A novel chemically modified carbon paste electrode based on a new mercury(II) complex for selective potentiometric determination of bromide ion

A new modified carbon paste electrode based on a recently synthesized mercury (II) complex of a pyridine containing proton transfer compound as a suitable carrier for Br⁻ ion is described. The electrode has a linear dynamic range between 3.00×10⁻² and 1.0×10⁻⁵ M with a near-Nernastian slope of 61.0±0.9 mV per decade and a detection limit of 4.0×10⁻⁶ M (0.32 ppm). The potentiometric response is independent of the pH of the solution in the pH range 4.0–8.3. The electrode possesses the advantages of low resistance, fast response and good over a variety of other anions. It was applied as an indicator electrode in potentiometric titration of bromide ions and for the recovery of Br⁻ from tap water.     

Voltammetric determination of isoprenaline in pharmaceutical preparations using a copper(II) hexacyanoferrate(III) modified carbon paste electrode

The electroanalytical determination of isoprenaline in pharmaceutical preparations of a homemade carbon paste electrode modified with copper(II) hexacyanoferrate(III) (CuHCF) was studied by cyclic voltammetry. Several parameters were studied for the optimization of the sensor such as electrode composition, electrolytic solution, pH effect, potential scan rate and interferences in potential. The optimum conditions were found in an electrode composition (in mass) of 15% CuHCF, 60% graphite and 25% mineral oil in 0.5 mol l⁻¹ acetate buffer solution at pH 6.0. The analytical curve for isoprenaline was linear in the concentration range from 1.96×10⁻⁴ to 1.07×10⁻³ mol l⁻¹ with a detection limit of 8.0×10⁻⁵ mol l⁻¹. The relative standard deviation was 1.2% for 1.96×10⁻⁴ mol l⁻¹ isoprenaline solution (n=5). The procedure was successfully applied to the determination of isoprenaline in pharmaceutical preparations; the CuHCF modified carbon paste electrode gave comparable results to those results obtained using a UV spectrophotometric method.     

Electrode reactions of palladium(II) in chloride solution at carbon paste electrodes modified with derivatives of N-benzoylthiourea

The study of the electrode reactions of palladium(II) at non-modified carbon paste electrodes (CPEs) in chloride solution has revealed the existence of a chloropalladate(II) complex at the electrode surface. The complex is formed during the application of anodic potentials after preceding palladium deposition. In the present paper the electrode reactions of Pd^II at CPEs modified with some N′,N′-disubstituted derivatives of N-benzoylthiourea [as selective ligands for palladium(II)] are studied in chloride solution by cyclic voltammetry. Two reduction peaks are observed in the cathodic scans recorded after deposition of palladium and anodization of the electrode. From the results it is concluded that [in addition to the chloropalladate(II) complex, observed at the non-modified electrode] a second palladium complex is formed at positive potentials. The formation of the palladium(II) complex of the N-benzoylthiourea derivatives by ligand exchange at the electrode surface is assumed. The ligand exchange itself occurs without charge transfer across the electrode|solution interface; therefore, it cannot be detected electrochemically. After palladium deposition and anodic treatment a pronounced "inverse" peak (i.e., an anodic peak in the cathodic scan) with peak currents up to 100 μA is observed at about +0.8 V. Its peak current increases with the amount of deposited palladium and the number of cycles. The reactions at the electrode surface are discussed. The results of the study reveal the existence of two different surface complexes of palladium(II) at ligand-modified CPEs, but the surface reactions could not be elucidated in detail. Electronic Publication     

Trace analysis of cefotaxime at carbon paste electrode modified with novel Schiff base Zn(II) complex

Cefotaxime a third generation cephalosporin drug estimation in nanomolar concentration range is demonstrated for the first time in aqueous and human blood samples using novel Schiff base octahedral Zn(II) complex. The cefotaxime electrochemistry is studied over graphite paste and Zn(II) complex modified graphite paste capillary electrodes in H₂SO₄ (pH 2.3) using cyclic voltammetry and differential pulse voltammetry. Cefotaxime enrichment is observed over Zn(II) complex modified graphite paste electrode probably due to interaction of functional groups of cefotaxime with Zn(II) complex. Possible interactions between metal complex and cefotaxime drug is examined by UV-vis and electrochemical quartz crystal microbalance (EQCM) techniques and further supported by voltammetric analysis. Differential pulse voltammetry (DPV) with modified electrode is applied for the determination of cefotaxime in acidified aqueous and blood samples. Cefotaxime estimation is successfully demonstrated in the range of 1-500 nM for aqueous samples and 0.1-100 μM in human blood samples. Reproducibility, accuracy and repeatability of the method are checked by triplicate reading for large number of samples. The variation in the measurements is obtained less than 10% without any interference of electrolyte or blood constituents.     

