Current scanning polarography at the dropping mercury electrode

An instrumentation for obtaining current-scanning polarograms is proposed and the resulting polarograms arc discussed. The proportionality of the limiting current to the concentration, the agreement of the half-wave potential with that of the ordinary polarography and the influence of external resistance on the relations between current, potential and time are described. In potentiometry at constant current with the dropping mercury electrode, the electrode potential should be measured at the maximum size of the mercury drops.     

Effect of methylviologen on the hydrogen evolution at mercury electrodes in aqueous buffer solutions

Methylviologen reduces the hydrogen overvoltage and gives a catalytic hydrogen wave at the dropping mercury electrode in aqueous buffer solutions (pH 3–7). The electrochemical behaviour of the catalytic hydrogen evolution was studied by dc and pulse polarography, potential-step chronoamperometry, and constant potential electrolysis. A possible reaction scheme has been proposed to explain the experimental results, in which the reduced neutral form of methylviologen MV⁰ is assumed to be oxidized to a monocation radical MV^·+ by the weak acid component in the buffer solution. Generation of MV^·+ during the hydrogen evolution is supported by the visual observation of a blue colour appearing at the surface and also in the vicinity of the electrode when a large catalytic current is flowing in dc polarography and constant potential electrolysis.     

Ellipsometry: The effect of atomic roughness on the optical properties of gold electrodes covered with a monolayer of thallium

The polarographic behavior of iodine in aprotic solvents has been investigated. Iodine gave two well-defined cathodic waves in aprotic solvents at either a platinum or a mercury electrode. At the dropping mercury electrode, the reduction wave of iodine corresponded to the reduction of mercuric iodide, formed at the electrode surface by the reaction of iodine with mercury. It was found that the difference in the half-wave potential between the first and second waves depends upon the stability of an intermediate, triiodomercurate ion in the solvents.In some aprotic solvents and water-mixed solvents, the solvent effects on the reduction potential of iodine and the formation of the complex ion, triiodomercurate ion, were discussed with the dropping mercury electrode.     

Application of the rotated dropping mercury electrode to the analysis of mixtures of electroactive substances

The use of poylacrylamide is recommended instead of gelatin as an ideal maximum suppressor to be employed with both the rotated dropping mercury electrode (RODE) and conventional dropping electrode (DME). It is retained at the mercury surface over the entire potential range and does not combine with heavy metals. Reproducibility of the measurement of the residual current and that of the limiting current at the RDME were studied, and it was found that accurate determinations of a single constituent (±2%) are possible at concentrations as low as 1 to 2.10^-5M. The variation of the limiting current with potential is much greater at the RDME than at the DME. This effect must be considered in the analysis of a mixture of electroactive species at the RDME. Method have been discussed for making the proper correction for a preceding wave when a limiting current of a mixture of constituents is measured. Because of its high sensitivity and good reproducibility of results, the RDME is recommended for the analysis of solutions containing one or more electroactive species at concentrations less than 10^-4M.     

Computer calculation of activation energies of irreversible electrode processes at a dropping mercury electrode

A computer program has been written for calculation of the activation energies of irreversible electrode processes at a dropping mercury electrode, based on the variation of current and half-wave potential with temperature. The program calculates the half-wave potential, the transfer coefficient, and the activation energies of the diffusion process and the irreversible electrode process. It has been written in both ALGOL and FORTRAN.     

Zur Frage Der Reversibilitát Der Sauerstoffreduktion an Quecksilberelektroden in Losungen von Alkalihydroxiden

Abstract

Oxygen formed electrolytically from hydrogen peroxide at the surface of the dropping mercury electrode was determined and the charge needed for its electrolysis measured. The yield is about 50% of the theory, which indicates oxide formation. This is supported by formation of black precipitate on dropped off mercury. Reduction of oxygen in alkaline media is thus not a simple reversible process, but complicated by mercury oxide formation, similarly as on solid electrodes.     

Inverse polarography with stationary amalgam anodes: Basic principles and technique

A new method of electrochemical analysis has been devised in which the metallic ions present in an electrolyte are discharged into a mercury cathode. The amalgam electrode so formed is then combined with a standard reference electrode in a suitable dissolution medium and the cell thus produced is allowed to supply current against an opposing voltage set up on a potentiometer. By decreasing the potentiometer voltage in stages a curve of current against potential can be constructed for the dissolution of the amalgam anode; in this curve each metal gives rise to a characteristic wave from the height of which the amount of metal originally present can be derived. Although the method is not so universally applicable as the conventional polarographic technique with the dropping mercury cathode, it possesses advantages in sensitivity and simplicity in suitable cases and may form a useful complementary method of analysis.     

