Voltammetric behavior of L-cysteine in the presence of CPB at a silver electrode

The voltammetric behavior of L-cysteine at a silver electrode is described. L-Cysteine can be anodically accumulated at a silver electrode surface as a sparingly soluble silver salt; at more negative potentials, the insoluble compound is stripped cathodically yielding a small current peak at about -1.10 V (vs. SCE). In the presence of cetyl pyridine bromide (CPB), the stripping peak shifts slightly to a more negative potential, and the peak height increases significantly. Thus, the peak becomes more useful for the determination of L-cysteine. In contrast to other surfactants, CPB can improve the accumulation and stripping of L-cysteine obviously. The voltammetric behavior of cysteamine, 3-mercaptopropionic acid and homocysteine is discussed as well.     

Voltammetric behavior of L-cysteine in the presence of CPB at a silver electrode

The voltammetric behavior of L-cysteine at a silver electrode is described. L-Cysteine can be anodically accumulated at a silver electrode surface as a sparingly soluble silver salt; at more negative potentials, the insoluble compound is stripped cathodically yielding a small current peak at about –1.10 V (vs. SCE). In the presence of cetyl pyridine bromide (CPB), the stripping peak shifts slightly to a more negative potential, and the peak height increases significantly. Thus, the peak becomes more useful for the determination of L-cysteine. In contrast to other surfactants, CPB can improve the accumulation and stripping of L-cysteine obviously. The voltammetric behavior of cysteamine, 3-mercaptopropionic acid and homocysteine is discussed as well.     

Stripping voltammetric determination of silver(I) at carbon paste electrode modified with 3-amino-2-mercapto quinazolin-4(3H)-one

A differential pulse anodic stripping voltammetric method was developed for the determination of Ag(I) at a 3-amino-2-mercapto quinazolin-4(3H)-one modified carbon paste electrode. The analysis procedure consisted of an open circuit accumulation step in stirred sample solution for 12 min. This was followed by medium exchange to a clean solution where the accumulated Ag(I) was reduced for 15 s in −0.6 V. Subsequently an anodic potential scan was effected from −0.2 to +0.2 V to obtain the voltammetric peak. The detection limit of silver(I) was 0.4 μg L⁻¹ and R.S.D. for 10, 100 and 200 μg L⁻¹ silver(I) were 2.4, 1.8 and 1.3%, respectively. The calibration curve was linear for 0.9-300 μg L⁻¹ silver(I). Many coexisting ions had little or no effect on the determination of silver(I). The procedure was applied to determination of silver(I) in X-ray photographic films and natural waters. In X-ray photographic film samples, the results have compared to those obtained by atomic absorption spectroscopy.     

Adsorptive voltammetric determination of copper with a benzoin oxime graphite paste electrode

A method for the determination of trace amounts of copper with a chemically modified carbon press-formed electrode is described. Copper could be accumulated at the electrode by complexing with benzoin oxime in ammonia buffer, then reduced at a constant potential of −0.4 V (vs. SCE) in nitric acid solution. Finally, a well-defined stripping peak could be obtained by scanning the potential in a positive direction. The response depends on the concentration of copper and accumulation time. For an accumulation time of 5 min the detection limit is about 1 ng/ml and the linear range is from 2 ng/ml to 4000 ng/ml, with a relative standard deviation of 5%. Many common metal ions have little or no effect on the determination of copper. The recommended procedure was applied to the determination of trace amounts of copper in natural water, and the results are in agreement with those of atomic-absorption spectrometry.     

Preconcentration and electroanalysis of silver at pt electrode modified with poly(2-aminothiazole)

Conducting poly(2-aminothiazole) (PAT) films were electrodeposited on a platinum disc electrode surface by constant potential electrolysis of 2-aminothiazole (AT) for the stripping voltammetric determination of Ag(I). Ag(I) was preconcentrated on the polymer matrix by dipping the modified Pt electrode (PAT-Pt electrode) into Ag(I)(aq) solution. Effects of the film thickness, reduction potential, pH, preconcentration time, Ag(I) concentration and the interference of some other metal ions on the oxidation peak current of silver were studied. Cu(II) interference observed to be significant for the stripping voltammetric determination of Ag(I). The detection limit was calculated on the basis of signal to noise ratio of 3 as 2 × 10^?7 mol L^?1.     