Voltammetric determination of N-acetylcysteine using a carbon paste electrode modified with copper(II) hexacyanoferrate(III)

The preparation and electrochemical characterization of a carbon paste electrode modified with copper(II) hexacyanoferrate(III) (CuHCF) as well as its behavior as electrocatalyst toward the oxidation of N-acetylcysteine were investigated. The electrochemical behavior of the modified electrode and the electrooxidation of N-acetylcysteine were explored using sweep linear voltammetry. The best voltammetric response was observed for a paste composition of 20% (w/w) copper(II) hexacyanoferrate(III) complex, acetate buffer solution at pH of 6.0 as the electrolyte and scan rate of 10 mV s^− 1. A linear voltammetric response for N-acetylcysteine was obtained in the concentration range from 1.2 × 10^− 4 to 8.3 × 10^− 4 mol L^− 1, with a detection limit of 6.3 × 10^− 5 mol L^− 1. The proposed electrode is useful for the quality control and routine analysis of N-acetylcysteine in pharmaceutical formulations.     

Thiocyanate-selective electrode based on N-salicylidene-benzylamineato copper(II) complex

Highly selective poly(vinyl chloride) (PVC) membrane electrode based on N-salicylidene-benzylamineato copper(II) complexes [Cu(SBA)₂] as new carriers towards thiocyanate-selective electrode was reported. The influence of membrane composition, pH and possible interfering anions were investigated on the response properties of the electrode. The resulting electrode exhibits anti-Hofmeister selectivity sequence: SCN⁻ > ClO₄⁻ > Sal⁻ > I⁻ > Br⁻ > NO₃⁻ > NO₂⁻ > SO₃²⁻ > H₂PO₄⁻ > Cl⁻ > SO₄²⁻, and a near-Nernstian potential linear range for thiocyanate from 1.0 × 10⁻¹ to 9.0 × 10⁻⁷ M with a detection limit of 7.0 10⁻⁷ M and a slope of , over a wide pH range of 3.0–9.0 in phosphate buffer solution at 20°C. The proposed electrode has a fast response time of about 5–10 s and can be used for at least 3 months without any considerable divergence in potential. The electrode was successfully applied to the determination of thiocyanate in waste water and human urine and saliva samples. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 3, pp. 293–299. The text was submitted by the authors in English.     

A selective modified carbon paste electrode for determination of cyanide using tetra-3,4-pyridinoporphyrazinatocobalt(II)

A chemically modified carbon paste electrode with 3,4-tetra pyridinoporphirazinatocobalt(II) (Co(3,4 tppa) was applied to the determination of free cyanide ion. The electrode has a linear range between 1.5 × 10⁻⁵ M and 1.0 × 10⁻² M with a Nernstian slope of 60 ± 1.5 mV/decade and its detection limit is 9 × 10⁻⁶ M. The response time of electrode is 5 min. The proposed electrode was applied successfully for the determination of cyanide in commercially available spring water. Some anions, such as SCN⁻, I⁻, Cl⁻, Br⁻ and oxalate that are usually serious interfering species for most of cyanide selective electrodes, did not have any interfering effect for this proposed electrode.     

Stripping voltammetric determination of mercury(II) at antimony-coated carbon paste electrode

A new procedure was elaborated to determine mercury(II) using an anodic stripping square-wave voltammetry at the antimony film carbon paste electrode (SbF-CPE). In highly acidic medium of 1 M hydrochloric acid, voltammetric measurements can be realized in a wide potential window. Presence of cadmium(II) allows to separate peaks of Hg(II) and Sb(III) and apparently catalyses reoxidation of electrolytically accumulated mercury, thus allowing its determination at ppb levels. Calibration dependence was linear up to 100 ppb Hg with a detection limit of 1.3 ppb. Applicability of the method was tested on the real river water sample.     

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.     

