Mercury electrode electrochemical oxidation

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

This reduction step can be readily observed at a mercury electrode in an aprotic solvent or even in aqueous medium at an electrode covered with a suitable surfactant. However, in the absence of a surface-active substance, nitrobenzene is reduced in aqueous media in a four-electron wave, as the first step (Eq. 5.9.3) is followed by fast electrochemical and chemical reactions yielding phenylhydroxylamine. At even more negative potentials phenylhydroxylamine is further reduced to aniline. The same process occurs at lead and zinc electrodes, where phenylhydroxylamine can even be oxidized to yield nitrobenzene again. At electrodes such as platinum, nickel or iron, where chemisorption bonds can be formed with the products of the... 

A number of reports have appeared concerned with the adsorption of purines at a dropping mercury electrode 77"80> but these are confined to studies at potentials far removed from those where electrochemical oxidation occurs. More recently some qualitative studies on the adsorption of certain purines at the PGE have appeared with a view to understanding the adsorption of these compounds at positively charged electrodes. Since many biological reactions occur at charged membrane or ribosomal surfaces it is of considerable interest to investigate these phenomena. 

Voltammetry is a part of the repertoire of dynamic electrochemical techniques for the study of redox (reduction-oxidation) reactions through current-voltage relationships. Experimentally, the current response (i, the signal) is obtained by the applied voltage (.E, the excitation) in a suitable electrochemical cell. Polarography is a special form of voltammetry where redox reactions are studied with a dropping mercury electrode (DME). Polarography was the first dynamic electrochemical technique developed by J. Heyrovsky in 1922. He was awarded the Nobel Prize in Chemistry for this discovery. 

Substrates DME = dropping mercury electrode FTO = fluorine-doped tin oxide G = graphite GC = glassy carbon GrC = graphic carbon ITO = indium tin oxide-coated glass SC = single crystals SS = stainless steel TCO = transparent conducting oxide VC = vitrious carbon. Miscellaneous ECALE = electrochemical atomic layer epitaxy ED = electrodeposition ML = monolayer RT = room temperature SMD = sequential monolayer deposition V = vacuum. 

The peptide fragments of metalloth-ioneins Lys-Cys-Thr-Cys-Cys-Ala [56-61] (FT) were studied by different electrochemical techniques. The cyclic voltam-metric behavior of the peptide fragment in the presence of Cd(II) indicated two reversible electrochemical processes due to the oxidation of the mercury electrode in the presence of CdFT and reduction of CdFT complex, both from the dissolved and adsorbed state [105]. The influence of the experimental conditions on electroreduction of Cd-metallothioneins... 

The oxidation of guanine (G) and adenine (A) follows a two-step mechanism involving the total loss of four electrons and four protons showing current peaks at approximately 0.9 and 1.2 V, respectively. However, the redox properties are dependent on the pH, the ionic strength of the electrolyte, and the electrode material.2 The reader is referred to a recent review by Palecek and coworkers for a more comprehensive discussion regarding the electrochemical mechanism of the oxidation and reduction of DNA bases on carbon and mercury electrodes.3 4 Guanine oxidation is irreversible and occurs in two consecutive steps (Fig 10.1).5... 

The amount of mercury formed on the gold electrode under different reduction conditions or that remained on the electrode after different storage conditions was measured by subsequent electrochemical oxidation (Fig. 12.8). First, the relation of the reduction (Qred) and... 

Fig. 12.8. Test of reproducibility of electrochemical oxidation/reduction and of completeness of thermoinjections. Dependence of the oxidative charge on the reduction charge is shown in (a, b). The oxidation was performed in pure water immediately after reduction (all data in (b) and in (c)), after 72 h storage at room temperature ( ) and after 96h storage at -20°C ( ). Kinetics of the limiting oxidation current of wire gold electrode with reduced mercury before (1) and after (2) thermoinjection is shown in (c) [45].

For reductions, hanging mercury drop electrodes or mercuryfilm electrodes are frequently used owing to their microscopic smoothness and because of the large overpotential for hydrogen evolution characteristic for this electrode material. Mercury film electrodes are conveniently prepared by the electrochemical deposition of mercury on a platinum electrode from an acidic solution of an Hg2+ salt, e.g. the nitrate. However, the oxidation of mercury metal to mercury salts or organomercurials at a low potential, 0.3-0.4 V versus the saturated calomel electrode (SCE), prevents the use of these electrodes for oxidations. 