Anodic stripping voltammetry of silver(I) using a carbon paste electrode modified with multi-walled carbon nanotubes

We describe a silver(I)-selective carbon paste electrode modified with multi-walled carbon nanotubes and a silver-chelating Schiff base, and its electrochemical response to Ag(I). Effects of reduction potential and time, accumulation time, pH of the solution and the stripping medium were studied by differential pulse anodic stripping voltammetry and optimized. The findings resulted in a method for the determination of silver over a linear response range (from 0.5 to 235 ng?mL^?1) and with a detection limit as low as 0.08 ng?mL^?1. The sensor displays good repeatability (with the RSD of ±?2.75 % for 7 replicates) and was applied to the determination of Ag(I) in water samples and X-ray photographic films.

Catalytic Adsorptive Stripping Chronopotentiometric Determination of Hexavalent Chromium at a Silver Amalgam Film Electrode of Prolonged Application

The paper reports on the application of stripping chronopotentiometry for the determination of chromium(VI) at a silver‐based amalgam film electrode. It is the first attempt to apply such a detection system for the speciation of chromium. The procedure utilized catalytic reduction of nitrate ions induced by the instantaneous chromium(III)‐DTPA complex, accumulated at the electrode surface. The chronopotentiometric step is realized in the constant current mode. Several key parameters, such as the deposition potential, deposition time, nitrate concentration and stripping current were optimized. The detection limit obtained for 15 s of accumulation time was estimated at 0.025 µg/L. The repeatability of the signal was 6.1 %.     

Preparation of a carbon ceramic electrode modified by 4-(2-pyridylazo)-resorcinol for determination of trace amounts of silver

A 4-(2-pyridylazo)-resorcinol (PAR)-modified carbon ceramic electrode (CCE) prepared by the sol-gel technique has been reported for the first time in this paper. By immersing the CCE in aqueous solution of PAR (0.001 mol L⁻¹), after a short period of time, a thin film of PAR was rapidly formed on the surface of the electrode due to its strong adsorption properties. A differential pulse anodic stripping voltammetric (DPASV) method was developed for determination of Ag(I) at the modified carbon ceramic electrode. The analysis procedure consisted of an open circuit accumulation step in a sample solution which was continuously stirred for 12 min. This was followed by replacing the medium with a clean solution where the accumulated Ag(I) was reduced for 15 s in −0.6 V. Then, the potential was scanned from −0.2 to +0.2 V to obtain the voltammetric peak. The detection limit of silver(I) was 0.123 μg L⁻¹, and for seven successive determinations of 10, 100 and 200 μg L⁻¹ Ag(I), the relative standard deviations were 2.1, 1.4 and 1.03%, respectively. The calibration curve was linear for 0.5-300 μg L⁻¹ silver(I). The procedure was applied to determine silver(I) in X-ray photographic films and super-alloy samples.     

Flow potentiometric and constant-current stripping analysis for silver(I) with carbon- and platinum-fibre electrodes

At concentrations above 50 μg l^?1, silver(I) is determined in nitric acid medium by means of potentiostatic deposition onto a platinum-fibre electrode and subsequent constant-current stripping in the sample or potentiometric stripping in a potassium permanganate medium. Interference from copper(II) is reduced by a pulsed potential procedure whereby copper deposited onto the fibre electrode is reoxidized intermittently. At concentrations below 50 μg l^?1, silver(I) is determined by using a mercury-coated carbon-fibre electrode and constant-current stripping in acetonitrile containing 0.20 M perchloric acid. Potentiostatic deposition for 30 min yielded a detection limit of 0.24 μg l^?1 silver(I) at the 3σ level.     

