Applications of small iridium-based mercury electrodes to high concentrations of depolarizers

Voltammetric and microscopic data on the analytical properties of small iridium-based (127- μm diameter) mercury film electrodes are presented. A method of estimating and controlling the size of the deposit is described. The voltammetric behaviour of electrodes with mercury deposits varying from a film to a near-hemisphere is discussed for linear-scan, differential- and normal- pulse voltammetry. The systems Pb²⁺/Pb, Fe³⁺/Fe²⁺ and Co²⁺/Co, are used to show that, by changing the size of the deposit, either time-dependent or almost time-independent (steady-state) voltammograms can be obtained under the same instrumental conditions. Possible applications to the determination of high concentrations of depolarizers are outlined; voltammetric data for the determination of lead in the range 10–120 mM are presented. The evaluation of diffusion coefficients is discussed.     

Voltammetric behaviour of ascorbic acid at a graphite—epoxy composite electrode chemically modified with cobalt phthalocyanine and its amperometric determination in multivitamin preparations

Cyclic voltammetry was used to investigate the electrochemical behaviour of ascorbic acid at a carbon—epoxy composite electrode modified with the electron mediator cobalt phthalocyanine. The modified electrode reduced the overpotential necessary for the oxidation of the vitamin by approximately 150 mV to 0.21 V vs. The saturated calomel electrode; the process was dependent on the pH of the supporting electrolyte, but independent of ionic strength over the range studied. The relative standard deviation (r.s.d.) of the peak heights of the cyclic voltammograms was 0.81% for a 1 × 10^?4 M ascorbic acid solution (n = 7). The optimum supporting electrolyte was found to be 0.05 M phosphate buffer (pH 5).Amperometry in stirred solutions was then done at an applied potential of +0.25 V. The limit of detection was 0.65 ng ml^?1 and the calibration graph was linear in the range 175 ng ml^?1?50 μg ml^?1. The method was used to determine ascorbic acid concentrations in single- and multivitamin preparations; the recovery was 97.86% for the vitamin added to one preparation. The r.s.d. for the analyses in these samples was about 5%. For comparison, the vitamin was also determined in these tablets using LC with UV detection at 254 nm; the correlation coefficient for the levels determined was 0.9989 (p = 0.0007).     

Analytical properties of the amidoxime group: Part VI. Electroanalytical behaviour of ethanediamidoxime

The electroanalytical behaviour of ethanediamidoxime (EDA) has been studied at 0.1 mol l^?1 ionic strength and several pH values (1.75–8.62), using several techniques (dc and DP polarography, chronocoulometry, dc and DP cyclic voltammetry). EDA undergoes an irreversible diffusion-controlled reduction in the entire pH range considered. The best conditions for identification and determination of EDA with the techniques mentioned are also studied.     

Electrooxidation of dextromethorphan on a carbon nanotube–carbon microparticle–ionic liquid composite: applied to determination in pharmaceutical forms

The electrooxidation of dextromethorphan on a composite constructed with carbon nanotube–ionic liquid–carbon microparticles was investigated by cyclic voltammetry in a 100 mM phosphate buffer solution, pH 7.40. In the voltammograms, an irreversible diffusion-controlled anodic peak appeared. The diffusion coefficient of dextromethorphan, the electron-transfer coefficient, and the standard rate constant of the electrooxidation process were found to be 3.45?×?10^?6 cm² s^?1, 0.65, and 1.67?×?10^?3 cm s^?1, respectively. A sensitive and timesaving determination procedure was developed for the analysis of dextromethorphan, and the corresponding analytical parameters were reported. Using this method, dextromethorphan was determined with an LOD and LOQ of 8.81 and 29.36 μM in a linear range of 2.5?×?10^?4 to 3.3?×?10^?3 M, respectively. The proposed amperometric method was successfully applied to the analysis of commercial pharmaceutical products (syrup and oral drop), and the results were in good agreement with the declared values.     

