Voltammetric,potentiometric and amperometric studies with a rotated aluminum wire electrode: The R.Al.E. as indicator electrode in potentiometric and amperometric acid-base titrations

The effect of varying concentrations of fluoride on the potential of the R.AI.E. in acid buffer solutions is reported. In the pH range between 1 and 5.5 the potential becomes 30 mV more negative per unit increase in pH at fluoride concentrations between 10^-3 and 10^-4M. At a given pH the potential becomes 100 mV more negative when the fluoride concentration is increased from 10^-4 to 10^-3it. No depolarization occurs and no reproducible potentials can be measured in phosphate buffers of pH 6 to 8, even in the presence of 10^-3M fluoride. This concentration of fluoride causes depolarization and establishment of reproducible potentials in veronal buffers of reproducible pH 6 to 8. At a pH greater than 9 fluoride has no effect on the electrode potential which now becomes determined by pH. In the presence of fluoride, oxygen shifts the electrode potential to less negative values (mixed potential). Examples of potentiometric and amperometric titrations of strong and weak acids are given with the R.AI.E. as indicator electrode.     

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

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

Redox Properties of the Iron Complexes of Orally Active Iron Chelators CP20, CP502, CP509, and ICL670

Redox cycling of iron is a critical aspect of iron toxicity. Reduction of a low‐molecular‐weight iron(III)‐complex followed by oxidation of the iron(II)‐complex by hydrogen peroxide may yield the reactive hydroxyl radical (OH^.) or an oxoiron(IV) species (the Fenton reaction). Complexation of iron by a ligand that shifts the electrode potential of the complex to either to far below ?350 mV (dioxygen/superoxide, pH=7) or to far above +320 mV (H₂O₂/HO^., H₂O pH=7) is essential for limitting Fenton reactivity. The oral chelating agents CP20, CP502, CP509, and ICL670 effectively remove iron from patients suffering from iron overload. We measured the electrode potentials of the iron(III) complexes of these drugs by cyclic voltammetry with a mercury electrode and determined the dependence on concentration, pH, and stoichiometry. The standard electrode potentials measured are ?620 mV, ?600 mV, ?535 mV, and ?535 mV with iron bound to CP20, ICL670, CP502, and CP509, respectively, but, at lower chelator concentrations, electrode potentials are significantly higher.     

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.     

Electrical Control of Alkanethiols Self-Assembly on a Gold Surface as an Approach for Preparation of Microelectrode Arrays

 It is shown by capacitive monitoring that the self-assembly of alkanethiols on gold electrodes and desorption of these self-assembled monolayers from the electrodes are controlled by the electrode potential. At neutral pH, chemical adsorption of alkanethiols was observed at an electrode potential of +300 mV vs SCE, but only physical adsorption was detected when the electrode potential was −1400 mV vs SCE. At electrode potentials between these values (−300 mV, −600 mV), chemical adsorption of alkanethiols occurred, but the alkanethiol monolayers were not stable in the absence of the alkanethiol in the bulk solution and were desorbed from the gold electrode. The desorption rate was higher at more negative electrode potentials. These results can be used in designing methods for electrically addressable immobilization of different receptors on (micro)electrode arrays. This has been demonstrated by deposition of two different types of alkanethiols onto a two-electrode array. Received June 24, 1998. Revision October 19, 1998.     

Differential Pulse Anodic Stripping Voltammetric Determination of Lead (II) Using Poly Xylenol Orange Modified Electrode

In this work a method for sensitive anodic stripping voltammetric determination of Pb(II) ions using a poly xylenol orange film modified electrode (PXOFME) has been proposed. Poly xylenol orange film (PXOF) was formed on a paraffin impegrenated graphite electrode (PIGE) using electro polymerization method by scanning the potential between −0.5 V to 1.3 V, at a scan rate of 50 mV/s for 30 segments in 0.1 M phosphate buffer solution (PBS) of pH 7. The PXOFME was characterized by scanning electron microscopy (SEM), ATR‐IR spectroscopy, cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The modified electrode has been used to develop a sensitive method for the determination of Pb(II) ions by anodic stripping voltmmetry (ASV). The PXOFME was used to preconcentrate Pb(II) ions through complexation, the complexed metal ions were reduced electrochemically and then stripped anodically from the surface of the electrode. A linear range of 5 μg/L to 413 μg/L with a limit of detection (S/N=3) of 1.6 μg/L was observed for the determination of Pb(II) ions. The method was applied to lead determination in sea water and tap water samples and the results were found to be satisfactory.     

