Reproducibility voltammetry

Electrodes. The Hanging Mercury Drop Electrode is traditionally associated with the technique of stripping voltammetry and its capabilities were investigated by Kemula and Kublik.51 In view of the importance of drop size it is essential to be able to set up exactly reproducible drops, and this can be done as explained in Section 16.8 for the S.M.D.E. 

FIGURE 2-4 Cyclic voltammetry of C60 and C70 in an acetonitrile-toluene solution. (Reproduced with permission from reference 2.)... 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

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. 

Fig. 4. Voltammograms in 0.1M Eti+NClOi+ZCH CNj rT = 1.2X1 O 8 mol/cm. Curve As Cyclic voltammetry of Pt/poly-Co -NI TPP at 20 mv/s = 200pA/cm. Curve B Four-electrode voltammetry of Pt/poly-Co( -NHi(,)TPP/Au sandwich electrode with E u = 0.0 Vj Ep scanned negatively at 5 mV/s = 400pA/cm. Curve Ci Surface profilometry of a poly-Co(o-NH2)TPP film on Sn02/glassj Tt = 7.6X10 9 mol/cm. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

Alemu et al. [35] developed a very sensitive and selective procedure for the determination of niclosamide based on square-wave voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate the electrochemical reduction of niclosamide at a glassy carbon electrode. Niclosamide was first irreversibly reduced from N02 to NHOH at —0.659 V in aqueous buffer solution of pH 8.5. Following optimization of the voltammetric parameters, pH and reproducibility, a linear calibration curve over the range 5 x 10 x to 1 x 10-6 mol/dm3 was achieved, with a detection limit of 2.05 x 10-8 mol/dm3 niclosamide. The results of the analysis suggested that the proposed method has promise for the routine determination of niclosamide in the products examined [35]. 

Fig. 6. Voltammetry and LEED patterns for a Pt(l 11)(V7 x x/7)M9.1°-I structure in 0.1 mM Ag+ solution, 1 M HCIO4, scan rate 2 mV/sec. Reproduced by permission from ref. [132], Figure 4.

Figure 1. Continued. C. Solid curve (—) immersion into 2 mM THBP followed by rinsing with 10 mM TFA. Dotted curve (.) as in solid curve, except 1 h in vacuum prior to voltammetry. (Reproduced with permission from ref. 3. Copyright 1984 Elsevier.) D. Solid curve (—) immersion into 0.03 mM HQ followed by rinsing with 10 mM TFA. Dotted curve (.) as in solid curve, except 1 h in vacuum prior to voltammetry.

In contrast to the successful implementation of the bead method in studying the anomalous features, the contributions from studies with UHV-electrochemical systems has been limited to just a few. Subsequent work from our apparatus following corroboration of Clavilier s results concentrated on the effect of potential cycling through "oxide formation potential on the surface structure (19). and later on the effect of pH and type of anion (Wagner, F.T. Ross, P.N., J. Electroanal. Chem.. in press) on the anomalous features. Using the system in Yeager s laboratory, Hanson (20) was able to reproduced Clavilier s voltammetry not only for the (111) surface, but also the (100) and (110) surfaces as well. In spite of the relatively small number of contributions to the literature that have come from the UHV-electrochemical systems, they have made and essential validation of the bead method of surface preparation, and have verified the structure sensitivity of the anomalous features inferred from purely electrochemical observations. 

Fig. 1. Cyclic voltammetry of 7% w/w iron phthalocyanine, FePc dispersed on Vulcan XC-72 carbon, after a heat treatment at 280°C in a flowing inert atmosphere. The measurement was conducted with the material in the form a thin porous Teflon bonded coating in 1 M NaOH at 25°. Sweep rate 5 mV/s. (Reproduced with permission from ref. 3. Copyright 1985 Elsevier.)...

Deposition of the mixed monolayer. Deposition solutions were prepared by dissolving octadecylmercaptan [ClsSH] and the respective bipyridinium in a mixture of chloroform and methanol. The electrode was cleaned by heating it in a gas-air flame. After cooling, the electrode was immersed in the deposition solution for 15 - 30 minutes, withdrawn, and rinsed in clean methanol or chloroform. Qualitatively the most reproducible surface redox waves and lowest charging currents during cyclic voltammetry were obtained with a freshly-prepared deposition solution containing 50 mM CiaSH and 10 mM of the bipyridinium in a 1 1 volume ratio of chloroform and methanol. 

Consideration of stirring. In stripping voltammetry, it is normal to employ a stationary electrode and a solution which is gently stirred. An alternative method is to have a still solution and an electrode that is rotated. If the solution is stirred, then the rate of stirring should be reproducible and controlled. Exhaustive electrolysis can be performed without stirring but the time required for deposition is likely to be quite long. 

