Voltammetry of biologically active species and surfactants on new miniaturized and contractible (compressible) mercury electrodes

The adsorption of the polyether-antibiotic monensin from an aqueous solution on mercury was used to investigate the effect of the decreasing size of a stationary mercury drop electrode on the shape of the voltammetric desorption peak of the surfactant. The change of the i-E curve indicated an acceleration of the transport of the surfactant to the electrode as well as of time-dependent changes in the adsorption layer. A decrease of the radius of the hanging mercury drop electrode from 220 μm to 80 μm at a constant accumulation time of t_ac = 70 s resulted in an about 4-fold increase of the evaluated signal (i-E pre-wave) of monensin. A 7-fold increase of the voltammetric desorption peak of monensin at conc. 5 · 10^–7 mol/L was observed as result of a compressive accumulation of the surfactant due to a contraction of the mercury drop electrode. A scheme of an apparatus for voltammetric/polarographic measurements by means of the contractible (compressible) mercury drop electrode is described. The controlled contraction of the electrode surface is presented together with preliminary results covering a new way of accumulation of surfactants, new accumulation effects, effective in adsorptive voltammetry, and other electroanalytical techniques.     

Determination of the pesticide Chlorpyrifos by cathodic adsorptive stripping voltammetry

The adsorption behavior and differential pulse cathodic adsorptive stripping voltammetry of the pesticide Chlorpyrifos (CP) were investigated at the hanging mercury drop electrode (HMDE). The pesticide was accumulated at the HMDE and a well-defined stripping peak was obtained at –1.2 V vs Ag/AgCl electrode at pH 7.50. A voltammetric procedure was developed for the trace determination of Chlorpyrifos using differential pulse cathodic adsorptive stripping voltammetry (DP-CASV). The optimum working conditions for the determination of the compound were established. The peak current was linear over the concentration range 9.90 × 10^–8– 5.96 × 10^–7 mol/L of Chlorpyrifos. The influence of diverse ions and some other pesticides was investigated. The analysis of Chlorpyrifos in commercial formulations and treated waste water was carried out satisfactorily Received: 10 July 1997 / Revised: 1 April 1998 / Accepted: 6 April 1998     

Speciation of arsenic(V) and arsenic(III) by cathodic stripping voltammetry in fresh water samples

A voltammetric stripping procedure is described for the determination of arsenic(V) in a mannitol-sulphuric acid medium. The arsenic is coprecipitated with copper and selenium and reduced to arsine at the hanging mercury drop electrode. Using an accumulation time of 240 s, the detection limit is 0.52 μg L^–1, the determination limit is 0.9 μg L^–1. The method has been applied to the determination of arsenic in water samples. By varying the composition of the supporting electrolyte it is possible to differentiate between arsenic(III) and arsenic(V). As both oxidation states have different toxicological characteristics, the ability to discriminate between both is an distinct advantage of the proposed method. Received: 25 October 1996 / Revised: 7 February 1997 / Accepted: 12 February 1997     

Determination of cobalt by catalytic-adsorptive differential pulse voltammetry in the presence of 2-aminocyclopentene-1-dithiocarboxylic acid and nitrite

A novel method for the determination of cobalt(II) by stripping voltammetry is described. It involves an adsorptive accumulation of the cobalt(II)-2-aminocyclopentene-1-dithiocarboxylic acid complex on a hanging mercury drop electrode, followed by a stripping voltammetric measurement of the catalytic reduction current of the complex at –1.4 V at pH = 9 (vs. Ag/AgCl). The effects of various experimental parameters on the catalytic current were investigated. An accumulation time of 60 s results in a low experimental limit of detection of 0.1 ng/mL of Co(II), and 0.50 to 40.0 ng/mL of cobalt can be determined. The relative standard deviation at 0.50 ng/mL is 2.8%. Possible interferences from co-existing ions were also investigated. Received: 17 August 1998 / Revised: 16 November 1998 / Accepted: 20 November 1998     

Determination of cobalt by catalytic-adsorptive differential pulse voltammetry in the presence of 2-aminocyclopentene-1-dithiocarboxylic acid and nitrite