Effect of fluoride anions on gramicidin photoinactivation sensitized by sulfonated aluminum phthalocyanines.

Interaction of potent photodynamic agents, sulfonated aluminum phthalocyanines (AlPcSn where n is a number of sulfonic groups), with biological membranes was studied here using model systems: sensitized photoinactivation of gramicidin channels in planar lipid bilayers and adsorption on lipid monolayers. Fluoride anions known to form complexes with aluminum were found to inhibit both the adsorption of aluminum phthalocyanines on lipid monolayers, as measured with a Langmuir trough by surface pressure and surface potential changes, and photodynamic efficacy of the dyes, as studied by gramicidin channel photoinactivation. The similar effects were caused by the alkalinization of the medium. Fluoride anions appeared to be much more effective in the case of AlPcS4 as compared to AlPcS3. The suppression of the photodynamic potency of aluminum phthalocyanines was attributed to desorption of the dyes from lipid bilayers induced by fluoride or hydroxyl ions. With AlPcS4 an enhancement of the dye aggregation leading to a decrease in the sensitizing activity was probably involved in the fluoride effect as revealed by absorption and fluorescence spectral measurements. Capillary electrophoresis was employed to understand the mechanism of the dye desorption. The results of these experiments indicated that the reduction in the membrane affinity was associated with an increase in the negative charge of the dye molecules due to the binding of fluoride or hydroxyl ions.     

Surface charge and zeta-potential of metabolically active and dead cyanobacteria

Zeta potential and acid-base titrations of active, inactivated, and dead Planktothrix sp. and Synechococcus sp. cyanobacteria were performed to determine the degree to which cell surface electric potential and proton/hydroxyl adsorption are controlled by metabolism or cell membrane structure. Surface OH(-) excess from potentiometric data, showed differences in surface charge between active and dead cyanobacteria from pH 3 to 10. Average zero salt effect pH (pH(pzse)) of 5.8+/-0.1 and 6.3+/-0.1 were obtained for active Planktothrix sp. and Synechococcus sp., respectively. Similarly for dead cyanobacteria pH(pzse) values of 5.8+/-0.1 and 4.6+/-0.1 were obtained. Zeta potentials of active Planktothrix sp. and Synechococcus sp. were positive at alkaline conditions, with a maximum of +13.7+/-1.5 mV at a pH of 9.0+/-0.1 for both species. This positive potential diminished in the presence of 1 mM HCO(-)(3). The zeta potential of Planktothrix sp. and Synechococcus sp. cells was negative at alkaline pH following their exposure to NaN(3), a metabolic inhibitor. The zeta potential of dead cyanobacteria was negative for Planktothrix sp., from pH 2.5 to 10.5, at -30 to -20 mV. Dead Synechococcus sp. exposed to a pH 2.5 solution recorded negative potentials to a minimum of -30 mV at pH 8, but positive potentials were found at higher pH reaching a maximum of +10 mV at pH 9.1. Zeta potentials for dead, but non-acidified Synechococcus sp. remained negative at -30 mV from an initial pH of 5.6 to 10.5, reflecting differences in cell wall structure between these species. These results indicate that Planktothrix sp. and Synechococcus sp. may metabolically control their surface charge to electrostatically attract bicarbonate anions at alkaline pH, required for photosynthesis.     

Voltammetric,potentiometric and amperometric studies with a rotated aluminum wire electrode : Amperometric titration of fluoride with aluminum

The rotated aluminum electrode is a suitable indicator electrode in the amperometric titration of fluoride, Fluoride in concentrations varying between 1.10^-4 and 2.10^-3M was titrated with a standard aluminum nitrate solution in aqueous buffer solutions (pH between 3.6 and 46) Equilibrium was established within 15 mim. The fluoride-aluminum ratio at the (true) end-point was found to increase from 2 2 in 10^-4M fluoride to 28 in. 2.10^-3M fluoride. The titration calculated from the stability constants of the various aluminum-fluoride complexes were found to be in excellent agreement with the experimental curves.From a practical point of view titration in an acetate buffer in 50% alcohol in the presence of 0 5 M potassium or sodium nitrate is recommended. The fluoride-aluminum ratio at the end-point was found to be 5 9 ± 0 1 and independent of the fluoride concentration.     

