A pyrolytic carbon film electrode for voltammetry-III Application to anodic-stripping voltammetry

The potential applicability of a pyrolytic carbon film electrode in the differential-pulse anodic-stripping voltammetric determination of cadmium and lead in sea-water is demonstrated. The performance at the 10⁻¹⁰M level is compared with that of a satisfactory glassy-carbon electrode. The two types of electrode display comparable behaviour in anodic-stripping voltammetry, but the pyrolytic carbon film electrode needs less pretreatment.     

Investigation by automated differential pulse anodic-stripping voltammetry of the problem of storage of dilute solutions

The storage of dilute solutions of metal ions before their laboratory analysis presents a difficult problem in the examination of many environmental samples. By utilizing the solution container as an electrochemical cell and employing the method of differential pulse anodic-stripping voltammetry at a hanging mercury drop electrode, it is shown that an automated read-out system in an enclosed environment can be developed for monitoring the solution-container interactions that occur over short or extended periods of time. In the present work, interactions of dilute solutions (1-10 mug/l.) of cadmium(II), lead(II), copper(II), zinc(II), and thallium(I) in glass, polyethylene and Teflon containers have been investigated at various pH values and in different ionic environments. The results demonstrate the importance of factors other than pH.     

Trace metal analysis by anodic-stripping voltammetry Effect of surface-active substances

The presence of cationic, anionic, and neutral surfactants and humic substances is shown to affect the peak heights of copper, lead and cadmium in synthetic sea-water analysed by differential pulse anodic-stripping voltammetry. At surfactant concentrations below 0.1 mg/l. the effect is insignificant, but at higher concentrations the peak heights usually decrease, although for copper an increase in the peak height was also observed. The peak heights do not depend to any great extent on the pH of the solution, except in alkaline solution and in the presence of humic substances. Adsorption and complex formation may account for the observed dependences.     

Determination of thallium in lead salts by differential pulse anodic-stripping voltammetry

The determination of trace levels of thallium in lead and lead salts by differential pulse anodic-stripping voltammetry has been made possible by using a surfactant as an electrochemical masking agent in addition to a complexing agent. In 0.2M EDTA at pH 4.5 as supporting electrolyte without surfactant, lead at concentrations below 0.5mM does not give a peak. When the electrolyte also contains tetrabutylammonium chloride (TBAC) at 0.01 M concentration, lead can be tolerated at concentrations up to 0.05M, while the height of the thallium peak is unaffected. It is thus possible to determine 5nM T1(I) in the presence of 0.05M Pb(II), i.e., Tl at the 1 x 10(-5)% level in lead. The precision of the determination (1-4%) and the recovery are satisfactory. Neither an 800-fold excess ratio of Cu(II) to Tl(I) nor a 10(7)-fold ratio of Bi(III) interferes in the determination. Thallium has been determined in a range of lead salts of various degrees of purity.     

Determination of arsenic in polluted waters by differential pulse anodic-stripping voltammetry

Experimental parameters affecting the analytical response of arsenic in differential pulse anodic-stripping voltammetry (DPASV) have been examined. DPASV offers higher sensitivity than linear-scan anodic-stripping voltammetry for similar analysis times. Both techniques have been applied to the NBS Standard Reference Water (SRM 1643) and some polluted water samples. The results on polluted waters compared favourably with those obtained by graphite-furnace atomic-absorption spectroscopy.     

Determination of trace impurities in high-purity reagents by mercury thin-film anodic-stripping voltammetry

Summary The use of an amalgamated silver-wire electrode, providing a relatively large rate of film area to volume of mercury, seems to present several advantages over the use of the hanging mercury drop electrode (HMDE) for the determination of trace impurities by anodic-stripping voltammetry (ASV). Peak resolution and sensitivity have both been improved by use of the mercury thin-film electrode (MTFE). Preparation, conservation and treatment of the electrode are simple, requiring brief etching of a silver wire with dilute nitric acid, followed by rinsing, drying and direct amalgamation with mercury. The MTFE is stored in mercury, the excess of which is wiped off before use; it is cleaned by anodic-stripping electrolysis at positive potentials. Under these conditions, ASV produces reproducible peak-heights, with excellent dependence on electrolysis time and concentration of impurities. Samples containing nanogram quantities or less of the impurities of interest, are analysed by a four-step procedure: (1) prolonged pre-electrolysis of the supporting electrolyte,in situ in the voltammetric cell, on an auxiliary MTFE; (2) ASV of the supporting electrolyte on a cleaned MTFE; (3) ASV after the dissolution of the sample; (4) ASV after the addition of standards. The method is suitable for the estimation of traces of Zn(II), Cd(II), Tl(I), Pb(II), Sb(III) and Cu(II) in various high-purity reagents. Most impurities have been adequately determined at the 10^–9–10^–8 M level by the regular procedure, and at the 10^–10–10^–9 M level may be determined if the preconcentration (electrolysis) time and rates of potential sweep are increased. The only disadvantage of the method is that the overvoltage of hydrogen evolution is lower for the MTFE than the HMDE.

