Adsorptive Voltammetric Study of Thorium–Alizarin Complexon Complex at a Carbon Paste Electrode

A new sensitive adsorptive voltammetric procedure is described for trace measurement of thorium. It is based on the cathodic stripping peak of the thorium–alizarin complexon (ALC) complex at a carbon paste electrode (CPE). The complex of Th(IV) with alizarin is adsorbed at a CPE in a mixed buffer solution (pH 5.0) which consists of 0.1mol·L^–1 sodium acetate and 0.04mol·L^–1 potassium biphthalate, yielding a sensitive cathodic voltammetric peak corresponding to the reduction of alizarin in the complex at –0.57V (vs. SCE). The second-order derivative peak current of the complex is linearly dependent upon the concentration of Th(IV) over the range of 3.0×10^–9 8.0×10^–7mol·L^–1. The detection limit is 1.0×10^–9mol·L^–1 for 180s accumulation. The molar ratio of each component in the complex was estimated as n_Th(IV):n_ALC=1:1 by a continuous variation method. The electrode processes of the Th(IV)–alizarin complex at a CPE were investigated. The procedure was successfully applied to the trace determination of thorium in ore and clay samples.     

Electroanalytical study of diethyl and dibutyl phthalate in micellar and oil-in-water emulsified media

A polarographic study was carried out of the reduction processes of diethyl and dibutyl phthalate in micellar solutions with the cationic surfactant Hyamine 1622, and in an emulsified medium from aliquots of the phthalates dissolved in a diethyl ether: ethyl acetate (1:9) mixture and Hyamine 1622 as emulsifying agent. The characteristics of the reduction processes in both media were established. The number of electrons involved was higher for the base form of the electroactive species. Using dpp at E=–50 mV, the detection limits obtained in the emulsified medium were 6.7×10^–8 and 7.4×10^–7 moll^–1 for diethyl phthalate and dibutyl phthalate, respectively. Interferences between the two phthalates were studied, and the possibility of carrying out the overall determination of both phthalates was demonstrated. Good results were obtained when applying the polarographic method developed in the emulsified medium to determine diethyl phthalate by dpp in spiked milk after extraction of the analyte with diethyl ether: ethyl acetate (1:9).     

Determination of propazine by differential pulse polarography in micellar and emulsified media

An electroanalytical study of the herbicide propazine's reduction process in micellar solutions and oil-in-water emulsions is reported. The anionic surfactant sodium pentanesulphonate was chosen as the most suitable. The differential pulse polarograms of micellar solutions had two reduction peaks below pH 2.0, whereas only one peak was obtained above pH 2.O. Ethyl acetate was chosen as the organic solvent to form propazine emulsions. Unlike in micellar solutions, the DPP polarograms of propazine emulsions showed only one peak even at pH < 2.0, suggesting that propazine hydrolysis was hindered in the emulsified medium. The limiting current is diffusion-controlled and the electrode process is irreversible. Propazine can be determined by differential pulse polarography over the 1.0 × 10^–1 – 1.0 × 10^–1moll^–1 and 1.0 × 10^–15 – 4.0 × 10^–1 moll^–1 concentration ranges and the limit of detection was 2.8 × 10^–1 moll^–1. Of the potential interferents simazine, methoprotryne and terbutryn (alls-triazines), thiram (a dithiocarbamate), dinoseb (nitrophenolic), and heptachlor (chlorinated cyclo-diene herbicide), only the first two were significant (10% error for equimolar concentrations). The method was applied to the determination of propazine in spiked drinking water. At a concentration level of 2.0 × 10^–1 moll^–1 a recovery of 94 ± 6% was obtained, after tenfold concentration on Sep-Pak.     

Preconcentration and voltammetric determination of the herbicide metamitron with a silica-modified carbon paste electrode

A carbon paste electrode incorporating silica (Si-MCPE) was fabricated to accumulate Metamitron at the electrode surface. Several electroanalytical techniques were used to explore its reductive behaviour. The results indicate that the system is irreversible and fundamentally controlled by adsorption. The adsorptive stripping response has been evaluated with respect to accumulation time, deposition potential, scan rate, pH and other variables, using differential pulse voltammetry (DPV) and square wave voltammetry (SWV) as redissolution techniques. In both cases a voltammetric peak is obtained, at –0.542 V (DPV) and –0.421 V (SWV) in Britton-Robinson buffer (pH 1.9). The detection limits were 3.66 × 10^–1 M and 4.22 × 10^–9 M for AdS-DPV and AdS-SWV, respectively. Under optimum conditions the Metamitron reduction peak gave two linear regions in the range from 4.0 × 10^–9 M to 8.0 × 10^–8 M by means of AdS-DPV, with a coefficient of variation of 2.19% (n = 10) for 1 × 10^–8 M herbicide solution. A method was developed for determination of Metamitron in soils, with a recovery of 98.8% and a coefficient of variation of 5.26% (0.01 g/g of soil).     