Voltammetric determination of dihydroxybenzenes at a mechanically renewed electrode made from a graphite-epoxy composite

Differences of potentials of anodic and cathodic peaks (ΔE _p) are determined in cyclic voltammograms of dihydroxybenzene/p-benzoquinone redox systems at an electrode made of a graphite-epoxy composite in a wide pH range. The data obtained (ΔE _p = 29 ± 1 mV) are close to the thermodynamic values for two-electron reversible reactions. This indicates that the electrode mechanically renewed by cutting a 0.5-μm surface layer directly in a test solution exhibits a high activity in such electrochemical reactions. The potentials of anodic and cathodic peaks are proportional to the pH of the supporting electrolyte solution in the range from 1.0 to 9.0. A change of 58 ± 1 mV in E _p per unit pH for all isomers shows that the first stage of the oxidation of each dihydroxybenzene isomer involves one electron and is accompanied by the detachment of one hydrogen ion, that is, an intermediate oxidation product, semiquinone, is formed. Despite the closeness of the potentials of hydroquinone and pyrocatechol peaks (ΔE = 100 mV), a scheme is proposed for the selective voltammetric determination of dihydroxybenzene isomers in a 0.1 M HCl solution in hydroquinone-pyrocatechol, pyrocatechol-resorcinol, and hydroquinone-resorcinol binary mixtures. The concentrations of hydroquinone and pyrocatechol are found from cathodic peaks and that of resorcinol, from the anodic peak. The results are well reproducible and contain no systematic error.     

Electrochemical investigation of a glassy carbon electrode modified with carbon nanotubes decorated with (poly)crystalline gold

Multiwalled carbon nanotubes with nanosized sputtered gold were used to modify a glassy carbon electrode (GCE). The substrate was characterized by scanning electron microscopy (SEM), X-ray diffraction, cyclic voltammetry and amperometry. SEM micrographs indicated an uniform coverage of the carbon nanotubes with nanosized (poly)crystalline gold. Cyclic voltammetry reveals that peak separation of the unmodified GCE in the presence of 1?mM ferricyanide is 131?mV, but 60?mV only for the modified GCE. In addition, the oxidation of NADH (1?mmol?L^?1 solution) begins at negative potentials (around ?100?mV vs. Ag/AgCl), and the anodic peak potential (corresponding to the irreversible oxidation of NADH) is found at +94?mV. The effect of pH on the electrocatalytic activity was studied in the range from 5.4 to 8.0. The relationship between the anodic peak potential and the pH indicated a variation of ?33.5?mV/pH which is in agreement with a two-electron and one-proton reaction mechanism. Amperometry, performed at either ?50 or +50?mV vs. an Ag/AgCl reference electrode, indicates that the modified electrode is a viable amperometric sensor for NADH. At a working potential of +50?mV, the response to NADH is linear in the concentration range from 1 to 100???mol?L^?1, with an RSD of 6% (n?=?4).

The electrooxidation of methanol and related compounds at ruthenium dioxide-coated electrodes

The underlying metal was observed to corrode when a ruthenium dioxide-coated titanium electrode was anodized in an aqueous methanol solution. With a similarly coated platinum electrode peaks were observed on the voltammogram below 1.0 V which were attributed to methanol oxidation on the underlying metal. This effect was more pronounced when the electrode was subjected during cycling to potentials close to 0 V. Rapid oxidation of methanol on RuO₂ was observed at potentials above 1.0 V, the rate at a given potential increasing in an approximately linear manner with increasing alcohol concentration. The rate of reaction also increased with increasing temperature and increasing surface roughness. Tafel slope values were rather high (>100 mV decade^?1) and a mechanism involving anodically generated species such as OH_ads was proposed to account for these results. The variation of activity with pH was similar to that reported earlier for oxygen evolution at these anodes and this was again explained in terms of partial deactivation of the surface due to a combination of proton loss and phosphate ion adsorption at intermediate pH values. The release of carbon dioxide from aqueous solutions of higher alcohols at 25°C confirmed the high oxidizing power of RuO₂ anodes.     

