Diffusion-controlled Electrode Reaction Research Articles

Novel disposable screen-printed sensors based on copper oxide nanoparticles for sensitive voltammetric assay of antazoline

The fabrication and electroanalytical validation of a novel homemade screen-printed carbon electrode enriched with copper oxide nanoparticles (CuONPs/SPCEs) were demonstrated for sensitive and selective voltammetric determination antazoline (ANT) in eye drop solution and spiked serum samples. With a reported electrocatalytic activity, CuONPs catalyzed the oxidation of ANT at the electrode surface showing an irreversible anodic oxidation peak at 0.9 V under a diffusion-controlled electrode reaction. Based on the electroanalytical and molecular orbital calculation studies, oxidation of ANT molecule undergoes via oxidation of the aliphatic nitrogen atom within the ANT moiety. Linear calibration curves were illustrated covering the ANT concentration ranged from 0.01 to 3.819 µg mL−1 with and limit of detection 3 ng mL−1. The fabricated sensors enriched with CuONPs exhibited high fabrication and measurement reproducibility with a prolonged shelf lifetime (6 months). The electroanalytical approach presented adequate sensitivity and selectivity with a short analysis time, applying low-cost instrumentation and simple pretreatment protocol avoiding handling hazardous organic solvents. Trace ANT was monitored in the presence of different interfering species, degradation products, and xylometazoline hydrochloride (ZYL) in the co-formulated ophthalmic pharmaceutical samples. The CuONPs based voltammetric sensors were introduced as a reliable eco-friendly analytical tool for monitoring ANT in ophthalmic solutions and biological fluids with acceptable recovery values.

Dynamic electrochemical impedance spectroscopy for the charge transfer rate measurement of the ferro/ferricyanide redox couple on gold

• Rate coefficients of charge transfer are determined by dynamic EIS. • Rate coefficients are presented in a 0.2 V broad range. • By using dEIS, it is easier to preserve cleanliness of the system. • No a priori assumptions on potential dependences of rates are involved. The use of dynamic electrochemical impedance spectroscopy, dEIS is shown in the context of diffusion-controlled electrode reactions. By this method, a number of audio-frequency impedance spectra were measured on a gold electrode in an aqueous solution of K 4 [Fe(CN) 6 ] while taking cyclic voltammograms (the CVs were taken with 50–200 mV/s scan-rate; the distance of potentials of impedance spectra was 16 mV). The Faradaic impedance elements were determined from the spectra, from them charge transfer rate coefficients were calculated; it was found to be 0.11 cm/s at the formal potential. This set of measurements demonstrates the main advantage of dEIS over the traditional steady state impedance measurements: dEIS characterization of an electrochemical system can be performed in seconds rather than minutes which makes possible to use freshly prepared (e.g., annealed) electrodes with reduced risk of contamination or modification of their surfaces.

Screen printed diamond electrode as efficient “point-of-care” platform for submicromolar determination of cytostatic drug in biological fluids and pharmaceutical product

This work presents advanced electrochemical platform based on screen printed diamond electrode (SPDE) system for the single drop “point-of-care” testing. Proposed approach was applied for the quantification of doxorubicin hydrochloride (DOX) in biological fluids and pharmaceutical product. Using a single drop (~30 μL) of the tested solution at the electrode surface, DOX showed high electroactivity over a wide range of pHs. In these conditions, single oval shaped, well-defined and pH dependent oxidation peak was observed in the potential range from 0.5 V to 1.3 V. In the reverse scan, two cathodic peaks, were noted – around 0.3 V and – 0.5 V. Similarly, first reduction peak was pH dependent, while second one was independent in the studied range. Experimental conditions for DOX quantification were optimized and natures of the electrode reactions were investigated. Working linear range obtained for DOX detection was from 0.1 to 2.5 μM. Diffusion controlled electrode reaction reveal long life time of the proposed electrode as well high potential for practical application. Developed procedure was successfully applied for the DOX analysis in biological fluids – urine and pharmaceutical formulation. Obtained results clearly indicated that given procedure can be easily implemented for pharmaceutical control and medical analysis, in both, laboratory and field conditions.