Iodide-selective carbon paste electrodes based on recently synthesized Schiff base complexes of Fe(III)

A new modified carbon paste electrode (CPE) based on a recently synthesized Schiff base complex of Fe(III) as a suitable carrier for I⁻ ion is described. The electrode exhibits a super Nernstian slope of 71.0±0.3 mV per decade for I⁻ ion over a wide concentration range from 1.0×10⁻⁶ to 5.0×10⁻¹ M, with a low detection limit of 6.5×10⁻⁷ M. It has a relatively fast response time, a satisfactory reproducibility and relatively long life time. The proposed sensor shows a fairly good selectivity toward I⁻ ion in comparison to other common anions. The potentiometric response is independent of the pH of the test solution in the pH range 3.5–10.0. Spectrophotometric studies confirmed the redox-type response mechanism of the electrode toward iodide ion. The proposed electrode was used as an indicator electrode in potentiometric titration of iodide ion.     

Simultaneous determination of lead,copper, and mercury at a modified carbon paste eletrode containing humic acid

The modified carbon paste electrode (CPE) responding simultaneously to lead(II), copper(II), and mercury(II) ions has been constructed by incorporating humic acid (HA) into the graphite powder with Nujol oil. Simple immerging of the electrode into the measuring solution containing these metal ions led to the chemical deposition of the ions onto the electrode through the complexation of the ions with HA. Cyclic and differential pulse voltammetry (DPV) characterized the modified electrode's surfaces. Several cyclings of the potential regenerated the electrode (from more positive than the stripping potential of reduced Hg to more negative than the reduction of Pb(II)ion), which was then used for another deposition. After five deposition/measurement/regeneration cycles, the peak current of voltammograns of the analyte decreased slightly. The response reproduced with a 5.1% relative standard deviation. We also applied ihe differential pulse technique to the previously mentioned system. Here, the detection limit tor Pb(II), Cu(II), and Hg(II) ions were 5.0 × 10⁻⁹ M 8.0 × 10⁻⁹ M, and 8.0 × 10⁻⁹ M, respectively, for 20 minutes of deposition time. After pretreatment of silver(I) ion with KC1, we could not observe any interference by other metal ions on the determination of the test ions in aqueous solution. Satisfactory results were acquired for the determination of the test metal ions in certified standard urine reference material SRM's 2670 (trace elements in urine).     

Voltammetric sensor for nitrite determination based on its electrocatalytic reduction at the surface of p-duroquinone modified carbon paste electrode

A p-duroquinone (tetramethyl-p-benzoquinone) modified carbon paste electrode (DMCPE) was employed to study the electrocatalytic reduction of nitrite in aqueous solutions using cyclic voltammetry (CV), double potential-step chronoamperometry, and differential pulse voltammetry (DPV). It has found that under an optimum condition (pH 1.00), the reduction of nitrite at the surface of DMCPE occurs at a potential of about 660 mV less negative than that of an unmodified carbon paste electrode (CPE). The catalytic rate constant, k′_h, based on Andrieux and Saveant theoretical model was calculated as for scan rate 10 mV s^-1. Also, the apparent diffusion coefficient, D _app, was found as 2.5 × 10^–10 and 3.61 × 10^–5 cm² s^-1 for p-duroquinone in carbon paste matrix and nitrite in aqueous buffered solution, respectively. The values for αn_α were estimated to be −0.65 and −0.19 for the reduction of nitrite at the surface of DMCPE and CPE, respectively. The electrocatalytic reduction peak currents showed a linear dependence on the nitrite concentration, and a linear analytical curve was obtained in the ranges of 5.0 × 10^–5 M to 8.0 × 10^–3 M and 6.0 × 10^–6 M to 8.0 × 10^–4 M of nitrite concentration with CV and DPV methods, respectively. The detection limits (2σ) were determined as 2.5 × 10^–5 M and 4.3 × 10^–6 M by CV and DPV methods. This method was also applied as a simple, selective and precise method for determination of nitrite in real samples (the weak liquor from the wood and paper factory of Mazandaran province in Iran) by using a standard addition method.     

