Rotating Disc Experiments Research Articles

Understanding the Voltammetry of Bulk CO Electrooxidation in Neutral Media through Combined SECM Measurements.

Recently, the bulk electrooxidation of CO on gold or platinum has been used to detect CO produced during CO2 reduction in neutral media. The CO bulk oxidation voltammetry may show two distinct peaks depending on the reaction conditions, which up to now have not been understood. We have used scanning electrochemical microscopy (SECM) to probe CO oxidation and pH in the diffusion layer during CO2 reduction. Our results show that the two different peaks are due to diffusion limitation by two different species, namely, CO and OH–. We find that between pH 7 and 11, CO oxidation by water and OH– gives rise to the first and second peak observed in the voltammetry, respectively. Additional rotating disc experiments showed that specifically in this pH range the current of the second peak is diffusion limited by the OH– concentration, since it is lower than the CO concentration.

Cathodic deposition of birnessite from alkaline permanganate solutions: Tools to control the current efficiency, morphology and adhesion

Abstract Birnessite deposition on Pt, Ni, and carbon supports resulting from electrochemical permanganate reduction in alkaline medium is optimized. The goal is to obtain the material for alkaline electrochemical devices under the same alkaline conditions. Optimization of electrodeposition mode assumes the choice of deposition potentials and permanganate concentration providing high enough current efficiency and the deposit mechanical stability. Deposition on carbon supports is accented. Voltammetric and rotating disc experiments discover the principle phenomenon responsible for deposition kinetics: solid oxide formed either in the course of Mn(V) disproportionation, or by means of cathodic reduction which inhibits the formation of soluble Mn-containing reduced species. Manifestations of inhibition for various electrode materials are discussed. Oxidative treatment of glassy carbon is found to favor the formation of homogeneous deposits with close to 100 % current efficiency. The specific features of birnessite coatings fabricated at different potentials are examined by XRD and SEM techniques and discussed in the context of collected kinetic information.

Studies on the catalytic reduction of iodate at glassy carbon electrodes modified by molybdenum oxides

The electroreduction of iodate at glassy carbon discs modified by molybdenum oxide films in sulfuric acid medium is discussed. Studies carried out in the pH range 1.8–3.0 with Mo(VI) in solution show that at more acidic conditions the catalytic response is much larger since protons participate in the oxygen abstraction step when the substrate is reduced. Rotating disc experiments performed in solutions containing both Mo(VI) and iodate at pH 1.8 confirmed that an immobilised form of molybdenum is responsible for the reduction of the substrate, even though at more acidic solutions the film is partially solubilised. The results of voltammetric experiments carried out with a glassy carbon electrode coated by the molybdenum oxide layer established that iodide is generated in the electroreduction of iodate.

Kinetics of electrocatalytic oxidation of formaldehyde on a nickel porphyrin-based glassy carbon electrode

Abstract Electrocatalytic oxidation of formaldehyde on a glassy carbon modified with electropolymerised films derived from tetrakis(4-hydroxy-3-methoxyphenyl)porphinato nickel(II) (THMPPNi(II)) was studied in 0.1 M NaOH on stationary and rotating electrodes by cyclic voltammetry and rotating disc amperometry, respectively. The data from rotating disc experiments were analysed according to Albery–Hillman theory. The findings showed that the poly-THMPPNi(II) behaves as an efficient electrocacalyst for the oxidation of formaldehyde with the cross-exchange reaction occurring throughout the layer at a low concentration of formaldehyde and for a thin modifier, i.e. the mediating process corresponds to the Lk kinetic case, with a change over to the LEk regime at higher substrate concentration or for thicker films.

Physical study of film-forming acrylate emulsion polymers for biosensor applications

A variety of alkyl acrylate emulsion polymers were investigated using rotating disc voltammetry and atomic force microscopy to assess their suitability as immobilisation matrices for biosensors. Continuous films with reproducible transport properties were the target. The rotating disc experiments showed that films formed from the emulsions Poly 14, Poly X1 and Poly X6 were continuous up to a thickness of 4.39 mg cm −2. These three different emulsions were assessed to determine the influence of entrapped glucose oxidase on the film characteristics. Poly X1 films were still observed to be continuous, and the film's permeability increased only slightly when enzyme was mixed with the polymer, which made these the best performing films. The enzyme was shown to disrupt Poly 14 most significantly, causing pin-holes in the enzyme/polymer films. All three emulsions gave similar glucose response curves when the concentration of the analyte, glucose, was monitored by the production of hydrogen peroxide. However, when the extent of the biochemical reaction was monitored by measuring the consumption of oxygen, the least permeable enzyme matrix (formed by Poly X1) was observed to have the shortest dynamic range.