The influence of fluoride on the physicochemical properties of anodic oxide films formed on titanium surfaces

The influence of fluoride (and its concentration) on the electrochemical and semiconducting properties of anodic oxide films formed on titanium surfaces was investigated by performing electrochemical measurements (potentiodynamic/pontiostatic polarization, open circuit potential (OCP), and capacitance measurements) for a titanium/oxide film/solution interface system in fluoride-containing 1.0 M HClO(4) solution. On the basis of the Mott-Schottky analysis, and with taking into account both the surface reactions (or, say, the specifically chemical adsorption) of fluoride ions at the oxide film surface and the migration/intercalation of fluoride ions into the oxide film, the changes in the electrochemical behavior of titanium measured in this work (e.g., the blocked anodic oxygen evolution, the increased anodic steady-state current density, the positively shifted flat band potential, and the positively shifted film breakdown potential) were interpreted by the changes in the surface and the bulk physicochemical properties (e.g., the surface charges, surface state density, doping concentration, and the interfacial potential drops) of the anodic films grown on titanium. The fluoride concentrations tested in this work can be divided into three groups according to their effect on the electrochemical behavior of the oxide films: < or =0.001 M, 0.001-0.01 M, and >0.01 M. By tracing the changes of the OCP of the passivated titanium in fluoride-containing solutions, the deleterious/depassive effect of fluoride ions on the titanium oxide films was examined and evaluated with the parameter of the film breakdown time. It was also shown that the films anodically formed on titanium at higher potentials (>2.5 V) exhibited significantly higher stability against the fluoride attack than that either formed at lower potentials (<2.5 V) or formed natively in the air.     

2,2‐Bis(3‐Amino‐4‐hydroxyphenyl)hexafluoropropane Modified Glassy Carbon Electrodes as Selective and Sensitive Voltammetric Sensors. Selective Detection of Dopamine and Uric Acid

2,2‐bis(3‐Amino‐4‐hydroxyphenyl)hexafluoropropane (BAHHFP) was electro‐polymerized oxidatively on glassy carbon by cyclic voltammetry. The activity of the modified electrode towards ascorbic acid (AA), uric acid (UA) and dopamine (DA) was characterized with cyclic voltammetry and differential puls voltammetry (DPV). The findings showed that the electrode modification drastically suppresses the response of AA and shifts it towards more negative potentials. Simultaneously an enhancement of reaction reversibility is seen for DA and UA. Unusual, selective preconcentration features are observed for DA when the polymer‐modified electrode is polarized at negative potential. In a ternary mixture containing the three analytes studied, three baseline resolved peaks are observed in DPV mode. At physiological pH 7.4, after 5 min preconcentration at ?300 mV, peaks positions were ?0.073, 0.131 and 0.280 V (vs. Ag/AgCl) for AA, DA and UA, respectively. Relative selectivities DA/AA and UA/AA were over 4000 : 1 and 700 : 1, respectively. DA response was linear in the range 0.05–3 μM with sensitivity of 138 μA μM^?1 cm^?2 and detection limit (3σ) of 5 nM. Sensitive quantification of UA was possible in acidic solution (pH 1.8). Under such conditions a very sharp peak appeared at 630 mV (DPV). The response was linear in the range 0.5–100 μM with sensitivity of 4.67 μA μM^?1 cm^?2 and detection limit (3σ) of 0.1 μM. Practical utility was illustrated by selective determination of UA in human urine.     

Voltammetric Determination of Adrenaline Using a Poly(1‐Methylpyrrole) Modified Glassy Carbon Electrode

A voltammetric method using a poly(1‐methylpyrrole) modified glassy carbon electrode was developed for the quantification of adrenaline. The modified electrode exhibited stable and sensitive current responses towards adrenaline. Compared with a bare GCE, the modified electrode exhibits a remarkable shift of the oxidation potentials of adrenaline in the cathodic direction and a drastic enhancement of the anodic current response. The separation between anodic and cathodic peak potentials (ΔEp) for adrenaline is 30 mV in 0.1 M phosphate buffer solution (PBS) at pH 4.0 at modified glassy carbon electrodes. The linear current response was obtained in the range of 7.5 × 10^?7 to 2.0 × 10^?4 M with a detection limit of 1.68 × 10^?7 M for adrenaline by square wave voltammetry. The poly(1‐methypyrrole)/GCE was also effective to simultaneously determine adrenaline, ascorbic acid and uric acid in a mixture and resolved the overlapping anodic peaks of these three species into three well‐defined voltammetric peaks in cyclic voltammetry. The modified electrode has been successfully applied for the determination of adrenaline in pharmaceuticals. The proposed method showed excellent stability and reproducibility.     