Indirect cathodic reduction of dispersed indigo by 1,2-dihydroxy-9,10-anthraquinone-3-sulphonate (Alizarin Red S)

The indirect cathodic reduction of dispersed indigo (Vat Blue 1) with 1,2-dihydroxy-9,10-anthraquinone-3-sulphonate (Alizarin Red S) as soluble mediator system was studied in 0.1 M NaOH by cyclic voltammetry, voltammetry in a flow cell and in galvanostatic reduction experiments. In cyclic voltammetry, the presence of 17.1 mM indigo led to an increase in the diffusion-controlled cathodic peak current (I _p)_d by a factor of 2. During the reverse scan of the voltammograms the oxidation of reduced indigo could be observed at −650 mV (vs. Ag/AgCl, 3 M KCl). In voltrammograms of 4.0 mM ALS in 0.1 M NaOH, recorded in a flow cell, a current density of 0.40–0.46 mA cm⁻² was determined for the diffusion-controlled cathodic current plateau, which appeared in the potential range of −850 to −1,050 mV. In galvanostatic batch electrolysis, solutions containing 2.5–3.8 mM reduced indigo were prepared and analysed by spectrophotometry and tested in dyeing experiments. The dyeing behaviour of the reduced indigo was independent of the reduction technique used. Energy consumption for electrochemical reduction of 1 kg of indigo could be estimated to 6.5 kWh kg⁻¹.     

A new experimental method to distinguish two different mechanisms for a category of oscillators involving mass transfer

We present new experimental evidence that further confirms that a combination of electrochemical reactions and diffusion–convection (ERDC) mass transfer accounts for the potential oscillations that appear under conditions of transport limited current. A typical example is given for the reduction of Fe(CN)₆³⁻ in alkaline solution accompanying periodic hydrogen evolution. No potential oscillations occur by simply replacing the hydrogen evolution with IO₃⁻ reduction as the second current carrier. That replacement removes only the convection mass transfer induced by the hydrogen evolution, and retains the negative differential resistance (NDR) from the Frumkin repulsive effect. The key role of hydrogen evolution is thus to restore the Fe(CN)₆³⁻ surface concentration after its depleting to zero by diffusion-limited reduction, rather than purely a second current carrier. Therefore, the other mechanism, which emphasizes the NDR from the Frumkin interaction due to electrostatic repulsion, is excluded because it does not have a direct connection with the oscillations. Moreover, a crossing cycle in cyclic voltammograms is a more convincible criterion for this category of electrochemical oscillators than the negative impedance.     

Electrochemical reduction and quantification of menadione in sodium dodecyl sulfate micellar media

Menadione cyclic voltammograms show a pair of redox steps on glassy carbon electrode in 0.1 M H₃PO₄ with potential separation of 343 mV. Cationic, nonionic, and anionic surfactants micellar media significantly decrease the menadione peak potential separation. Statistically significant increase of menadione reduction current (3- and 4.4-fold) has been observed in Triton X 100 and sodium dodecyl sulfate (SDS) micellar media, respectively. Electrochemical reduction of menadione in 9 mM SDS micellar media is reversible diffusion-controlled one-electron process corresponding to formation of relatively stable semiquinone anion radical. The linear dynamic ranges of menadione determination are 7–560 and 600–2,550 μM with the limits of detection and quantification of 1.66 and 5.53 μM, respectively. The current concentration sensitivity is (8.6?±?0.2)?×?10³ μA μM^?1. The voltammetric method for the determination of menadione in pharmaceutical “Aekol” based on preliminary extraction with ethanol has been developed.     

Determination of chlorite at very low levels by using differential pulse polarography

A method is reported for the determination of μgl^?1 levels of chlorite by using differential pulse polarography. The electrochemical reduction of chlorite was studied between pH 3.7 and 14 and in an ionic strength range of 0.05–3.0 M. The optimum conditions are pH 4.1–4.4 and an ionic strength of 0.45 M. The current under these conditions is diffusion-controlled and is a linear function of chlorite concentration ranging from 2.77×10^?7 to 2.80×10^?4 M (19 μgl^?1 to 19 mg l^?1). The imprecision is better than ±1.0% and ±3.4% at concentrations of 2.87×10^?5 M and 1.74×10^?6M, respectively, with a detection limit of 1×10^?7 M (7μgl^?1). An interference study and the application of this method for determining chlorite in drinking water are reported.     

Electrochemical determination of paraquat using a DNA-modified carbon ionic liquid electrode

Deoxyribonucleic acid (DNA) was electrochemically deposited on a carbon ionic liquid electrode to give a biosensor with excellent redox activity towards paraquat as shown by cyclic voltammetry and differential pulse voltammetry. Experimental conditions were optimized with respect to sensing paraquat by varying the electrochemical parameters, solution pH, and accumulation time of DNA. Under the optimized conditions, a linear relation exists between the reduction peak current and the concentration of paraquat in the range from 5?×?10^?8 mol L^?1 to 7?×?10^?5 mol L^?1, with a detection limit of 3.6?×?10^?9 mol L^?1. The utility of the method is illustrated by successful analysis of paraquat in spiked real water samples.