Catalytic adsorptive stripping voltammetric determination of copper(II) on a carbon paste electrode

A catalytic adsorptive stripping voltammetric method for the determination of copper(II) on a carbon paste electrode (PCE) in an alizarin red S (ARS)-K₂S₂O₈ system is proposed. In this method, copper(II) is effectively enriched by both the formation and adsorption of a copper(II)-ARS complex on the PCE, and is determined by catalytic stripping voltammetry. The catalytic enhancement of the cathodic stripping current of the Cu(II) in the complex results from a redox cycle consisting of electrochemical reduction of Cu(II) ion in the complex and subsequent chemical oxidation of the Cu(II) reduction product by persulfate, which reduces the contamination of the working electrode from Cu deposition and also improves analytical sensitivity. In Britton-Robinson buffer (pH 4.56±0.1) containing 3.6×10⁻⁵ mol L⁻¹ ARS and 1.6×10⁻³ mol L⁻¹ K₂S₂O₈, with 180 s of accumulation at −0.2 V, the second-order derivative peak current of the catalytic stripping wave was proportional to the copper(II) concentration in the range of 8.0×10⁻¹⁰ to ∼3.0×10⁻⁸ mol L⁻¹. The detection limit was 1.6×10⁻¹⁰ mol L⁻¹. The proposed method was evaluated by analyzing copper in water and soil.     

Diamond paste based electrodes for the determination of Pb(II) at trace concentration levels

Three types of monocrystalline diamond: natural diamond 1 μm, synthetic diamond 50 μm (synthetic-1), and synthetic diamond 1 μm (synthetic-2) were used for electrodes’ construction. The linear concentration ranges recorded for Pb(II), when natural diamond, synthetic-1 and synthetic-2 based electrodes were used were between 10⁻⁹ and 10⁻⁶; 10⁻¹⁰ and 10⁻⁷; and between 10⁻¹⁰ and 10⁻⁸ mol l⁻¹, respectively. Low detection limits which lie between 10 and 100 pmol l⁻¹ proves the sensitivity of the electrodes. It was found that Pb(II) yielded a peak at about +0.3±0.02 V (versus Ag/AgCl) for all the electrodes. Lead was determine with high reliability from water and tea samples at trace concentration levels using the proposed diamond paste based electrodes.     

Synthesis and characterisation of copper(II) hydroxide gels

The formation of copper(II) hydroxide gels from aqueous precursors requires very critical conditions. Gels have been obtained by adding ammonia to aqueous solutions of copper(II) acetate, in the presence of a small amount of sulphate ions. Other salts (chloride, nitrate, sulphate) or bases (NaOH, KOH) lead to precipitation rather than gelation. These gels are actually made of an intimate mixture of acetate-based organic/inorganic polymers and nanometric posjnakite crystals Cu₄(OH)₆(SO₄) · H₂O. Acetate ions and ammonia can be partially removed upon washing, which after drying leads to crystalline copper(II) hydroxide needles deposited on a strongly oriented layer of posjnakite crystals. A theoretical model based on the electronegativity equalisation principle is used to describe these experimental results. It provides a better understanding of the role of complexing anions during the formation of condensed phases.     

Water hyacinth modified carbon paste electrode for simultaneous electrochemical stripping analysis of Cd (II) and Pb (II)

In this study, we demonstrated a highly sensitive electrochemical sensor for the simultaneous detection of Pb (II) and Cd (II) in aqueous solution using carbon paste electrode modified with Eichhornia crassipes powder by square wave anodic stripping voltammetry. The effect of modifier composition, pH, preconcentration time, reduction potential and time, and type of supporting electrolyte on the determination of metal ions were investigated. Pre-concentration on the modified surface was performed at open circuit. The modified electrode exhibited well-defined and separate stripping peaks for Pb (II) and Cd (II). Under optimum experimental conditions, a linear range for both metal ions was from 10 to 5000 μg L^?1 with the detection limits of 4.9 μg L^?1, 2.1 μg L^?1 for Cd(II) and Pb (II), respectively. The modified electrode was found to be sensitive and selective when applied to determine trace amounts of Cd (II) and Pb (II) in natural water samples.     

Glucose biosensors based on the immobilization of copper oxide and glucose oxidase within a carbon paste matrix

The performance of amperometric glucose biosensors based on the dispersion of glucose oxidase (GOx) and copper oxide within a classical carbon (graphite) paste composite is reported in this work. Copper oxide promotes an excellent electrocatalytic activity towards the oxidation and reduction of hydrogen peroxide, allowing a large decrease in the oxidation and reduction overpotentials, as well as an important enhancement of the corresponding currents. Therefore, it is possible to perform the glucose biosensing at low potentials where there is no interference even in large excess of ascorbic acid, uric acid or acetaminophen. The influence of the copper oxide and glucose oxidase content in the paste on the analytical performance of the bioelectrode is discussed. The resulting biosensor shows a fast response, a linear relationship between current and glucose concentration up to 1.35 × 10⁻² M (2.43 g L⁻¹) and a detection limit of 2.0 × 10⁻⁵ M. The effect of the presence of the enzyme in the composite material on the dispersion of the copper oxide particles is also discussed.     

