Square wave voltammetry at disc microelectrodes for characterization of two electron redox processes

Analytical explicit solutions are presented for the use of square wave voltammetry (SWV) at disc microelectrodes to study two-electron reversible redox processes. This combines the advantages of SWV (minimization of capacitative effects, peak-shaped response and quick experiments) with those of microelectrodes (reduction of capacitative and ohmic drop effects, enhanced mass transport and measurements of small volumes). Further, the analytical expressions are very easy to implement in comparison with the numerical methods usually employed for simulation of electrochemical experiments at microdisc electrodes. From the theory, the effects of the technique parameters (frequency, pulse amplitude) are examined and procedures are given for the characterization of the redox system from the values of the peak current, peak potential and half-peak width. Finally, the theory is applied to the experimental study of the two-electron reduction of anthraquinone-2-sulfonate in aqueous media. For this system, the formal potentials of the redox centres in aqueous solutions can be tuned by means of the electrolyte cation.     

Detector for organophosphorus compounds in liquid chromatography based on the cholinesterase inhibition reaction

A sensitive method for the post-column reaction detection of organophosphorus compounds is described. The method relies on cholinesterase and is particularly suitable for the analysis of potent inhibitors such as sarin, soman and tabun. The compounds are separated by reversed-phase chromatography with methanol-water as the mobile phase in a linear gradient system. The reactor used for the detection comprises conventional autoanalyzer equipment with air segmentation of the reactor stream. The detection limits are 10 pg for sarin and soman and 60 pg for tabun. A quantitation method is presented, based on the linear correlation between the residual enzyme activity and the inhibitor concentration. The repeatability is +/- 1%. As a test of the system, the model compounds were detected against a background of urban air.     

Square wave voltammetry of nitrofurans in aqueous media using a carbon fiber microelectrode

The nitrofuran compounds exhibit a wide spectrum of biological action, and the understanding of their reduction mechanism can contribute to a better comprehension of their biological action. In this sense, the electrochemical reduction of nitrofurans has been studied through square wave voltammetry (SWV) in aqueous media, having a carbon fiber microelectrode as working electrode. One single cathodic peak in acidic media is registered for all studied drugs (nitrofurazone, nitrofurantoin, and furazolidone), being the hidroxylamine derivative formation linearly dependent on pH. In alkaline media, the peaks reduction is not pH-dependent, but a split of the original into two distinct peaks is observed. The evaluation of the results obtained in unprecedented form by SWV for different alkaline pH media, suggests that the charge transfer process for the R-NO₂ ^?? formation is followed by irreversible chemical reaction, which chacraterizes an EC mechanism.     

Optimisation of FIA system for detection of organophosphorus and carbamate pesticides based on cholinesterase inhibition

The sensitivity of the bioanalytical FIA system containing different immobilised cholinesterases (AChEs from electric eel, human erythrocytes, bovine erythrocytes and BuChE from horse serum) for determination of organophosphorus and carbamate pesticides was tested. Responses to some frequently used organophosphorus (paraoxon, oxydemeton-methyl, triazophos) and carbamate (carbofuran, propoxur) pesticides were found to be dependent on the origin of cholinesterases. The highest sensitivity was obtained by bioanalytical columns prepared with electric eel AChE while the lowest sensitivity was shown by the bioanalytical columns prepared with horse serum BuChE. The differences in responses for different enzymes were found to be less pronounced when the contact time between the enzyme and the pesticide is long enough (low flow rates). The optimal flow rate was chosen as a compromise between the duration of analysis and reasonably low limits of detection.     

Phase behavior of the lecithin/water/isooctane and lecithin/water/decane systems

The isothermal pseudo-ternary-phase diagram was determined at 25 degrees C for systems composed oflecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (Lalpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with Lalpha (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than 1 year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and Lalpha, (ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.     

Square wave anodic stripping voltammetry determination of eco-toxic metals in samples of biological and environmental importance