A novel method for the determination of cobalt(II) by stripping voltammetry is described. It involves an adsorptive accumulation of the cobalt(II)-2-aminocyclopentene-1-dithiocarboxylic acid complex on a hanging mercury drop electrode, followed by a stripping voltammetric measurement of the catalytic reduction current of the complex at –1.4 V at pH = 9 (vs. Ag/AgCl). The effects of various experimental parameters on the catalytic current were investigated. An accumulation time of 60 s results in a low experimental limit of detection of 0.1 ng/mL of Co(II), and 0.50 to 40.0 ng/mL of cobalt can be determined. The relative standard deviation at 0.50 ng/mL is 2.8%. Possible interferences from co-existing ions were also investigated. Received: 17 August 1998 / Revised: 16 November 1998 / Accepted: 20 November 1998     

Speciation of trace metals in natural waters: the influence of an adsorbed layer of natural organic matter (NOM) on voltammetric behaviour of copper

The influence of an adsorbed layer of the natural organic matter (NOM) on voltammetric behaviour of copper on a mercury drop electrode in natural water samples was studied. The adsorption of NOM strongly affects the differential pulse anodic stripping voltammogram (DPASV) of copper, leading to its distortion. Phase sensitive ac voltammetry confirmed that desorption of adsorbed NOM occurs in general at accumulation potentials more negative than −1.4 V. Accordingly, an application of negative potential (−1.6 V) for a very short time at the end of the accumulation time (1% of total accumulation time) to remove the adsorbed NOM was introduced in the measuring procedure. Using this protocol, a well-resolved peak without interferences was obtained. It was shown that stripping chronopotentiogram of copper (SCP) in the depletive mode is influenced by the adsorbed layer in the same manner as DPASV. The influence of the adsorbed NOM on pseudopolarographic measurements of copper and on determination of copper complexing capacity (CuCC) was demonstrated. A shift of the peak potential and the change of the half-peak width on the accumulation potential (for pseudopolarography) and on copper concentration in solution (for CuCC) were observed. By applying a desorption step these effects vanished, yielding different final results.     

Silica-modified electrode for the selective detection of mercury

Pure silica particles were dispersed within carbon paste and the resulting modified electrode was applied to the selective voltammetric detection of mercury(II) species after their accumulation at open circuit. The remarkable selectivity observed between pH 4 and 7 was attributed to the intrinsic adsorption mechanism which involves a condensation reaction between mercury(II) hydroxide and hydroxyl groups on the silica surface, leading to the formation of an inner-sphere-type surface complex. After optimization with respect to the electrode composition, the detection medium, and the voltammetric scan mode, a linear response was obtained in the concentration range between 2 × 10⁻⁷ M to 1 × 10⁻⁵ M, by applying anodic stripping square wave voltammetry. Various silica samples were used and their sorption behavior was discussed in relation to their specific surface area and porosity. The effect of chloride and pH on the accumulation of mercury(II) on silica was also investigated. Received: 4 September 1999 / Accepted: 5 January 2000     

Flow injection amperometric sensing of uric acid and ascorbic acid using the self-assembly of heterocyclic thiol on Au electrode

Au electrode modified with the self-assembled monolayer of a heterocyclic thiol, mercaptotriazole (MTz), is used for the electroanalysis of uric acid (UA) and ascorbic acid (AA). MTz forms a less compact self-assembly on Au electrode. The self-assembly of MTz on Au electrode favors the oxidation of UA and AA at less positive potential. Significant decrease (∼400 mV) in the overpotential and enhancement in the peak current for the oxidation of interfering AA with respect to the unmodified electrode is observed. The negative shift in the oxidation peak potential of AA favors electrochemical sensing of UA without any interference. Two well-separated voltammetric peaks for AA and UA are observed in their coexistence. The large separation between the two voltammetric peaks allows the simultaneous or selective sensing of the analytes without compromising the sensitivity. Linear response is obtained for a wide concentration range. This electrode could sense as low as 1 μM of UA in the presence of 10-fold excess of interfering AA. No change in the sensitivity (0.012 μA/μM) of the electrode toward UA in the presence and absence of AA is observed. Reproducible and stable amperometric flow injection response was obtained upon repetitive injection.     

