Contribution a l'etude des effets catalytiques exerces par les composes soufres a l'electrode a gouttes de mercure. Cas de la thiouree

The influence of thiourea and some of its N-alkyl-derivatives on the reduction of various metallic cations (Ni²⁺, Co²⁺, Co²⁺) has been studied in neutral medium at the dropping mercury electrode. The polarographic curves exhibit kinetic prewaves reaching a 2^?_e amplitude at high concentration. In addition fairly large extra-currents can be detected at verynegative potentials. By resorting to classical and pulse polarography and to macro-electrolysis, it has been demonstrated that these currents are related to both the electroreduction of thiourea with production of sulphide and cyanide ions and catalytic discharge of water itself. The reaction mechanism is discussed in terms of the structure of the organic compound and the properties of the electrode metal surface, in connection with similar phenomena observed in the presence of thiocyanate ion (reduction of the thione group) and sulfur proteins (catalytic hydrogen evolution).     

Alternating current chronopotentiometry—III. Electroreduction of silver(I), cadmium(II), and thallium(I) in calcium nitrate tetrahydrate melt

The electroreduction of silver(I) on a platinum electrode and of cadmium(II) and thallium(I) on a mercury electrode has been studied by alternating current chronopotentiometry. Expected relations for the diffusion controlled, one-electron deposition of an insoluble substance are obeyed for silver(I); but reduction of cadmium(II) on mercury is more complex. Nucleation of metallic thallium is the rate controlling process during the early stages of the reduction of thallium(I).     

Separation of Co(II) and Ni(II) from thiocyanate media by hollow fiber supported liquid membrane containing Alamine300 as carrier — investigation on polarity of diluent and membrane stability

The separation of cobalt(II) and nickel(II) ions by HFSLM has been presented. The feed solution is 0.5M thiocyanate containing 300 ppm each of cobalt(II) and nickel(II) ions, whereas extractant is Alamine300 and the stripping solution is ammonia. Cobalt(II) is more preferable with Alamine300 than nickel(II). The effects of pH, Alamine300 concentration and ammonia concentration were investigated. Seven diluents were used: hexane, decanol, chlorobenzene, benzene, dichloromethane, ethylene dichloride and chloroform with different polarity indexes, from 0.1–4.1. Nickel(II) ion which is unpreferable with Alamine300 was used as a tracer to determine the membrane stability. The polarity of the diluents was found to be the main factor influencing the extraction performance and stability of a liquid membrane. The decreasing of polarity of the diluent can prolong the membrane stability, but the percentages of extraction and stripping decreased. The longest lifetime, 200 minutes, was obtained by using hexane as a diluent with the polarity index of 0.1.     

Thermodynamics of complex formation: nickel—caprolactam system

Polarographic techniques have been used to investigate the complexing reactions in aqueous solution between ε-caprolactam and nickel ion. The coordination number and stability constants of the complex have been determined polarographically. The method of Ringbom and Eriksson using cadmium as indicator ion was used on the basis of reversible and irreversible character of electrode reactions at dropping mercury electrode. The stability constants obtained at various temperatures were used to calculate various thermodynamic functions by graphical method.     

The reactions between mercury ions and silver: dissolution and displacements

The results of polarization measurements for the reduction of mercury (I) and mercury (II) on a rotating platinum electrode, and the oxidation of silver and amalgamated silver at a rotating silver electrode are presented. For mercury (I) and mercury (II) diffusion coefficients were estimated from limiting current measurements as a function of rotation speed and temperature. The Values obtained for the diffusion coefficients at 25°C, and activation energies were:

The kinetics of the reactions between a rotating silver disc and solutions of mercury (I) and mercury (II) are also reported. The mercury (I) reaction is a displacement in which mercury is deposited and silver goes into solution. The kinetics are first order-first order reversible, and can be quantitatively accounted for by assuming that the diffusion of mercury (I) to the surface and silver (I) away from the surface are jointly rate controlling. At temperatures below 35°C, the mercury (II) reaction is a dissolution, controlled by the speed at which mercury (II) ions diffuse to the reacting surface. Above 35°C, the mercury (II)-silver reaction is a displacement, with somewhat irreproducible kinetics. The changeover between the two reactions at 35°C apparently occurs because the displacement reaction is thermodynamically favoured above 35°C.     

