Stripping voltammetric

Figure 6.5 CO ds voltammetric stripping peak (symbols) and the fits obtained from (6.11) (lines) for the Pt(lll) electrode in 0.5 M H2SO4 at different temperatures. The scan rate was 50mV/s.

The technique of voltammetric stripping analysis is one of the most sensitive techniques available for the determination of metal ions in complex sample matrices. Traditionally such techniques have been undertaken at Hg electrodes, but in recent years, a large number of reports have focused on the use of SPCEs in this area. Reports prior to 2003 have been reviewed recently by the present authors [3]. 

Hanging mercury drop electrode (HMDE) A microelectrode that can concentrate traces of metals by electrolysis into a small volume the analysis is completed by voltammetric stripping of the metal from the mercury drop. 

Voltammetric Stripping of Electroactive Microparticles from a Solid Electrode... 

Eriksen (1979) compared different wet digestion procedures for voltammetric stripping analysis of urine and preferred the use of a mixture of nitric, sulfuric, and perchloric acids. Satisfactory recovery (92-100%) of lead and cadmium from spiked samples was obtained. An effective procedure based on a rapid (20 min.) digestion of freeze-dried urine samples coupled with voltammetric analysis was reported (Golimowski et al., 1979). 

Stripping voltammetry is an important, but limited technique, which uses a pre-concentration step to enhance the sensitivity of the voltammetric analysis step. This pre-concentration step is electrochemical using the same instrumentation as the subsequent voltammetric analysis. The simplest polarographic equipment can be used to obtain the highest sensitivity which depends on the length of the pre-concentration step as well as on the sensitivity of the voltammetric stripping step. Since the pre-concentration step requires the production of an insoluble product which can be reproducibly stripped from the electrode surface in the determination step, the use of stripping voltammetry is limited to a few analytes, eg transition metal ions, halides and pseudo-halides. 

In trace analysis the voltammetric stripping methods are popular because of their sensitivity - ranging down to sub ppb concentration. These methods are accurate and precise and the instrumentation is of low cost. The stripping methods are based on previous accumulation of the ion or compound to be determined on the working electrode. In addition to the electrolytical accumulation - used in anodic or cathodic stripping voltammetry (ASV or CSV resp.) also adsorptive accumulation of the species on the working electrode can be exploited, as many organic compounds exhibit surface active properties that are manifested by their adsorption from solution onto the surface of the solid phase. 

Figure 7.25 Principles of anodic voltammetric stripping to study the influence of additives on electrodeposition.

In the conductivity, potentiometric, and voltam-metric measurements the response is correlated to concentration or activity of the analyte usually by using calibration curves. In coulometry, however, the charge measured gives directly the amount of substance and therefore no calibration is needed. However, in coulometry the sample is consumed in the measurements and the problem is that the method requires 100% current efficiency to be reliable. Conductimetry and potentiometry are sample nonconsuming methods. In voltammetry, only an insignificant amount of the sample is consumed and therefore the measurement can be repeated. Only in voltammetric stripping methods of very low concentrations of the analyte the amount consumed at the electrode reaction has to be considered if repeated measurements are to be done. 

Whereas the measured CO voltammetric stripping charge indicated the population of metal surface sites under DMFC operation conditions, the stripping peak potential provided some information on PtRu surface alloy composition. From the correlation found between activity and stripping peak potential, it was su ested that metal surfaces closest in composition to Pto.sRuo.s apparently enabled most effective stripping of CO and were also the ones most active in anodic methanol oxidation. The latter strong correlation shows that steps (54) + (55) are rate determining for methanol concentrations and anode potentials most relevant in DMFCs [103]. 

Fig. 65. Comparison of three voltammetric stripping responses under equal accumulation conditions. (A) linear sweep voltammetry, (B) differential pulse voltmametry, (C) square-wave technique. Conditions 5 x 10 M vitamin K3 in 0.3 M HCIO3, static MDE, accumulation potential, Eacc=-0.1V (vs. Ag/AgCl), tacc = 60s, rest time tr=10s. Linear sweep rate v = 0.02Vs , differential pulse scanning 0.01 Vs , SW-scanning 0.20Vs pulse amplitudes dEop = JEsw = 0.02 V, SW frequency 100 Hz. Adapted according to [162].

Responses obtained at mediator modified electrodes can be complicated if some of the analyte diffuses through to the bare electrode surface. Conventional voltammetric stripping procedures introduce another step into conventional voltammetry, namely, preconcentration of the analyte on or into the electrode surface. Similarly, with preconcentrating modified electrodes (Figure 5.7, iii) the analytical performance depends on ... 

M), which is suitable for monitoring ultralow levels of trace metals in natural water. Other advantages include multielements measurement, trace element speciation and distribution, and continuous in situ detection. The sensors for two voltammetric stripping techniques are discussed below anodic stripping voltammetry (ASC) and adsorptive cathodic stripping voltammetry (AdCSV). 

Although the stopped-flow technique is of little use with current CCD and diode array U V—Vis spectrophotometers, which allow the spectrum at each point along a profile to be obtained without halting the flow, it continues to be useful for other scanning techniques, e.g. fluorimetry, polarogra-phy, and voltammetric stripping. 

Figure 2.35. Controlling waveform of a voltammetric stripping analysis. Top polarization voltage reference as function of time. Bottom recorded signal...

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