Anodic stripping voltammetric determination of Zn, Pb and Cu traces in whisky samples

The simultaneous “in natura” determination of trace Zn, Pb and Cu in whisky samples by anodic stripping voltammetry (ASV), using a hanging mercury drop electrode, without previous treatment or addition of supporting electrolyte is described. The choice of an appropriate stripping voltammetric method and deposition potential minimizes the influence of the organic content and ensures a good reproducibility of the measurements. The reliability of the method was tested comparing the results with those of atomic absorption spectroscopy (AAS), with differences of about 10%. The method allows the determination of heavy metal ions in the μg L^–1 range. Received: 14 August 1997 / Revised: 10 December 1997 / Accepted: 11 December 1997     

Increased sensitivity of anodic stripping voltammetry at the hanging mercury drop electrode by ultracathodic deposition

An improved approach to the anodic stripping voltammetric (ASV) determination of heavy metals, using the hanging mercury drop electrode (HMDE), is reported. It was discovered that using very cathodic accumulation potentials, at which the solvent reduction occurs (overpotential deposition), the voltammetric signals of zinc(II), cadmium(II), lead(II) and copper(II) increase. When compared with the classical methodology a 5 to 10-fold signal increase is obtained. This effect is likely due to both mercury drop oscillation at such cathodic potentials and added local convection at the mercury drop surface caused by the evolution of hydrogen bubbles.     

Adsorptive Stripping Voltammetric Determination of Ultra Trace of Zinc and Lead with Carbidopa as Complexing Agent in Food and Water Samples

In the present work, the cathodic stripping voltammetric methodology using a hanging mercury drop electrode was described for simultaneous determination of lead and zinc in different real samples. The method is based on adsorption of metal ions on mercury electrode using carbidopa as a suitable complexing agent. The potential was scanned to the negative direction and the differential pulse stripping voltammograms were recorded. Optimal conditions were found to be: accumulation time; 70 s, accumulation potential; 50 mV versus Ag/AgCl, scan rate; 40 mV s^?1, supporting electrolyte; 0.01 M ammonia buffer at pH 8.5, and concentration of carbidopa; 8.0 μM. The relationship between the peak current versus concentration was linear over the range of 0.1–210 and 0.2–170 nM for lead and zinc, respectively. The detection limits are 0.09 and 0.15 nM for lead and zinc ions respectively. The relative standard deviations at a concentration level of 70 nM of both metal ions are found 1.08 and 1.24% for lead and zinc ions respectively.     

High sensitive determination of trace amount of cobalt by catalytic adsorptive stripping voltammetry

A very sensitive and selective catalytic adsorptive cathodic stripping procedure for trace measurements of cobalt is presented. The method is based on adsorptive accumulation of cobalt-CCA (calcon carboxylic acid) complex onto a hanging mercury drop electrode followed by reduction of the adsorbed species by voltammetric scan using differential pulse modulation. The reduction current is enhanced catalytically by nitrite. The effect of various parameters such as pH, concentration of CCA, concentration of nitrite, accumulation potential and accumulation time on the selectivity and sensitivity were studied. The optimum condition for the analysis of cobalt, include pH 5.2 (Acetate buffer), 2.1 μM clacon carboxylic acid, 0.032 M sodium nitrite and an accumulation potential of 0.05 V (versus Ag/AgCl). Under these optimum conditions and for an accumulation time of 60 s, the measured peak current at −0.480 V is proportional to the concentration of cobalt over the entire concentration range tested 0.003–2.0 ng ml⁻¹ with a detection limit of 1 pg ml⁻¹ for an accumulation time of 60 s and 2.0–10.0 ng ml⁻¹ for an accumulation time of 40 s. The relative standard deviations for ten replicate measurement of 0.5 ng ml⁻¹ of cobalt were 3.1%. The main advantage of this new system is the microtrace Co(II) determination by ASV. The method was applied to determination of cobalt in a water sample and some analytical grade salts with satisfactory results. Published in Elektrokhimiya in Russian, 2009, Vol. 45, No. 2, pp. 221–228. The article is published in the original.     