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Zusammenfassung Die Verwendung einer amalgamierten Silberdrahtelektrode mit einer im Vergleich zum Quecksilbervolumen großen Oberfläche bietet offenbar verschiedene Vorteile bei der Bestimmung von Spurenbeimengungen durch Anodic-stripping Voltammetry (ASV) gegenüber einem hängenden Quecksilbertropfen (HMDE). Peakauflösung und Empfindlichkeit wurden mit Hilfe der Quecksilberdünnschicht-Elektrode (MTFE) verbessert. Herstellung, Aufbewahrung und Behandlung solcher Elektroden sind einfach; sie erfordern eine kurze Anätzung eines Silberdrahtes mit verd. Salpetersäure, Spülung, Trocknung und Amalgamierung mit Quecksilber. Diese Elektroden werden in Quecksilber aufbewahrt, dessen Überschuß vor der Verwendung weggewischt wird. Sie werden durch Anodic-stripping-Elektrolyse bei positivem Potential gereinigt. Unter diesen Bedingungen gibt ASV reproduzierbare Peakhöhen, die zu der Elektrolysezeit und der Konzentration der Beimengungen in gutem Abhängigkeitsverhältnis stehen. Proben, die Nanogrammengen solcher Beimengungen enthalten, werden nach einem Vierstufenverfahren analysiert: 1. verlängerte Vorelektrolyse des Trägerelektrolyten mittels einer Hilfs-MTFE im Voltammetriegefäß; 2. ASV des Trägerelektrolyten mit einer gereinigten MTFE; 3. ASV nach Auflösung der Probe; 4. ASV nach Zugabe der Standards. Die Methode eignet sich zur Bestimmung von Spuren Zn(II), Cd(II), Tl(I), Pb(II), Sb(III) und Cu(II) in verschiedenen hochreinen Reagenzien. Die meisten dieser Verunreinigungen wurden in der Größenordnung 10^–9 bis 10^–8 M mit Hilfe des regulären Verfahrens bestimmt. In der Größenordnung 10^–10 bis 10^–9 M können sie bei Verlängerung der Anreicherungszeit (elektrolytisch) und Erhöhung des Potentialbereichs bestimmt werden. Der einzige Nachteil des Verfahrens ist, daß die Überspannung zur Wasserstoffentwicklung geringer ist als mit dem hängenden Quecksilbertropfen.

     

A contribution to the use of chelating agents in anodic stripping voltammetry

The E(p)/(2), values of the ions Pb(2+), Bi(3+), Cu(2+), Cd(2+), Tl(+) were determined by means of cyclic voltammetry in solutions of acetate and Britton-Robinson buffers and in solutions containing EDTA and DCTA. DCTA was then utilized in the determination of small amounts of bismuth, down to 10(-7)M, in the presence of 10(4) times as much lead, and of traces of thallium down to 5 x 10(-9)M in the presence of 2 x 10(5) times as much cadmium by anodic stripping voltammetry. The precision was good. Further, the trace copper concentration in analytical grade DCTA was determined. A new flow-through vessel for stripping analysis with solution exchange is also described and the reproducibility of results without solution exchange, with manual solution exchange, and with the flow-through vessel is evaluated for a model determination of thallium in a DCTA solution.     

Evaluation of differential pulse anodic-stripping voltammetry at a stationary mercury-film electrode with stirred solution

Differential pulse anodic-stripping voltammetry at a stationary mercury-film electrode with the solution stirred during the deposition step has been investigated. The sensitivity achieved by using such a simple set-up is similar to that obtained with a mercury-film rotating disk electrode. The effects of various experimental parameters on the peak current are described. Lead and cadmium were used as test systems, and gave detection limits of around 1 x 10(-10)M with 5-min deposition times.     