Wall-to-bed mass transfer in a three-phase fluidized bed with coaxially placed string of spheres internal

Turbulent promoters are generally used to realize enhancement in heat and mass transfer rates. With a view to exploit this advantage for the favor of obtaining magnitudes of improvements in mass transfer rates in electrochemical processes, in the present study experiments were conducted in three phase fluidized bed electrochemical cells in the presence of coaxially placed string of spheres. Diffusion controlled electrode reactions were employed to obtain limiting current densities, and hence, mass transfer coefficients. An equimolar potassium ferrocyanide and potassium ferricyanide solution in the presence of sodium hydroxide supporting electrolyte was used as the liquid phase. Glass spheres of different sizes were employed as fluidizing solids and nitrogen was used as the inert gas phase. In three phase fluidized bed, mass transfer coefficient increased with increasing sphere diameter and particle diameter. The parametric effect of gas and liquid velocities on mass transfer coefficient was found to be marginal. The mass transfer coefficient was found to decrease with increase in pitch. A correlation to predict mass transfer coefficient has been developed by using least squares regression analysis.

IONIC MASS TRANSFER IN THREE-PHASE FLUIDIZED BEDS IN THE PRESENCE OF DISC PROMOTERS

Three-phase fluidized beds have wide applications in process industries. The present investigation is carried out to identify the enhancement of ionic mass transfer coefficients due to the presence of a disc promoter in a three-phase fluidized bed. A diffusion-controlled electrode reaction—reduction of ferricyanide ion—was employed to obtain mass transfer coefficient data. The mass transfer coefficient data were obtained by varying the geometric variables of the disc promoter (disc diameter, disc spacing) and dynamic variables (superficial liquid velocity, superficial gas velocity). The effect of particle diameter was also investigated. The investigations revealed that the mass transfer coefficients were enhanced with decreased disc spacing, increased disc diameter, increased superficial gas velocity, increased superficial liquid velocity, and increased particle diameter.

Voltammetry and Redox Charge Storage Capacity of Ferrocene-Functionalized Silica Nanoparticles

We describe the electrochemistry of 15 nm diameter silica nanoparticles densely functionalized with ferrocene (FcSiO(2)) through siloxane couplings. Each nanoparticle bears approximately 600 Fc sites, as measured by potentiometric titration (590 Fc) and diffusion-controlled voltammetry (585 Fc) and estimated by XPS (630 Fc). The nanoparticle ferrocene coverage amounts to ca. a complete monolayer of ferrocene sites, which react electrochemically without mutual interactions and which are apparently fully accessible for diffusion-controlled electrode reactions. Diffusion-controlled voltammetry of the FcSiO(2) nanoparticles was observed in dilute methanol dispersions and in more concentrated slurry phases formed in methanol/acetonitrile mixtures. Electrochemical studies reveal interesting behavior in the dilute and more concentrated solutions. Because of the large nanoparticle surface area/volume ratio, the ferrocene-coated silica nanoparticles are capable of storing up to 5 x 10(7) C/m(3) of redox charge as dry phases and 6 x 10(5) C/m(3) in the concentrated slurries.

The reduction of dispersed indigo by cathodically formed 1,2,4-trihydroxynaphthalene