HPLC online reductive scanning voltammetric detection of diquat, paraquat and difenzoquat with mercury electrodes

To establish an electrochemical HPLC detection system which is suitable for the voltammetric characterisation of unknown contaminants and food components at working potentials lower than –1 V, a modified flow-through cell for the use of a hanging mercury drop electrode (HMDE) is described. The introduction of silanised glass capillaries and a new flow-channel design provide a high HMDE lifetime, which is recommended in HPLC detection. As test system the herbicides diquat, paraquat and difenzoquat and ethylviologen as internal standard were measured using differential pulse voltammetry (DPV) detection to improve selectivity. Spiked water samples were analysed with voltammetric and UV-detection and results agreed well. Received: 12 November 1998 / Revised: 21 January 1999 / Accepted: 26 January 1999     

Square-wave adsorptive stripping voltammetric determination of an antihistamine drug astemizole

The square-wave voltammetric technique was used to explore the adsorption properties of the astemizole drug. The analytical methodology used was based on the adsorptive preconcentration of the drug on a hanging mercury drop electrode (HMDE), followed by the electrochemical reduction process which yielded a well-defined cathodic peak at −1.184 V (vs. the Ag/AgCl electrode). To achieve high sensitivity, various experimental and instrumental variables were investigated such as the supporting electrolyte, pH, accumulation time and potential, drug concentration, scan rate, SW frequency, pulse amplitude, convection rate, and the working electrode area. Under the optimized conditions, the AdSV peak current was proportional over the analyte concentration range of 5 × 10⁻⁷ to 2.5 × 10⁻⁶ mol L⁻¹ (r = 0.998) with the detection limit of 1.4 × 10⁻⁸ mol L⁻¹ (6.4 ng mL⁻¹). The precision of the proposed method in terms of RSD was 2.4 %, whereas the method accuracy was indicated by the mean recovery of 100.1 %. Possible interferences of several substances usually present in the pharmaceutical tablets and formulations were also evaluated. The applicability of this electroanalytic approach was illustrated by the determination of astemizole in tablets and biological fluids.     

Simultaneous voltammetric detection of dopamine and uric acid in the presence of high concentration of ascorbic acid using multi-walled carbon nanotubes with methylene blue composite film-modified electrode

An electrochemically functional nanocomposite through the adsorption of methylene blue onto the multi-walled nanotubes (MB-MWNTs) was prepared, and a sensitive voltammetric sensor was fabricated. The modified electrode showed excellent electrocatalytic activity toward dopamine (DA) and uric acid (UA) in 0.1 M phosphate solution medium (pH 3.0). Compared to the bare electrode, the MB-MWNTs film-modified electrode not only remarkably enhanced the anodic peak currents of DA and UA, i.e., shifted the anodic peak potential of DA negatively, but also avoided the overlapping of the anodic peaks of DA and UA. The interference of ascorbic acid (AA) was eliminated. Under the optimized conditions, the peak separation between AA and DA and between DA and UA was 219 and 174 mV, respectively. In the presence of 1.0 mM AA and 10.0 μM UA, the anodic peak current was linear to the concentration of DA in the range of 0.4–10.0 μM with a detection limit of 0.2 μM DA. The anodic peak current of UA was linear to the concentration in the range of 2.0–20.0 and 20.0–200.0 μM with a lowest detection limit of 1.0 μM in the presence of 1.0 mM AA and 1.0 μM DA.     