Pre-column chelation liquid chromatographic separation and determination of trace V,Cu, Co,Pd, Fe and Ni

Conditions for the separation by reversed-phase liquid chromatography (LC) of V(V), Cu(II), Co(III), Pd(II), Fe(III) and Ni(II) chelates with 2-(5-bromopyridylazo)-5-diethylaminophenol (5-Br-PADAP) were studied. Six species of metal chelates were separated successfully with methanol-acetonitrile-water (72:12:16, v/v/v) containing 0.13 M NaCl and 0.29 mM cetyltrimethylammonium bromide (pH 5.0) as the mobile phase on a Nucleosil C₁₈ (5 μm) column (250 × 4 mm i.d.).The conditions of the determination of these metal chelates are discussed. A simple and rapid method for the determination of trace amounts of V(V), Cu(II), Co(III), Pd(II) and Ni(II) simultaneously by reversed-phase LC has been developed. The detection limits are 5 × 10^?12, 1 × 10^?10, 3 × 10^?11, 5.3 × 10^?9 and 2 × 10^?10 g, respectively. The method is applied to the determination of these metals in natural waters and mineral samples.     

Study on the growth,defects, and optical properties of Tm:YAP crystal

Tm:YAP crystal was grown by the Czochralski method. Color center absorption phenomenon is significantly weakened in the ultraviolet and visible region. Dominated forced convection or increased crystal diameter may reduce the crystal-like scattering dispersion. The average value of absorption coefficient and cross-section are calculated to be 2.09 cm^?1 and 6.03 × 10^?21 cm², respectively. The described techniques may be useful for the further improvement of the quality of Tm:YAP crystals.     

A coated tubular solid-state chloride-selective electrode in flow-injection analysis

The construction, suitability and behaviour of a coated tubular solid-state chloride ion-selective electrode in continuous flow analysis are discussed. The flow-through cell is constructed easily from Tygon tubing and silver foil, which is treated to give a silver/silver chloride electrode. The optimum contact area is obtained with a tube 5 mm long and of 2 mm internal diameter, for a total flow rate of 4.7 cm³ min^?1; the carrier streams are 1.0 mol dm³ potassium nitrate. Response times are fast (3–4 s for 95% response). With 30-μl samples, the working range covers 5–5000 mg dm³ chloride with a reproducibility better than 1.7% (relative standard deviation); the stability of the system is excellent. Interferences are similar to those found in batch analysis with Ag/AgCl electrodes, but are less severe in the flow system; pH has no effect in the range 2–12. Results obtained for chloride in waters at a sample throughput rate of about 120 h^?1 agree well with results by a standard spectrophotometric automated method.     

Lead adsorption on silver single crystal surfaces

Twin-electrode thin layer experiments have been carried out in the system Ag(hkl)/Pb²⁺, ClO₄^? in order to determine independently the charge and Pb²⁺ coverage fluxes of the lead underpotential adsorption. The electrosorption valency was found to be equal to the charge number of the lead ion. Therefore, the sorption behaviour of the system can be described by the metal monolayer model. The peak potentials of the cyclic voltammograms as well as the saturation coverages are only slightly dependent on the orientation of the silver single crystal surfaces used. Hypothetical superlattice structures for the monolayer formation are discussed. The Kolb-Gerischer linearity rule is considered critically.     

A Polarographic Study on Dipyridamole and its Determination in Commercial Tablets

Abstract

The direct current polarographic behaviour of dipyridamole, a coronary vasodilator, in the supporting electrolyte consisting of 0.2 M KCl, 20 % (v/v) ethanol and buffer solutions is described in this study. Morphologically well-defined DC polarographic curves have appeared within the pH range of 7–8 and the optimum polarographic conditions found were, a potential rate of 4 m V.s^?1, drop time of 1 s and a pressure of 1000 dyne.cm^?2. The factor effecting the polarographic current was diffusion-controlled and the reduction reaction was highly reversible. The effect of concentration on the limiting current was examined using DC, SCAP, SIAP and DP polarographic modes at pH 7.8 and optimum conditions. Linear calibration equations with good correlations were obtained for all techniques. DP polarography was used among the modes for the determination of dipyridamole in the tablets. The results obtained were compared with the results of the UV-spectrophotometric method. Using the Mann-Whitney U-test, it was concluded that there is an insignificant variance between the two methods. The relative standard deviations are 75±1% and 75.3±0.7% for DP polarographic and UV-spectrophotometric methods, respectively. The determination method proposed in this study seems to be accurate, rapid and practical. Therefore, it may be suitable for the content uniformity test (USP XXI).     