Polarographic study of hydrogen peroxide anodic current and its application to antioxidant activity determination

Behavior of hydrogen peroxide in alkaline medium has been studied by direct current (DC) polarography with dropping mercury electrode (DME) aiming to apply it in antioxidant (AO) activity determination. Development of a peroxide anodic current having form of a peak, instead of common polarographic wave, has been investigated. As a base for this investigation the interaction of H₂O₂ with anodically dissolved mercury was followed. Formation of mercury complex [Hg(O₂H)(OH)] has been confirmed. The relevant experimental conditions, such as temperature, concentration and pH dependence, as well as time stability of hydrogen peroxide anodic current, have been assessed. Development of an AO assay based on decrease of anodic current of hydrogen peroxide in the presence of antioxidants (AOs) has been described. Under optimized working conditions, a series of benzoic acids along with corresponding cinnamate analogues have been tested for hydrogen peroxide scavenging activity. In addition, the assay versatility has been confirmed on various complex samples.     

Determination of critical micelle concentration of non-ionic surfactants by electrocapillary curves

A method is described for the determination of the critical micelle concentration of non-ionic surfactants by measurement of their effect on the electrocapillary curves for a dropping mercury electrode. The method was compared with the iodine-solubilization, surface tension, and polarographic maximum suppression methods.     

Digital Harmonic Analysis of Cadmium Ion Reduction on a Dropping Mercury Electrode

The cadmium ions reduction on a mercury electrode is investigated. A sinusoidal voltage perturbation signal, superimposed on a linearly changing dc potential, is used. A total time–frequency analysis of the polarographic current of cadmium ions reduction on a dropping mercury electrode is performed. Classical Fourier transform leads to decomposition of the polarographic current into its harmonics. The current harmonics determined by Fourier transform are potential-independent. The STFT transform of the polarographic current also leads to its decomposition into harmonics. However, application of the STFT transform correctly reflects changes in the current harmonics as the function of potential.     

Prototypical Nonelectrochemical Method for Surface Regeneration of an Integrated Electrode in a PDMS Microfluidic Chip

Abstract

A prototypical method for surface regeneration of an integrated electrode in a microfluidic chip is demonstrated. A platinum wire as working electrode was mounted in a polydimethylsiloxane chip vertical to the chip through the channel. The regeneration of the electrode was easily achieved by drawing the platinum wire out for 5 mm, because the area exposed to the channel or the stream would be altered. With continuous motion, the wire electrode can maintain a fresh surface just like a dropping mercury electrode. The current–time curve and open circuit potential (OCP) of dopamine solution show the performance of this prototypical system.     

Determination of Tellurium by Gas Phase Molecular Absorption Spectrometry After Hydrogen Telluride Generation

Abstract

A method for determination of tellurium(IV) or tellurium(VI) is described that involves hydrogen telluride generation by reduction with sodium tetrahydro-borate(III), evolution of hydride with HCl solution, transport into a flow-cell placed in a UV-visible molecular absorption spectrophotometer and measurement of gas at 190 nm. Hydride generation and determination procedures are optimized, based on height and area of absorbance versus time profile of hydrogen telluride generated. Using the best experimental conditions found the calibration curve is linear from 5 to 100 μg/ml of tellurium, 1.1 μg/ml of tellurium(IV) or 3.1 μg/ml of tellurium(VI) can be detected, and relative standard deviation ranges from 4 to 7%. The method is applied to the analysis of additives for synthetic rubber making.     

Electrochemical behavior of kryptand 222 on a mercury electrode

The electrochemical behavior of kryptand 222 at the interface between a mercury electrode and an aqueous solution is studied by the impedance spectroscopy, polarography, and cyclic voltammetry methods. It is established that kryptand 222 possesses high surface activity at this interface. Adsorption parameters of kryptand 222 are found on the basis of the model of two parallel capacitors supplemented by the Frumkin isotherm by the regression analysis method. The potential dependences of the differential capacitance (C,E curves), calculated using these parameters satisfactorily agree with experiment. It is shown that the maximum in the C,E curves in the region of negative potentials is caused by catalytic process of hydrogen evolution.     

Factors to be considered in quantitative polarography with the rotated dropping mercury electrode

The various factors which affect limiting currents at the rotated dropping mercury electrode (RDME) are reviewed. The current at the RDME in the absence of surface active substances involves maxima of the second kind, even under conditions where these maxima are not observed at the conventional dropping mercury electrode (DME). For most practical purposes, it is necessary to eliminate the maxima completely by adding a suitable surface active compound. The presence of 0.01% polyacrylamide is recommended as a maximum suppressor in all polarographic work. At a suitable speed of rotation (200 to 300 r.p.m.) the sensitivity of the RDME in the presence of a surface active substance is about ten times as great as that of the DME.Equation (3), derived in a previous paper² accounts quantitatively for the relation between limiting current in the presence of a suitable maximum suppressor on the one hand and the concentration and diffusion coefficient of electroactive species, speed of rotation, kinematic viscosity and the characteristics of the electrode (m and t) on the other. The limiting current is practically indepndent of the pressure of mercury, but varies with t^a, where a ? 0 5.For reversible reactions half-wave potentials at the RDME are of the same order of magnitude as observed at the DME. The slight difference observed in the reduction of metal ions to metal amalgams is accounted for quantitatively by the larger rate of transport of electroactive species to the RDME.The RDME is recommended for the analysis of solutions containing one or more electroactive species in concentration of less than about l0^-4N. For markedly higher concentrations the DME ib generally preferred.     