Determination of thallium in bismuth by differential pulse anodic-stripping voltammetry without preliminary separation

The determination of trace levels of thallium in bismuth and bismuth salts by differential pulse anodic-stripping voltammetry has been made possible by using a surfactant as an electrochemical masking agent, in addition to a complexing agent. In 0.2M EDTA at pH 4.5 as supporting electrolyte in the absence of surfactant, bismuth at concentrations below 10(-4)M does not interfere. When the electrolyte also contains tetrabutylammonium ions at 0.01 M concentration, bismuth can be tolerated at concentrations up 0.05M, and the height of the thallium peak is unaffected. It is thus possible to determine 1 nM Tl(I) in the presence of 0.05M Bi(III), i.e., Tl at the 1 x 10(-6)% level in bismuth. The precision of the determination and the recovery are satisfactory. Neither an 800-fold ratio of Cu(II) nor a 10(7)-fold ratio of Pb(II) to Tl(I) interferes in the determination. Other cations such as Zn(2+), Cd(2+), In(3+), Hg(2+), Fe(3+), Sb(3+) and Sn(4+) in 10(4)-fold molar ratio to Tl(I) have no effect on the determination. Thallium has been determined in bismuth metal and in bismuth nitrate of various degrees of purity.     

Determination of cadmium by differentialn pulse anodic-stripping voltammetry after salting-out extraction into acetonitrile

A selective and specific method is presented for anodic-stripping voltammetric determination of cadmium after extraction with 0.1Mtetrabutylammonium iodide solution in acetonitrile from aqueous ammonium sulphate solutions. The detection limit of this method is 0.2 ng ml (in the acetonitrile extract). Interference from matrices or large amounts of elements reduced at more positive potentials can be eliminated by prior extraction. The method has been applied to trace analysis for cadmium in zinc, lead and indium metals, and some inorganic salts.     

Determination of nanomolar concentrations of lead and cadmium by anodic-stripping voltammetry at the silver electrode

Lead and cadmium have been determined by subtractive anodic-stripping voltammetry (SASV) in the square-wave mode at a silver electrode without removal of oxygen. The sensitivities and detection limits for the two metals differ considerably. Detection limits of 0.05 nM for lead and 1 nM for cadmium have been achieved following 90 s electrodeposition. The repeatability of consecutive SASV runs is good (for lead 0.5% at 20 nM for 30 s electrolysis, 5% at 0.3 nM for 60 s electrolysis; for cadmium 2.5% at 20 nM for 30 s electrolysis, 5% at 5 nM for 60 s). Hundreds of runs can be carried out without any pretreatment of the electrode. The high stability is attributed to renewal of the electrode surface that takes place during the electrodeposition step in a two-electrode cell: the silver counter/quasi-reference electrode generates silver ions that codeposit with lead and cadmium at the Ag-RDE, thus ensuring a continuity of the latter. Underpotential deposition (UPD) plays a central role in anodic-stripping voltammetry (ASV). During the deposition step, the adatom coverage of trace elements is in the range of 0.01-1% and no bulk deposition is invoked for metals that exhibit UPD. The UPD properties and, as a result, the ASV signals are strongly affected by the type and concentration of the supporting electrolyte. The effects of Cl⁻, Br⁻, SO₄²⁻ and NO₃⁻ are shown. The analysis of lead and cadmium in natural waters has been performed. Surfactants distort the SASV signal. In order to ensure surfactant-free solutions, the samples were pretreated by wet ashing.     

Determination of mercury by differential-pulse anodic-stripping voltammetry with various working electrodes Application to the analysis of natural water sediments

Four types of working electrode (glassy-carbon and gold rotating-disk electrodes and two types of gold-film electrode) have been used in determination of traces of mercury by differential-pulse anodic-stripping voltammetry, and the analytical parameters of the procedures compared. The technique has been applied to the analysis of river sediments. The lowest limit of detection (0.02 μg/l.) was obtained with the gold rotating-disk electrode. Two procedures have been found optimal for analyses of sediment samples; determination with the gold rotating-disk electrode and solution-exchange after the preelectrolysis, and determination with the gold-film electrode prepared in situ in the sample extract. The sample pretreatment involved a separation of the 0.45–63 μm fraction, mineralization with a mixture of hydrochloric and nitric acids (3:1 or 1:3) under atmospheric pressure in a fused silica vessel, followed by irradiation with ultraviolet light, after addition of hydrogen peroxide (to destroy organic matter). The most serious interference is from iron; this can be prevented by adding fluoride or pyrophosphate. The procedure is an alternative to the AAS determination of the total mercury content in sediments, especially with heavily polluted samples (mercury concentrations up to 0.01%).     

