Electrochemical techniques, anodic

Electrochemical techniques anodic stripping voltammetry (ASV) and cathodic stripping voltammetry (CSV) for determining trace elements, and potentiometric sensors for determining dissolved gases (C02, N02, S02, NH3, H2S, HCN, and HF) as well as chloride, fluoride, cyanide, and sulfide. 

To obtain the corrosion current from Rp, values for the anodic and cathodic slopes must be known or estimated. ASTM G59 provides an experimental procedure for measuring Rp. A discussion or the factors which may lead to errors in the values for Rp, and cases where Rp technique cannot be used, are covered by Mansfeld in Polarization Resistance Measurements—Today s Status, Electrochemical Techniques for Corrosion Engineers (NACE International, 1992). 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

The formation of colloidal sulfur occurring in the aqueous, either alkaline or acidic, solutions comprises a serious drawback for the deposits quality. Saloniemi et al. [206] attempted to circumvent this problem and to avoid also the use of a lead substrate needed in the case of anodic formation, by devising a cyclic electrochemical technique including alternate cathodic and anodic reactions. Their method was based on fast cycling of the substrate (TO/glass) potential in an alkaline (pH 8.5) solution of sodium sulfide, Pb(II), and EDTA, between two values with a symmetric triangle wave shape. At cathodic potentials, Pb(EDTA)2 reduced to Pb, and at anodic potentials Pb reoxidized and reacted with sulfide instead of EDTA or hydroxide ions. Films electrodeposited in the optimized potential region were stoichiometric and with a random polycrystalline RS structure. The authors noticed that cyclic deposition also occurs from an acidic solution, but the problem of colloidal sulfur formation remains. 

XPS investigations of the composition of the anodically grown passive layer on Ti electrodes were performed by Armstrong and Quinn [123, 124], The formation of a suboxide layer between the underlying Ti metal substrate and the stoichiometric Ti02 on top was demonstrated using XPS, AES and electrochemical techniques. 

Therefore, achievement of this desired substitution, particularly the formation of a carbon-carbon bond at the a position is one of most important goals of modern organofluorine chemistry. Although anodic substitution is a characteristic of certain electrolytic reactions, no results pertaining to the electrolytic substitution of trifluoromethylated compounds have been reported. Recently, the use of the electrochemical technique has opened new avenues for the realization of such nucleophilic substitution [40-42] and construction of a carbon-carbon bond [43-45]. 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

Heterocycles are of great interest in organic chemistry due to their specific properties. Many of these cycles are widely present in natural and pharmaceutical compounds. Electrochemistry appears as a powerful tool for the preparation and the functionalization of various heterocycles because anodic oxidations and cathodic reductions allow the selective preparation of highly reactive intermediates (radicals, radical ions, cations, anions, and electrophilic and nucleophilic groups). In this way, the electrochemical technique can be used as a key step for the synthesis of complex molecules containing heterocycles. A review of the electrolysis of heterocyclic compounds is summarized in Ref. [1]. 

For this reason, the dissolution of hydrous oxides does not require a high energy of activation. If hydrous oxides are dehydrated, they become dry oxides, which therefore acquire higher resistance to anodic dissolution. The most straightforward way to obtain dry oxides is to subject hydrous oxides to thermal treatments or better to prepare them as thin surface films by a non-electrochemical technique (thermal decomposition, chemical vapor deposition, reactive sputtering, etc.). 

The study of the effect of the adsorptions of various additives on the anodic dissolution has been the subject of several studies. For instance, the influence of the adsorption of N species on the anodic dissolution of Ni was studied in [43]. The dissolution and passivation of Ni in nitrite-containing acid solutions were investigated by Auger spectroscopy, AFM, and conventional electrochemical techniques. It was found that the dis-solution/passivation of the Ni surface is consistent with a competition between adsorbed OH and nitrogen-containing... 