Polarographic analysis of some cephalosporin antibiotics

Summary The cephalosporin derivatives moxalactam, cefazolin, cefuroxime, ceftriaxone and cefotaxime were studied by direct current, sampled direct current, differential pulse polarography and cyclic voltammetry. While moxalactam and cefazolin give rise to one reduction wave, cefuroxime, ceftriaxone and cefotaxime exhibit two reduction waves. The characteristics of corresponding electrode reactions and their analytical parameters are presented and discussed. Optimum pH-ranges for the determination of the five cephalosporins are given. Linear concentration ranges varied from 0.1 to 200 g/ml and limits of determination were of the order of 0.01 to 0.04 g/ml depending on the compound of interest. The precision of the proposed method is excellent with relative standard deviation around 1.3% at a concentration of 0.5 g/ml for all investigated cephalosporins.

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Polarographische Analyse einiger Cephalosporin-Antibiotica

     

Electrochemical studies and square-wave voltammetric determination of fenofibrate in pharmaceutical formulations

The electrochemical reduction of fenofibrate at a hanging mercury drop electrode (HMDE) was investigated by cyclic voltammetry, square-wave voltammetry, and chronoamperometry. Different buffer solutions were used over a wide pH range (3.0–10.0). The best definition of the analytical signals was found in borate buffer (pH 9.0)–tetrabutylammonium iodide mixture containing 12.5% (v/v) methanol at –1.2 V (versus Ag/AgCl). According to cyclic voltammetric studies, the reduction was irreversible and diffusion controlled. The diffusion coefficient was 2.38×10^–6 cm² s^–1 as determined by chronoamperometry. Under optimized conditions of square-wave voltammetry, a linear relationship was obtained between 0.146–4.96 g mL^–1 of fenofibrate with a limit of detection of 0.025 g mL^–1. Validation parameters such as sensitivity, accuracy, precision, and recovery were evaluated. The proposed method was applied to the determination of fenofibrate in pharmaceutical formulations. The results were compared with those obtained by a published high-performance liquid chromatography method. No difference was found statistically.     

Thiazolylblue tetrazolium bromide — a new spectrophotometric reagent for determination of manganese

A method has been developed for the micro-determination of Mn(VII) by the selective extraction of the MnO₄ ^– anion with thiazolylblue tetrazolium bromide from acidic medium. The molar absorptivity of the complex is 1 × 10⁴lmol^–1cm^–1 at 250 nm and the system obeys Beer's law in the range 0.1–1.75 gml^–1 Mn(VII). The composition and stability of the complex are discussed. The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase are: extraction constantK _ex = 3.16 × 10⁴, distribution constantK _D = 21.85 and association constant =1.45 × 10³. Manganese(VII) was determined in soils and plants. The determination was carried out without preliminary separation of manganese.     

Voltammetric study of glibenclamide at carbon paste and Sephadex-modified carbon paste electrodes

The oxidative behaviour of the antidiabetic agent glibenclamide on a bare carbon paste electrode (CPE) and a Sephadex-modified carbon paste electrode (SMCPE) was explored by cyclic and differential pulse voltammetry (DPV). The analysis procedure consisted of an open circuit accumulation step in stirred sample solution of Britton-Robinson buffer (0.04 mol L^–1, pH 2.0). This was followed by medium exchange to a clean solution of Britton-Robinson buffer (0.04 mol L^–1, pH 5.0), and subsequently an anodic potential scan was effected to obtain the voltammetric peak. The glibenclamide oxidation peak current obtained by DPV was proportional to the concentration of the glibenclamide in the range of 1.0×10^–9 mol L^–1 to 5.0×10^–8 mol L^–1 for 180 s accumulation time, with a detection limit of 4.0×10^–10 mol L^–1. A method was developed for the determination of glibenclamide in formulation and spiked human serum. Moreover, the proposed procedure was used to estimate the serum concentrations after oral administration of a 5 mg tablet of glibenclamide to three diabetic subjects.     

