing agent at the electtode surface, often at an applied potential chosen so as to help the adsorption process. Either the metal ion or the ligand can be measured in the presence of an excess of the other component in bulk solution. Usually, the determination step involves reduction of the metal ion from the adsorbed complex. Adsorption follows an isotherm, usually of Langmuir type [3]. 

Rudjer Boskovic Institute, 10001 Zagreb, Croatia e-mail mlovric irb.hr 

The preconcentration, i.e., accumulation on the electrode surface, can be achieved by the following methods  

The accumulation is a dynamic process that may turn into a steady state in stirred solutions. Besides, the activity of accumulated substance is not in a time-independent equilibrium with the activity of analyte in the bulk of the solution. All accumulation methods employ fast reactions, either reversible or irreversible. The fast and reversible processes include adsorption and surface complexation, the majority of ion transfers across liquid/liquid interfaces and some electrode reactions of metal ions on mercury. In the case of a reversible reaction, equilibrium between the activity of accumulated substance and the concentration of analyte at the electrode surface is established. It causes the development of a concentration 

The reduction of metal ions on solid electrodes is a process consisting of three main steps a deposition of adatoms, a two- and three-dimensional nucleation and a three-dimensional crystal growth [10]. The formation of the first monolayer of metal atoms on the foreign substrate follows a quasi-Nernst equation  

The difference Em - E is related to the binding energy of the first monolayer. In cyclic voltammetry this is the difference between the potentials of the most positive peak and the peak corresponding to the three-dimensional metal phase. The latter difference is linearly correlated with the difference of work functions of the deposited metal and the electrode material [11]  

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Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Albery W J and Brett C M A 1983 The wall-]et ring disc electrode. 2. Collection efficiency, titration curves and anodic stripping voltammetry J. Electroanal. Chem. 148 201... 

Stripping Voltammetry One of the most important quantitative voltammetric techniques is stripping voltammetry, which is composed of three related techniques anodic, cathodic, and adsorptive stripping voltammetry. Since anodic strip-... 

Anodic stripping voltammetry consists of two steps (Figure 11.37). The first is a controlled potential electrolysis in which the working electrode, usually a hanging mercury drop or mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the electrode. For example, with Cu + the deposition reaction is... 

Potential-excitation signal and voltammogram for anodic stripping voltammetry at a hanging mercury drop electrode. 

The experimental design for cathodic stripping voltammetry is similar to that for anodic stripping voltammetry with two exceptions. First, the deposition step in-... 

Representative Examples of Analytes Determined by Stripping Voltammetry... 

Table 11.11 lists several analytes that have been analyzed successfully by cathodic stripping voltammetry. 

In adsorptive stripping voltammetry the deposition step occurs without electrolysis. Instead, the analyte adsorbs to the electrode s surface. During deposition the electrode is maintained at a potential that enhances adsorption. For example, adsorption of a neutral molecule on a Hg drop is enhanced if the electrode is held at -0.4 V versus the SCE, a potential at which the surface charge of mercury is approximately zero. When deposition is complete the potential is scanned in an anodic or cathodic direction depending on whether we wish to oxidize or reduce the analyte. Examples of compounds that have been analyzed by absorptive stripping voltammetry also are listed in Table 11.11. 

When either pulse polarography or anodic stripping voltammetry can be used, the selection is often based on the analyte s expected concentration and the desired... 

The concentration of copper in a sample of sea water is determined by anodic stripping voltammetry using the method of standard additions. When a 50.0-mL sample is analyzed, the peak current is 0.886 )J,A. A 5.00-)J,L spike of 10.0-ppm Cu + is added, giving a peak current of 2.52 )J,A. Calculate the parts per million of copper in the sample of sea water. 

Peak currents in anodic stripping voltammetry are a linear function of concentration... 

Differential pulse polarography and stripping voltammetry have been applied to the analysis of trace metals in airborne particulates, incinerator fly ash, rocks. 

Miscellaneous Samples Besides environmental and clinical samples, differential pulse polarography and stripping voltammetry have been used for the analysis of trace metals in other samples, including food, steels and other alloys, gasoline, gunpowder residues, and pharmaceuticals. Voltammetry is also an important tool for... 

Precision Precision is generally limited by the uncertainty in measuring the limiting or peak current. Under most experimental conditions, precisions of+1-3% can be reasonably expected. One exception is the analysis of ultratrace analytes in complex matrices by stripping voltammetry, for which precisions as poor as +25% are possible. 

Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions. For example, in stripping voltammetry, sensitivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that obtained with a linear potential scan. 

Time, Cost, and Equipment Commercial instrumentation for voltammetry ranges from less than 1000 for simple instruments to as much as 20,000 for more sophisticated instruments. In general, less expensive instrumentation is limited to linear potential scans, and the more expensive instruments allow for more complex potential-excitation signals using potential pulses. Except for stripping voltammetry, which uses long deposition times, voltammetric analyses are relatively rapid. 

In stripping voltammetry the analyte is first deposited on the electrode, usually as the result of an oxidation or reduction reaction. The potential is then scanned, either linearly or by using potential pulses, in a direction that removes the analyte by a reduction or oxidation reaction. 

This experiment describes the application of cathodic stripping voltammetry to the analysis of Ni and Co in fresh water and sea water. 

Wang, J. Anodic Stripping Voltammetry, /. Chem. Educ. 1983, 60, 1074-1075. 

The application of anodic stripping voltammetry for the quantitative analysis of Cd, Pb, and Cu in natural waters is described in this experiment. 

Anodic stripping voltammetry at a mercury film electrode can be used to determine whether an individual has recently fired a gun by looking for traces of antimony in residue collected from the individual s hands, fn a typical analysis a sample is collected with a cotton-tipped swab that had been wetted with 5% v/v HNO3. When returned to the lab, the swab is placed in a vial containing 5.00 mb of 4 M HCl that is 0.02 M in hydrazine sulfate. After allowing the swab to soak overnight,... 

X f0 ppb Sb was added, anodic stripping voltammetry is repeated, giving a peak current of f.i4 pA. How many nanograms of Sb is collected from the individual s hand ... 

The speciation scheme of Batley and Florence requires eight measurements on four samples. After removing insoluble particulates by filtration, the solution is analyzed for the concentration of anodic stripping voltammetry (ASV) labile metal and the total concentration of metal. A portion of the filtered solution is passed through an ion-exchange column, and the concentrations of ASV metal and total metal are determined. A second portion of the filtered solution is irradiated with UV light, and the concentrations of ASV metal... 

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. 

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