Application of a copper-based mercury film electrode in cathodic stripping voltammetry

The utility of a copper-based mercury film electrode (MFE) in cathodic stripping voltammetry (c.s.v.) is tested by comparing the cyclic and stripping voltammograms obtained with this electrode for thiocyanate, tryptophane, cysteine and benzotriazole against those obtained with the hanging copper-amalgam drop electrode (HCADE) and the HMDE. The cathodic stripping peaks obtained at the copper-based MFE and the HCADE are usually narrower and higher and are located at more negative potentials than the peaks obtained at the HMDE. Lower detection limits and better separations of adjacent peaks are thus achieved, and useful peaks can be separated from the mercury waves obtained with the conventional HMDE. The advantage of the copper-based MFE over the HCADE is its simplicity of preparation and maintenance. Thiocyanate, tryptophane, cysteine and benzotriazole can be determined at the copper-based MFE by c.s.v. with detection limits of 1 × 10^?8, 1 × 10^?8, 5 × 10^?8 mol dm^?3, respectively.     

Determination of traces of streptomycin and related antibiotics by adsorptive stripping voltammetry

Adsorptive stripping voltammetry provides sensitive determinations of trace amounts of the saccharide-related antibiotics, streptomycin, erythromycin and novobiocin. A static mercury drop electrode is immersed in a stirred alkaline solution of the drug for a fixed time (60–300 s) at a suitable potential, and the adsorbed species is then stripped in the linear-scan or differential-pulse mode. The preconcentration potentials and stripping peak potentials (vs. Ag/AgCl) are, respectively, ?1.0 V and ?1.58 V for streptomycin, ?0.9 V and ?1.2 V for erythromycin, and ?1.0 V and ?1.38 V for novobiocin. The interfacial behavior is discussed. Short preconcentration periods suffice to quantity streptomycin, novobiocin, and erythromycin down to the 7 × 10^?10 M, 2.5 × 10^?9 M, and 1.3 × 10^?8 M levels, respectively. Streptomycin added to urine can be quantified after simple dilution.     

Catalytic Effect on Silver Electrodeposition of Gold Deposited on Carbon Electrodes

A new methodology, based on silver electrocatalytic deposition and designed to quantify gold deposited onto carbon paste electrode (CPE) and glassy carbon electrode (GCE), has been developed in this work. Silver (prepared in 1.0 M NH₃) electrodeposition at ?0.13 V occurs only when gold is previously deposited at an adequate potential on the electrode surface for a fixed period of time. When a CPE is used as working electrode, an adequate oxidation of gold is necessary. This oxidation is carried out in both 0.1 M NaOH and 0.1 M H₂SO₄ at oxidation potentials. When a GCE is used as working electrode, the oxidation steps are not necessary. Moreover, a cleaning step in KCN, which removes gold from electrode surface, is included. To obtain reproducibility in the analytical signal, the surface of the electrodes must be suitably pretreated; this electrodic pretreatment depends on the kind of electrode used as working electrode. Low detection limits (5.0×10^?10 M) for short gold deposition times (10 min for CPE and 5 min for GCE) were achieved with this novel methodology. Finally, sodium aurothiomalate can be quantified using silver electrocatalytic deposition and GCE as working electrode. Good linear relationship between silver anodic stripping peak and aurothiomalate concentration was found from 5.0×10^?10 M to 1.0×10^?8 M.     

Enhancement effect of cysteine on anodic stripping voltammetry of copper

The anodic stripping behaviour of copper in the presence of compounds with a mercapto group, such as cysteine, was investigated. In the presence of cysteine, a copper stripping wave at ?0.12 V vs. SCE decreased, and instead a new sharp wave was observed at more positive potential. Its peak height increased with increasing concentration of cysteine, and at 1 × 10^?5M cysteine it became about seven times as large as that observed in the absence of cysteine. Then the method using this enhanced wave was studied for the determination of trace cupric ion. The results were that the relative standard deviation for five repetitive determinations was about 4% at 10^?8M Cu(II) and the detection limit was 6 × 10^?10M Cu(II). From the investigation by means of cyclic voltammetry, it was found that this enhanced wave was due to the transformation from a cupric—cysteinate complex to a mercuric—cysteinate complex.     