The indirect cathodic reduction of the vat dye indigo (C.I. Vat Blue 1) by cathodically reduced Lawsone (2-hydroxy-1,4-naphthoquinone; C.I. Natural Orange 6) was studied in aq. solution at different pH values. Cyclic voltammetry and spectroelectrochemistry were used to investigate the electrochemical behavior of 2-hydroxy-1,4-naphthoquinone at a hanging mercury drop electrode. The cathodic peak potential ( E p) d measured at 0.1 mM lawsone solution at a scan rate of 50 mV s −1 changed from −425 mV at pH 7, to −730 mV at pH 11.5 and −750 mV at pH 13 (vs. Ag/AgCl, 3 M KCl). Particularly at pH values of 8–9 and 11.5–13 voltammograms indicated successful, indirect cathodic reduction of the dye in which the cathodically reduced 2-hydroxy-1,4-naphthoquinone acted as soluble mediator. The linear relationship obtained for ( I p) d vs. v 1/2 is indicative of a diffusion-controlled electrode reaction mechanism. In the presence of dispersed indigo, the overall cathode reaction is similar to the E cat process with continuous regeneration of the electroactive species. Spectrochemical experiments were used to prove the indirect cathodic reaction of dispersed vat dyes by 2-hydroxy-1,4-naphthoquinone.

Bilirubin oxidase bound to multi-walled carbon nanotube-modified gold

We report on direct electron transfer (DET) reactions of bilirubin oxidase at multi-walled carbon nanotube-(MWCNT) modified gold electrodes. MWCNTs are very suitable for protein immobilisation and provide surface groups that can be used for the stable fixation on electrodes. They can also effectively replace the natural substrate of BOD – bilirubin, the electron donor for oxygen reduction. The bioelectrocatalytic oxygen reduction was recorded using linear sweep voltammetry (LSV) with BOD covalently linked to the nanotubes. The start potential of the bioelectrocatalytic oxygen reduction at pH 7 and a scan rate of 10 mV/s was determined to be 485 ± 10 mV vs. Ag/AgCl, 1 M KCl (720 mV vs. SHE). Current densities up to 500 μA/cm 2 were detected in an air-saturated buffer at room temperature (25 ± 5 °C). Experiments with a rotating disk electrode (RDE) indicate a diffusion controlled electrode reaction. A k s value in the range of 80–100 s −1 could be estimated. The DET could also be observed directly by the redox conversion of a copper centre of BOD under anaerobic conditions. A peak pair with a formal potential of 680 ± 10 mV vs. SHE was found. The T1 site is probably addressed by the electrode as indicated by several experimental studies.

Redox processes in a tin doped melt with the basic composition 20Na 2O · 80SiO 2 studied by square-wave voltammetry and impedance spectroscopy

Redox processes in a tin doped melt with the basic composition 20Na 2O · 80SiO 2 were studied by square-wave voltammetry and impedance spectroscopy. At temperatures of 1350 and 1450 °C, the recorded square-wave voltammograms exhibited a distinct peak attributed to the reduction of Sn 4+ to Sn 2+. Impedance spectra recorded at the same temperatures could be simulated with a simple equivalent circuit assuming a diffusion controlled electrode reaction. At temperatures T ⩽ 1250 °C, and small step times, the voltammograms exhibited a second peak which was much stronger affected by the frequency (step time) applied than expected from a diffusion controlled electron transfer reaction. In order to fit the impedance spectra at these temperatures, a more complex equivalent circuit was necessary, taking into account adsorption phenomena. In all impedance spectra, the Warburg parameter obtained from simulation, which stands for the diffusion from and to the electrode, exhibited a minimum which was equal to the voltammetric peak at more negative potentials. The voltammetric peak at less negative potentials is caused by the reduction of Sn 4+ to an adsorbed state. Within this reduction step, a monolayer of Sn 2+ species is formed.