Voltammetric Determination of 2′‐Deoxyadenosine and Adenine in Urine of Patients with Hepatocellular Carcinoma Using Fullerene‐C60‐modified Glassy Carbon Electrode

A fullerene‐C₆₀‐modified glassy carbon electrode has been examined for the simultaneous determination of 2′‐deoxyadenosine (2′‐dAdo) and adenine in human blood and urine using Osteryoung square‐wave voltammetry (OSWV) at pH 7.2. Compared to bare glassy carbon electrode (GCE), the modified electrode displays a shift of the oxidation potential in the negative direction with significant increase in the peak current for both the analytes. At modified electrode well‐defined anodic peaks at potential of 1248 mV and 994 mV are observed for 2′‐dAdo and adenine respectively. Linear calibration curves were obtained within the concentration range 10 nM to 100 μM for both the compounds in 0.1 M phosphate buffer solution (PBS) with the limit of detection 0.8×10^?8 M and 0.95×10^?8 M for 2′‐dAdo and adenine respectively. The analytical utility of the present method is demonstrated by quantitative detection of 2′‐dAdo and adenine in human urine of normal subjects as well as in patients with hepatocellular carcinoma. Interfering effect of some coexisting metabolites has also been reported.     

The Use of Buffers in the Determination of Fluoride by an Ion-Selective Electrode at Low Concentrations and in the Presence of Aluminum

Abstract

The influence of the composition of buffer solutions on fluoride analyses by the fluoride ion selective electrode was studied. Residual fluoride content of reagents was observed to restrict the use of some buffers and reagents in low-level work. The optimum pH for low-level fluoride determinations was found to be around 5, which can be maintained by a sodium acetate-acetic acid buffer. Of the complexing agents studied, citrate was observed to be the most efficient masking agent for aluminum, but this ability depended strongly on ligand concentration. Citrate was also effective in masking iron (III) and magnesium ions. Tris (hydroxymethyi) aminomethane showed a similar ability to complex aluminum at a pH around 8. However, at this pH the hydroxyde ion interferes in fluoride analysis below 0.1 ppm [fbar].     

Electrochemical Characterization of Pyrocatechol Sulfonephthalein-Modified Glassy Carbon Electrode and Separation of Electrocatalytic Responses for Ascorbic Acid and Dopamine Oxidation

A pyrocatechol sulfonephthalein- (PS-) modified glassy carbon (PS/GC) electrode has been prepared by adsorption of PS on a glassy carbon electrode surface. Cyclic voltammograms of the PS/GC electrode indicate the presence of a couple of well-defined redox peaks, and the formal potential shifts in the negative direction with increasing solution pH. The relation between formal potential,E^0′, and solution pH can be fit to the equationE^0′(mV) = −51.4 pH + 538.7. The PS/GC electrode shows high electrocatalytic activity toward ascorbic acid oxidation, with an overpotential ca. 380 mV less than that of the bare electrode and a drastic enhancement of the anodic currents. The electrocatalytic reaction rate constant (k), which was decreased with increasing concentration of H₂A, was determined using rotating disk electrode measurements. The values ofkwas also affected by the solution pH. The electrode can also separate the electrochemical responses of ascorbic acid and dopamine. The separation between the anodic peak potentials of ascorbic acid and dopamine is more than 50 mV by the differential pulse voltammetry.     

Studies on the application of Fe-Zr mixed hydrous oxide membranes in ion-selective potentiometry

A solid membrane electrode selective to chloride ions has been prepared from Fe-Zr mixed hydrous oxide (with iron as a major constituent) with polystyrene as binder. Although the response of the electrode is non-Nernstian it can be utilized to estimate chloride in the concentration range 10(-4)-10(-1)M. The potentials generated across the membrane are reproducible within +/-0.2 mV and the response time is a few seconds. The standard deviation of the potential measurements is 0.4 mV at the 0.1M level. The useful pH range is 4-7 and the electrode can also be used in partially non-aqueous systems. The electrode exhibits fairly good selectivity for chloride.