Electrochemical behaviour and analytical detection of ammonium ions in alkali nitrate melts

The voltammetric behaviour of ammonium ions was investigated in the (Na, K)NO₃ equimolar melt at 518 K. The voltammograms recorded at platinum, gold and vitreous carbon rotating-disc electrodes under one atmosphere of anhydrous nitrogen are characterized by three consecutive cathodic waves. The rapid disappearance of ammonium ions from the solution can be ascribed mainly to the evaporation of the salt, which is a more rapid process than thermal decomposition under the given experimental conditions. In order to be able to neglect the effect of evaporation and decomposition, the voltammograms were recorded within a few minutes from the addition of NH₄NO₃ to the melt. The experimental ratios between the limiting currents of the three waves recorded at a vitreous carbon rotating-disc electrode, the most suitable for analytical purposes, were found to have a constant value within the explored ammonium concentration range, i.e 8×10^?5 mol kg^?1<[NH₄⁺]<5.0×10^?3 mol kg^?1. A simple mechanistic model is proposed which permits a reasonable interpretation of the experimental results and, in particular, explains the origin of the three waves and their limiting current ratios. The diffusion coefficient of ammonium ions was found to be D=(4.0±0.4) 10^?6 cm² s^?1 at 518 K.     

Differential pulse adsorptive stripping voltammetric determination of methotrexate using a functionalized carbon nanotubes-modified glassy carbon electrode

A glassy carbon electrode (GC) containing multiwalled functionalized carbon nanotubes (MWCNTs) immobilized within a dihexadecylhydrogenphosphate film (DHP) is proposed as a nanostructured platform for determination of methotrexate (MTX) concentration (a drug used in cancer treatment) using differential pulse adsorptive stripping voltammetry (DPAdSV). The voltammograms for a MTX solution using MWCNTs-DHP/GC electrode presented an oxidation peak potential at 0.98 V vs. Ag/AgCl (3.0 mol L^?1 KCl) in a 0.1 mol L^?1 sulphuric acid. The apparent heterogeneous electron transfer rate constant of 0.46 s^?1 was calculated. The recovery area of 2.62×10^?9 mol cm² was also obtained. Under the optimal experimental conditions, the analytical curve was linear in the MTX concentration range from 5.0×10^?8 to 5.0×10^?6 mol L^?1, with a detection limit of 3.3×10^?8 mol L^?1. The MWCNTs-DHP/GC electrode can be easily prepared and was applied for the determination of MTX in pharmaceutical formulations, with results similar to those obtained using a high-performance liquid chromatography comparative method.      

496—Cyclic voltammetric studies on the application of carbon elctrodes in the determination of oxalic acid

The application of carbon paste and glassy carbon electrodes in the analysis of oxalic acid was investigated by comparing the characteristics of cyclic voltammograms of oxalic acid obtained in various supporting electrolytes (acetate, borate, citrate, phosphate, etc.). When a semi-micro carbon paste electrode (area 0.49 cm²) was used, the oxalic acid was oxidized at +1.0 to +1.2 V vs Ag|AgCl yielding current in the μA range (scan rate 50 mV/s) for oxalic acid concentration of approximately 10^?4M. Oxalic acid oxidation was observed under both acidic and alkaline conditions. The presence of chloride ions or oxygen did not have any deleterious effect on the electrode response. The peak current was reproducible for repeated scans obtained with the same electrode after brief stirring. The glassy carbon electrode was found to be less suitable for oxalic acid oxidation studies because the peaks in the voltammograms were poorly defined and the current response was markedly reduced. These investigations suggest that carbon paste electrodes are sensitive and stable for oxalic acid oxidation studies. A plot of the peak currents obtained with carbon paste electrode for different concentrations of oxalic acid between 1×10^?4M to 1×10^?3M was linear and reproducible. It is suggested that a flow through carbon paste electrode coupled to a chromatographic column can be used in the development of a sensitive method for oxalic acid analysis in biological samples.     