Thermosolutal Convection in an Inclined Rectangular Enclosure with a Partition

AbstractAn experimental study of thermosolutal convection in an inclined rectangular enclosure with a partition is presented in this article. Aspect ratio, partition ratio, and inclination angle were kept constant at Ar =0.5, Ap = 0.25 and φ = 30°, respectively. The convective flow is generated by both inclined temperature and concentration gradients under limiting current condition. Both the thermal and solutal buoyancies, which either cooperated or opposed one another, were induced from the copper plates. The temperature gradient was maintained and controlled using two separate constant temperature baths that circulated heated or cooled water through a heat exchanger. We used copper sulphate-sulfuric acid solution as both the working fluid and the electrolyte. An electrochemical method based on a diffusion-controlled electrode reaction was employed to create the concentration gradient. We used the shadowgraph recording technique to visualize and analyze the flow field phenomenon. Thermal Grashof numbers ranging from 8.16 × 105 to 16.32 × 105 and a solutal Grashof number Grm = 4.36 × 106 were investigated. It is demonstrated that the mass transfer rate increases with the increasing thermal Grashof numbers within our experimental ranges. Multilayer structures are found in the cooperating case or the opposing case.

Dependence of the Electrochemical Response of Ferritin on the Number of Iron Atoms at the Ferritin Core

Abstract The well-defined redox reaction of ferritin was observed at an indium oxide electrode with a fully hydrophilic surface at a low concentration of ferritin solution (0–7 μmol dm−3). This was a diffusion-controlled electrode reaction process. Conversely, at a higher concentration of ferritin (7 μmol dm−3 <), ferritin was weakly adsorbed on the electrode surface. The electrochemical redox response of ferritin was strongly affected by the number of iron atoms at the core of the ferritin molecule.

DOUBLE-DIFFUSIVE CONVECTION IN A TWO-UPPER-PARTITION RECTANGULAR ENCLOSURE

Experiments were conducted to obtain better criteria for the temperature and concentration distributions caused by double-diffusive convection under an electrochemical system in a two-upper-partition enclosure whose aspect ratio (Ar) was 0.5. Both the thermal and solutal buoyancies, which either cooperated or opposed one another, were induced from the sides. The temperature gradient was maintained and controlled using two separate constant-temperature baths that circulated heated or cooled water through a heat exchanger. We used a copper sulfate–sulfuric acid solution as both the working fluid and the electrolyte. An electrochemical method based on a diffusion-controlled electrode reaction was employed to create the concentration gradient. We used the shadowgraph recording technique to visualize and analyze the flow. Thermal Grashof numbers ranging from 4.08 × 105 to 8.16 × 105 and solutal Grashof numbers ranging from 2.55 × 106 to 6.15 × 106 were investigated. Cooperating and opposing cases with a fixed partition ratio (Ap = 0.25) and Grm are discussed in this work. Given the Sherwood number correlation, it is demonstrated that the mass transfer rate increased with the increasing thermal Grashof numbers. Results showed that the Sherwood number in the cooperating case is much higher than that in the opposing case, by 23.7% for the fixed Grm and that in the opposing case, the Sherwood number decreases by 11.1% with increasing Grm.

DOUBLE-DIFFUSIVE CONVECTION IN A TWO-UPPER-PARTITION RECTANGULAR ENCLOSURE

Experiments were conducted to obtain better criteria for the temperature and concentration distributions caused by double-diffusive convection under an electrochemical system in a two-upper-partition enclosure whose aspect ratio (Ar) was 0.5. Both the thermal and solutal buoyancies, which either cooperated or opposed one another, were induced from the sides. The temperature gradient was maintained and controlled using two separate constant-temperature baths that circulated heated or cooled water through a heat exchanger. We used a copper sulfate–sulfuric acid solution as both the working fluid and the electrolyte. An electrochemical method based on a diffusion-controlled electrode reaction was employed to create the concentration gradient. We used the shadowgraph recording technique to visualize and analyze the flow. Thermal Grashof numbers ranging from 4.08 × 105 to 8.16 × 105 and solutal Grashof numbers ranging from 2.55 × 106 to 6.15 × 106 were investigated. Cooperating and opposing cases with a fixed ...