The electrochemical behaviour of polycrystalline nickel electrodes in different carbonate-bicarbonate ion-containing solutions

The dissolution and passivation of polycrystalline nickel in carbonate-bicarbonate ion-containing solutions covering wide ranges of pH and electrolyte concentration were investigated by employing voltammetric, galvanostatic and potentiostatic transient techniques. Results obtained with a rotating disc electrode allow the competing reactions related to the active-passive transition to be distinguished through the influence of the potential sweep rate and the rotation speed on the electrochemical behaviour of the system at fixed concentrations of either carbonate or bicarbonate ion. The first oxidation level of nickel corresponds mainly to Ni(OH)₂ formation, the chemical dissolution of the surface layer and the precipitation of NiCO₃ and Ni(OH)₂. The partial removal of the prepassive layer is predominantly assisted by both the bicarbonate ion concentration and the electrode rotation. In the presence of chloride ions the formation of soluble Ni(II) species and NiCo₃ in the potential range of the first oxidation level appears to be enhanced. This effect can be interpreted by taking into account competitive adsorption processes at the base metal between Cl^– and OH^– ions.     

New electrochemical oscillator based on the cation-catalyzed reduction of nitroaromatic radical anions

Nitroaromatic compound bifenox (methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate) is used as a model system for a new NDR oscillator in the aprotic solvent acetonitrile. The reduction of bifenox yields the anion radical, which is further reduced by an overall addition of three electrons and four protons to the corresponding phenylhydroxylamine. The second reduction step is strongly influenced by the nature and concentration of the tetraalkylammonium salts used as the supporting electrolytes. At the low concentration of tetrahexylammonium hexafluorophosphate the anion radical reduction occurs at extremely negative potentials due to the double-layer effect on the negatively charged reactant. An addition of small amounts of the alkali metal cations (Li⁺, Na⁺ and K⁺) to such a system has a remarkable acceleration effect on the electron transfer rate since these ions undergo pairing with in situ generated anion radicals and the double-layer effect on the resulting particles of a diminished charge is much weaker. At potentials where alkali metal cations are reduced to the corresponding amalgams the acceleration ceases. As a result the current-potential curves show a negative slope. Electrochemical impedance spectroscopy sensitively indicates the presence of the negative differential charge transfer resistance (NDR). The addition of a high external resistor in series with the electrochemical cell leads to the current oscillations at the stationary mercury electrode and to the current instabilities on the dropping mercury electrode. Described system is a new example of the NDR oscillator, in which the double-layer effect in connection with cationic catalysis rather than anionic catalysis plays a crucial role in the observation of NDR.     

ZnCl2–NH4Cl溶液中锌配合离子的阴极还原

采用多种电化学方法探讨了ZnCl2–NH4Cl溶液配制液中精炼锌的阴极过程. 结果表明：Zn(II)离子在ZnCl2–NH4Cl溶液配制液中阴极还原生成金属锌的电极过程，具有两电荷放电一步完成和电荷转移控制的不可逆电极过程动力学规律，Zn(II)离子在此溶液配制液中主要以[Zn(NH3)4]2+配合离子存在，而直接在阴极放电的配合离子物种是[Zn(NH3)2]2+.     