Shear-induced changes in chain configuration of poly(acrylic acid) in dilute solution

Small amounts (less than 30 ppm) of polymer dissolved in solution may significantly reduce the work required to pump fluids through a pipe at a given rate. In other words, the drag of the solution along the pipe walls is reduced. Drag reduction by polymers has been well characterized, however the molecular origin of the phenomena is not fully understood. Polymers that exhibit drag reduction characteristics typically have high molecular weight, have predominantly linear, flexible chains, and have an expanded molecular configuration in solution.Work in this laboratory has focused on the drag reduction behavior of poly(acrylic acid), PAA, in recent years. This polymer is one of the most shear stable water-soluble polymers and due to the ionic groups in the polymer chain its conformation in solution changes with pH and ionic strength. In a recent work, PAA solutions of 18 ppm, pH=8.1, showed an initial drag reduction of over thirty-five percent in rotating disc experiments. Over four minutes of shearing the drag reduction decreased to ten percent. This was surprising because of the known shear stability of PAA. When the sheared solution was left undisturbed for two weeks, it did not recover its drag reduction performance. However, the addition of NaCl to the solution during the shearing immediately restored drag reduction to its initial level. It was hypothesized that the shear flow induced interchain association that was possibly stabilized by hydrogen bonding and that the addition of the NaCl caused dissociation and drag reduction recovery. In additional work, fluorescence probe studies showed that shear flow induced local chain rigidity in the originally flexible polymer chains. In this study, the drag reduction experiments were repeated and the configurations of the sheared and unsheared polymer chains were viewed using electron microscopy.

Passivation of zinc in concentrated alkaline solution—II. Role of various experimental factors and the distinction between the solid-state and dissolution—precipitation mechanisms

Pseudo steady-state current—potential measurements have been made during the dissolution and passivation of zinc electrodes in 6.8 mol dm−3 (30% w/w) KOH solutions over a wide potential range. Thepre-passive characteristics are very sensitive to impurities,viz. iron. Similarly,ir drop has an appreciable effect in this region necessitating full correction if measured potentials are to have any quantitative significance. Amalgamation exerts a considerable influence but only in the pre-passive region where the pre-passive film is considerably more inhibiting than that formed on unamalgamated zinc. It is suggested that this film could be derived from intermediates of the form Zn (OH)−2 when the potential reaches a critical value. A high degree of reproducibility observed in thetransition region upon sweep reversal indicates that the potential is also the important parameter controlling the initiation of thepassive Zno/Zn (OH)2 film and not supersaturation/solubility effects. Active—passive characteristics are identical on single crystal and polycrystalline electrodes except in the transition region, where cosiderably less stability exists in the case of the polycrystal. This is interpreted in terms of an electrochemical spectrum for passive film formation/removal on different crystal planes. Polarization data from rotating disc experiments have been analysed in respect of the formation of both thepre-passive andpassive films. Evidence has been sought which would support a solid-state mechanism (constant potential of passivation under conditions where the supersaturationdecreases with increase in electrode rotation speed) or a dissolution—precipitation mechanism (constant supersaturation at the point of passivation under conditions where the passivation potentialincreases with increase in electrode rotation speed). Only evidence in favour of the former mechanism has been obtained for both film types.

Program for evaluating drug dissolution kinetics in preformulation.

The in vitro dissolution kinetics of various drugs were studied at different experimental conditions using both a centrically rotating disc method and a modified excentrically rotating disc method. The combination of these two methods provides a preformulation program that includes many important pharmaceutical and biopharmaceutical parameters related to drug dissolution. The influence of the hydrodynamic conditions on the dissolution rate was demonstrated and the intrinsic dissolution tendency was obtained. Further, the acid dissociation constant, diffusion coefficient and enthalpy of dissolution can be calculated from data obtained from the rotating disc experiments. The relationship between dissolution rate and aqueous solubility of each drug tested is also considered.