Determination of folic acid by adsorptive stripping voltammetry at the static mercury drop electrode

Folic acid can be determined at nanomolar concentrations by controlled adsorptive accumulation of folic acid on a static mercury drop electrode held at ?0.3 V vs. Ag/AgCl followed by reduction of the surface species. In 0.1 M sulfuric acid, a cathodic scan gives peaks at ?0.47 v and ?0.75 V vs. Ag/Agcl; the latter peak provides greater sensitivity. Differential-pulse stripping is shown to be superior to normal-pulse and d.c. stripping. After a 5-min preconcentration, the detection limit is about 1 × 10^?10 M folic acid. The adsorptive stripping response is evaluated with respect to concentration dependence, preconcentration time and potential, solution acidity and the presence of gelatin and bromide. The relative standard deviation at the 5 × 10^?8 M level is 1.2%. This method is applied to the determination of folic acid in pharmaceutical tablets.     

Application of semidifferential electroanalysis to anodic stripping voltammetry

A new voltammetric technique, Semidifferential electroanalysis, in which the semiderivative, e, of the current, i, is measured as a function of electrode potential, has been applied for detection in anodic stripping voltammetry. The semiderivative of the current is defined by [fx131-1.tif] Cd²⁺, Pb²⁺, and Tl⁺ in 0.1 M KNO₃ at different pH values were tested as samples on a hanging mercury drop working electrode. Symmetrical sharp peaks were observed for the re-dissolution processes of metal amalgams formed during pre-electrolysis at -1.0 V vs. SCE. The peak potentials of e vs. E curves for the above three amalgams agreed well with the literature values for d.c. polarographic half-wave potentials. The peak heights were proportional to the pre-electrolysis time up to about 5 min, to the potential scan rate in the range 60–160 mV s^-1, and to the concentrations of Cd²⁺, Pb²⁺, and Tl⁺ in the original solution in the range 10^-6–10^-9 M. The relative standard deviation for the determination of Cd²⁺ was about ±4% at the 2 × 10^-5 M level.     

Cathodic stripping voltammetry: Part I. Determination of organic sulfur compounds,flavins and porphyrins at the sub-micromolar level

The experimental parameters of cathodic stripping voltammetry have been studied and optimised, and the use of a cell with a mercury pool electrode has been proposed. The technique is valuable for the determination of several classes of organic compounds, including thiols, disulfides, flavins, flavones, pterins and porphyrins at concentrations as low as 1×10^?8M. In most cases the measurement precision at the 2×10^?7M level is ±3–5%, which is similar to that of anodic stripping voltammetry. Detailed investigations were made of the electrode reaction mechanisms of cysteine, cystine, riboflavin and hemoglobin. All thiol compounds were found to adsorb strongly on mercury and chemically react with it to form a film of a mercury-thiol compound. Some closely-related thiols, e.g. 2- and 6-mercaptopurine, produced stripping peaks at well-separated potentials. Selectivity could be achieved with some thiol mixtures by adjustment of the deposition potential.     

Cathodic stripping voltammetry of the peptide felypressin in trace amounts

Felypressin, a peptide containing eight amino acids including cystine, is studied by cathodic stripping voltammetry (c.s.v.) at a mercury drop electrode at pH 4.6 in the concentration interval 5 × 10^?9-7 × 10^?7 M. Excess of copper(II) ions is required to obtain the c.s.v. activity. The stripping peak potential is ?0.55 to ?0.70 V vs. SCE depending on the excess of copper(II). The accumulated product is adsorbed both in its oxidized and reduced state. Interference from c.s.v.-active substances which desorb in the reduced state can be eliminated by applying a repetitive cyclic scan and evaluating the second or third scan. Lypressin and somatostatin, two other cystine-containing peptides, are also c.s.v.-active.     

