Theory of a.c. voltammetry at a rotating disk electrode: Part III. Redox-electrode reactions coupled with first-order chemical reactions

A theory of a.c. voltammetry at a rotating disk electrode is developed for systems with coupled first-order chemical reactions. A general expression for a.c. waves, together with that for d.c. waves, is derived. It is shown that the effect of hydrodynamics on the phase angle is negligible under the condition of p≤0.35, where p is a parameter defined by the frequency of of a.c. wave, the rotation speed of the electrode and Schmidt number.     

Electrochemical and microgravimetric studies of poly[3,4-ethylenedioxythiophene]-tyrosinase biocomposite material electrodeposited onto gold electrodes by a sinusoidal voltages method

The electrodeposition of poly(3,4-ethylenedioxythiophene)-tyrosinase (PEDOT-Ty) biocomposite material onto gold electrode has been achieved by means of a sinusoidal voltages (SV) method. The SV method consists in the superimposition of a sinusoidal voltage (sin wave) with fixed frequency and amplitude onto a d.c. potential. The influence of electrochemical parameters like frequency and amplitude of SV signal, d.c. potential value, deposition time, on the electrodeposition process has been investigated. The biocomposite material has been prepared as a thin layer onto quartz crystals coated with gold. The frequency change of the quartz crystal during the electrodeposition of the biocomposite material was recorded simultaneously with the measured current response. The morphology of the deposited PEDOT-Ty coatings was investigated by scanning electron microscopy. The electrochemical behavior of the biocomposite material in aqueous solution was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The use of d.c. potential value of 0.6 V revealed the contribution of the SV component to the electrodeposition process. The PEDOT-Ty modified electrode was used as an electrochemical biosensor in the voltammetric determination of dopamine in the presence of hydroquinone. The analytical performances of the prepared biosensors were also investigated.     

A.C. Cyclic voltammetry: A digital simulation study of the slow scan limit condition for a reversible electrode process

Rate laws presented to date for analysis of a.c. cyclic voltammetric data have invoked the so-called “slow scan limit approximation” which requires that ΔEω ? v, where Δ E and ω are the applied a.c. potential amplitude and angular frequency, respectively, and v is the d.c. potential scan rate. To provide a more useful guideline for the experimentalist than this qualitative condition, a pure digital simulation approach has been used to compute the a.c. cyclic time domain waveform for a reversible process under small amplitude conditions. The a.c. content of this waveform is extracted by the digital FFT alogirthm. Results of this study are presented here. Among the conclusions reached are more quantitative limitations for the slow scan limit rate laws describing the fundamental and second harmonic responses (approximately 128 a.c. cycles/d.c. cyclic sweep and 512 a.c. cycles/d.c. cyclic sweep, respectively) and an interesting prediction that the latter limitations can be relaxed further by a current waveform subtraction strategy, to as low as about 16 a.c. cycles/d.c. cyclic sweep for the fundamental and second harmonics. The cycles/sweep values assume one triangular wave potential scan of ±200 mV is encompassed.     

Control of d.c. bias current in radio-frequency glow discharge source and its emission characteristics

A novel technique to control r.f.-powered Grimm-style glow discharge plasmas is described. A new channel of d.c. current driven by the self-bias potential is opened by using a low-pass filter circuit and a load resistor; as the result, a large number of electrons can flow along the d.c. circuit channel including the plasma body from the grounded electrode to the sample electrode. This phenomenon is effective for improvement of the detection sensitivity in the optical emission spectrometry. Atomic emission lines having lower excitation energies are predominantly enhanced by a factor of 10–20. The conduction of the d.c. bias current could promote these excitations.     

Polarographic determination of chlorhexidine in pharmaceutical preparations

The electroreduction of chlorhexidine has been studied by d.c, a.c. and pulse polarography. Polarograms of the drug recorded from ammonium acetate buffers exhibit a single well-defined wave. The current is diffusion-controlled and proportional to the concentration. The reduction wave is due to an irreversible 8-electron reduction of the four> C=NH groups in the molecule to amino groups. The drug is strongly adsorbed on the electrode surface over a considerable potential range. Hence, the drug can be determined by polarography in the presence of other weaker surfactants often present in pharmaceutical formulations. Procedures have been proposed for pulse-polarographic determination of the drug in antiseptic cream and liquids. The proposed method is simple and accurate and does not involve time-consuming separation of the drug from insoluble constituents present in the sample.     

Polarographic currents in potassium peroxydisulphate-alkali halide systems and the determination of peroxydisulphate ion

Potassium peroxydisulphate-alkali halide systems have been studied by d.c. and a.c. (sine wave) polarography. When the mercury drop of the mercury electrode becomes covered by a film of adsorbed bromide or iodide ions, a peak due to peroxydisulphate ion is found in the a.c. polarogram at a potential more negative than that of the halide peak. This peak is probably due to a product of a chemical reaction between the peroxydisulphate ion and halide ion close to the surface of the DME. The determination of the peroxydisulphate ion by a.c. polarography is described.     