Stripping voltammetric determination of copper(II) in natural waters and human hairs based on the adsorption of its complex with Kryptofix 22 on the carbon paste electrode

Traces of copper(II) can be determined by adsorptive stripping voltammetry using Kryptofix 22 as complex forming reagent. First, copper(II)-Kryptofix 22 complex was adsorbed on the carbon paste electrode in phosphate buffer pH 5.3 with an accumulation potential of −0.5 V. Following this, the adsorbed complex was oxidized and detected by anodic differential pulse voltammetric scan from −0.2 to 0.1 V. The effective parameters of sensor response were examined. The detection limit of copper(II) was 1.1 μg/L and relative standard deviations for 50 and 150 μg/L of Cu(II) were 1.2 and 0.96%, respectively. The calibration curve was linear for 2–150 μg/L of copper(II). This technique does not use mercury and therefore is environmentally beneficial. The method, which is reasonably sensitive and selective, has been successfully applied to the determination of trace amounts of copper(II) in water and human hair samples.     

Development of renewed mercury multi-purpose (multi-mode, versatile) microelectrodes, a part of PC-controlled measuring systems (including the PC-ETP)

Information is given about the development of renewed miniaturized mercury multi-purpose (multi-mode) electrodes as an inseparable part of the development of modern polarography and voltammetry; casette-type, pen-type and PC-controlled systems, incl. the polarographic/voltammetric set. According to the experimental behavior and ranges of their radii the regimes of renewed stationary mercury electrodes are divided into three categories – mini-, semimicro- and microelectrodes (mE, sμE and μE). Their spontaneously growing, stepwisely growing, stationary and contracting (compression) modes are schematically outlined. Information on basic functional properties of the discussed renewed mercury semimicro- or microelectrodes using meniscus and microdrop-electrodes, by means of the d. c., anodic stripping and adsorptive stripping voltammetry is given. Received: 25 March 1998 / Accepted: 20 May 1998     

Surfactant analysis with differential pulse tensammetry

Differential pulse polarograms of surfactants exhibit tensammetric (adsorption/desorption) peaks. The dependence of peak current on bulk concentration of a surfactant is at first linear but reaches a saturation limit at higher concentrations. Even with commerical instrumentation, in which the current measuring sequence is such that charging current is minimized, differential pulse tensammetry is a viable analytical method. The optimum pulse amplitude depends on the surfactant. Sensitivity can be increased by using a long drop time or a stationary mercury electrode. Sensitivity and resolution from interfering faradaic processes were improved by modifying the current measuring sequence of a commercial instrument. This modification involved decreasing the delay time between application of the pulse and the second current sampling period and decreasing the second current sampling period itself.     

Simultaneous determination of copper, zinc and lead by adsorptive stripping voltammetry in the presence of Morin

A sensitive and selective method for the simultaneous determination of copper, zinc and lead is presented. The method is based on the adsorptive accumulation of 2′,3,4′,5,7-pentahydroxyflavone (Morin) complexes of these elements onto a hanging mercury drop electrode, followed by reduction of adsorbed species by voltammetric scan using differential pulse modulation. Optimal analytical conditions were found to be Morin concentration of 2.0 μM, pH of 4.0, and an adsorption potential at −500 mV versus Ag/AgCl. With an accumulation time of 60 s, the peak currents are proportional to the concentration of copper, lead and zinc over the 1 to 60, 0.3-80 and 1-70 ng ml⁻¹ range with detection limits of 0.06, 0.08 and 0.06 ng ml⁻¹, respectively. The procedure was applied to the simultaneous determination of copper, lead and zinc in some real and synthetic artificial real samples with satisfactory results.     

Differential pulse polarography and voltammetry of di-2-pyridyl ketone benzoylhydrazone

Summary The differential pulse polarographic (DPP) and voltammetric (DPV) behaviour of di-2-pyridyl ketone benzoylhydrazone (DPKBH), has been investigated over a wide pH range (2.1–11.5). The effect of various operational parameters on the reduction current are discussed and the mechanism of the electrode reaction is also included. Both methods DPP and DPV are applied for the analytical determination of this reagent and the detection limits are found to be 1.24 mol/l and 0.86 mol/l, respectively. The DPP behaviour of DPKBH in the presence of Pb(II), Cd(II) and Cu(II) is studied and the behaviour of the Cu(II)-DPKBH complex is fully investigated. The voltammetric determination of Cu(II), based on the accumulation of its complex at the hanging mercury drop electrode, is described. Statistical analysis of the calibration curve data is included.On leave from Assuit University, Egypt