Simultaneous Determination of Dopamine and Ascorbic Acid Using the Nano‐Gold Self‐Assembled Glassy Carbon Electrode

Electrochemical behavior of dopamine (DA) was investigated at the gold nanoparticles self‐assembled glassy carbon electrode (GNP/LC/GCE), which was fabricated by self‐assembling gold nanoparticles on the surface of L ‐cysteine (LC) modified glassy carbon electrode (GCE) via successive cyclic voltammetry (CV). A pair of well‐defined redox peaks of DA on the GNP/LC/GCE was obtained at E_pa=0.197 V and E_pc=0.146 V, respectively. And the peak separation between DA and AA is about 0.2 V, which is enough for simultaneous determination of DA and AA. The peak currents of DA and AA were proportional with their concentrations in the range of 6.0×10^?8–8.5×10^?5 mol L^?1 and 1.0×10^?6–2.5×10^?3 mol L^?1, with the detection limit of 2.0×10^?8 mol L^?1 and 3.0×10^?7 mol L^?1 (S/N=3), respectively. The modified electrode exhibits an excellent reproducibility, sensibility and stability for simultaneous determination of DA and AA in human serum with satisfactory result.     

A.C. polarography and faradaic impedance of strongly adsorbed electroactive species: Part I. Theoretical and experimental study of a quasi-reversible reaction in the case of a Langmuir isotherm

The expression of the faradaic impedance is calculated in the case of a quasi-reversible system O+ne R under the following conditions: (a) both the oxidized and the reduced forms are strongly adsorbed; (b) the adsorption rate is large, and does not control the kinetics of the system; (c) the adsorption of both O and R obeys a Langmuir isotherm. The results show that the tangent of the phase angle is proportional to κ_s/ω (κ_s=rate constant of the electrochemical reaction). When ω→o, the phase angle tends towards 90°: the faradaic impedance becomes purely capacitive. The equation of the a.c. polarogram has been derived; whatever κ_s/ω, the peak height is proportional to the bulk concentration of the reactant, to τ^7/6 (τ=drop time), and to h^?1/2 (h=height of the mercury reservoir). When κ_s/ω→∞, the shape of the a.c. wave is identical to that of a “diffusion-controlled” a.c. wave. The experimental results obtained by a.c. polarography for the three systems azo-hydrazobenzene, benzo(c)cinnoline-dihydrobenzo(c)cinnoline and phenazine-dihydrophenazine are in excellent agreement with the theoretical predictions.     

Coupled convection of solution near the surface of ion-exchange membranes in intensive current regimes

Mechanisms responsible for the overlimiting ion transfer in membranes systems are discussed. The overlimiting transfer is shown to be due largely to the action of four effects coupled with the concentration polarization of the system. Two of these are connected with the water dissociation near the membrane/solution interface: the emergence of additional charge carriers (ions H⁺ or OH^?) in the depleted solution layer and the exaltation of transfer of salt counterions. The latter effect is connected with the perturbation of electric field caused by the water dissociation products. The other two effects are two versions of coupled convection, which leads to partial destruction of the depleted diffusion layer. These include gravitational convection and electroconvection. The former is caused by the emergence of the solution’s density gradient. The latter develops via a mechanism of electroosmotic slip. In this work, methods of voltammetry and chronopotentiometry and pH measurements are used to study the transfer of ions through homogeneous membranes Nafion-117 and AMX as a function of the concentration of sodium chloride solutions in the underlimiting and overlimiting current regimes. In a 0.1 M NaCl solution, gravitational convection makes a considerable contribution to the transfer of salt ions near the membrane surface in intensive current regimes. The influence of this effect on the electrochemical behavior of membrane systems weakens with the solution dilution and with increasing relative transfer of the H⁺ and OH^? ions that are generated at the membrane/solution interface. In conditions where gravitational convection is suppressed and the water dissociation near the membrane/solution interface is not great, the major contribution to the overlimiting growth of current is made by electroconvection. Topics for discussion in the paper include the mutual influence of effects on one another, in particular, the effect the rate of generation of the H⁺ and OH^? ions exerts on the gravitational convection and electroconvection and the reasons for the different behavior of cation-and anion-exchange membranes in intensive current regimes.