Comparison of fundamental and second-harmonic a.c., and normal,derivative and differential pulse linear-sweep and stripping voltammetric methods

Linear-sweep and stripping a.c. and pulse voltammetric methods have been compared for a variety of electrodes and electrode processes. Each of the linear-sweep techniques is readily used systematically because, in contrast to d.c. linear-sweep voltammetry, the theory for reversible electrode processes is basically analogous to that for polarography at a dropping mercury electrode. In stripping analysis, some departures are found at a hanging mercury drop electrode because of spherical diffusion effects. For reversible electrode processes, the limits of detection for a.c. and pulse methods are comparable. However, a.c. methods offer advantages over pulse methods in discriminating against irreversible electrode processes and permit the ready use of faster scan rates. Pulse methods are more sensitive for irreversible electrode process. Normal pulse polarography is particularly favourable in minimizing undesirable phenomena arising from adsorption or deposition of material on electrodes.     

Direct determination of pentachlorophenol by differential pulse polarography

Differential pulse polarography at the dropping mercury electrode and differential pulse voltammetry at the carbon paste electrode are used for direct determinations of pentachlorophenol at concentrations down to 0.27 ppm. PCP is electrochemically reduced in phosphate buffers of pH 8 to produce a concentration-dependent current peak at —0.8 V vs. Ag/AgCl. The procedure requires only 15 min. Cyclic voltammetry at the hanging mercury drop electrode is used to evaluate the electrochemical reaction and to establish the reversibility of the PCP electrode reaction.     

Voltammetric study of the electrochemical reduction of lucigenin in aqueous medium

The electrochemical reduction of lucigenin (bis-N-methylacridinium nitrate) in aqueous solution was studied by d.c. Tast polarography at a dropping mercury electrode, by cyclic voltammetry at an amalgamated gold electrode and at a hanging mercury drop electrode, and by microcoulometry. The effects of pH, lucigenin concentration and temperature were studied, and special methods were applied to study the suspected adsorption and catalytic (regeneration) currents. A spectrophotometric study is also reported. It was found that lucigenin is reduced in two separate one-electron steps. An adsorption prewave accompanies the first step, while the second, below pH 3.5, is catalytic, owing to the chemical regeneration of the intermediate reduction product at the electrode surface.     

High flow-rate cells for continuous monitoring of low concentrations of electroactive species by polarography and stripping voltammetry at the static mercury drop electrode

A high-capacity flow-through cell which can be used at a maximum flow rate of 300 ml min^-1 has been developed for continuous monitoring of electroactive substances. The cell is compatible with the recently developed static mercury drop electrode. Comparative studies with a cell employing a conventional dropping mercury electrode are described. A wide range of polarographic techniques is applied, and it is demonstrated that the static mercury drop electrode improves the limits of detection, that laminar flow conditions are essential for low noise levels of operation, and that solution flow through a sulphite bed is a more effective method of oxygen removal than nitrogen bubbling. The combination of a microprocessor-controlled polarographic system, static mercury drop electrode and high-volume flow cell is very versatile for the determination of trace levels of electroactive species in flow streams. Preliminary results on anodic stripping voltammetry in flow streams are reported.     

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.     

Evaluation of mercury coated glassy carbon electrodes for oscillographic polarography

The suitability of a mercury film electrode formed over a glassy carbon substrate for oscillographic polarography has been studied. The data obtained show that it is possible to get well-developed oscillograms characteristic of the depolariser (in the concentration range 10^∮5–3×10^?4M) in solution which are similar to those obtained with a dropping mercury electrode. For quantitative analysis the mercury film electrode has been found to have limited applicability since the indentations have been found to be time-dependent.     

The flowing mercury electrode in polarography. part I

A polarisable electrode of constant surface area is described in which the surface layer of a column of mercury is continuously renewed from an internal mercury supply. Reproducible diffusion currents are obtainable which do not exhibit maxima. Wave heights are proportional to the concentration of reducible ion. The mercury consumption is similar to that of the dropping mercury electrode. The electrode and cell are constructed throughout in polyethylene.     