Square wave anodic stripping voltammetry was used in simultaneous determinations of eco-toxic metals (Pb, Cd, Cu and Zn) on bismuth film electrodes. The electrodes were prepared in situ on a glassy-carbon electrode (GCE) from 0.1 M acetate buffer (pH 4.5) containing 200 μg L^?1 of bismuth (III), as well ex situ on electrochemically oxidized graphitized polyacrylonitrile carbon fibres from 200 mg L^?1 Bi(NO₃)₃ in 1% HNO₃ (aqueous) solution. Preparation of a Bi-modified carbon fibre electrode (CFE) was by cation exchange of Bi⁺³ ions for H⁺ of the acidic surface groups of the electro-oxidized carbon fibres, followed by electrochemical reduction to Bi⁰. For the Bi-GCE the linear range was 20–280 μg L^?1 for zinc, 10–100 μg L^?1 for lead, 10–80 μg L^?1 for copper, and 5–50 μg L^?1 for cadmium. For the Bi-CFE it was 20–160 μg L^?1 for zinc, 10–100 μg L^?1 for lead, 10–100 μg L^?1 for copper, and 2–120 μg L^?1 for cadmium. For both kinds of bismuth modified carbon electrodes, low limits of detection and satisfactory precision were achieved. The method was successfully applied to certified reference materials of biological (bovine liver) and environmental (mussel tissue) importance.      

Solvolysis of benzoyl halides in water/NH4DEHP/isooctane microemulsions

A study was carried out on the solvolysis reactions of different benzoyl halides in microemulsions of water/NH4DEHP/isooctane, where NH4DEHP is ammonium bis(2-ethylhexyl) phosphate. Because of the low solubility of benzoyl halides in water, they are distributed between the continuous medium and the interface of the microemulsion, where the reaction takes place. The application of the pseudophase model has allowed us to obtain the distribution constants and the rate constants at the interface for the benzoyl halides. Reaction mechanisms and the changes in these mechanisms in terms of the water content of the microemulsion have been determined on the basis of kinetic data. The influence of the substituent and the leaving group on the reaction rate has been investigated. A comparison of kinetic results with those previously obtained in water/AOT/isooctane microemulsions allows a kinetic evaluation of the change in the microemulsion properties with the surfactant.     

Fluorescent behavior of B-phycoerythrin in microemulsions of aerosol OT/water/isooctane

Taking advantage of its unusual fluorescent properties, the incorporation of B-phycoerythrin (B-PE) in aerosol OT (AOT, sodium bis-(2-ethylhexyl) sulphosuccinate)/water/isooctane microemulsions was investigated by following their steady-state and time-resolved fluorescence as a function of the water-to-surfactant molar ratio, w(0). The fluorescent intensity at 575 nm increased continuously with increasing water content, saturating at a w(0) around 35 and staying practically constant at w(0)> or =40. The steady-state anisotropy showed an initial increase with increasing water content until w(0)=23 and then decreased strongly, staying practically constant when w(0)> or =40. The values of the fluorescent parameters, anisotropy and fluorescent intensity, were unchanged when the water content of the system increased in the range between w(0)=40 to 50. This implies the effective incorporation of B-PE in the microemulsion droplets with w(0)> or =40, as well as the equilibrium of the dispersion at these water/surfactant ratios, since higher water content does not affect the main surrounding microenvironment of the protein. The overall incorporation in the microemulsion droplets caused minor spectroscopic changes with respect to biliprotein in aqueous solution of 20 mM sodium phosphate buffer, pH 7.0, such as a blue absorption shift of 3 nm and an emission shift of 1.5 nm, as well as a slight increase in excitation anisotropy spectrum mainly caused by a decrease in protein mobility. Therefore, there are no important interactions between the chromophores and the AOT sulfonate head groups. Emission intensity decays followed complex kinetics in both aqueous and dispersion media. The stability with time and temperature of the biliprotein in the microemulsion was higher than in the aqueous solution. All the results can be explained in terms of B-PE inclusion in the water droplets of AOT microemulsions where the protein has similar configuration and conformation to that in aqueous solution but with the chromophores more protected.     

MesoDyn Simulation Study on Phase Diagram of Aerosol OT/isooctane/water System

There is a growing interest in the study of surfactant self-assemble in oil/water/surfactant system because of their applications not only in traditional colloid chemistry but in analytical, synthetic, and medicinal chemistry as well1,2. In these systems, one of the most commonly studied surfactants which can form reverse micelles is sodium bis(2-ethylhexyl) sulfosuccinate, i.e. Aerosol OT (AOT)3. The properties of the AOT reverse micelles have been discussed by some experimental methods4…     

Application of flow microcalorimetry to analytical problems-I. Determination of organophosphorus pesticides by inhibition of cholinesterase

Flow microcalorimetry has been applied to the determination of organophosphorus pesticides by inhibition of cholinesterase. One direct inhibitor (TEPP) and one latent inhibitor (parathion) were investigated. The former is determinable at concentrations of about 10(-6)M and the latter at about 10(-4)M. The inhibitory power of parathion is increased if methanol is used as solvent.     