Reduction of cytochrome c at a lipid bilayer modified electrode

The reduction of horse heart cytochrome c has been investigated at a platinum electrode modified with a lipid bilayer membrane (BLM) which immobilized vinyl ferrocene as an electron mediator. The current—voltage curves show that the direct electrochemistry of cytochrome c at the metal electrode occurs quite efficiently. An adsorption equilibrium constant for cytochrome at the BLM surface, as well as an electron transfer rate constant between the protein and the modified electrode have been estimated from these results. The values of both parameters are much higher than those reported with other types of electrode modifications, indicating that a lipid bilayer-modified platinum electrode system using vinyl ferrocene as a mediator provides substantial improvements in electrochemical activity of cytochrome c at metal electrodes. The potential for modifying and utilizing this new class of “biomembrane-like” electrode surface for metalloprotein electrochemistry is briefly discussed.     

Electrochemical reduction of the lanthanide ions: Part I. First reduction step of ytterbium(III) in acidic 1 M NaClO4 solution

The reduction of Yb(III) to Yb(II) in 1 M NaClO₄ in the pH range 1.9–6.6 was studied by d c. polarography, cyclic voltammetry and electrode impedance measurements as a function of frequency and electrode potential. It results that the d.c. reversible reduction is followed by a homogeneous chemical reaction and is accompanied, by an irreversible process which is attributed to a lowering of the overpotential of the reduction of the hydrogen ions. Values of the rate constant and transfer coefficient pertaining to the charge transfer step were deter mined.     

Study of space charge phenomena in d.c. electron-capture detectors

The influence of space charge phenomena on the field and the potential in a d.c. electron-capture detector (ECD) under simplified conditions was studied. The results make it possible to explain the shape of the current-voltage characteristics of a d.c. ECD and the dependence of the sensitivity of such a detector on the electrode distance with positively and negatively polarized sources. All of these dependences studied using an ECD with variable geometry (electrode distances between 0.25 and 30 mm) and a changeable radioactive source (³H, ⁶³Ni, ²⁴¹Am).     

Components of the Net Current in Differential Pulse Polarography. Part 2. Kinetics and Adsorption

Components of the net response in differential pulse polarography were studied theoretically, assuming gradually changing electrode rate constant or strength of the reactant adsorption. The difference between the maximum potential of the peak component and the half‐wave potential of the wave component appear as an important parameter. From its value, the electrode rate constant can be calculated even when the standard potential is not known. In a reversible process, effects of the reactant adsorption on the mentioned separation are less pronounced. The results were compared with experimentally obtained effects. For deeper insight into the applicability of this approach, additional experimentally obtained polarograms (that reflect different potential/timing parameters or changing character of the electrode process) should be studied in terms of their components.     

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.     

Alternating current polarography of some complexes in presence of ionic surfactants

Well defined a.c. polarographic waves are obtained in the presence of certain ionic surfactants. The current is brought about by an electron-transfer through the adsorbed layer, probably by the formation of a bridge which accelerates the electrontransfer between the electrode and the depolarizer. The resulting a.c. wave is accompanied by a d.c. polarographic step and the height of the wave increases proportionally to the bulk concentration of the depolarizer. In contrast to ordwary a.c, waves, the a.c. current of these waves increases linearly with increasing height of the mercury column above the capillary.     

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.     

Determination of nimesulide in human serum using a glassy carbon electrode modified with SiC nanoparticles

A glassy carbon electrode (GCE) was modified with silicon carbide nanoparticles and used to investigate the electrochemistry of the drug nimesulide via voltammetry and chronoamperometry. The structure of the modified electrode was studied by field emission scanning electron microscopy. Nimesulide undergoes electroreduction at pH 2 at a potential that is shifted from ?526 mV (at the bare GCE) to ?387 mV at the modified electrode. Simultaneously, sensitivity is increased by a factor of 5.8. The charge transfer coefficient, diffusion coefficient, standard heterogeneous rate constant and catalytic reaction rate constant were determined. A plot potential vs. pH revealed a voltammetric pKa value of about 6.5–7.0. The differential pulse voltammetric calibration plot for nimesulide is linear in 0.09–8.7 μM concentration range, and the detection limit and sensitivity are 30 nM and 512 nA.μM^?1, respectively. The modified electrode was applied to the determination of nimesulide in acidic solution and human blood serum samples without further pretreatment. The recoveries, as determined by the standard addition method, range from 95.7 to 98.7%, with an RSD of around 1.6%.