Polarography��past, present, and future

Polarography was first developed as an automated method of voltage-controlled electrolysis with dropping mercury electrode. The spontaneously renewed pure electrode surface provided reproducible electrochemical results which enabled scientists to work out adequate theory and rich analytical applications. The original method was then instrumentally modified in various ways. Later, hanging mercury drop was added as an alternative indicator electrode??in this way, polarography turned formally into voltammetry with mercury drop electrodes. Beside, in potential-controlled electrolysis, the mercury drop electrodes have been also used in current-controlled electrolysis (chronopotentiometry)??there, it has provided new experimental effects. Polarography has thus gradually covered a wide field of electrolytic methods based on the use of mercury electrodes, in which it continues developing.     

Differential pulse polarography and voltammetry with a microprocessor-controlled polarograph and a pressurized mercury electrode

The performance of a microprocessor-controlled polarograph with a pressurized mercury electrode system has been evaluated. For the technique of differential pulse polarography, the theory applying to the pressurized mercury electrode in the dropping mercury format is shown to be the same as for a conventional gravity-controlled mercury electrode system. At the short drop times used (0.2–0.4 s), faradaic distortion terms are shown to influence the shape of the observed differential pulse polarograms. A substantial decrease in sensitivity is also incurred in using these short drop times, compared with the longer ones generally employed in differential pulse polarography. Results for differential pulse anodic stripping conform to the usual expectations.     

Reduction of complexes in unbuffered solutions a unified treatment of the reduction of complexes at a stationary planar electrode or at an expanding mercury electrode

A general treatment is given of an electrode reaction connected with complexation at low ligand concentrations in unbuffered media. The reduction of the simple or complex metal ions occurs at an electrode expanding in accord with some power law; the stationary (solid or mercury) electrode and the dropping mercury electrode are special cases. The ligand is added to the solution as a j-protic acid. Linear semi-infinite diffusion is regarded as the sole mode of transport of all dissolved substances.The solution found, being of a very wide generality, was applied to the case of a potentiostatic regime of electrolysis on a dropping mercury electrode, as well as to the case of a galvanostatic regime on a stationary electrode. The voltammetric relationships obtained embrace all known equations of polarographic and chronopotentiometric reduction of simple and complex metal ions.The theoretical polarographic I—E curves in buffered and unbuffered solutions are presented graphically and compared. Apparently, the changes in the surface proton concentration cause a stretching of the wave.     

Mechanism and analytical aspects of the polarographic maximum wave of tellurium

The polarogram of tellurium(IV) in weakly basic solution has a sharp maximum on the diffusion current plateau. The electrode process causing this maximum has been examined by means of various techniques such as semiconductor catalysis, "block" polarography and ultraviolet irradiation of the dropping mercury electrode. The maximum was found to be caused by the catalytic wave of hydrogen produced by the tellurium deposited on the mercury surface. Part of the tellurium is reduced to hydrogen telluride by nascent hydrogen formed catalytically. The brown mist which usually appears in the vicinity of the surface of the dropping mercury cathode consists of elemental tellurium produced by the decomposition of the hydrogen telluride near the electrode. The sudden decrease in the current maximum at a potential more negative than the peak potential of the maximum is due to desorption of tellurium, caused by movement of mercury.     

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.     

On the shape of the reversible d.c. polarogram complicated by reactant and/or reaction product adsorption at the surface of the dropping mercury electrode

An elaboration is made of the consequences of reactant or product adsorption for the shape and situation of d.c. polarographic waves at the dropping mercury electrode. The treatment is based on the diffusion-layer model. As an example the Langmuir isotherm is adopted, but the results will be representative in a more qualitative way also for other isotherms. Detailed features of the polarograms under various circumstances are given. It is concluded that the polarograms can be analyzed precisely if only one component is adsorbed. If both components are adsorbed, it may occur that the information concerning the adsorption cannot be obtained from the polarogram. The adsorption may even seem to be absent.     