Anodic stripping voltammetry (ASV). This is an electrochemical technique in which the element to be analyzed is first deposited on an electrode and then redissolved, that is, stripped, from the electrode to form a more concentrated solution. For example, a drop of mercury hanging from a platinum electrode in a solution containing the species to be measured has been used as the deposition electrode. A potential slightly more negative than the half-wave potential for the ion of interest is applied to deposit the element on the electrode. After deposition of the metal for a given... 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

Energetics of oxidation-reduction (redox) reactions in solution are conveniently studied by arranging the system in an electrochemical cell. Charge transfer from the excited molecule to a solid is equivalent to an electrode reaction, namely a redox reaction of an excited molecule. Therefore, it should be possible to study them by electrochemical techniques. A redox reaction can proceed either by electron transfer from the excited molecule in solution to the solid, an anodic process, or by electron transfer from the solid to the excited molecule, a cathodic process. Such electrode reactions of the electronically excited system are difficult to observe with metal electrodes for two reasons firstly, energy transfer to metal may act as a quenching mechanism, and secondly, electron transfer in one direction is immediately compensated by a reverse transfer. By usihg semiconductors or insulators as electrodes, both these processes can be avoided. 

The electrochemical technique can be used also for direct synthesis of bimetallic alkoxides. For instance, the anodic dissolution of rhenium in the methanol-based electrolyte that already contained MoO(OMe)4, permitted to prepare with a good yield (60%) a bimetallic complex RevMov,02(OMe)7, with a single Re-Mo bond [904], Application of the same procedure permitted the preparation of complex alkoxide solutions with controlled composition for sol-gel processing of ferroelectric films [1777]. 

The field of cathode activation, as well as that of anode activation, requires the use of complementary physical techniques to evaluate systems otherwise difficult to understand. Electrochemical techniques are sufficient to evaluate the kinetic parameters and the state of intermediates, especially if digital acquisition of open-circuit potential-decay transients, coupled with computer processing of the data, is used [104-106]. But the chemical and physical characterization of the surface remains essential. The literature shows that such an approach is becoming more accepted, so that there are hopes that the real situation of a number of systems will become clarified in the near future. 

It is generally assumed that electrons are transferred one by one (cases reported to be direct two-electron transfers 109 110 may actually be two very closely spaced one-electron transfers 11 ). This postulate immediately tells us that the first intermediate formed from a neutral substrate must be a cation radical in anodic oxidation and an anion radical in cathodic reduction and a neutral radical from oxidation of an organic anion and reduction of an organic cation. This has been amply verified both by electrochemical techniques and by ESR studies in inert SSE s 29,65) unfortunately, such results do not allow us to draw conclusions regarding the future reactions of these types of intermediates, as will be outlined in the following section. 

Electrochemical technique (also electrocoagulation) is a simple and efficient method for the treatment of drinkable water. Recent results reported by Part-hasarathy and Yang [54,55] have demonstrated that electrocoagulation (EC) using aluminium anodes is effective in defluoridation. In the EC cell, the aluminium electrodes sacrifice themselves to form aluminium ions first. Afterwards the aluminium ions are transformed into AI(OH)3 before being polymerized to Aln(OH)3n. The AI(OH)3 floe is believed to adsorb F strongly as illustrated by the equation. 

Anodic and cathodic stripping voltammetry electrochemical techniques, combined with metal titration analyses, for obtaining ligand concentrations and conditional stability constants, can be used to determine how many ligands are involved in metal complexation as well as their relative differences in strength. 

A number of coal-derived liquids were examined by cyclic-voltammetry and other electrochemical techniques and found to show some activity. At anodic potentials films form on glassy carbon electrodes. It is suggested that this film formation is caused by oxidative coupling of radical cationic species with neutral ring structures through a mechanism similar to that which causes charring and coking in coal conversion processes. 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

The most commonly used hard templates are anodic aluminum oxide (AAO) and track-etched polycarbonate membranes, both of which are porous structured and commercially available. The pore size and thickness of the membranes can be well controlled, which then determine the dimension of the products templated by them. The pores in the AAO films prepared electrochemically from aluminum metals form a regular hexagonal array, with diameters of 200 nm commercially available. Smaller pore diameters down to 5 nm have also been reported (Martin 1995). Without external influences, capillary force is the main driving force for the Ti-precursor species to enter the pores of the templates. When the pore size is very small, electrochemical techniques have been employed to enhance the mass transfer into the nanopores (Limmer et al. 2002). 

Abrasive stripping voltammetry — Technique where traces of solid particles are abrasively transferred onto the surface of an -> electrode, followed by an electrochemical dissolution (anodic or cathodic dissolution) that is recorded as a current-voltage curve [i]. It allows qualitative and quantitative analysis of metals, alloys, minerals, etc. The technique is a variant of - voltammetry of immobilized particles [ii]. 

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