The strength of acids in alcohols as determined by steady-state voltammetry

Steady-state voltammograms for reduction of acids of various strengths in alcohols with excess supporting electrolyte and without any supporting electrolyte can be used to infer charge type and strength of the acid on the basis of the phenomenon of migration. For strong and moderately weak acids (K(a)/[Formula: see text] > 10(-)(3)) in alcohols, the ratio of steady-state transport-limited current to diffusion-limited current, corrected appropriately for ion-ion interactions, the presence of ionic impurities, and changes in viscosity, for hydrogen ion reduction without supporting electrolyte and with excess supporting electrolyte equals 2. For acetic acid, which is very weak (K(a)/[Formula: see text] < 10(-)(6)), the value of the steady-state transport-limited current is, under the experimental conditions applied here, independent of supporting electrolyte concentration. In the case of a homogeneous acid-base equilibrium, a novel analytical procedure yields diffusion coefficients of both hydrogen ion and undissociated weak acid molecules from the diffusional and migrational currents. Limiting currents obtained in alcohols with excess supporting electrolyte and without supporting electrolyte are compared by means of an extended formula that incorporates the ionic strength dependence of diffusion coefficients.     

Neutral reagents in solutions with low content of supporting electrolyte: how to determine the steady-state conditions

Cyclic voltammograms obtained at ultramicroelectrodes in the electrochemical systems where an uncharged reactant is significantly more concentrated than the supporting electrolyte show an unusual feature. The forward and the backward waves cross over, forming a hysteresis loop. The width of the hysteresis increases with the relative concentration of the reactant, with the electrode size, and with the scan rate. We show that the reason for this hysteresis is the slow transport of supporting electrolyte ions that are necessary to compensate the charge of the reaction product. As a result, the steady-state concentration profile of counterions is reached significantly slower than the steady-state concentration gradient of the reactant, and the counterion transport determines how rapidly the steady state for the whole system is approached. The scan rate yielding near-steady-state voltammograms can differ by more than 1 order of magnitude for systems with high and low concentrations of supporting electrolyte. Experimental evidence for this, supported by digital simulation results, is presented. The appropriate criterion for assessing the steady state in such systems is thus the identity of the forward and backward scans, without hysteresis. If this condition is not fulfilled, the formal potentials and the related parameters determined from the obtained voltammograms may be erroneous.     

Steady-state voltammetry of hydroxide ion oxidation in aqueous solutions containing ammonia

An oxidation process observed in dilute aqueous solutions of ammonia was investigated under steady-state conditions with gold microelectrodes with radii in the range 2.5-30 microm. Over the ammonia concentration range 0.1-10 mM, a well-defined voltammetric wave was observed at approximately 1.4 V versus Ag/AgCl. It was attributed to the oxidation of hydroxide ions that arise from the dissociation of the weak base. The steady-state limiting current was found to depend on the concentration of supporting electrolyte, and in solution with low electrolyte, it was enhanced by migration contribution, as expected for a negatively charged species that oxidizes on a positively charged electrode. In addition, the steady-state limiting current was proportional to both the ammonia concentration and the electrode radius. The overall electrode process was analyzed in terms of a CE mechanism (homogeneous chemical reaction preceding the heterogeneous electron transfer) with a fast chemical reaction when measurements were carried out in solutions containing NH3 at < or = 5 mM and with electrodes having a radius of > or = 5 microm. This was ascertained by comparing experimental and theoretical data obtained by simulation. The formation of the soluble complex species Au(NH3)2+ was also considered as a possible alternative to explain the presence of the oxidation wave. This process however was ruled out, as the experimental data did not fit theoretical predictions in any of the conditions employed in the investigation. Instead, the direct oxidation of NH3, probably to N2O, was invoked to explain the anomalous currents found when the CE process was strongly kinetically hindered. Throughout this study, a parallel was made between the CE mechanism investigated here and that known to occur during the hydrogen evolution reaction from weak acids.     