Determination of cobalt by adsorptive stripping voltammetry with double accumulation and stripping steps

A double accumulation and stripping steps were proposed to increase the sensitivity of Co(II) determination by catalytic adsorptive stripping voltammetry (AdSV). Electrodes with large and small surface area were used for the first and second accumulation step, respectively. As the accumulation of Co(II) complex at the first electrode was finished, the electrode was placed at a short distance opposite the second one. Then the Co(II) complex desorbed from the first electrode was accumulated at the second electrode. Taking into account the small volume of space between the electrodes, the concentration of the Co(II) complex in solution between the electrodes was drastically higher than that in the bulk solution. The accumulation step at the second electrode was performed from the solution with higher concentration of Co(II) and therefore the detection limit was lowered. The calibration graph of Co(II) determination for accumulation time of 120 s at both electrodes was linear from 1.18 to 58.9 ng L^?1. The detection limit for Co(II) was equal to 0.47 ng L^?1 and it is so far the lowest detection limit obtained for Co(II) using mercury-free electrodes. The proposed method was applied to Co(II) determination in water certified reference material.     

Microanalysis of oligodeoxynucleotides by cathodic stripping voltammetry at amalgam-alloy surfaces in the presence of copper ions

The application of gold amalgam-alloy electrode (AuAE) for a sensitive voltammetric detection of different oligodeoxynucleotides (ODNs) containing the purine units within the ODN-chains in the presence of copper is described. The detection of ODNs is based on the following procedure: (i) the first step includes an acidic hydrolysis of the ODN (ahODN) samples performing the release of the purine bases from ODN-chain; (ii) the second step includes an electrochemical accumulation of the complex of the purine base residues released from ODN-chain with copper ions Cu(I) (ahODN-Cu(I) complex) at the potential of reduction of copper ions Cu(II) on the amalgam-alloy electrode surfaces; (iii) finally followed the cathodic stripping of the electrochemically accumulated ahODN-Cu(I) complex from the electrode surface. The proposed electrochemical method was used for: (a) detection of different ODN lengths containing only adenine units (the number of adenine units within the ODN-chains was changed from 10 to 80), and (b) determination of the number of purine units within the 30-mer ODNs containing a random sequence segments involving both the purine and pyrimidine units. The intensity of the cathodic stripping current density peak (j_CSP) of the electrochemically accumulated ahODN-Cu(I) complex increased linearly with the increasing number of purine units within the ODN-chains. We observed a good correlation between the percentage content of purine units to the whole length of different 30-mer ODNs and the percentage content of the intensity of the j_CSP of the electrochemically accumulated 30-mer ahODN-Cu(I) complexes. The detection of acid hydrolysed 80-mer (A₈₀) in the bulk solution and in a 20-μl volume is possible down to 200 pM and 2 nM at the AuAE, respectively. For the shortest 10-mer (A₁₀) a detectable value of 5 nM in the bulk solution on the AuAE was observed. The sensitive detection of different ODNs containing the purine units in their chains in the presence of copper can be also performed at the platinum amalgam-alloy (PtAE) and copper amalgam-alloy (CuAE) contrary to a lower sensitivity at the silver amalgam-alloy (AgAE) electrode.     

New Protocol for Determination of Rifampicine by Adsorptive Stripping Voltammetry

For determinations of organic compounds by adsorptive stripping voltammetry till now the same material of the electrode has been used for the accumulation and stripping steps. This paper describes a new protocol for extending the range of organic compounds, which can be determined by adsorptive stripping voltammetry. In the proposed procedure the accumulation step was performed on the electrode modified by a lead film, which assures adsorption of the studied species on the modified electrode and then the stripping step of the accumulated substance was performed on the support of the lead film electrode. As an example rifampicine was accumulated by adsorption at the lead film electrode while in the stripping step lead film and then the accumulated rifampicine were oxidized at a glassy carbon electrode. Using an acetate buffer as a supporting electrolyte a calibration graph for rifampicine in the range from 2.5×10^?10 to 1×10^?8 mol L^?1 was obtained. The detection limit for rifampicine following 60 s accumulation time was equal to 9×10^?11 mol L^?1. The obtained detection limit was comparable or lower than reported previously for other stripping voltammetric procedures. The proposed procedure was applied to rifampicine determination in pharmaceutical preparation.     