Figure 45. Cyclic voltammetry of (a) 4-halo-1,2-dimethoxy-benzenes (Reproduced with permission from ref 432 (Figure 8). Copyright 1999 The Electrochemical Society.) and (b) 1,3-dimethoxybenzenes in LiPFe/PC/DMC on Pt (Reproduced with permission from ref 432 (Figure 9). Copyright 1999 The Electrochemical Society.)...

Figure 48. Anodic stability as measured on a spinel LL-Mn204 cathode surface for EMS-based electrolytes (a) Lilm (b) LiC104 (c) LiTf. In all cases, 1.0 m lithium salt solutions were used, and slow scan voltammetry was conducted at 0.1 mV s with lithium as counter and reference electrodes and spinel LiJV[n204 as working electrode. (Reproduced with permission from ref 75 (Figure 3). Copyright 1998 The Electrochemical Society.)...

Figure 51. Cathodic and anodic stability of LiBOB-based electrolytes on metal oxide cathode and graphitic anode materials Slow scan cyclic voltammetry of these electrode materials in LiBOB/EC/EMC electrolyte. The scan number and Coulombic efficiency (CE) for each scan are indicated in the graph. (Reproduced with permission from ref 155 (Eigure 2). Copyright 2002 The Electrochemical Society.)...

Figure 56. Slow scan (10 /iV s ) voltammetry on a graphite working electrode (a) 1.0 M LiFAP in EC/DEC/ DMC (b) LiPEe in EC/DEC/DMC. Solid line pristine graphite. Dashed line after 1 week of cycling. (Reproduced with permission from ref 499a (Figure 4). Copyright 2003 The Electrochemical Society.)...

Figure 18. Schematic model of the structure of Pt particles on an voltammogram in relation to the electrode potential. The hydrogen, double layer, and oxide regions are based on cyclic voltammetry. The lattice disorder decreases in the order D>A>C>B." (Reproduced with permission from ref 40. Copyright 1993 ElsevierSequoia S.A., Lausanne.)...

Figure 26. Comparison of the resonance scattering from H atoms or H+ obtained by fitting the Fano line shape in HCIO4 (open squares) and H2SO4 (closed squares) with the adsorbed hydrogen coverage (closed circles) and sulfate adsorption (open circles) obtained by cyclic voltammetry. (Reproduced with permission from ref 50. Copyright 2001 The Electrochemical Society, Inc.)...

The electrochemical behavior of chlorpromazine hydrochloride in 0.2M H2SO4 was studied by cyclic and linear sweep voltammetry at an oxidized and a non-oxidized ruthenium wire electrode [173]. Preparation of a stable and permanent coating of RuOj on the electrode was very time-consuming, but the resulting curves were highly reproducible. The... 

FIGURE 2.24 PCA scores for Pt electrode in ferrocyanide solution. Data represents six series of pulse voltammetry measurements performed with a self polishing device. A reproducible drift pattern is shown (reproduced from Olsson et ai, 2006, with permission). 

Fig. 7 Cyclic voltammetry of O2 in pyridine, at a stationary mercury drop electrode. Reference electrode aqueous SCE scan rate 45 mV s potential scale from —0.6 to —1.05 V (Reproduced with permission from Ref. 35).

The improvement in the preparation of renewable and reproducible mercury film-covered carbon paste electrode for use in anodic stripping voltammetry has been reported in Ref. 39. Mercury salts (mercuric oxalate) distributed in the electrode bulk served as a source of mercury. 

Upon repetitive cycling of the potential scan, the voltammetric record is reproduced, but an additional cathodic peak near to -0.45 V appears. This is due to the reduction of Pb + ions electrochemically generated by the previous oxidation of lead metal. The reduction of lead ions occurs at a potential different from that at which the reduction of PbCOs takes place. In repetitive voltammetry, additional anodic peaks appear at -0.40 and -0.28 V. These are due to the oxidative dissolution of different lead deposits generated in reductive scans [130, 131]. 

Figure 8.4 Differential pulse voltammetry of the major (a) and minor isomer (b) of La C82 and Y C82 (c) in o-DCB + 0.1M of ( -Bu)4NPF6 pulse amplitude 50 mV, pulse width 50 ms, pulse period 200 ms, scan rate 20 mV/s. Reproduced from Ref. 26, with permission from Elsevier.

Voltammetry is a collection of methods in which the dependence of current on the applied potential of the working electrode is observed. Polarography is voltammetry with a dropping-mercury working electrode. This electrode gives reproducible results because fresh surface is always exposed. Hg is useful for reductions because the high overpotential for H+ reduction on Hg prevents interference by H+ reduction. Oxidations are usually studied with other electrodes because Hg is readily oxidized. For quantitative analysis, the diffusion current is proportional to analyte concentration if there is a sufficient concentration of supporting electrolyte. The half-wave potential is characteristic of a particular analyte in a particular medium. 

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