Electrochemical processes amalgam electrode

Electrochemical processes involving metals, such as metal ion discharge and metal atom ionization, can be studied without the complications of structural changes when electrodes of the molten metal (at elevated temperatures and in non-aqueous electrolytes) or of the metal s liquid amalgam are used instead of the solid metal. 

The alkali metal is deposited at the electrode surface and diffuses at a certain velocity into the interior of mercury. The course of the electrochemical process merely depends upon the composition of the phases at the boundary between the electrolyte and the electrode. The quicker the diffusion of the alkali metal from the surface of the mercury to the interior, the less risk there is of too concentrated amalgam being formed at the surface of the cathode and, therefore, less probability of hydrogen deposition. 

As has been shown, this kind of reaction does take place on the mercury electrode in the presence of cystine. The i-E curves measured on an amalgamated electrode in the presence of peroxidase apoenzyme in the solution also have anodic and cathodic maxima, similar to those obtained in the peroxidase solution. Thus, in the potential region investigated, the S—S groups of the protein globule of peroxidase are electrochemically active. Heme iron in the active center does not take part in the observed redox process. 

Vetter summarized the main results obtained up to 1967. The principal difficulty in the experimental investigation of metal electrodes in the past was the poorly reproducible preparation of the electrode surface. This problem could be avoided for some electrodes by dissolving metals in hquid mercury. Therefore, many results about the mechanism were measured with amalgam electrodes. On such an electrode no crystallization overvoltage is expected. When it was possible to suppress diffusion depletion, the charge transfer process could be investigated. The charge transfer mechanism could then be determined from the electrochemical reaction orders of the complex molecules or ions. 

Jacquinot and Hauser reported preparation of an amalgamated Pb, Tl, Au-poly(tetrafluoroethylene) gas diffusion electrode and an internal electrolyte containing [Ni (cyclam)] +. This preparation involves the formation of various metal films by electrolytic deposition or in the case of Hg by dipping the Au disk electrode into elemental Hg. For concentrations between 0.1 and 1% the electrochemical cell showed a sensitivity of 3.58 mA% and the detection limit is 500 ppm. The optimum pH range was determined between 3.5 and 6 and a selectivity ratio of the catalyst for CO2/ H+ of 5 1 was found. The electrochemical process is limited by diffusion of CO2 demonstrated by rotating disk experiments it means that the relationship between reduction current and the square root of the angular speed was linearFigure 5.8 presents the response of a thin mercury film electrode (TMFE) when exposed to solutions equilibrated with CO2. As can be seen a well established reduetion wave for CO2 is only obtained in presence of the [Ni (cyclam)] + catalysts. 

An example of the simplest (in the sense of the number of kinetic parameters) electrochemical reaction is reduction of silver ions (Ag+) from a dilute aqueous solution of a well soluble silver salt (e.g., nitrate) in the presence of excess of an indifferent salt (e.g., potassium nitrate) on a liquid silver-mercury alloy (also called amalgam) electrode. Besides the transfer of a single electron, only diffusion steps are involved in this process. The entire reaction can be very well modeled and the kinetic parameters are determined experimentally with high level of accuracy. The information gleaned while analyzing the mechanism of silver ion reduction can be used in elucidating more complex, multi-step, multiphase processes, such as the electrochemical reaction in a lithium-ion cell. 

Direct, nonmediated electrochemical reduction of NADIP)" " at modified electrode surfaces has been used to produce the en2ymatically active NAD(P)H and even to couple the NAD(P)H regeneration process with some biocatalytic reactions [228]. The modifier molecules used for these purposes are not redox active and they do not mediate the electron-transfer process between an electrode and NAD(P)+ however, they can effectively decrease the required overpotential and prevent formation of the nonenzymatically active dimer product [228]. For example, the efficiency of the direct electrochemical regeneration of NADH from NAD" " was enhanced by the use of a cholesterol-modified gold amalgam electrode that hinders the dimerization of the NAD-radicals on its modified-surface [228]. This direct electrochemical NAD+ reduction process was used favorably to drive an enzymatic reduction of pyruvate to D-lactate in the presence of lactate dehydrogenase. The turnover number for NAD" " was estimated as 1400 s k Other modifiers that enhance formation of the enzymatically active NAD(P)H include L-histidine [229] and benzimidazole [230], immobilized as monolayers on silver electrodes. CycKc voltammetric experiments demonstrated that these modified electrodes can catalyze the reduction of NAD+ to enzymatically active NADH at particularly low overpotentials. 

Radiopolarography measurements for the cathodic reduction of Bk(III) to Bk(0) at a dropping mercury electrode in 0.1 M LiCl at pH 2 give an amalgamation halfwave potential value of 1.63 V versus SHE and an estimated of 2.18 V [169]. Analysis of the electrochemical data leads the authors to conclude that the Bk(III)/Bk(0) electrode process is irreversible. 

Sugars with a potential aldehyde function can be reduced electrochemically at electrodes of mercury or amalgamated lead [23]. The rate of this process is controlled by the rate for the conversion of the cyclic to the open-chain form of the sugar. A technical-scale plant [24] for the conversion of glucose to either sorbitol or mannitol was operated in the past, but this method has largely been ousted by other processes. Glucose is converted to... 

The generic process for electrochemical synthesis of sp-carbon chains was electrochemical reductive carbonization (corrosion) of poly(tetrafluoro-ethylene) (PTFE) by alkali metal amalgams, pioneered by Jansta and dousek [6 9] (for review see Reference 3). The reaction occurs at the interface of a dry contact between PTFE and alkali metal amalgams, hence, it does not seem to recall an electrochemical synthesis in its classical sense. The purely electrochemical carbonization of PTFE on a Pt electrode in aprotic electrolyte solution is also possible [3], but the amalgam-driven process is superior, presenting a clean and well-defined alternative to classical (wet) electrochemistry. 

The electrode elements of an electrochemical sensor are often metallic, semiconductive, or inert. For metallic electrode elements, noble metals such as gold, platinum, and silver are often used in conventional electrochemical sensors. Mercury or amalgam as electrode materials for microfabricated electrochemical sensors are seldom used due to the difficulty involved. Because mercury has a relatively high vapor pressure, it does not lend itself well in any fabrication process using a vacuum or low-pressure environment. The formation of amalgam requires the use of mercuric ion containing reagent. This step can be elaborate and complicated. 

Similar data were obtained on the amalgamated gold and pyrographite electrodes. The half-wave potentials are equal to E /2 = -0.03 V and E /2 = -0.37 V and are practically independent of the nature of the electrode. The anode-cathode polarization curves obtained in the presence of a mixture of the oxidized and reduced forms of MV are given in Figure 14. An analysis of the kinetics of mediator oxidation and reduction at the electrode reveals that the process proceeds on the carbon electrode under close to reversible conditions and is controlled by concentration polarization. Thus, MV fully satisfies the above-formulated requirements of mediators for electron transport in electrochemical systems with the participation of enzymes. 

However, there are some electrochemical experiments where the volume of the electrolytic solution is comparable to that of the depletion layer such as the case of porous electrode surfaces, microvolumetric cells and amalgamation processes. In these cases, the domain of the solution phase is confined to a distance d from the electrode surface at which no flux of species takes place so that the limit condition is given by... 

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