Square wave voltammetry for analytical determination of paracetamol using cobalt microparticles film modified platinum electrode

Cobalt microparticles (Co MPs) modified Pt electrode is simply and conveniently fabricated. The electrochemical properties of paracetamol (PCT) at the prepared modified electrode are investigated using cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements. Based on these techniques, a sensitive and rapid electrochemical method is developed for the determination of PCT. The result indicates that the oxidation of PCT is strongly improved at the Co MPs/Pt electrode as compared with the bare Pt electrode, with relatively high sensitivity, stability and life time. The determination of PCT on the Co MPs/Pt with square wave voltammetry displays a high sensitivity of 101 μA/mM and a low detection limit of 0.42 μM (S/N = 3) in the range (0.5–100 μM). The sensitivity of the modified electrode for the detection of PCT is almost 17 times greater than on the bare Pt electrode. The proposed method is successfully applied to the PCT determination in tablets.     

Square wave voltammetry based on determination of copper (II) ions by polyluteolin- and polykaempferol-modified electrodes

Applicability of square wave voltammetry for the determination of Cu(II) ions by PolyLut/GC and PolyKae/GC electrodes was evaluated in this study. For this luteolin and kaempferol were electrochemically polymerized on glassy carbon (GC) electrode surface in order to get polyluteolin and polykaempferol-modified glassy carbon electrodes (PolyLut/GC and PolyKae/GC, correspondingly). The formation of polyphenol layer on the GC electrode surface was evidenced by atomic force microscopy. Square wave voltammetry was found to be more sensitive in comparison with differential pulse voltammetry. It was determined that PolyLut/GC and PolyKae/GC electrodes offered great sensitivity towards Cu(II) ions with very low limit of detection, good reproducibility, sufficient stability and excellent selectivity of analytical signal.     

Influence of anionic surfactants on the electric percolation of AOT/isooctane/water microemulsions

A study was carried out concerning the influence of sodium alkyl sulfonates on the electric percolation of AOT/isooctane/water microemulsions ([AOT] = 0.5 M and W = [H2O]/[AOT] = 22.2). An important effect was observed with regard to the percolation temperature caused by the addition of small quantities of alkyl sulfonates (rho = [alkyl sulfonate]/[AOT] = 0.01). The short chain alkyl sulfonates (C3-C5) cause an increase in the percolation temperature, which in turn is reduced as we increase the chain length of the additive until we obtain a percolation temperature which is lower than that which is observed in the absence of an additive (C6-C8). For hydrocarbon chains of a greater length we can observe a new increase in the percolation temperature (C10-C18). This behavior has been explained as a consequence of (i) the incorporation of the additives at the interphase of the microemulsion and (ii) the geometric parameters of the different surfactants added to the microemulsion.     

Square wave adsorptive cathodic stripping voltammetry automated by sequential injection analysis Potentialities and limitations exemplified by the determination of methyl parathion in water samples

This paper describes the development and evaluation of a sequential injection method to automate the determination of methyl parathion by square wave adsorptive cathodic stripping voltammetry exploiting the concept of monosegmented flow analysis to perform in-line sample conditioning and standard addition. Accumulation and stripping steps are made in the sample medium conditioned with 40 mmol L⁻¹ Britton-Robinson buffer (pH 10) in 0.25 mol L⁻¹ NaNO₃. The homogenized mixture is injected at a flow rate of 10 μL s⁻¹ toward the flow cell, which is adapted to the capillary of a hanging drop mercury electrode. After a suitable deposition time, the flow is stopped and the potential is scanned from −0.3 to −1.0 V versus Ag/AgCl at frequency of 250 Hz and pulse height of 25 mV. The linear dynamic range is observed for methyl parathion concentrations between 0.010 and 0.50 mg L⁻¹, with detection and quantification limits of 2 and 7 μg L⁻¹, respectively. The sampling throughput is 25 h⁻¹ if the in line standard addition and sample conditioning protocols are followed, but this frequency can be increased up to 61 h⁻¹ if the sample is conditioned off-line and quantified using an external calibration curve. The method was applied for determination of methyl parathion in spiked water samples and the accuracy was evaluated either by comparison to high performance liquid chromatography with UV detection, or by the recovery percentages. Although no evidences of statistically significant differences were observed between the expected and obtained concentrations, because of the susceptibility of the method to interference by other pesticides (e.g., parathion, dichlorvos) and natural organic matter (e.g., fulvic and humic acids), isolation of the analyte may be required when more complex sample matrices are encountered.     