Electrochemical study and detection of perphenazine using a gold electrode modified with decanethiol SAM

A novel method for the determination of perphenazine has been developed. The method is based on the accumulation of perphenazine at a gold electrode modified with decanethiol (DEC) self-assembled monolayer (SAM) and its oxidation at about 0.6 V (vs. saturated calomel electrode (SCE)). Because some coexistent electroactives were blocked and perphenazine was selectively accumulated by the SAM, the electrode exhibited good selectivity and sensitivity. Various conditions were optimized for practical application. Under the selected conditions (i.e. 0.05 M pH 10 sodium borate buffer, accumulation time: 120 s, accumulation potential: −0.4 V, scan rate: 100 mV s⁻¹), the anodic stripping peak current was linear to perphenazine concentration in the ranges of 6×10⁻⁹–5×l0⁻⁷ and 5×10⁻⁷–5×10⁻⁶ M with correlation coefficients of 0.998 and 0.995, respectively. For a 1.0×10⁻⁶ M perphenazine solution, the relative standard deviation of peak height was 2.3% (n=8). This method was applied to the determination of perphenazine in some drugs and the recovery was 92–101%. In addition, it was found that in the presence of perphenazine, the SAM structure changed a little and more needle holes appeared. However, the SAM could recover the original form when perphenazine and its redox product were removed from the monolayer by repeatedly cycling the electrode in a blank solution for a minute. The modified electrode was characterized by alternating current impedance and electrochemical probe.     

Stripping voltammetry of silver ions at polythiophene-modified platinum electrodes

The present work describes the development of a modified platinum electrode for stripping voltammetric determination of silver. The deposition of films based on electropolymerisation of the monomer thiophene was carried out by cycling the potential towards positive values between 0 and 1.6 V.The preconcentration process of silver ions was initiated on the surface of the modified electrode by complexing silver with polythiophene (PTH) when a negative potential (−0.5 V) was applied; then the reduced products was oxidized by means of differential pulse stripping voltammetry and the peak was observed at 0.17 V. Parameters such as pH, supporting electrolyte and number of electropolymerisation cycles were studied. A linear relation between current peak and concentration of Ag(I) was obtained in the range 0.07-1.0 mg L⁻¹. The detection limit for Ag(I) was evaluated to be 0.06 mg L⁻¹. The reproducibility was tested carrying out 11 measurements at different electrodes and the relative standard deviation was 1.5%. The interference of several metals was investigated and showed negligible effect on the electrode response.     

Silver(I) ion-selective membrane based on Schiff base-p-tert-butylcalix[4]arene

A PVC membrane electrode for silver(I) ion based on Schiff base-p-tert-butylcalix[4]arene is reported. The electrode works well over a wide range of concentration (1.0 x 10(-5)-1.0 x 10(-1) mol dm-3) with a Nernstian slope of 59.7 mV per decade. The electrode shows a fast response time of 20 s and operates in the pH range 1.0-5.6. The sensor can be used for more than 6 months without any divergence in the potential. The selectivity of the electrode was studied and it was found that the electrode exhibits good selectivity for silver ion over some alkali, alkaline earth and transition metal ions. The silver ion-selective electrode was used as an indicator electrode for the potentiometric titration of silver ion in solution using a standard solution of sodium chloride; a sharp potential change occurs at the end-point. The applicability of the sensor to silver(I) ion measurement in water samples spiked with silver nitrate is illustrated.     

Preconcentration and voltammetric determination of trace myoglobin at a 6-mercaptopurine modified silver electrode

6-Mercaptopurine(6-MP) is covalently modified onto a silver electrode to construct a chemically modified electrode (CME). It exhibits the capability of selectively complexing myoglobin and can be applied as a selective biosensor for this compound. Myoglobin is accumulated onto the CME at 0.32 V (vs. SCE); after exchanging the medium it is determined by differential pulse stripping voltammetry. A cathodic stripping peak is obtained at 0.15 V (vs. SCE) by scanning the potential from 0.35 to –0.1 V. The peak currents are linearly proportional to the concentration of myoglobin in the range of 0.2 ～ 4 μg/mL with a relative standard deviation of 7.8%. The detection limit is about 0.1 μg/mL. The mechanism of the complexation is also discussed.     