Charge neutralization process of mobile species at any distance from the electrode/solution interface. 1. Theory and simulation of concentration and concentration gradients developed during potentiostatic conditions

The theoretical framework of a general model for the simulation of concentration profiles of electroactive and nonelectroactive species, driven by an electrochemical process under potentiostatic conditions, is presented. Based on this analysis, finite differences simulations are performed to calculate the actual profiles under different experimental conditions. Furthermore, the effect of experimental parameters (diffusion coefficients of the ions of the redox couple or the supporting electrolyte, charge of the different species, etc.) on the concentration profiles is also examined. The results obtained when low and high concentrations of supporting electrolyte are compared aid understanding of the effect of the electrolyte on the measurements. The presented model also underlines the role of supporting electrolyte species when nonspecific techniques are employed to measure the concentration changes produced by electroactive species. On the other hand, if a highly specific technique were used to detect changes in the concentration or concentration gradient of a given species, then it would be possible to estimate the respective profiles of the other species. The simulations suggest that techniques measuring concentration gradients are more sensitive to determining concentration changes than those involving a measurable linearly related to concentration.     

An experimental evaluation of cyclic voltammetry of multicharged species at macrodisk electrodes in the absence of added supporting electrolyte

The reversible reduction of [S2Mo18O62]4- to [S2Mo18O62]5- and [S2Mo18O62]6- at a glassy carbon macrodisk electrode has been studied by cyclic voltammetry in acetonitrile as a function of the concentration of [(C6H13)4N]4[S2Mo18O62] in the absence and presence of [(C6H13)4N]ClO4 as the added supporting electrolyte. Consideration is given to the influence of scan rate, reference-working electrode distance, [(C6H13)4N]4[S2Mo18O62], and electrolyte concentrations. Experimental data confirm theoretical predictions that cyclic voltammetry at a macrodisk electrode is a viable technique for studies of multiply charged electroactive species without added electrolyte, provided the influence of enhanced complexities associated with effects of increased solution resistance, the mass transport contribution from migration, and convection arising from enhanced density gradients are considered. Enhanced density gradients present in the absence of added supporting electrolyte give rise to a more marked dependence of voltammograms on the angle of the electrode and hence lead to significant distortion of wave shapes at low scan rates. The summation of all these obstacles implies that quantitative evaluation of cyclic voltammograms of multiply charged species requires significantly greater care in the absence than in the presence of added supporting electrolyte.     

Ion chromatographic determination of acidity

The practice of determining acid concentrations by titrations has remained unchanged for more than a century. We introduce a new approach to the determination of acid concentrations based on cation exchange chromatography. We demonstrate the ability of sulfonated styrene-divinylbenzene based stationary phases to separate the hydrogen ion from other monovalent cations. The eluent is a dilute solution of a neutral salt, sometimes containing a small concentration of the corresponding acid, e.g., sodium ethanesulfonate, pH adjusted with ethanesulfonic acid. The high equivalent conductance (approximately 350 S.cm2/equiv) of H+ and relatively low eluent concentration allows sensitive conductometric detection of H+, down to the 50 microM level under favorable conditions. The conductometric response to H+ can be linear over a wide range of H+ concentrations, from sub-millimolar to several molar concentrations. The system allows the rapid quantitation of strong acids; weak acids can also be determined depending on pKa and injected concentration. The determinations of several strong and weak acids are presented along with factors that govern their chromatographic analysis.     

Steady-state limiting currents determined by coupled diffusion, migration, and chemical equilibrium

A simple theoretical model is presented for the reduction of a singly charged cation under conditions where migration is important and the cation is coupled to a neutral species through a chemical equilibrium, AB = A(+) + B(-). Only the steady-state transport-limited current, I(l), is considered. Simple algebraic equations describe the ratio of I(l) to the diffusion-limited current, I(d), as it depends on the degree of dissociation, determined by the ratio of equilibrium constant to formal concentration, K(AB)/C*(AB). The ratio I(l)/I(d) is found to depend on the ratio of electrolyte to equilibrium concentration of A(+) in bulk solution just as for the well-known result for the case without the equilibrium (i.e., K(AB) → ∞). The results are in accord with published experimental data for weak acids. Agreement and disagreement with other theoretical treatments of this problem are discussed. The main results are for 1:1 supporting electrolytes; extensions are made to 2:1, 1:2, and 2:2 supporting electrolytes.     