Square wave voltammetric determination of hydrazine and 4-chlorophenol as two important water pollutants using nanostructure-amplified sensor

Research on Chemical Intermediates - Hydrazine and 4-chlorophenol are two major water pollutants, and determination of them is very important for human health. A new hydroquinone derivative, i.e....     

Square wave adsorptive stripping voltammetry of molybdenum(VI) in continuous flow at a wall-jet mercury film electrode sensor

An adsorptive stripping voltammetry method for the determination of traces of molybdenum(VI) in flowing solution at a wall-jet electrode sensor has been developed. After adsorption of a molybdenum complex on a wall-jet mercury film electrode, the complex is reduced by a square wave scan. More satisfactory results were obtained using 8-hydroxyquinoline as a complexing agent in nitrate medium than using Toluidine Blue in oxalic acid. Enhanced sensitivity was achieved by optimizing adsorption time and square wave parameter values. The detection limit of Mo(VI) was found to be at the nanomolar level. Interference of some other metallic species in the determination of nanomolar Mo(VI) was also investigated: Cu(II), Zn(II), Mn(II) do not interfere at 10 muM, whereas 1 muM FeEDTA(-) causes an increase in peak current. This iron interference was removed effectively with citric acid.     

Dielectric model and theoretical analysis of cationic reverse micellar solutions in CTAB/isooctane/n-hexanol/water systems

Dielectric relaxation spectra of CTAB reverse micellar solutions, CTAB/isooctane/n-hexanol/water systems with different concentrations of CTAB and different water contents, were investigated in the frequency range from 40 Hz to 110 MHz. Two striking dielectric relaxations were observed at about 10(4) Hz and 10(5) Hz, respectively. Dielectric parameters were obtained by fitting the data using the Cole-Cole equation with two Cole-Cole dispersion terms and the electrode polarization term. These parameters show different variation with the increase of the concentration of CTAB or the water content. In order to explain the two relaxations systematically and obtain detailed information on the systems and the inner surface of the reverse micelles, an electrical model has been constituted. On the basis of this model, the low-frequency dielectric relaxation was interpreted by the radial diffusion of free counterions in the diffuse layer with Grosse model. For the high-frequency dielectric relaxation, Hanai theory and the corresponding analysis method were used to calculate the phase parameters of the constituent phases in these systems. The reasonable analysis results suggest that the high-frequency relaxation probably originated from the interfacial polarization. The structural and electrical information of the present systems were obtained from the phase parameters simultaneously.     

Spectrometric study of AOT-hydrolysis reaction in water/AOT/isooctane microemulsions using phenolphthalein as a chemical probe

The kinetics of the alkaline hydrolysis of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in water/AOT/isooctane microemulsions has been studied by monitoring the absorbance change of the phenolphthalein in the system with time. The apparent first-order rate constant k(obs) has been obtained and found to be dependent on both the molar ratio of water to AOT ω and the temperature. The dependences of k(obs) on ω have been analyzed by a pseudophase model which gives the true rate constants k(i) of the AOT-hydrolysis reaction on the interface and the partition coefficients K(wi) for the distribution of OH(-) between aqueous and interface pseudophases at various temperatures; the latter is almost independent of the temperature and ω. The temperature dependences of the reaction rate constants k(obs) and k(i) have been analyzed to obtain enthalpy ΔH(≠), entropy ΔS(≠), and energy E(a) of activation, which indicate that the distribution of OH(-) between aqueous and interface pseudophases increases ΔS(≠) but makes no contribution to E(a) and ΔH(≠). The influence of the overall concentration of AOT in the system on the rate constant has been examined and found to be negligible. It contradicts with what was reported by García-Río et al. (1) but confirms that the first-order reaction of the AOT-hydrolysis takes place on the surfactant interface. The study of the influence of AOT-hydrolysis on the kinetics of the alkaline fading of crystal violet or phenolphthalein in the water/AOT/isooctane microemulsions suggests that corrections for the AOT-hydrolysis in these reactions are required.     

Micelles and reverse micelles in the nickel bis(2-ethylhexyl) sulfosuccinate/water/isooctane microemulsion

The ternary system Ni(2+)(AOT)(2) (nickel 2-bis[2-ethylhexyl] sulfosuccinate)/water/isooctane presents w/o and o/w microemulsions with a Winsor progression (2Phi-3Phi-2Phi), without the addition of salt; the "fish diagram" was obtained for alpha=0.5 and gamma=0.02-0.22. Using static and dynamic light scattering the micellar size, the ratio of water to surfactant, and the density of micelles for this system were estimated. In addition, the mean interfacial curvature as a function of temperature was obtained.