Activated carbon adsorption of cyanide complexes and thiocyanate ion from petrochemical wastewaters

Experimental isotherms indicate that thiocyanate and nickel cyanide complex (tetracyanonicklate (II)) adsorb to an equal extent from synthetic and actual waste solutions at loadings below 10 mg/g carbon. Loading of nickel hexammine complex is an order of magnitude less. Adsorption of the divalent nickel cation is strongly pH dependent and is believed to be a chemisorption by precipitation or reaction on the carbon surface. In pure solution, ferrocyanide (hexacyanoferrate (II)) adsorption was comparable to that of thiocyanate or nickelocyanide, but adsorption from the waste liquor tested was negligible. This difference may be due to solvation of ferrocyanide with methanol in the wastewater to form a species with entirely different adsorption characteristics. Chemical oxygen demand (COD) values were obtained for several of the pure materials (SCN^?, 1.1; Fe(CN)₆^?4, 0.5; Ni(CN)₄^?2, 0.4 mg $\text{COD}\text{mg}$ ion) and the wastewater to monitor adsorption of organic background materials.     

Kinetics of discharge of zinc at the dropping mercury electrode

The rate of the discharge reaction of zinc ions at the dropping mercury electrode is diminished if the concentration of the indifferent surface inactive electrolyte is increased, because of the decrease of the potential difference in the diffuse double layer. It diminishes also with increase of radius of the alkali-metal cations present in the electrolyte and with increase of charge of the cation of the electrolyte. A strong retarding effect is exhibited by tetraalkylammonium cations, which increases with the size of the alkyl group. The electrode reaction rate increases in presence of iodide anions, as manifested by formation of a new, more positive wave. In presence of iodide and tetraalkylammonium cations together the process is accelerated at more positive potentials and is retarded at more negative potentials.

All these phenomena are analogous to those observed with the hydrogen-evolution reaction at the mercury electrode, and are the opposite of those observed with anions of the persulphate type.

Abstract

La vitesse de décharge de Znl²⁺ sur électrode à mercure gouttante diminue quand s'accroît la concentration superficielle d'un électrolyte inactif, par suite de l'abaissement de la tension électrique dans la couche diffuse. Elle diminue aussi quand s'accroîssent le rayon on la charge des cations de l'électrolyte. Les cations tétraalkylammonium exercent un effet fortement retardateur, qui augmente avec l'encombrement du groupe alkyl. La vitesse s'élève par centre en présence d'anions I⁻. Si cet anion est accompagné d'un des cations précédents, le processus s'accélère aux tensions plus positives et se ralentit aux tensions plus négatives. Tons ces phénomènes, contrastant avec ceux manifestés avec les anions du type persulfate, sont comparables à ceux observés dans l'évolution d'hydrogène sur électrode de mercure.

Abstract

The influence shown by the composition of the electrode double layer on the electrode reaction rate of the zinc ion is quite similar to analogous effects observed with the hydrogen-evolution reaction at mercury electrodes. The decrease of the rate of the latter process with increase of the radius of alkali-metal cations, of the charge of the cation of the electrolyte and of the concentration of the electrolyte was described by Herasymenko and lendyk.²¹ The strong retardation of this process by quaternary cations and the acceleration by iodide was found by Yofa, Kabanov and co-workers.²² Tsa Chuan Sin and Yofa²³ observed the combined effect of iodide and tetraalkylammonium cations which at positive potentials accelerate the hydrogen evolution reaction slightly over the value in presence of iodide only, but retard it at more negative potentials.

From the similar features of both reactions it may be concluded that in spite of a different total reaction path, the rate-determining step is identical, i.e. the transfer of the charged particle from its hydration sheath to the electrode. It is expected that analogous phenomena will be observed with other cations which are not adsorbed too strongly at the electrode (cf. the influence of surface-active cations on the discharge of T1⁺ and Cd^{2+ 24,25}).