Electrochemical behaviour of inorganic redox substances dispersed into Nafion® films

The voltammetric response of a Nafion^® film deposited on an indium tin oxide (ITO) electrode was studied. It was observed that the response is affected by the thickness of the Nafion^® layer. Also, the voltammograms show differences depending on the concentration and composition of the supporting electrolyte solution. The ITO–Nafion^® system can be stabilised by repetitive cyclic voltammetry, and then the inner hydrogen reduction is not detected by electrochemical impedance spectroscopy. Different substances were dispersed into the Nafion^® matrix in order to study their electrochemical response.     

Voltammetric reduction and determination of hydrogen peroxide at an electrode modified with a film containing palladium and iridium

Cyclic voltammetry of a mixture containing 0.2 mM Na2IrCl6, 0.1 mM PdCl2, 0.2 M K2SO4, and 0.1 M HCl between 1.2 and -0.3 V vs Ag/AgCl for five cycles at 50 mV s-1 yields a stable film on a glassy carbon electrode. The reduction of hydrogen peroxide in 0.1 M KCl is diffusion controlled at that modified electrode. Calibration curves obtained at a 100 mV s-1 scan rate are linear in the range 0.2-1.8 mM H2O2. The slope, 28 microA L mmol-1, is independent of film thickness. Since dissolved oxygen is reduced at about the same potential as H2O2, -0.3 V, at the modified electrode, it will act as an interferent in solutions that are not deaerated; however, the currents are additive. A second limitation of the described procedure is that with the KCl electrolyte the immobilized film must be reoxidized prior to each measurement. Preliminary data are described which suggest that this problem is alleviated by switching to a basic supporting electrolyte.     

用阴极电解法沉积铝合金铈转化膜

在铈盐溶液中阴电极解的方法沉积铝合金铈转化膜,用电化测试,腐蚀实验,ＳＥＭ和ＥＤＡＸ等分析手段研究了转化膜的性能,结果表明,钪转化膜的形貌垢形貌组成和耐蚀与铝合金表面状态和阴极是否发生析氢反应关系密切,要想获得性能良好的转化膜必须抑制氢气在阴极上析出。     

Experimental and theoretical investigations on the adsorption of 2'-deoxyguanosine oxidation products at oxidized boron-doped diamond electrodes

Electrochemical oxidation of 2'-deoxyguanosine has been performed on boron-doped diamond (BDD) electrodes, resulting in a strong adsorption of the formed oxidized products onto the BDD surface. The adsorption behavior has been investigated by studying the electrochemical behavior of a redox probe ([IrCl6]3-) using cyclic voltammetry. The most probable situations are the formation of (A) an insulating adsorbed film resulting in a partially blocked electrode behavior, (B) a porous film, or (C) an overall conductive film. Different parameters such as the standard rate constant, the charge-transfer coefficient, the electrode/adsorbed products/solution interface resistance, and the formal potential of the redox couple were determined. Through comparison of theoretical current-potential curves obtained by analytical calculations with experimental cyclic voltammograms, we found that the oxidized products of 2'-deoxyguanosine form a continuous conductive film on BDD.     

Layer-by-layer assembly of poly(p-xylyleneviologen)–DNA multilayers and their electrochemical properties

Multilayers of poly(p-xylyleneviologen) (PXV) and calf thymus DNA were constructed on the gold surface by layer-by-layer (LBL) method. The assembly process was examined by quartz crystal microbalance (QCM) measurements. According to the frequency change, the average mass increase was estimated to be about 97 and 110 ng/cm² for each assembly of PXV and DNA layer, respectively. Cyclic voltammograms of the multilayer modified gold electrodes showed a couple of redox peaks, the potentials of which were closely dependent on the layer numbers and nature of the outmost layer. The alternatively assembled DNA layers could hinder formation of π-complex dimer of viologen groups due to the strong interlayer electrostatic interaction. The charge transfer process was discussed by the chronocoulometry method. The assembled PXV–DNA multilayers showed high long-term stability at ambient conditions and in electrolyte solution.     