The latter process would be expected to occur at a potential more positive than that for the oxidation of 02. Processes similar to the reactions of Eqs. (9.40) and (9.41) have been observed with the electrochemical reduction of HOOH at mercury electrodes in DMF and MeCN. The almost reversible cyclic vol-tammogram for 02 at a glassy-carbon electrode in MeCN indicates that specific reactions occur between metal electrodes and 02 in a poorly solvating medium for anions, such as MeCN. 

A wide variety of quinones spontaneously adsorb onto various electrodes, including gold, platinum, carbon, and especially mercury. On mercury electrodes, these quinonoid monolayers often exhibit nearly ideal electrochemical responses in low-pH electrolytes, so making them attractive model systems for probing the thermodynamics of adsorption. In low-pH electrolytes, both the oxidized and... 

Dithiocarbamates and thioureas are included in this section because of their useful electrochemical behavior at mercury and mercury amalgam electrodes. The formation of mercury complexes results in an easy oxidation at the mercury electrode. On the other hand, carbon electrodes are not well suited for the detection of these compounds because the oxidation occurs beyond the usual scope of carbon detector cells. 

The electrochemical behavior of thiocarbamates has been studied by several investigators (35,36,38,74-78). At mercury electrodes, thiocarbamates are oxidized in a one-electron process to form insoluble mercury (II) salts. Thioureas undergo a similar process. 

Electrochemical (EC) techniques provide an alternative way to detect sulfur containing molecules. Earlier methods of EC detection involve the application of a gold/mercury electrode.15 Platinum and gold electrodes have also been used for anodic detection of thiols,16 but this requires high oxidation potentials, which complicates analytical applications. Thus, chemically modified electrodes with inorganic or organic mediators have been employed to facilitate electron-transfer between the electrode and the analyte, and therefore reduce the oxidation potential. Recently, pyrroloquinoline quinone (PQQ) modified electrodes have been developed for detection of endo- and exogenous thiols.17... 

Adsorptive accumulation — Organic substances which exhibit -> surface activity and electroactivity can be electrochemically analyzed by adsorptive accumulation on the surface of a an electrode, e.g., mercury electrode, followed by the reduction, or oxidation of the adsorbate using -> voltammetry [i,ii]. Also, the adsorption of highly stable and inert -> complexes of metal ions with surface-active organic ligands is utilized for the determination of trace metals [iii]. In all these methods the maximum voltammetric response is linearly proportional to the surface concentration of the adsorbed analyte at the end of the accumulation period [iv]. In the majority of cases, the adsorption on mercury can be described by the -> Frumkin isotherm /icx=o = 0exp(ad)/(1- 9), where f is the adsorption constant, cx=o is the concentration of the dissolved compound at the electrode surface, 6 = T/rmax is the surface coverage, T is the surface concentration of the adsorbed compound, rmax is the maximum surface concentration and a is the Frumkin... 

The oscillographic polarography data of 4-nitro- and 6-nitro-2-aminobenzothiazole (in aqueous-alcoholic solutions) are reported [1000], The electrochemical behavior of dyes is investigated into which 5-nitrobenzothiazole or 5-nitrobenzoselenadiazole is added [1001], The authors of this work have utilized a RDER, glass-graphite, and mercury-dropping electrode. The oxidation potentials of 2-alky lsubstituted 6-nitroben-zimidazoles are determined using Pt electrode [1002],... 

Other careful electrochemical measurements of the oxidation potentials of 2,4,6-tri-t-butylphenol and 2,6-di-t-butyl-4-methylphenol in acetate buffered ethanol or acetonitrile have been measured by Mauser et al.184). They determined the static potentials using a boron carbide indicator and a mercury/mercury-acetate reference-electrode. Since in this case the oxidation of the phenols and not the phenolates to the phenoxyls has been determined the oxidation potentials cannot be compared with those in Table 12. For other electrochemical oxidations of phenols in buffered aqueous solutions using a graphite electrode see Ref. 185 186>. 

Guanine and guanosine can be electrochemically oxidized at a pyrolytic graphite electrode [86, 96,97] with Epn = -f-1.02 V in 2 M H2SO4. Uracil and thymine. Uracil and thymine are not reducible polarographically under normal conditions [78, 86,95], but in alkaline media they yield an anodic wave caused by the formation of a mercury salt [98]. 

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