Deposition time, stripping analysis

Anodic stripping voltammetry (ASV) is the oldest, and still the most widely used version of stripping analysis [3]. The technique is applicable to metal ions that can be readily deposited at the working electrode, and particularly for those metals that dissolve in mercury. In this case, the metals are being preconcentrated by electrodeposition into a small-volume mercury electrode (a thin mercury film or a hanging mercury drop). The preconcentration is done by cathodic deposition at a controlled potential and time. The deposition potential is usually ca. 0.3 Y more negative than E° for the least easily reduced metal ion to be determined. The metal ions reach the mercury electrode by diffusion and convection, where they are reduced and concentrated as amalgams ... 

Anodic stripping voltammetry (ASV) was applied to the determination of copper traces present as Cu(dik)2. The differential pulse technique was used to strip the amalgamated copper from a hanging mercury drop electrode. The experimental variables such as scan rate of electrode potential, deposition potential, deposition time and stirring speed of the solution could be optimized. The linear range of the calibration plot was 0.05-1 (xM and the LOD was 0.014 fiM Cu(II). A method was used for the determination of copper in breast milk and beer as typical examples of application, consisting of minerahzation of the sample, extraction of Cu(II) from the aqueous solution with a 1 M solution of acacH in chloroform and ASV end analysis . 

As electrochemical stripping analysis has developed, new techniques have been introduced with a view to overcoming the problems of lengthy deposition times and improving sensitivity. 

The potential/time profile for anodic stripping voltammetry and a typical experimental curve for the determination of a mixture of heavy metal ions is shown in Fig. 11.14. The method is clearly limited to the determination of metals which form simple amalgams (inter-metallic compounds must also be avoided). This limitation, however, introduces some desirable selectivity and most organic compounds will not interfere with the determination of the metals. Using acceptable deposition times, analysis of very low concentrations is possible. Certainly for heavy metal ions, the sensitivity of anodic stripping analyses compares well with that of atomic absorption spectroscopy even with non-flame atomization (see Table 11.4). Moreover, these data do not represent the ultimate detection limit since the plating time can be extended. 

Anodic stripping analysis The accumulation is usually carried out by the electrolysis at controlled potential for a definite time, t cc under reproducible mass-transport conditions. The deposition potential, E, imposed to the working electrode should be adjusted at the potential adequate to the plateau of the reduction voltammetric wave characteristic for the test metal (see Figs. 62A and 62B for a mixture of two tested species). Before adjusting the experiment, it is recommended to know the solubility data and diffusion coefficients of metals in mercury and their half-wave potentials (see Table 4). 

Fig. 62. Scheme of potential-time and current-time dependences in anodic stripping analysis, (a) Ed potential during the deposition period E 1/2 and E"/2 half-wave potentials of two test substances, Ef the final potential t p rest period, tj stripping period. (B) Current-time dependence during the LSV stripping step, IJ, and I" peak heights of the test substances. 

The combination of ASV with the flow injection analysis (FIA) improves the reproducibility and versatility of the anodic stripping procedure. For a conventional operation the detection limit is of 10" M. The disadvantage of the FIA, namely the short deposition time, can be compensated by repeated reversals of the flow direction [174]. The multiple passage of the same sample along the mercury-plated carbon fiber electrode increases the effective deposition time and thus the stripping current response. 

Potentiometric stripping analysis is carried out in several stages. After electrochemical generation of the mercury film on a graphite substrate, the elements to be determined are accumulated by electrolysis at constant potential. The next stage is the oxidation of the deposited elements by the oxidant present in the. solution. For this, the current circuit is disconnected. The deposited analytes are stripped in the order of their electrochemical jxitentials. Anodically deposited precipitates can similarly be stripped by chemical reduction. In all cases, potential - time curves with transition times proportional to concentration result [39]-[41]. 

By changing the deposition time, the oxidant concentration, or the electrode size, the method can be adjusted to the different analytical tasks. Most often, the trace levels of Cu, Pb, Bi, Sn, and Cd ion concentrations are measured with potentiometric stripping analysis. 

As a result of that reductive process, a deposit of copper metal (denoted in Eq. 2.2 by s for solid ) is formed on the carbon electrode surface. The prominent anodic peak recorded in the reverse scan corresponds to the oxidative dissolution of the deposit of copper metal previously formed. The reason for the very intense anodic peak current is that the copper deposit is dissolved in a very small time range (i.e., potential range) because, in the dissolution of the thin copper layer, practically no diffusion limitations are involved, whereas in the deposition process (i.e., the cathodic peak), the copper ions have to diffuse through the expanding diffusion layer from the solution to the electrode surface. These processes, labeled as stripping processes, are typical of electrochemically deposited metals such as cadmium, copper, lead, mercury, zinc, etc., and are used for trace analysis in solution [84]. Remarkably, the peak profile is rather symmetrical because no solution-like diffusive behavior is observed. 

Stripping is the most sensitive voltammetric technique (Table 17-2), because analyte is concentrated from a dilute solution. The longer the period of concentration, the more sensitive is the analysis. Only a fraction of analyte from the solution is deposited, so deposition must be done for a reproducible time (such as 5 min) with reproducible stirring. 

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