Carbon nanotubes grown on stainless steel to form plate and probe electrodes for chemical/biological sensing

This paper describes the fabrication and evaluation of carbon nanotube (CNT) electrodes grown on stainless steel (SS) plate and wire for electrochemical sensor applications. Multi-wall carbon nanotubes with different diameters were grown on the SS plate and wire by chemical vapor deposition from an ethylene precursor. The SS provides a good electrical and mechanical connection to the CNT, and the SS is a tough substrate. The SS part of the electrode was electrically insulated from the analyte so that only the CNT were active in sensing. Cyclic voltammetry for the reduction of 6 mM K3Fe(CN)6 in a 1.0 M KNO3 supporting electrolyte was performed to examine the redox behavior of the CNT-SS electrode. The cyclic voltammograms showed sigmoidal-like shapes, indicating that mass transport around the electrodes is dominated by radial diffusion. Based on the cyclic voltammograms, the effective area of the CNT-SS electrodes and the number of individual CNTs were estimated. These results indicate that the CNT-SS plate and wire electrodes are good candidates to develop practical in vivo biosensors.     

Effect of supporting electrolyte on the polymerization of electrochemically deposited poly(o-anisidine) films

Poly(o-anisidine) films deposited galvanostatically exhibit the formation of different oxidation states of the polymer with varying pH of the solution. The influence of supporting electrolyte on the extent of formation of the conducting phase at pH 2.1 has been investigated with the help of optical, thermal and potential cycling techniques. The optical spectra of the films reveal the proportionate increase in the fraction of the conducting emeraldine salt phase with the amount of supporting electrolyte (NaCl) added to the solution. Similarly, clarity in the decomposition steps of the thermal patterns and redox peaks in the cyclic voltammograms are found to increase as a function of NaCl concentration. This revised version was published online in November 2006 with corrections to the Cover Date.     

Dissolution Model for a Weak Carboxylic Acid

A chemical reaction, diffusion, and ionization model is presented for the dissolution of solid, weak acids in reactive media. The model assumes an equilibrium plane, which is located within the diffusion layer and in which ionization equilibrium is established. The model, which proposes that the aqueous diffusion layer is divided into three zones, is tested by use of experimental dissolution rates of p-aminobenzoic acid and is compared to a two-zone model.     

Limiting Currents for Steady-State Electrolysis of an Equilibrium Mixture, with and without Supporting Inert Electrolyte

An exact treatment derives the steady-state limiting current of a one-electron reduction for the N ? O(+) + A(-) mixture at a hemispherical microelectrode. Either or both of the neutral N and cationic O(+) species may be electroactive. A supporting salt is present at any concentration, including zero or excess; its ions are electropassive and do not interact with the other solutes or each other. The various species are treated as having distinct diffusivities, linked to their mobilities through the Nernst-Einstein relationship. Universal electroneutrality is assumed. The predictions of the model are compared with published experimental data on the reduction of aqueous weak acids; agreement is excellent at intermediate, but poor at low, support ratios. Analysis of the unsupported case shows that the neutral N species dissociates in a narrow zone close to the electrode, and the injection of ions there serves to increase the electric field in the outer region of the transport zone. This enhances cationic migration enormously, leading to an unsupported limiting current that is much more than double the supported value. However, the limiting current is drastically diminished by traces of foreign electrolyte. Curiously, the limiting current with full support adopts the same value when equilibration is fast as when it is very slow, although the mechanisms are totally different.     