All the effects of the double-layer structure connected with the electrode reaction of the zinc ion are just the opposite of those observed with the reduction of anions of persulphate type.²⁰     

Simultaneous determination of lead, copper and cadmium onto mercury film supported on wax impregnated carbon paste electrode: Assessment of quantification procedures by anodic stripping voltammetry

The deposition and stripping processes of lead and copper and cadmium ions over the wide concentrations range of 1 × 10⁻⁵ to 5 × 10⁻⁹ M, have been studied at mercury film deposited on wax impregnated carbon paste electrode, using cyclic voltammetry, linear sweep anodic stripping voltammetry and differential pulse anodic stripping voltammetry. The carbon paste electrode modified with the mercury film was characterized for its physical and electrochemical properties. The parameters of deposition and stripping processes of the analytes have been investigated using standard solution of the metal ions at various concentrations and different supporting electrolytes and different pH. The linear sweep anodic stripping has been adopted for the determination of analytes at higher concentration whereas the analytes at lower concentrations were determined using DPASV. The DPASV behavior for the ions studied dependent on concentrations of the analyte as well as on the time used in the pre-concentration step. The method developed using standard solutions have been successfully applied for the determination of Cu(II), Pb(II) and Cd(II) in Fin Fish muscles and water samples.     

Photocurrents at irradiated mercury electrodes in solutions of nickel ions

The influence of concentration and potential on photocurrents produced at mercury electrodes in solutions of nickel ions have been investigated using a pulsed xenon lamp. It is concluded that the mechanism involves scavenging of hydrated electrons by the nickel ions followed by an oxidative regeneration, rather than the reduction of excited nickel ions.     

Selective Sorption of Mercury(II) from Aqueous Solution with an Organically Modified Clay and its Electroanalytical Application

Abstract

The organo‐clay used in this work was prepared from a Na‐montmorillonite (Wyoming‐USA deposit) by treatment with water solution of hexadecyltrimethylammonium cations. As organo‐clays exhibit strong sorptive capabilities for organic molecules, 2‐mercapto‐5‐amino‐1,3,4‐thiadiazole organofunctional groups, with potential usefulness in chemical analysis, were incorporated on its solid surface. The physically adsorbed reagent did not present any restrictions in coordinating with several metal ions on the surface. The resultant organo‐clay complex exhibited strong sorptive capability for removing mercury ions from water in which other metals and ions were also present. The purpose of this work is to study the selective separation of mercury(II) from aqueous solution using the organo‐clay complex, measured by batch and chromatographic column techniques, and its application as preconcentration agent in a chemically modified carbon paste electrode for determination of mercury(II) in aqueous solution.     

Electrochemical behaviour of metals in micellar medium and in water—tributylphosphate emulsions. Application to electrowinning of metals in waste water—I. Micellar medium

The inhibition of copper(II), nickel(II), cadmium(II), lead(II), cobalt(II), zinc(II) reduction by a non-ionic surfactant is studied in NaClO₄ as a non-complexing medium and in KSCN as a complexing medium. The magnitude of inhibition of these reactions appears very different according to pH in the perchlorate medium. In the thiocyanate medium, it seems that the stability of the complex has a very significant role in the inhibition process.     

Gas purification by removal of atomic mercury using electrochemical cells

Electrochemical cells are described, which allow the oxidative removal of gaseous atomic mercury from gas streams. The gas stream is fed into an electrochemical cell containing an electrolyte solution and a three-dimensional anode with a large surface. The results can be explained by assuming that the atomic mercury is first transferred from the gas to the electrolyte solution followed by oxidation to mercury(II) ions at the electrode surface. Once mercury(II) has reached a sufficient concentration, it reacts with the atomic mercury to form mercury(I). The dissolved mercury(I) is than oxidized to mercury(II) at the electrodes. Therefore the efficiency of mercury removal from the gas stream can be enhanced by adding mercury(II) to the electrolyte at the very beginning of the process.     

INDIVIDUAL AND SIMULTANEOUS BIOSORPTION OF CHROMIUM (VI) AND NICKEL (II) ONTO DRIED ACTIVATED SLUDGE

The biosorption of chromium(VI) and nickel(II) ions, both singly and in combination, by dried activated sludge was investigated in a batch system as a function of initial pH and single- and dual-metal ion concentrations. The working pH value for single chromium(VI) and nickel(II) biosorption was determined as 1.0 and 4.5, respectively, due to the selective uptake of biomass. Multi component biosorption studies were performed at these two initial pH values. It was observed that the biosorption kinetics and equilibrium uptakes of chromium(VI) and nickel(II) ions in binary mixtures changed due to the initial biosorption pH and the presence of the other component.     

