Anodic stripping methods

Figure 25-.14a illustrates the voltage excitation program that is followed in an anodic stripping method for determining cadmium and copper in an aqueous solution of these ions. A linear scan method is often used to complete the analysis. Initially, a constant cathodic potential of about -1 V is applied to the working electrode, which causes both cadmium and copper ions to be reduced and deposited as metals. The electrode is maintained at this potential for several minutes until a significant amount of the two metals has accumulated... 

Thermodynamically it is the total charge balance of the adsorbed ion and co-adsorption of counter ions. The value can be obtained by independent measurements of adsorbed charge and adsorbed mass. The charge is usually determined by chronoamperometry and integration of potential scans either in the negative direction (adsorption) or the positive one (desorption and anodic stripping method). The latter method is usually preferred because the adsorption film reaches a stable condition. The mass can be determined by several methods. The experimental techniques and examples will be described in Section 4.2. 

In this chapter only the sensitive polarographic methods for rare earths will be reviewed other electroanalytical methods, such as indirect analysis by displacement reactions and anodic stripping methods, will not be included. 

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. 

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. 

Square-wave anodic stripping voltammetry was employed by Komorsky-Lovric [107] for the determination of bismuth in seawater. A bare glassy-carbon rotating disk electrode was preconditioned at -0.8 V versus Ag/AgCl, prior to concentration of bismuth. The method was applied to seawater in the 12 ng/1 range. 

Halliday et al. [396] have described a simple rapid graphite furnace method for the determination of lead in amounts down to 1 xg/l in polluted seawater. The filtered seawater is diluted with an equal volume of deionised water, ammonium nitrate added as a matrix modifier, and aliquots of the solution injected into a tantalum-coated graphite tube in an HGA-2200 furnace atomiser. The method eliminates the interference normally attributable to the ions commonly present in seawater. The results obtained on samples from the Firth of Forth (Scotland, UK) were in good agreement with values determined by anodic stripping voltammetry. 

A method described by Florence and Farrer [584] separated tin from its associated lead by distillation from an aqueous sulfuric acid medium into which the vapour from boiling 50% hydrobromic acid is passed. The distillate provides an ideal supporting electrolyte for the determination of tin (II) (produced by reduction with hydrazinium hydroxide) by anodic stripping at a rotating vitreous-carbon electrode in the presence of codeposited mercury [585,586]. The tin is deposited at -0.70 V versus the SCE for 5 minutes, and then stripped at -0.50 V during a sweep from -0.70 V to -0.45 V at 5 V per minute. Tin in seawater is coprecipitated on ferric hydroxide, and the precipitate is then dissolved in the aqueous sulfuric acid, and subjected to the above procedure. The average content for Pacific coastal waters was found to be 0.58 xg/l. 

Van den Berg [620] also reported a direct determination of sub-nanomolar levels of zinc in seawater by cathodic stripping voltammetry. The ability of ammonium pyrrolidine dithiocarbamate to produce a significant reduction peak in the presence of low concentrations of zinc was used to develop a method capable of achieving levels two orders of magnitude below those achieved with anodic stripping voltammetry. Interference from nickel and cobalt ions could be overcome by using a collection potential of 1.3 V, and interference from... 

Analysis of total zinc by anodic stripping voltammetry is problematic because of interference by the hydrogen wave in acidified samples, and due to the inability to detect organically complexed zinc at natural pH values near 8 [ 185]. An improved understanding of zinc in marine systems now requires rapid, sensitive analytical methods that are less prone to contamination, and that can be performed at sea [624],... 

Brugmann et al. [680] compared three methods for the determination of copper, cadmium, lead, nickel, and zinc in North Sea and northeast Atlantic waters. Two methods consisted of atomic absorption spectroscopy but with preconcentration using either freon or methyl isobutyl ketone, and anodic stripping voltammetry was used for cadmium, copper, and lead only. Inexplicable discrepancies were found in almost all cases. The exceptions were the cadmium results by the two atomic absorption spectrometric methods, and the lead results from the freon with atomic absorption spectrometry and anodic scanning voltammetric methods. 

Earlier work on the application of anodic stripping voltammetry to the determination of methods in seawater is reviewed in Table 5.9. 

Although anodic stripping voltammetry is one of the few techniques suitable for the direct determination of heavy metals in natural waters [310,756-764], it is not readily adaptable to in situ measurements. Lieberman and Zirino [623] examined a continuous flow system for the anodic stripping voltammetry determination of zinc in seawater, using a tubular graphite electrode predeposited with mercury. A limitation of the approach was the need to pump seawater to the measurement cell, while the method required the removal of oxygen with nitrogen before measurements. 

Nygaard et al. [752] compared two methods for the determination of cadmium, lead, and copper in seawater. One method employs anodic stripping voltammetry at controlled pH (8.1,5.3 and 2.0) the other involves sample pretreatment with Chelex 100 resin before ASV analysis. Differences in the results are discussed in terms of the definition of available metal and differences in the analytical methods. 

Bond et al. [24] examined interferences in the stripping voltammetric method determination of trimethyl lead in seawater using polarography and mercury 199 and lead 207 NMF. It was shown that Hgn reacts with ((CH3) Pb)+ in seawater. Consequently, anodic stripping voltammetric methods for determining ((CH3)3Pb)2+ and inorganic Pb11 maybe unreliable. 

A method for the determination of organotin compounds by anodic stripping polarography has been published [121]. It has yet to be applied to seawater. Since the sensitivity permits the measurement of 0.01 ppm of the tin compounds, it is likely to be not quite sensitive enough for seawater and would require a preconcentration step. 

Method 3130 Metals by anodic stripping voltammetry (proposed by American Public Health Association, American Water Works Association, and the Water Environment Federation, 1995)... 

Most of our understanding of the marine chemistry of trace metals rests on research done since 1970. Prior to this, the accuracy of concentration measurements was limited by lack of instrumental sensitivity and contamination problems. The latter is a consequence of the ubiquitous presence of metal in the hulls of research vessels, paint, hydrowires, sampling bottles, and laboratories. To surmount these problems, ultra-clean sampling and analysis techniques have been developed. New methods such as anodic stripping voltammetry are providing a means by which concentration measurements can be made directly in seawater and pore waters. Most other methods require the laborious isolation of the trace metals from the sample prior to analysis to eliminate interferences caused by the highly concentrated major ions. 

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