The applicability of phase-sensitive alternating current measurements in flow-through detection

The applicability of phase-sensitive alternating current (a.c.) measurements in flowthrough detection is discussed. The differences between flow and non-flow conditions are examined for copper(II) and thiourea at a horizontal dropping mercury electrode in an “open system”. The possibility of using this detection system in flow-injection procedures is indicated. A glassy-carbon electrode cell is used to test the feasibility of a.c. measurements with hydroquinone as the example.     

The analytical application of square-wave voltammetry

The effects of changing certain parameters of the applied potential waveform at a dropping mercury electrode (DME) in square-wave voltammetry (s.w.v.) were investigated and compared with theory. Optimum parameters of the waveform are determined for a reversible system, lead(II) in 0.10 M perchloric acid. Current sensitivities for Pb(II) at a static mercury drop electrode by both the integrating and instantaneous current-measurement schemes available in a laboratory computer-based system are compared to those obtained from a commercial instrument suitable for s.w.v. An irreversible system, Ni(II) in 0.10 M HCl, was investigated at the static mercury drop electrode and optimum parameters determined as for Pb(II).     

Determination of alkylphenol- and alkylalcohol-based non-ionic surfactants by a derivative chronopotentiometric method

The adsorption of surfactants at a dropping mercury electrode and at a hanging mercury drop electrode has been studied. The DME is more suitable for analytical purposes. For quantitative work the size of the indentation in the dE/dt-curve at the potential of desorption is used. A crude classification of surfactants is possible on the basis of the form of the dE/dt-curve.     

The analytical performance of direct current,normal pulse and differential pulse polarography with static mercury drop electrodes

The recently developed static mercury drop electrode (SMDE) provides a fundamentally new approach to electrodes for polarography. An analytical evaluation of the electrode is presented. For a range of electrode processes, current-sampled d.c. polarography at the SMDE is useful down to at least the 10^-7 M concentration level when short drop times and fast potential scan rates are used. The improvement in the limit of detection for d.c. polarography is therefore very substantial. Improvements in sensitivity associated with normal pulse and differential pulse polarography at the SMDE compared with the dropping mercury electrode (DME) are marginal. It is concluded that at the SMDE, the analytical performance and response characteristics of d.c., normal pulse and differential pulse polarography tend to converge.     

Étude du comportement polarographique du zirconium en milieu acide et en présence. d'lons nitrate

In the presence of zirconium, one observes a catalytic reduction wave of nitrate ions at the mercury dropping electrode. This wave can be used for the quantitative determination of nitrate ions. The mechanism of the reduction is explained.     

Study of the electrochemical behaviour of amphetamine and its derivatives in aqueous solution

The polarographic behaviour of amphetamine-type drugs in acid and alkaline solutions was investigated. Differential pulse polarography (DPP) with a dropping mercury electrode (DME) was used to monitor these drugs in aqueous solution. DPP and cyclic voltammetry (CV) were also utilised to study the electrode process of some 2-phenylethylamines derivatised with 4-N,N-dimethylaminobenzaldehyde (Ehrlich’s reagent) and 2,4-dinitrofluorobenzene, known as Sanger’s reagent, respectively.     

The polarographic and voltammetric behaviour of acetylacetonato and hexafluoroacetylacetonato complexes in acetonitrile

The polarographic behaviour of Ce(acac)₄, Ce(acac)₃, Eu(acac)₃, Fe(acac)₃, Cr(acac)₃, Co(acac)₃, Mn(acac)₃, NaMn(acac)₃, Mn(acac)₂, Ni(acac)₂, Cu(acac)₂, VO(acac)₂, Fe(hfacac)₃, Cr(hfacac)₃ and Cu(hfacac)₂ has been studied in acetonitrile on the dropping mercury electrode. Half-wave potentials versus bisbiphenylchromium(I)/(0), the reversibility of the electrode reaction and the number of electrons participating in the electrode processes measured by coulometry are reported. Cyclovoltammetric measurements have been performed on the hanging mercury drop electrode and on the stationary platinum electrode, the data of these studies are given. quite different behaviour has been observed on the platinum electrode compared to the dropping mercury electrode. Large scale electrolysis was employed to obtain information on the reaction products. The influence of the electrode material and the reaction mechanisms are discussed.