Parameters affecting the determination of mercury by anodic stripping voltammetry using a gold electrode

The electrochemical determination of aqueous Hg(II) by anodic stripping voltammetry (ASV) at a solid gold electrode is described. The aim of this work is to optimise all factors that can influence this determination. Potential wave forms (linear sweep, differential pulse, square wave), potential scan parameters, deposition time, deposition potential and surface cleaning procedures were examined for their effect on the mercury peak shape and intensity. Five supporting electrolytes were tested. The best responses were obtained with square wave potential wave form and diluted HCl as supporting electrolyte. Electrochemical and mechanical surface cleaning, aimed at removing the amount of mercury deposited onto the gold surface, were necessary for obtaining a good performance of the electrode. Response linearity, repeatability, accuracy and detection limit were also evaluated.     

Para-phenylenediamine oxidation at a platinum electrode in acetonitrile solutions

p-Phenylenediamine oxidation at platinum electrodes in acetonitrile solutions has been studied under a very wide range of experimental conditions. Chronopotentiometry, rotating disc electrode and cyclic voltammetry were used as electrochemical techniques. Coulometry at constant potential and product analysis were also performed.The electrochemical reaction appears as a fast and reversible one electron exchange per molecule of PPD. The electrode reaction is further complicated by follow-up chemical reactions giving unknown products in the bulk of the solution.The whole polarization curve under steady state conditions shows two waves, while under non-steady state conditions a small wave at intermediate potentials is also apparent.The reaction pathway for the first wave was interpreted as a non-conventional e.c.e. mechanism where the parent molecule acts as a base in the chemical step.These assumptions were confirmed through experiments performed with pyridine or water addition.     

An integrated instrumental and theoretical approach to quantitative electrode kinetic studies based on large amplitude Fourier transformed a.c. voltammetry: A mini review

Herein, we emphasise the main advantages provided by Fourier transform assisted a.c. voltammetry over d.c. methods in quantitative electrode kinetic studies and report on the progress with integration of the a.c. approach with contemporary computer-assisted automated data analysis methods. In the mechanistically sensitive version of Fourier transformed a.c. voltammetry emphasised in this mini review, a large amplitude sinusoidal or another time dependent periodic function is superimposed onto the d.c. potential ramp. Using a sequence of Fourier Transform and filtering operations, both experimental and simulated data for the assumed mechanism are resolved into the aperiodic (d.c.) and a.c. harmonic components. Comparison of experiment and theory by heuristic or computer-assisted automated methods allows quantitative estimates to be provided of the relevant thermodynamic and kinetic parameters as well as of uncompensated resistance, double layer capacitance and other parameters present in the electrode kinetics model.     

A study of potentiostat noise

This paper deals with a generalized version of the galvanostatic reversed coulostatic pulse method proposed by van Leeuwen and Sluyters. The potential of the working electrode is stepped to an appropriate potential by any kind of d.c. perturbation and then allowed to relax. The relaxing potential is perturbed again at an arbitrary time by a coulostatic pulse with polarity opposite to that of the first perturbation. The charge of the coulostatic pulse is adjusted so that the potential vs. time curve may have a horizontal tangent at an appropriate time after the end of the pulse. The overpotential at that time is known from the potential vs. time curves obtained with and without the coulostatic pulse. Analysis of experimental results is made on the basis of the rigorous solution for the boundary value problem of diffusion. The proposed procedure is tested with success by experiments on the trisoxalatoferrate(III)/(II) electrode and the corresponding equivalent circuit of Randles type. Its applicability to such an electrode reaction as involves an unstable product is suggested.     

Kinetic-coulometric determination of mercury in biological samples

A simple approximate calculation method is given which permits determina-tion of the maximal scan rates of potential allowable for distortion-free recording of current-voltage curves with X-Y recorders. For the calculations only the response time of the recorder, the wave shape and the scan rate of potential need be known. Stationary mercury electrodes and rapid polarography with a dropping mercury electrode at controlled drop times were examined. Electroanalytical implications are discussed, with particular emphasis on the rapid a.c. polarographic method with short controlled drop times and on stationary-electrode fundamental and second harmonic a.c. voltammetry. Theoretically and experimentally it has been shown that an X-Y recorder with 0.5–1.0-s response time can be used for scan rates up to about (50/nt') mV s^-1 with a.c. techniques and about (100/nt') mV s^-1 with d.c. polarography (t'= response time of recorder, n = number of electrons).     

Radial particle distributions in a d.c. arc—maximum off-axis

It is shown that for some elements evaporated from the anode of a d.c. arc the position of the maximum concentration of atomic particles (i.e. atoms + ions) occurs not on the axis of the arc column but some distance away from the centre, this distance being largely determined by the inside diameter of the electrode crater. It is shown that the magnitude of the off-axis peak increases with increasing volatility of the element concerned, but decreases with increasing electrical power generated in the plasma as well as with decreasing ionisation potential of the buffer metal. The proposed mechanism for this phenomenon is based on the fact that, due to thermal repulsion, cool vapours do not readily mix with hot gases. Thus volatile sample components evaporating from regions not immediately beneath the anode spot would tend to diffuse laterally around the arc rather than vertically into the hot plasma.