Dissolution Model for a Weak Carboxylic Acid

Abstract

A chemical reaction, diffusion, and ionization model is presented for the dissolution of solid, weak acids in reactive media. The model assumes an equilibrium plane, which is located within the diffusion layer and in which ionization equilibrium is established. The model, which proposes that the aqueous diffusion layer is divided into three zones, is tested by use of experimental dissolution rates of p-aminobenzoic acid and is compared to a two-zone model.     

Generalized theory for nanoscale voltammetric measurements of heterogeneous electron-transfer kinetics at macroscopic substrates by scanning electrochemical microscopy

Here we report on a generalized theory for scanning electrochemical microscopy to enable the voltammetric investigation of a heterogeneous electron-transfer (ET) reaction with arbitrary reversibility and mechanism at the macroscopic substrate. In this theory, we consider comprehensive nanoscale experimental conditions where a tip is positioned at a nanometer distance from a substrate to detect the reactant or product of a substrate reaction at any potential in the feedback or substrate generation/tip collection mode, respectively. Finite element simulation with the Marcus-Hush-Chidsey formalism predicts that a substrate reaction under the nanoscale mass transport conditions can deviate from classical Butler-Volmer behavior to enable the precise determination of the standard ET rate constant and reorganization energy for a redox couple from the resulting tip current-substrate potential voltammogram as obtained at quasi-steady state. Simulated voltammograms are generalized in the form of analytical equations to allow for reliable kinetic analysis without the prior knowledge of the rate law. Our theory also predicts that a limiting tip current can be controlled kinetically to be smaller than the diffusion-limited current when a relatively inert electrode material is investigated under the nanoscale voltammetric conditions.     

Electrodeposited silicate films: importance of supporting electrolyte

Silica and hybrid organic-inorganic films, ca. 100-200 nm thick, can be grown on glassy carbon electrodes through reactions initiated by electrogenerated hydroxide or hydronium ions in water under reductive and oxidative conditions, respectively. A variety of different alkoxysilanes (tetramethoxysilane and organoalkoxysilanes) and supporting electrolytes were used to evaluate whether film formation takes place on glassy carbon electrodes. The results of the study indicate that the acid-base properties of the supporting electrolyte are an important factor in determining whether film formation will take place. For cathodic electrodeposition, thin films can be formed using supporting electrolytes that are close to neutral, such as KCl, KNO3, and NaClO4. For anodic electrodeposition, thin films can be formed using supporting electrolytes that are acidic, such as, KH2PO4, HNO3, H2SO4, etc. The acidity/basicity effects of the electrolytes arise in part from the strong dependence of the hydrolysis and condensation rates of the silicon alkoxide precursors on pH.     

Hydrogen-related defects in crystalline semiconductors: a theorist''s perspective

Hydrogen is a common impurity in all semiconductors. Although it is sometimes deliberately introduced, hydrogen often penetrates into the crystal during device processing. It interacts with broken or weak covalent bonds, such as those found at extended and localized defect centers. The main results of these covalent interactions are shifts of energy levels out of (or into) the gap and new optical activity (infrared absorption and Raman scattering). The shifts in energy levels lead to the passivation (or activation) of the electrical activity of various centers. Hydrogen can also interact with the perfect crystal and with itself, sometimes leading to the formation of extended structures known as platelets. Finally, H also acts as a catalyst, dramatically enhancing the diffusivity of interstitial oxygen in Si. The consequences of these interactions are substantial changes in the electrical and optical properties of the crystal, and in the lifetime of charge carriers. The thermal stability of the complexes containing hydrogen varies from room temperature up to several hundreds of degrees Celsius, and the diffusion of H is trap-limited up to rather high temperatures. Hydrogen normally exists in more than one configuration and charge state in semiconductors. A range of experimental and theoretical techniques have been used to investigate the rich properties of hydrogen in semiconductors, and several extensive reviews focusing mostly on the experimental side of these issues have been published in the past five years. The present review focuses mostly on the theoretical work performed in this field. However, the most recent experimental results are also discussed, and the current understanding of hydrogen interactions in semiconductors summarized.