Determination of medecamycin by adsorptive stripping voltammetry with an activated electrode

The adsorptive and electrochemical behaviors of medecamycin were investigated on a glassy carbon electrode (GCE) pretreated by anodic oxidation at +1.8 V for 5 min in 0.025 mol l^–1 NH₃-NH₄Cl (pH 8.6) solution. An adsorptive stripping voltammetric method for the determination of medecamycin at the pretreated glassy carbon electrode has been developed. Medecamycin was accumulated in NH₃-NH₄Cl buffer (pH 9.0) at a potential of –0.7 V (vs. saturated calomel electrode (SCE)) for a certain time, and then determined by second-order differential anodic stripping voltammetry. The second-order differential anodic stripping peak current at +0.72 V was proportional to the concentration of medecamycin in the range 2.0 g ml^–1 to 50.0 g ml^–1. The detection limit (three times the signal-to-noise) was 1.0 g ml^–1 and the relative standard deviation of the results was 3.28% for eight successive determinations of 10.0 g ml^–1 medecamycin. This method has been applied to the direct determination of medecamycin in commercial tablets and spiked urine samples with satisfactory results.     

The New Fluorescence Enhancement System Eu<Superscript>3+</Superscript>-ARADE-HMTM-Al<Superscript>3+</Superscript> and its Analytical Application

A new bis-Schiff base ligand, N,N-bis-(4-N-aminothiourea-2-amylidene)-4,4-diaminodiphenyl ether (ARADE), was synthesized. Its complex with Eu³⁺ in DMF emits the intrinsic fluorescence of Eu³⁺. The fluorescence intensity of the Eu³⁺-ARADE system was enhanced about 60-fold by the addition of hexamethylene tetramine (HMTM) and potash alum. This is a new fluorescence enhancement phenomenon. The excitation and emission wavelengths are 375nm and 615nm, respectively. Under optimal conditions, the fluorescence intensities vary linearly with the concentration of Eu³⁺ in the range of 7.8×10^–9–1.75×10^–5mol·L^–1 with a detection limit of 5.2×10^–9mol·L^–1, or with the concentration of Al³⁺ in the range of 3.1×10^–7–9.7×10^–5mol·L^–1 with a detection limit of 2.7×10^–7mol·L^–1. The interferences of some rare earth metals and other inorganic ions were described. This method was applied to the determination of Eu³⁺ in high purity yttrium oxide and the analysis of Al³⁺ in alloy steel standard samples. The mechanism of fluorescence enhancement was also studied.     

Determination of ametryn in soils via microwave-assisted solvent extraction coupled to anodic stripping voltammetry with a gold ultramicroelectrode

An extraction-anodic adsorptive stripping voltammetric procedure using microwave-assisted solvent extraction and a gold ultramicroelectrode was developed for determining the pesticide ametryn in soil samples. The method is based on the use of acetonitrile as extraction solvent and on controlled adsorptive accumulation of the herbicide at the potential of 0.50 V (vs. Ag/AgCl) in the presence of Britton-Robinson buffer (pH 3.3). Soil sample extracts were analysed directly after drying and redissolution with the supporting electrolyte but without other pre-treatment. The limit of detection obtained for a 10 s collection time was 0.021 g g^–1. Recovery experiments for the global procedure, at the 0.500 g g^–1 level, gave satisfactory mean and standard deviation results which were comparable to those obtained by HPLC with UV detection.     

Adsorptive Catalytic Voltammetry of Physcion in the Presence of Dissolved Oxygen at a Carbon Paste Electrode

A novel electroanalytical method for the determination of physcion is described for the first time. Physcion yields an adsorption catalytic voltammetric peak at –0.74V (vs. SCE) in 0.4molL^–1 NH₄Cl–NH₃·H₂O buffer solution (pH 10.5) at a carbon paste electrode (CPE). The experimental results indicated that physcion is efficiently accumulated at a CPE by adsorption. In the subsequent potential scan, physcion was reduced to a homologous anthrahydroquinone compound. The compound was then immediately oxidized to physcion by the dissolved oxygen in the solution, and then physcion was again reduced at the CPE. As a result, a cyclic catalytic reaction was established. The second-order derivative peak current is proportional to the physcion concentration in the ranges of 2.0×10^–104.0×10^–9molL^–1 (accumulation 90s) and 4.0×10^–92.0×10^–8molL^–1 (accumulation 60s). The limit of detection is 8×10^–11molL^–1 (S/N=3) for a 120s accumulation time. The method was applied to the direct determination of physcion in the medicinal plant polygonum multiflorum Thumb with satisfactory results.     