Electrogenerated chemiluminescence in mechanistic investigation of electroorganic reactions: Part IV. Luminescence oscillations at the falling mercury drop

In the intensity-potential curves of the electrogenerated chemiluminescence at the dropping mercury electrode spikes are observed, which are due to the reaction of excessive anion radicals A^?. at the falling mercury drop. The luminescence trace of the drop exhibits a characteristic intensity pattern. These oscillations arise from an interplay of a concentration difference of A^?. between the front and back sides or the falling drop, which is hydrodynamically caused and leads to a corresponding difference in the electrochemical potential and in the surface tension, and a surface movement of the mercury as the retroactive effect. The influence of the experimental conditions and the structural effect of the investigated cathodic luminescence systems on the oscillating behavior are studied.     

Catalytic wave of hydrogen in systems based on iron(II) nitrosyl complexes

The electroreduction of Fe(III) at a dropping mercury electrode in an acetate buffer solution containing NO ₂ ^- and NH ₄ ⁺ ions and some hydroxy acids was studied. Based on the fact that the current depends on a number of factors, it was concluded that the wave observed was catalytic wave of hydrogen. The proposed mechanism of the process includes the electroreduction of the Fe(III) complex, the formation of a mixed-ligandFe(II) complex, and its protonation and reduction at a dropping mercury electrode with the liberation of hydrogen. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EMA-99), Moscow, December 6–8, 1999.     

Polarographlc study of adsorbed tellurium at the hanging and dropping mercury electrodes in 1 m hydrochloric or perchloric acid solutions

Studies of tellurium(IV) reduction at a mercury drop electrode are difficult because the two-step electrochemical scheme is complicated by adsorption of elemental tellurium at the mercury surface. Anomalous i—t curves at the HMDE are presented in order to confirm the important rôle of the primary adsorbed monolayer and the unusual behaviour of elemental tellurium when the deposit is increased by more than this mono-layer.     

Kinetics and Mechanism of Electroreduction of Ammonia and Hydroxyammonia Complexes of Zinc(II) on a Dropping Mercury Electrode

It is found that the equilibrium potential of the Zn(Hg)/Zn(II) system depends on the concentration of ammonia molecules and solution pH. The dependence conforms to the literature data on the stability constants for ammonia and hydroxyammonia complexes of zinc. Their reduction on a dropping mercury electrode in solutions of pH 9.2–12 and [NH₃] = 0.05–2 M yields one irreversible cathodic wave with a diffusion limiting current. In dilute supporting electrolytes, the plateau of the latter is preceded by a maximum due to accumulation of insoluble reduction products on the surface of the mercury drop. The pH and [NH₃] dependences of the half-wave potential of waves that are undistorted by a maximum are analyzed with allowance made for a change in the composition of zinc(II) complexes in the bulk solution. According to the analysis, the slow two-electron electrochemical stage involves complexes Zn(NH₃)₂ ²⁺ that form from complexes present in solution in preceding reversible chemical reactions. The effect the supporting-electrolyte concentration has on the electroreduction rate of zinc(II) complexes and the mechanism of the electrochemical stage is discussed.     

Amperometric determination of thioacetamide with copper(I)

A method is given for the amperometric determination of thioacetamide (TAA) based on the reaction of TAA with copper(I) in ammoniacal solution. The reduction current due to the copper(I) is used to determine the end-point. The determination is carried out at constant temperature (25°), pH 8.0-9.8 (ammonia-urotropine buffer), and at constant potential (−0.6 V vs. S.C.E.), with a dropping mercury electrode as cathode and a saturated calomel electrode as anode. The solution is deaerated by a stream of electrolytically generated hydrogen. The titrations are carried out simply and rapidly and the results are fairly reproducible. The error is generally 1.0% for amounts of TAA ranging from 0.07 to 1.8 mg.     

The reaction of hydrogen chloride with bis (p-ethoxyphenyl)telluride,a possible route to pure tellurium

The reaction of hydrogen chloride with bis(p-ethoxyphenyl)telluride is investigated and shown to give essentially phenetole (C₆H₅COEt), tellurium, and bis(p-ethoxyphenyl)tellurium dichloride. Spectroscopic methods (UV–visible, ¹²⁵Te NMR) show that some bis(p-ethoxyphenyl)ditelluride is produced. This is believed to arise from a side reaction of organic telluride with an intermediate organyltellurenyl chloride which is considered to arise from initial proton attack at the Te–C bond of the telluride. The ditelluride reacts with HCl to deposit 80% of its tellurium content as the element; phenetole is the other major product. Use of a spin-trap reagent gave no evidence of radical intermediates. An attempt to develop a reaction sequence to prepare high-purity tellurium is described. Although a purity of 99.985% could be achieved, overall yields are not economic, unless very highpurity tellurium is required. The method described is, however, very effective for the removal of traces of tin, lead, arsenic and antimony and can significantly reduce the copper, iron and selenium content; for example, one treatment of an alloy (Te, 75%; Se, 25%) gave a sample of tellurium containing 40 ppm selenium.