THERMOSOLUTAL CONVECTION IN A RECTANGULAR ENCLOSURE WITH TWO LOWER PARTITIONS IN COOPERATING CASE

The purpose of this study was to gain a better understanding of the temperature and concentration distributions caused by thermosolutal convection in an electrochemical system in a two-lower-partition enclosure whose aspect ratio was 0.5. The temperature gradient was maintained and controlled using two separate constant-temperature baths, which circulated heated or cooled water through a heat exchanger. The working fluid was aqueous copper sulfate. An electrochemical method based on a diffusion-controlled electrode reaction was employed to create the concentration gradient. We used the shadowgraph recording technique to visualize and analyze the flow. Thermal Grashof numbers ranging from 4.08 2 10 5 to 8.16 2 10 5 and solutal Grashof numbers ranging from 2.55 2 10 6 to 6.15 2 10 6 were investigated. It was demonstrated that the two partitions affected the partition ratio (Ap = 0.25) and the flow pattern. For fixed Gr m and Ap, a correlation of the Sherwood number was given and it was demonstrated that the mass transfer rate increased with increasing thermal Grashof numbers. Results showed that increasing temperature differences by two times would increase the Sherwood number by 14% in this system.

Hard chromium plating on cold swaged Cr-Mo steel using rotating cylinder electrode

Hard chromium plating is widely used owing to its good wear and corrosion resistance. This plating technology has been practiced and well developed for many decades [1, 2]. Since reduction of hydrogen ions occurs at electrode surface during chromium plating, the current efficiency for the plating is normally under 25% [1]. However, Hoare [3] shows that rapid flow of the electrolyte over the cathode surface is essential for highspeed chromium plating due to the reduction of the diffusion path in a diffusion-controlled electrode reaction. Plating by using rotating disc electrode (RDE) method is useful to study this effect. Cross-section microstructure of the plated hard chromium was studied with optical microscope and scanning electron microscope (SEM) [4, 5]. The results showed that the grain size of as-plated chromium was so fine that the grain size cannot be estimated, and only the grains after annealing can be observed. Pina [5] showed that a fiber texture existed with 〈111〉 axis perpendicular to the chromium deposit surface by using X-ray diffraction method. The interface microstructure between Cr-Mo steel and chromium deposit has not been studied yet. Especially, little work has been done on the current efficiency and TEM-cross section microstructure of chromium plating using rotating cylinder electrode (RCE), by which a uniform mass transport over all of the electrode surface can be obtained. In this letter the aspects mentioned above are reported using RCE specimens. Cold swaged Cr-Mo steel was the chosen specimen with its chemical composition shown in Table I. The cylinder specimen dimension was 16 mm φ× 5 mm with an exposing plating area of 2.51 cm2. Platinized-Ti mesh with dimension of 25× 35 mm was used as anode. All chromium plating was performed in a conventional bath containing 250 g/l of CrO3 and 2.5 g/l of H2SO4 at 50 ◦C. Cathodic current densities applied were 30, 40, 50 and 60 A/dm2, respectively. Rotating speed of RCE was controlled in the range of 5–1000 rpm. Before electroplating, the RCE specimens were anodically polarized with current density of 40 A/dm2 at 200 rpm for 30 s. The current efficiency was evaluated with three RCE specimens after a constant applied cathodic charge of 1.33 A h under same plating condition. The weight increment of chromium plating was measured and compared with theoretical weight increment according to Faraday’s law. After plating, the RCE specimen was rinsed in acetone, dried with hot air, then prepared for current efficiency evaluation; TEM-cross section specimens were prepared as mentioned elsewhere [6] and their microstructure examined. The current efficiency of applied cathodic current density with respect to logarithmic rotating speed of RCE is shown in Fig. 1. The results show that all current efficiency increases with the increasing of cathodic current density, suggesting that higher electric field tends

Improved ionic mass transfer in an electrochemical cell in presence of turbulence promoters