Voltammetric behavior of rutin at a boron-doped diamond electrode. Its electroanalytical determination in a pharmaceutical formulation

This paper examined for the first time, the possibilities of the usage of a boron-doped diamond electrode for the redox behavior of rutin using cyclic and adsorptive stripping voltammetry. The cyclic voltammograms showed a pair of redox peaks at lower potentials followed by an irreversible oxidation peak at higher positive potential. Using square-wave adsorptive stripping voltammetry, the compound yielded a well-defined voltammetric response in Britton-Robinson buffer, pH 4.0 at +0.48 V (vs. Ag/AgCl) (after 60 s accumulations at a fixed potential of 0.2 V). The calibration curve was linear in the concentration range from 0.01 µg mL^?1 to 0.1 µg mL^?1 (1.64×10^?8 M ? 1.64×10^?7 M). A detection limit of 0.0017 µg mL^?1 (2.78×10^?9 M) was observed without any chemical modifications and electrochemical surface pretreatments. As an example, the practical applicability of boron-doped diamond electrode was tested with the measurement of rutin in dietary supplement products.      

Electrochemical investigation and determination of ceftazidime in pharmaceutical dosage forms and human urine

A voltammetric study of the oxidation of Ceftazidime (CEFT) has been carried out at the glassy carbon electrode by cyclic, differential pulse (DPV) and square wave (SWV) voltammetry. The oxidation of CEFT was irreversible and exhibited diffusion controlled process depending on pH. The oxidation mechanism was proposed and discussed. According to the linear relationship between the peak current and concentration, DPV and SWV voltammetric methods for CEFT assay in pharmaceutical dosage forms and human urine were developed. For analytical purposes, a well resolved diffusion controlled voltammetric peak was obtained in 0.1 M H₂SO₄ at 1.00 and 1.02 V for differential pulse and square wave voltammetric techniques, respectively. The linear response was obtained within the range of 4 × 10^?6?8 × 10^?5 M with a detection limit of 6 × 10^?7 M for differential pulse and 4 × 10^?6–2 × 10^?4 M with a detection limit of 1 × 10^?6 M for square wave voltammetric technique. The determination of CEFT in 0.1 M H₂SO₄ was possible over the 2 × 10^?6–1 × 10^?4 M range in urine sample for both techniques. The standard addition method was used for the recovery studies.     

Electrode Modified with Langmuir–Blodgett (LB) Film of Calixarenes for Preconcentration and Stripping Analysis of Hg(II)

A new type of current sensor, Langmuir–Blodgett (LB) film of calixarene on the surface of glassy carbon electrode (GCE) was prepared for determination of mercury by anodic stripping voltammetry (ASV). An anodic stripping peak was obtained at 0.15 V (vs. SCE) by scanning the potential from ?0.6 to +0.6 V. Compared with a bare GCE, the LB film coated electrode greatly improves the sensitivity of measuring mercury ion. The fabricated electrode in a 0.1 M H₂SO₄+0.01 M HCl solution shows a linear voltammetric response in the range of 0.07–40 μg L^?1 and detection limit of 0.04 μg L^?1 (ca. 2×10^?10 M). The high sensitivity, selectivity, and stability of this LB film modified electrode demonstrates its practical application for a simple, rapid and economical determination of Hg²⁺ in a water sample.     

Determination of Cysteine in the Presence of Copper in Diluted Alkaline Electrolyte by Adsorptive Stripping Voltammetry at the Mercury Film Electrode

Abstract

A stripping method for the determination of cysteine in the presence of copper at the submicromolar concentration levels is described. The method is based on controlled adsorptive accumulation of cysteine at mercury film electrode followed by linear cyclic voltammetry scan measurement of the surface species. Optimum experimental conditions were found to be the use of a 1×10^?3 M NaOH solution, an accumulation potential of ?0.50 V and a scan rate of 200 mV. s^?1. The response of cysteine is linear over the concentration range 0.04–0.20 ppm. For an accumulation time of 15 minutes, the detection limit was found to be 0.9 ppb (7.4×10^?9 M). The more convenient relation to measuring the cysteine in presence of metals, and others amino acids were also investigated. The utility of the method is demonstrated by presence of casein and ATP.     