Voltammetric method for the determination of H2O2 in rainwater

Summary Presented below is an electrochemical method for the determination of hydrogen peroxide levels in the atmospheric liquid water. The hydrogen peroxide concentration is determined by a voltammetric method involving a rotating disk electrode. The oxidation limiting current at +0.4 V/SCE is proportional to the concentration of hydrogen peroxide found in 1 mol/l KNO₃ containing a phosphate buffer, pH 7.5. An analytical blank is prepared in situ by addition of catalase enzyme to avoid interferences. The detection limit obtained is 5×10^–9 mol/l.     

Electrochemical behavior of a covalently modified glassy carbon electrode with aspartic acid and its use for voltammetric differentiation of dopamine and ascorbic acid

Aspartic acid was covalently grafted on to a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation of the amino-containing compound. X-ray photoelectron spectroscopic (XPS) measurement and cyclic voltammetric experiments proved the aspartic acid was immobilized as a monolayer on the GCE. Electron transfer to Fe(CN)₆^4– in solution of different pH was studied by cyclic voltammetry. Changes in solution pH resulted in the variation of the charge state of the terminal group; surface pK_a values were estimated on the basis of these results. Because of electrostatic interactions between the negatively charged groups on the electrode surface and dopamine (DA) and ascorbic acid (AA), the modified electrode was used for electrochemical differentiation between DA and AA. The peak current for DA at the modified electrode was greatly enhanced and that for AA was significantly reduced, which enabled determination of DA in the presence of AA. The differential pulse voltammetric (DPV) peak current was linearly dependent on DA concentration over the range 1.8×10^–6–4.6×10^–4 mol L^–1 with slope (nA mol^–1 L) and intercept (nA) of 47.6 and 49.2, respectively. The detection limit (3) was 1.2×10^–6 mol L^–1. The high selectivity and sensitivity for dopamine was attributed to charge discrimination and analyte accumulation. The modified electrode has been used for determination of DA in samples, in the presence of AA, with satisfactory results.     

Reactions of superoxide anion radical with antioxidants and their use in voltammetry

Kinetic parameters were calculated for the electrochemical reduction of oxygen at a glassy-carbon electrode with the generation of superoxide radical anions in a 0.05 M solution of (C₂H₅)₄NI in dimethylformamide in the presence of fat-soluble antioxidants, retinol and -tocopherol. A procedure based on the protonation of the radical anion with antioxidant molecules is proposed for the voltammetric determination of antioxidants to determine milligram amounts of retinol and -tocopherol in model solutions (RSD = 1–2%). The calibration graphs for retinol and -tocopherol are linear in the concentration ranges 9.7 × 10^–5–2.3 × 10^–3 and 6.2 × 10^–4–3.1 × 10^–3 M, respectively. The detection limits for retinol and -tocopherol are 4.8 × 10^–5 and 4.1 × 10 ^–4 M, respectively. The procedure was applied to the determination of the active component (retinol and -tocopherol) in pharmaceuticals.Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 1, 2005, pp. 56–59.Original Russian Text Copyright © 2005 by Ziyatdinova, Gilmetdinova, Budnikov.     

Spectroscopic and voltammetric studies of Pefloxacin bound to calf thymus double-stranded DNA

Spectral and electrochemical studies have been carried out on the interaction of pefloxacin with calf thymus double-stranded dsDNA. The voltammetric behavior of pefloxacin was investigated at glassy carbon, carbon paste and dsDNA-modified carbon paste electrodes using cyclic voltammetry. Pefloxacin was oxidized, yielding one irreversible oxidation peak. The modification of the carbon paste surface with dsDNA allowed an accumulation process to take place for pefloxacin such that higher sensitivity was achieved compared with the bare surface. The response was characterized with respect to ionic strength, accumulation time, pefloxacin concentration, and other variables. The stripping differential pulse voltammetric response showed a linear calibration curve in the range 1.0×10^–7–1.0×10^–5 mol l^–1 with a detection limit of 5.0×10^–8 mol l^–1 at the dsDNA modified electrode. The method was applied to the direct determination of pefloxacin in diluted urine samples.     