Selective retention properties to silver(I) and mercury(II) ions of poly(4‐vinylpyridine) methyl iodide studied by the liquid‐phase polymer‐based retention technique

The retention of various metal ions by water‐soluble poly(4‐vinylpyridine) methyl iodide in conjunction with ultrafiltration membrane was investigated. The method is based on the retention of inorganic ions by this polymer in a membrane filtration cell and subsequent separation of low‐molecular weight species from the polymer metal ion complex formed. It is shown that the polychelatogen can bind silver(I) and mercury(II) ions in aqueous solution at pH 1. At higher pH, the water‐soluble polymer can be applied to the separation and preconcentration of silver metal ions. Therefore, this polychelatogen is highly selective to Hg(II) at pH 1 with respect to metal ions such as Cd(II) and Zn(II). © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2578–2582, 2001     

Cathodic processes in the leaching and electrochemistry of covellite in mixed sulfate–chloride media

The cathodic processes that occur on a covellite (CuS) surface in mixed sulfate–chloride solutions in the absence and presence of copper(II) ions have been studied using potentiostatic transients and cyclic voltammetry at rotating disk electrodes in the potential range 0.3–0.7 V (versus SHE). This range is relevant to the oxidative leaching of this copper mineral in sulfate and chloride lixiviants. Variations in the concentrations of sulfate and chloride ions had a small effect on the cathodic reduction of covellite in the potential range of 0.5–0.3 V, although the presence of chloride ion resulted in a significant increase in the anodic current on the reverse sweep. On the other hand, addition of copper(II) ions resulted in enhanced cathodic currents and subsequent anodic currents in both sulfate and chloride solutions due to reduction of covellite to an undefined reduced copper sulfide species. Reduction of copper(II) to copper(I) ions becomes the preferred cathodic reaction as the concentration of chloride ions increases, becoming mass transport controlled at a rotating disc electrode at potentials below about 0.4 V. Potentiostatic measurements at potentials negative to the mixed potential in acidic chloride solutions have shown that reduction of copper(II) ions is reversible and have been used to estimate the rate of oxidative dissolution of the mineral which value agrees reasonably well with previously reported leaching rates under similar conditions. Reduction of dissolved oxygen has been found to be very much slower that that of copper(II) ions under ambient conditions.     

黑镍镀液中硫氰酸根和氯离子的自动电位滴定

研究了用自动电位滴定测定硫氰酸根和氯离子的条件及方法.结果表明:选用银电极为指示电极,217型双液接电极为参比电极,在硝酸介质中既能连续滴定也能分步滴定这两种离子.应用于黑镍镀液分析,方法简便快捷,分析的准确度和精密度完全能满足生产需要.     

Catalytic pre-wave of nickel(II) in presence of cocarboxylase (disulfide form) in borate medium

A brief study is made of the nickel(II) polarography in borate medium. The nickel(II) in borate medium gives one or two waves depending on its concentration as well as the medium composition.In a borate medium, cocarboxylase in its disulfide form gives a nickel(II) pre-wave. The effects of the cocarboxylase (disulfide form) concentration, nickel(II) concentration and mercury column height on the pre-wave are described.The effect of neutral salts on the pre-wave is studied and it is seen that an increase in the ionic strength is accompanied by a decrease in pre-wave current, being of decreasing effect in the order K⁺ < Na⁺ < Li⁺. The electrocapillary curves are present, and it is shown that the presence of cocarboxylase (disulfide form) in the solution causes a decrease in the interfacial tension.The results are discussed and a mechanism for the electrode process of the pre-wave is proposed. Using the approximate method for the catalytic currents, nickel(II) and cocarboxylase-thiol are shown to form the catalytic complex in the relationship 1:1.