Simultaneous voltammetric analysis of lead,copper and mercury ions by carbon paste electrode based on 1-(3-aminopropyl) imidazole modified polymer

In this study, a carbon paste electrode modified with a novel 1-(3-aminopropyl) imidazole functionalised crosslinked chlorosulfonated poly(styrene)-divinyl benzene polymer was used for selective and sensitive determination of the trace amounts of Pb²⁺, Cu²⁺ and Hg²⁺ ions by square wave anodic stripping voltammetry. The effect of some parameters such as paste composition, pH, preconcentration time, reduction potential and time, type of supporting electrolyte and potential scan rate on the determination of metal ions were investigated to find the optimal conditions. The effective open-circuit accumulation of the studied metal ions was succeeded only by the modification of the carbon paste electrode with functional polymer. For 6 min open-circuit preconcentration, the detection limit of Pb²⁺, Cu²⁺ and Hg²⁺ was found to be 5, 9 and 14 µgL^?1, respectively at 100 mVs^?1. The results confirmed that the lower concentration levels of these trace metal ions can be determined with the increase of preconcentration time and/or potential scan rate. Good detection limits and large dynamic concentration ranges were also obtained for their binary and ternary mixtures. The optimised method was successively applied to determine the concentration of Pb²⁺, Cu²⁺ ions in the tap water sample and Cu²⁺ ion in the waste water sample in the presence of possible interfering species (RSD<1%, recoveries 96–110% for 4 min preconcentration).     

Voltammetric determination of pyridoxine (Vitamin B6) at a carbon paste electrode modified with vanadyl(IV)-Salen complex

The preparation and electrochemical characterization of a carbon paste electrode modified with the N,N-ethylene-bis(salicylideneiminato)oxovanadium(IV) complex [VO(Salen)] as well as its behavior in relation to the oxidation of pyridoxine (Vitamin B₆) are described. The electrochemical behavior of the modified electrode and the electrooxidation of pyridoxine were investigated using cyclic voltammetry. The best voltammetric response was obtained for an electrode composition of 15% (m/m) [VO(Salen)] in the paste, KCl solution of pH 5.5-8.0 and scan rate of 25 mV s⁻¹. A sensitive linear voltammetric response for pyridoxine was obtained in the concentration range of 4.5×10⁻⁴ to 3.3×10⁻³ mol l⁻¹ with a slope of 42.5 μA mmol⁻¹ l, and a detection limit (3σ/slope) of 3.7×10⁻⁵ mol l⁻¹ using linear sweep voltammetry. Among several compounds tested only Vitamin B₁ seems to interfere in the analyte signal. The concentrations of pyridoxine in pharmaceutical formulations using the proposed electrode and an official spectrophotometric method based in the reaction with N,N-diethyl-p-phenylenediamine are in agreement at the 95% confidence level and within an acceptable range of error.     

Voltammetric determination of ciprofloxacin based on the enhancement effect of cetyltrimethylammonium bromide (CTAB) at carbon paste electrode

Herein, an electrochemical method was developed for the determination of ciprofloxacin based on the enhancement effect of cetyltrimethylammonium bromide (CTAB). In pH 7.0 phosphate buffer, a poorly-defined oxidation peak is observed at carbon paste electrode (CPE) for ciprofloxacin. However, the oxidation peak current remarkably increases in the presence of low concentration of CTAB, suggesting that CTAB exhibits obvious enhancement effect to the determination of ciprofloxacin. All the experimental parameters, such as supporting electrolyte, pH value, concentration of CTAB, and accumulation time, were optimized for ciprofloxacin analysis. This new method possesses high sensitivity (detection limit is 5.0 × 10⁻⁸ mol l⁻¹), wide linearity (1.0 × 10⁻⁷−2.0 × 10⁻⁵ mol l⁻¹), rapid response, low cost and simplicity. Finally, this method was successfully employed to detect ciprofloxacin in drugs. The text was submitted by the authors in English.