Determination of copper in the presence of a large excess of bismuth by differential-pulse anodic-stripping voltammetry without preliminary separation

Conditions have been found which make possible the determination of copper in the presence of a large excess of bismuth by differential-pulse and anodic-stripping voltammetry without preliminary separation. The electrochemical activity of the bismuth, which usually interferes in the determination of copper, is inhibited by using tetrabutylammonium chloride (TBAC) as surfactant. In 0.2M EDTA and 0.01M ascorbic acid at pH 4.5 as supporting electrolyte without the surfactant present, trace levels of copper (1.5 x 10(-8)M) can be determined accurately if the molar ratio of bismuth to copper is not higher than 3, but if the electrolyte also contains TBAC at 0.01M concentration, bismuth can be tolerated in concentrations up to 10(-4)M, and the height of the copper peak is unaffected.     

Hydrodynamic voltammetry at tubular electrodes-III Determination of traces of bismuth by differential-pulse anodic-stripping voltammetry at a glassy-carbon tubular electrode with in situ mercury plating

An equation for the current in differential-pulse anodic-stripping voltammetry at tubular electrodes is derived. Application of a glassy-carbon tubular electrode to determination of traces of bismuth in environmental water samples by differential-pulse anodic-stripping voltammetry is described. In hydrochloric acid medium, the stripping peak current is proportional to the concentration of bismuth in the range 2-100 ng/ml, with a deposition time of 3-10 min. The detection limit is 0.5 ng/ml.     

Determination of trace metals by anodic-stripping voltammetry with a new kind of mercury electrode: The long-lasting sessile-drop electrode

A new kind of semi-stationary mercury drop electrode is described, which can be used for the determination of trace metals in natural waters by differential-pulse anodic-stripping voltammetry. Results are reported for the determination of zinc in potassium chloride and in samples of sea-water. The reproducibility of the electrode and of the results obtained with it for zinc at the 10(-8)M level are very satisfactory.     

Determination of thallium at subtrace level in rocks and minerals by coupling differential pulse anodic-stripping voltammetry with suitable enrichment methods

Ten international reference samples have been analysed for thallium content by differential pulse anodic-stripping voltammetry. Two separation techniques, solvent extraction and ion-exchange, were employed to preconcentrate the thallium: the results were critically compared to establish which was the better separation technique. The values found were quite satisfactory and confirmed the wide scope for application (not yet fully investigated) of voltammetry in geochemical studies.     

A non-conventional way to perform voltammetry

In a conventional voltammetric experiment, the electroactive species is dissolved in solution, and then diffuses from the solution phase to the electrode phase. In our proposed non-conventional voltammetric experiment, the electroactive species is trapped in the electrode phase instead of being dissolved in solution. A non-aqueous solvent was first used to trap the organic species in a porous surface layer and the modified electrode then transferred to an aqueous buffer to conduct voltammetry measurements.We tested the non-conventional voltammetric mode using a modified multi-walled carbon nanotube electrode containing mono-, di- and tri-nitroaromatic compounds trapped in the porous three-dimensional network of the CNTs. From these experiments, we conclude that the non-conventional mode produces higher peak currents and displacement of the peak potentials, yielding lower overpotentials. Furthermore, it is possible to obtain more selective voltammograms in the non-conventional mode, showing peaks that could not be resolved in the conventional mode.These results are due to a change in the mass transport regime, with thin layer diffusion being the main transport method in the non-conventional mode, compared to semi-infinite diffusion in the conventional mode.The proposed approach is an excellent alternative for performing voltammetric studies on insoluble or slightly soluble organic compounds.     

Determination of accurate electrode contribution during voltammetry scan of electrochemical capacitors

This paper introduces the method which allows determining the accurate electrode contributions during cyclic voltammetry (CV) scan of electrochemical capacitor. As a result of theoretical considerations, a calculation method which reveals voltammetry response of both electrodes during CV of two-electrode cell with reference is developed. The technique is based on the preservation of charge neutrality where the accurate potential sweep rate of individual electrode is dynamically assigned based on its total contribution to the total two-electrode cell voltage ramp. This practice should be used in the research with CV scans of energy storage devices in order to improve their precision. The technique is not an alternative to real three-electrode measurements, where constant sweep rate of working electrode is applied and an oversized auxiliary electrode is used, but it is rather a supplement, which allows observing the true electrode behavior during operation of the capacitor. The paper provides comparison of CV scans obtained with fixed scan rates of both electrodes with dynamic CV scan for electrochemical capacitors operating in aqueous media of 1 mol L^?1 Li₂SO₄ and 7 mol L^?1 KSCN. For the first time, the simple procedure is proposed to visualize the real qualitative electrode responses.