Spectrophotometric and first-derivative spectrophotometric determination of cadmium witho-hydroxybenzenediazoaminoazobenzene (HDAA)

The synthesis ofo-hydroxybenzenediazoaminoazobenzene (HDAA) is described. Cadmium forms with HDAA in the presence of Triton X-100 a 13 complex, which gives a maximum absorption at 520nm with an apparent molar absorptivity of 1.97 × 10⁵1 · mol^–1 · cm^–1 in pH 10 borax buffer solution and 1.52 × 10⁵1 · mol^–1 · cm^–1 in ammoniacal medium. In both media, Beer's law is followed in the range of 0 –10 g of cadmium in 25ml of solution and the coefficients of variation do not exceed 1.5%. A derivative method has been employed to determine cadmium in certain waste water samples without separation.     

Studies on the voltammetric behaviour of salicyl fluorone and gallium-salicyl fluorone complex

In the NH₄Cl supporting electrolyte, within the pH range from 1 to 5, an irreversible adsorptive reducing wave of salicyl fluorone(SAF) was obtained. The electrode process was verified as follow: On the surface of mercury electrode, the adsorption of SAF obeys Frumkin isotherm.In 0.2 mol/l potassium hydrogen phthalate/HCl buffer solution, at pH 3.0, the sensitive adsorptive complex wave of Ga-SAF was obtained by linear sweep voltammetry. The composition of the electroactive complex was determined as Ga:SAF = 11. The peak height of the complex is proportional to the concentration of Ga(III) in the range of 1.5 × 10^–9 to 6.0 × 10^–7 mol/l, the detection limit is 1.0× 10^–9 mol/l. The proposed method has been applied to the determination of gallium content in aluminium alloys.     

Membrane sensors for batch and flow injection potentiometric determination of ethamsylate (cyclonamine) in pharmaceutical preparations

The lipophilic nickel(II) and iron(II) bathophenanthroline derivatives of ethamsylate are used as ion-exchangers with high selectivity characteristics for ethamsylate. Poly(vinyl chloride) membrane sensors incorporating these electroactive materials display fast linear response for 1 × 10^–1–1 × 10^–4 M ethamsylate under static and hydrodynamic modes of operation. In an acetate buffer of pH 4, the calibration slope is 51–53 mV/concentration decade and the lower limit of detection is 5.3 g/ml. Except for salicylate and nitrate, most common anions, organic sulfonates, carboxylates, phenolates and various pharmaceutical excipients and diluents do not interfere. Determination of ethamsylate in various dosage forms shows an average recovery of 98.9% of the nominal and a mean standard deviation of 0.7%.     

Linear scan stripping voltammetry of copper(II) at the chemically modified carbon paste electrode

A new chemically modified electrode (CME), -benzoinoxime (CUPRON) modified carbon paste electrode, for determining copper(II) is reported because of its excellent selectivity and sensitivity. The electrode is made by mixing a quantity of CUPRON (25%, w/w) with graphite powder (50%, w/w) and paraffin oil (25%, w/w). The CME preferentially deposits copper from the pH 8.5 NH₃-NH₄Cl buffer solution containing copper(II) under an open circuit and most of metal ions do not interfere with the measurements. The detection limit (S/N of three) for determining Cu(II) is 3 × 10^–10 g/ml after 10 min accumulation in fast linear scan stripping voltammetric measurement. Linear calibration curves are obtained for Cu(II) concentration ranged from 1 × 10^–8 M to 1 × 10^–6 M. The response can be maintained with relative standard deviation of 6.0% in a 5 × 10^–6 M Cu(II) solution after eight accumulation/measurement/ regeneration cycles at the same electrode surface. The effect resulted from carbon paste preparation, reduction potential, electrode renewal, electrolyte and solution pH, preconcentration time, concentration dependence, possible interference and other variables has been evaluated. As for application, the CME demonstrates its high sensitivity and copper-selectivity in complex composition samples, such as anodic mud and polluted water.