Voltammetry, typical analysis

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,... 

The concentration of the transferred ion in organic solution inside the pore can become much higher than its concentration in the bulk aqueous phase [15]. (This is likely to happen if r <5c d.) In this case, the transferred ion may react with an oppositely charged ion from the supporting electrolyte to form a precipitate that can plug the microhole. This may be one of the reasons why steady-state measurements at the microhole-supported ITIES are typically not very accurate and reproducible [16]. Another problem with microhole voltammetry is that the exact location of the interface within the hole is unknown. The uncertainty of and 4, values affects the reliability of the evaluation of the formal transfer potential from Eq. (5). The latter value is essential for the quantitative analysis of IT kinetics [17]. Because of the above problems no quantitative kinetic measurements employing microhole ITIES have been reported to date and the theory for kinetically controlled CT reactions has yet to be developed. 

Cyclic voltammetry was conducted using a Powerlab ADI Potentiostat interfaced to a computer. A typical three electrode system was used for the analysis Ag/AgCl electrode (2.0 mm) as reference electrode Pt disc (2.0 mm) as working electrode and Pt rod (2.0 mm) as auxiliary electrode. The supporting electrolyte used was a TBAHP/acetonitrile electrolyte-solvent system. The instrument was preset using a Metrohm 693 VA Processor. Potential sweep rate was 200 mV/s using a scan range of-1,800 to 1,800 mV. 

These studies have been mainly carried out using cyclic voltammetry and frequency response analysis as experimental tools. As a typical example. Fig. 9.12 illustrates the voltammogram related to the p-doping process of a polypyrrole film electrode in the LiClQ -propylene carbonate electrolyte, i.e. the reaction already indicated by (9.16). 

It is possible to use cyclic voltammetry in the presence of ferrocene carboxylic acid to confirm the presence of micro-electrodes due to the typical sigmoidal-shaped profile produced [2] (Fig. 24.3). Twenty different sensors comprising micro-electrode arrays formed by this technique were analysed for reproducibility. This analysis can be performed by holding the sensors at a potential of +100 mV for 60s and recording the... 

As in the case of differential double potential pulse techniques like DDPV, slow electrochemical reactions lead to a decrease in the peak height and a broadening of the response of differential multipulse and square wave voltammetries as compared with the response obtained for a Nemstian process. Moreover, the peak potential depends on the rate constant and is typically shifted toward more negative potentials (when a reduction is considered) as the rate constant or the pulse length decreases. SWV is the most interesting technique for the analysis of non-reversible electrochemical reactions since it presents unique features which allow us to characterize the process (see below). Hereinafter, unless expressly stated, a Butler-Volmer potential dependence is assumed for the rate constants (see Sect. 1.7.1). 

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 . 

Most of the work reported before the Second World War was carried out in aqueous electrolyte solutions. Since 1945 the focus has shifted to include the application of nonaqueous solvents. This has allowed for the detection of the primary intermediates, typically radical anions and radical cations, and for the study of their reactions. The theoretical foundations, for the analysis of kinetics and mechanisms by, for instance, cyclic voltammetry and related techniques were mostly published in the 1960s and 1970s. The application of such techniques has resulted in a steadily increasing understanding of the kinetics and mechanisms of organic electrochemical processes. The current trend is to return to water-like conditions reflecting the need to substitute organic solvents with environmentally friendlier electrolyte systems. 

The use of these labels for DNA detection typically requires covalent attachment because the probe molecules lack intrinsic DNA-binding capabilities. However, access to probes whose structural and electrochemical properties can be chosen for optimal efficiency amply justifies the added sample preparation time. In many of these cases, the electrochemical processes used for readout do not involve the DNA/electrode interface per se. Rather, the molecular-recognition properties of DNA are exploited to recruit the DNA-bound redox probes to the surface for analysis by more traditional electrochemical techniques, such as enzymatic catalysis or stripping voltammetry. 

A microelectrode is an electrode with at least one dimension small enough that its properties are a function of size, typically with at least one dimension smaller than 50 pm [28, 29, 30, 31, M and 33]. If compared with electrodes employed in industrial-scale electrosynthesis or in laboratory-scale synthesis, where the characteristic dimensions can be of the order of metres and centimetres, respectively, or electrodes for voltammetry with millimetre dimension, it is clear that the size of the electrodes can vary dramatically. This enormous difference in size gives microelectrodes their unique properties of increased rate of mass transport, faster response and decreased reliance on the presence of a conducting medium. Over the past 15 years, microelectrodes have made a tremendous impact in electrochemistry. They have, for example, been used to improve the sensitivity of ASV in environmental analysis, to investigate rapid... 

Fast linear sweep voltammetry can give a complete current potential curve in times of a second or much shorter and is thus suitable for following the kinetics of chemical reactions or where ever very rapid analysis is necessary. For more conventional polarographic methods scan times of 10 to 20 minutes are more typical. However the fast scan rates of fast linear sweep voltammetry result in large charging or capacitive currents necessary to charge up the electrode surface to the potential required. This results in a loss of sensitivity etc,in consequence fast linear sweep voltammetry is not a popular analytical technique. 

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. 

Impedance analysis is used to study the response of electrochemical systems to sinusoidal perturbations about a steady state or equilibrium condition. In contrast to cyclic voltammetry which is a large amplitude technique, impedance measurements are carried out with small amplitude (voltage) perturbations. The voltage is typically 3-5 mV peak-to-peak about a d.c. voltage level so that the (current) response is linear. The frequency of perturbation is varied in order to separate the individual electrochemical relaxation processes which occur with different time constants. 

---