Dissolution at porous interfaces II. A study of pore effects through rotating disc experiments

The intrinsic dissolution rate of 4 substances with different solubility (borax, nicotinic acid, disodium oxalate and theophylline) was measured by means of a rotating disc technique. Compressed pellets, mounted in a holder, were rotated in the dissolution medium at various speeds. The obtained results allow the calculation of the effective diffusion coefficients of the substances under examination. Pores drilled into the surface of the pellets (pore diameter 0.20-2.00 mm) caused an increase of the dissolution rate because of changing hydrodynamics. The diameter of a drilled pore must exceed a critical value to bring about this rise in dissolution rate. Apart from the diameter, the depth of the pore is an important factor determining to what extent the dissolution rate is increased. The influence of the pore depth decreases with an increasing depth : diameter ratio. The critical pore diameter was found to be the same (about 0.2 mm) for all the substances investigated. This implies that its size depends only on the hydrodynamic conditions (that are affected by the geometric change of the solid/liquid interface), and not on the physicochemical properties of the dissolving solid. Therefore these results have a general applicability for all compounds dissolving under similar hydrodynamic conditions.

Dissolution kinetics and etch pit studies of potassium aluminium sulphate

The dissolution process of the {111} faces of potash alum is studied, both by microtopographic examinations of the etch pit patterns and by measurement of the dissolution kinetics in a rotating disc crystallizer. Both methods showed that the Cabrera-Levine dissolution theory holds for the two most common dislocation types ending on the {111} faces of potash alum. On the basis of the rotating disc experiments, the interfacial supersaturation of the etch pit experiments was roughly estimated. Using this, it was found that at interfacial supersaturations below -0.6% (dislocations with 〈110〉 Burgers vector) or below -0.85% (dislocations with 〈100〉 Burgers vector) numerous etch pits related to those dislocation types appeared. Below those undersaturations the dissolution process is mainly determined by volume diffusion. From the critical undersaturation, determined in the rotating disc crystallizer, the value of the edge free energy of a step was found to be approximately 0.01 J/m 2.

Kinetics and equilibria of tea infusion. Part 3.—Rotating-disc experiments interpreted by a steady-state model

The kinetics of the extraction of a soluble constituent from tea leaf have been treated by a steady-state model. This leads to an overall rate constant made up of 3 main contributions which arise from the diffusion of the constituent through the leaf, its transfer across the leaf/water interface, and its diffusion away through the Nernst layer. That the last step was not the rate-determining one was shown by rotating-disc experiments. Koomsong tea dust was glued on to large discs and the rate of caffeine extraction at 80 °C measured at various rotation speeds. The rate was found to be independent of the speed. The viability of the experimental procedure was checked using discs coated with similarly-sized copper powder: here the rate of attack by dilute dichromate increased with increasing rotation speed in semi-quantitative agreement with the Levich equation.

Dynamic spring analysis and its application to the polystyrene melt relaxation

The technique of dynamic spring analysis (DSA) was employed to obtain the storage and loss moduli of polystyrene samples in the glassy and rubbery regions of viscoelastic response. These results were compared with dynamic shear moduli from eccentric rotating disc experiments by time–temperature superposition to determine the degree and range of applicability of DSA. The storage moduli gave semiquantitative agreement over the rubbery region, while the loss moduli agreed over a narrower region of response. An apparent relaxation above the glass transition temperature of polystyrene was attributed to the evolution of solvent from the polymer–spring composite samples studied.

The electrochemical reduction of maleimide: The synthesis of succinimide

The electrochemical reduction of maleimide has been investigated in the pH range 0–14 at a variety of electrode materials. The reaction scheme was found to fall into three distinct sections, depending upon the pH at which the reduction was performed. At low pH the reduction product was found to be succinimide, and the mechanism of its formation is described. At higher pH values, the hydrolysis of starting materials complicates the interpretation of the electrochemical measurements, but nevertheless the overalls reduction mechanism could be deduced. Polarographic data complement information obtained from rotating disc experiments, and comparison is made with the electrochemical reduction of phthalimide.

The electrochemical reduction of maleimide: The synthesis of succinimide

The electrochemical reduction of maleimide has been investigated in the pH range 0–14 at a variety of electrode materials. The reaction scheme was found to fall into three distinct sections, depending upon the pH at which the reduction was performed. At low pH the reduction product was found to be succinimide, and the mechanism of its formation is described. At higher pH values, the hydrolysis of starting materials complicates the interpretation of the electrochemical measurements, but nevertheless the overalls reduction mechanism could be deduced. Polarographic data complement information obtained from rotating disc experiments, and comparison is made with the electrochemical reduction of phthalimide.