In industrial operations, reduction in equipment size can increase productivity, lower energy inputs and improve purity of the product. The presence of turbulence promoters in an electrochemical cell can increase the intensity of local turbulence, which enhances the mass transfer coefficient at the transfer surface and the limiting current density. The regularity of their geometrical configuration induces a pseudo-uniformity of the spatial distribution of the local mass transfer coefficients. High mass transfer coefficients can be achieved at much lower flow rates when turbulence promoters are placed in the cell. Limiting current data were obtained using a diffusion-controlled electrode reaction at point copper electrodes fixed on the wall of the electrolytic cell in the presence of rectangular turbulence promoters. The improvement in limiting current density, and hence mass transfer coefficient, due to the presence of turbulence promoters is assessed. The effects of the flow rate of the electrolyte, height of the promoters and spacing between the promoters were studied.

Ionic mass transfer measurements in the case of submerged multijets impinging on to a flat surface

AbstractIn the case of submerged multijets impinging on to a flat surface, the limiting current densities were measured at microelectrodes, fixed flush with the target surface using diffusion controlled electrode reactions (i.e. reduction of ferricyanide ion). The height of the multijet distributor from the target surface (Y) was varied from 5 to 9 cm. The numbers of holes of the distributor (N) studied were 76 and 141. The variation of mass transfer data with radial distance reveals the existence of two different regions, i.e. the impingement region and the wall jet region. The mass transfer rate increased with increase in Y in the impingement region. The effect of Y is negligible in the wall jet region. Decrease in N decreases the mass transfer rate in both the regions. The data are correlated separately for the two regions.

Time scanning pulse polarography, a new approach to chronoamperometry

A new approach has been developed for discriminating at the dropping mercury electrode (DME) between currents controlled by various types of rate processes, including diffusion and chemical kinetics of reactions coupled with electron transfer. The crux of time scanning pulse polarography (t.s.p.p.) is a combination of chronoamperometry with pulse polarography. Typically, pulses of 200 ms were used and applied instantaneously (i.e. with the aid of a potentiostat whose rise-time was 2 ms) at a time, t 0 (on the order of 1 s) which corresponded to an appreciable fraction of the total drop time of the DME. Limiting currents, ( i) were subsequently “sampled” on successive drops as function of a time-shift, 1<θ<200 ms. These current measurements were taken at times ( t 0+θ) using sampling “time-windows” of 5 ms or less. Diagnostic plots of log i versus long θ were obtained whose slopes were characteristic of the controlling rate process, viz, ∂ log i/∂ log θ=−0.47, for a diffusion controlled electrode reaction; ∂ log i/∂ log θ=zero, for an electrode process whose kinetics are controlled by a preceding first order chemical reaction. Similar diagnostic plots were found to characterize “catalytic current” sequences. The significance of t.s.p.p. is inherent in the feature that it is applicable to trace concentration levels (10μ M or less) which are intractable by classical polarography.

Current potential characteristics of Zn/electrolyte and ZnO/electrolyte contacts

With regard to corrosion phenomena of zinc in solutions containing 0.1 M borax, potentiodynamic current-potential characteristics and open-circuit potentials of rotating disk electrodes made of Zn and ZnO have been investigated. Measurements were carried out at a constant temperature of 25 °C with various Zn 2+ concentrations, O 2 partial pressures and rotation speeds. Below Zn 2+ saturation and without dissolved oxygen it is observed that corroding Zn electrodes behave like metal electrodes in equilibrium with ZnO, and that with Zn 2+ saturation they behave like pure n-conducting ZnO electrodes. In the presence of dissolved oxygen two different diffusion controlled electrode reactions are observed, which are due to reduction of O 2 to H 2O 2 and O 2 to OH -. Because of the low electron density at the surface with Zn 2+ saturation the first reaction degenerates to a small current peak and is probably caused by a kind of O 2 adsorption.

Resonance Raman spectroelectrochemistry V. Intensity transients on the millisecond time scale following double potential step initiation of a diffusion controlled electrode reaction

Resonance Raman spectroelectrochemistry V. Intensity transients on the millisecond time scale following double potential step initiation of a diffusion controlled electrode reaction