Determination of traces of palladium by adsorptive stripping voltammetry of the dimethylglyoxime complex

Preconcentration is achieved by adsorption of a palladium-dimethylglyoxime complex on a hanging mercury drop electrode. Optimal conditions area stirred acetate buffer solution (pH 5.15) containing 2 × 10^?4 M dimethylglyoxime and an accumulation potential of —0.20 V. The height of the stripping peak in a negative-going linear scan is linearly dependent on palladium concentration and preconcentration time (over the ranges 0–16 μg l^?1 and 0–300 s, respectively). For a 10-min preconcentration time, the detection limit is 20 ng l^?1 (2.1 × 10^?10 M). Possible interferences by other trace metals are investigated. Palladium added to seawater samples was easily quantified.     

Study on the determination of trace rhenium (VII) by the adsorption differential pulse polarography

The determination of trace rhenium (VII) by differential pulse polarography in the system of H₂SO₄-(NH_sOH)₂ · H₂SO₄-TeO^2?₄ is markedly improved by the addition of Nitron, which is adsorbed on the surface of mercury electrode. The limit of detection is down to 2 × 10¹⁰ M. The adsorptive peak potential is ?0.80 V (vs. SCE). In the ranges of 5 × 10¹⁰—10^?8, 1 × 10^?5—10^?7 and 1 × 10^?7—10^?6M, there are good linear relationships between the peak current increment and the concentration, of which the relative standard deviations are 9.5, 6.6, 1.8% respectively with the correlation coefficients of linear regression of 0.995–0.999. The results relating to this polarographic wave show that it is an adsorption-catalytic wave. The mechanism of the electrode reaction is discussed.     

Simultaneous determination of tin and lead by a.c. anodic stripping voltammetry at a hanging mercury drop electrode sensitized by cetyltrimethylammoniu

The influence of cetyltrimethylammonium bromide (CTAB) on the simultaneous determination of tin(IV) and lead(II) by anodic stripping voltammetry at a hanging mercury drop electrode (HMDE) in a 0.1 M hydrochloric acid—0.1 M oxalic acid medium was studied using d.c. and a.c. stripping. In the presence of CTAB, tin and lead show voltammetric peaks separated by 100 mV, the sensitivity depending on the concentration of CTAB. The best conditions for the simultaneous determination of both elements (2 × 10^-3 M) were found. A method is proposed for the determination of tin in the presence of lead and three procedures are given for the determination of lead in the presence of tin.     

Electrochemical studies of homocysteine and homocystine at mercury electrodes

The behaviour of homocysteine and cysteine at mercury electrodes is compared. The one-electron oxidation associated with thiols is shown to be the same for both compounds in acidic phosphate buffer, giving rise to an adsorbed thiol—mercury complex, (RS)₂Hg, at the electrode surface. Formation of this complex is utilized in the cathodic stripping voltammetric determination of homocysteine; the detection limit is 10^?9 M after a deposition time of 90 s at a hanging mercury drop electrode. The similar E_{$\text{1}\text{2}$} values for homocysteine and cysteine mean that prior separation is needed for their individual determination. Amperometric detection with a mercury-coated goal electrode after separation by cation-exchange liquid chromatography provides a method for the simultaneous determination of both compounds. Reduction of homocystine at the mercury electrode is also compared to that of cystine. The more negative reduction potential, and the maximum observed for homocystine on d.c. polarograms, which is not seen for cystine, is attributable to different reaction kinetics at the mercury electrode; the products of both the 2-electron reductions are the corresponding thiol-containing amino acids.     