Anodic stripping voltammetric determination of silver ion at a carbon paste electrode modified with carbon nanotubes

A carbon paste electrode (CPE) was modified with multi-wall carbon nanotubes and successfully applied to the determination of silver ion by differential pulse anodic stripping voltammetry. Compared to a conventional CPE, a remarkably improved peak current response and sensitivity is observed. The analytical procedure consisted of an open circuit accumulation step for 2?min in ?0.4?V, this followed by an anodic potential scan between +0.2 and?+?0.6?V to obtain the voltammetric peak. The oxidation peak current is proportional to the concentration of silver ion in the range from 1.0?×?10^?8 to 1.0?×?10^?5?mol?L^?1, with a detection limit of 1.8?×?10^?9?mol?L^?1 after an accumulation time of 120?s. The relative standard deviation for 7 successive determinations of Ag(I) at 0.1???M concentration is 1.99%. The procedure was validated by determining Ag(I) in natural waters.

Chelate adsorption for trace voltammetric measurements of 2-thiouracil and 4-thiouridine

A very sensitive electrochemical stripping method for trace measurements of 2-thiouracil and 4-thiouridine in presence of Cu(II) is described. The chelate of Cu(II) with 2-thiouracil and 4-thiouridine is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated complex is measured by cathodic stripping voltammetry. The adsorption and redox behaviour are indicated by cyclic voltammetry. Optimum experimental conditions include a preconcentration potential of 0.0 V, solution of pH 7.2, adsorption time 5 min, pulse amplitude 100 mV, and a linear scan mode. The sharp chelate peak, associated with the effective interfacial accumulation, coupled with the flat baseline, facilitates measurements at the nanomolar and submicromolar concentration levels.     

Anodic stripping voltammetric determination of bismuth(III) using a Tosflex-coated mercury film electrode

A Tosflex-mercury film electrode (TMFE) was prepared by spin-coating a solution of the perfluorinated anion exchange polymer Tosflex onto a glassy carbon electrode surface followed by electrodeposition of mercury film on this electrode. This electrode was used for the determination of trace bismuth(III) which was preconcentrated onto the TMFE as anionic bismuth(III) complexes with chloride in a chloride medium. The preconcentration was carried out at a potential of-0.2 V, and the preconcentration of the bismuth(III) was enhanced significantly by the anion-exchange feature of Tosflex. The accumulated bismuth(III) was then determined by anodic square-wave stripping voltammetry (SWSV). Various parameters influencing the determination of bismuth(III) were examined in detail. With 2 min accumulation, the analytical signal versus concentration dependence was linear up to 50 ppb, and the detection limit was 0.58 ppb. This modified electrode showed good resistance to the interferences from surface-active compounds and common ions.     

Studies on the voltammetric behaviour of a 2-mercaptoimidazole containing carbon paste electrode: determination of traces of silver

The voltammetric behaviour of a 2-mercaptoimidazole (2-MI) containing carbon paste electrode was studied. When mixed to carbon paste as an electrode modifier, 2-MI can be reduced at negative potentials (–1 V vs. SCE), but it does not give a response in the potential range where Ag(0) is oxidized to Ag(I). Silver could be accumulated from 0.1 mol l^–1 acetate buffer onto a 2-MI modified carbon paste electrode without a potential applied; after medium exchange, it was reduced at –1 V vs. SCE in 0.1 mol l^–1 acetate buffer solution and determined by differential pulse anodic stripping voltammetry. With suitable preconcentration times, the detection limit was 0.1 g l^–1; a linear relation between current and concentration was found to exist within a range of 0.5 to 1000 g l^–1. In the presence of EDTA, common metal ions have no or only little effect on the voltammetric determination of silver.