A polarographic and voltammetric study of some primary,secondary and tertiary thioamides of pharmaceutical importance

The d.c. polarographic, and cyclic and cathodic stripping voltammetric behaviour of some primary, secondary and tertiary thioamides based on tetrahydroquinoline, is described. Catalytic reduction occurs in all cases; the primary and secondary thioamides undergo anodic oxidation and cathodic stripping, with the formation of mercury(II) sulphide. Tertiary thioamides are not amenable to anodic electrolysis/stripping. Mechanisms for these oxidation and reduction processes are postulated. Differential pulse polarography has a limit of detection of 5 × 10^?7 M. Cathodic stripping voltammetry can be applied with a detection limit of 2 × 10^?8 M.     

Sensitive and selective determination of mercury by differential pulse stripping voltammetry after accumulation of mercury vapour on a gold plated graphite electrode

 A selective and sensitive method is proposed for the determination of mercury by anodic stripping voltammetry after its preconcentration from the gas phase. Mercury from the sample solution is reduced to elemental Hg by SnCl₂ and volatilized by the bubbles of a carrier gas. The gas containing mercury vapour is dried and passed through a capillary onto a gold coated graphite electrode. An anodic stripping voltammogram is recorded from 0.1 mol/l HClO₄+3×10^-3 mol/l HCl solution. The calibration curve is linear from 1×10^-9 to 4×10^-8 mol/l Hg(NO₃)₂. The absolute detection limit is 0.46 ng Hg. The relative standard deviations for 4×10^-9 mol/l and 2×10^-8 mol/l Hg(NO₃)₂ are 9.8% and 6.1%, respectively (n=5). Received: 18 December 1995/Revised: 16 April 1996/Accepted: 20 April 1996     

Determination of Methimazole and Carbimazole Using Polarography and Voltammetry

Abstract

Anodic waves of methimazole (I) (1-methylimidazole-2-thiol) and carbimazole (II) (1-ethoxycarbonyl-3-methyl-2-thio-4-imidazoline) on mercury electrodes correspond to mercury salt formation. Both compounds form in the thiono form a soluble complex at pH < 6, compound (I) at higher pH-values a slightly soluble salt of the thiol form. Electrode processes involving the thiol form are complicated by adsorption. Oxidation at solid electrodes occurs only at potentials more than 0.5 V more positive. For compound (I) spectrophotometry indicated pK_a=12.0 ± 0.2. By d.c. polarography in 0.1 M H₂SO₄ containing 10% ethanol the determination of both compounds is possible between 4 × 10^? and 1 × 10^?3 M, by differential pulse polarography between 1 × 10^? and 1 × 10^?4 M, by differential pulse voltammetry at HMDE between 5 × l0^?7 and 6 × 10^? M.     

High Sensitivity and Reproducibility of a Bismuth/Poly(bromocresol purple) Film Modified Glassy Carbon Electrode for Determination of Trace Amount of Cadmium by Differential Pulse Anodic Stripping Voltammetry

This work described a novel type of bismuth/poly(bromocresol purple) film modified glassy carbon electrode (denoted as Bi/Poly(BCP)/GCE) for anodic stripping analysis of trace Cd²⁺. The Bi/Poly(BCP)/GCE was fabricated in situ by depositing simultaneously bismuth and cadmium by reduction at ?1.20 V on the poly(BCP) film using a differential pulse voltammetry. Under the optimum conditions, the anodic stripping peak current response increased linearly with the Cd²⁺ concentrations in a range of 2.0×10^?8–1.0×10^?7 M and 1.0×10^?7–6.0×10^?6 M in 0.1 M NaAc‐HAc buffer solution (pH 5.0) with the detection limit of 6.5×10^?9 M (S/N=3). The Bi/poly(BCP)/GCE performed good reproducibility and high sensitivity. Finally, this proposed method was successfully applied to determine the concentration of Cd²⁺ in water samples.