Effective thickness of the diffusion layer during hydrogen ion reduction in aqueous hydrochloric acid solutions

The process of mass transport during hydrogen ion reduction in aqueous hydrochloric acid solutions is examined both with and without excess supporting electrolyte. The study of this process is based on a numerical solution to a system of equations of material balance and the movement of particles in solution under the influence of forces for diffusion, migration, and convection. The homogeneous chemical reaction of water dissociation is also taken into account. The results of calculations show that a diffusion layer forms near the electrode during the passage of current in these solutions and that the effective thickness of this layer is the same at any instant for all particles participating in mass transport in solution in spite of differences in their diffusion coefficients. The value of the diffusion coefficient measured in these multicomponent solutions by the methods of chronopotentiometry and rotating disk electrode should differ little from that of hydrogen ions in spite of the fact that other particles with different diffusion coefficients participate in the mass transport.     

Diffusion with hydrolysis and ion association in aqueous solutions of beryllium sulfate

Taylor dispersion is used to measure mutual diffusion coefficients for aqueous solutions of beryllium sulfate at concentrations from 0.005 to 1 mol-L^–1 at 25°C. Least-squares analysis of the dispersion profiles shows that diffusion of the partially hydrolyzed salt produces a small additional flow of sulfuric acid, about 0.04 mol sulfuric acid per mole of total beryllium sulfate. Ternary diffusion coefficients measured for the aqueous BeSO₄–H₂SO₄ system are qualitatively consistent with Nernst-Planck predictions based on the formation of beryllium sulfate ion pairs, bisulfate ions, and the hydrolysis equilibria 2Be²⁺+H₂O= Be₂OH³⁺+H⁺, 3Be²⁺+2H₂O=Be₃(OH) ₂ ⁴⁺ +2H⁺. Except for very dilute solutions, the predicted flow of sulfuric acid is small compared to the flow of beryllium sulfate because most of the beryllium ions are protected from hydrolysis by the formation of BeSO₄ ion pairs, and most of the hydrogen ions produced by hydrolysis are converted to less-mobile bisulfate ions.     

Diffusion of ionic species labeled with35S in various aqueous electrolyte solutions at 25°C

Composite diffusion coeffcients have been measured for the various species labeled with³⁵S which are present in a number of aqueous solutions due to the introduction of the labeled material as³⁵SO ₄ ^2– . The solutions were of two components consisting of water and either sodium sulfate. The diffusion coeffcient measured for sodium chloride solutions is similar to literature data for the corresponding diffusion in sodium sulfate solutions. The results for sulfuric acid and ammonium hydrogen sulfate have been interpreted using literature data for the relative concentrations of the hydrogen sulfate and sulfate ions to obtain estimates for the diffusion coefficents of those ions. The results for perchloric acid, regarded as representing the diffusion coefficient of the hydrogen sulfate ion, have a much different concentration dependence to that observed for the estimates for that ion in sulfuric acid and ammonuim hydrogen sulfate. The difference is attributed to the effect of the perchlorate ion on the water structure.     

Modelling of the migration effect occurring at an ISFET-based coulometric sensor-actuator system

The migration effect, in addition to diffusion, occurring at an ion-selective field-effect transistor (ISFET)-based coulometric sensor-actuator system has been studied. A diffusion-migration model is presented, based on the numerical solution of the Nernst-Planck equations of which a digital simulation is realized. Corresponding experiments were carried out and compared with the simulation. The results are in good agreement with the simulation.Typical titration times of this system were found to be 0.5–10 s, corresponding to fully dissociated acid concentrations of 0.5×10^?3?6.5×10^?3 mol 1^?1 with excess of supporting electrolyte. Both the simulation and experimental results show that if the concentration of the supporting electrolyte is ? 20 times higher than that of the species to be titrated, the deviation caused by migration is less than 5% and within the experimental error when pure diffusion is considered. At relatively low concentrations of supporting electrolyte, the migration effect should be taken into account to determine the concentrations of titrated species.     

Polarography in hexamethylphosphoramide: Effect of supporting electrolyte cations

Polarographic reductions of various metal ions such as the silver, cupric, zinc, cobaltous, nickel, ferric, ferrous ions and hydrogen ion in hexamethylphosphoramide (HMPA), have been investigated in the supporting electrolytes with various perchlorates. The reduction of most of these ions is strongly influenced by the cation of the supporting electrolyte. In the presence of the tetraethylammonium ion, when the size of the cation of the supporting electrolyte is small and easily adsorbed on the negatively charged electrode surface, the reductions of metal ions are controlled by some preceding processes and are naturally irreversible. The rate of reduction becomes more rapid with the increase of the size of the cation. Thus, in Hex₄NClO₄ or LiClO solutions, the reduction of these various metal ions takes place almost totally under diffusion control, although the waves of most of metal ions show a maximum. These effects of the cation of the supporting electrolytes on reduction can be explained as a phenomenon occurring on the electrode surface. This phenomenon has been reported in previous papers [1] on the reductions of the alkali and alkaline earth metal ions. The difference in the electrocapillary curves in these solutions is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential. The difference in the effect of the size of the cation of the supporting electrolyte on reduction of metal ion coincides well with the difference in the electrocapillary curves in these solutions: the effect of the size of the supporting electrolyte cation on the polarographic reduction is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential; therefore this effect is due to the electrode double-layer difference.     

Mechanism of oxidation of cyclohexanehexol (inositol) by quinquevalent vanadium

The oxidation of inositol by quinquevalent vandadium in acid medium is a first-order reaction both in vanadium (V) and inositol. The stoichiometry of the reaction is consistent with the use of two equivalents of vanadium (V) per mole of inositol with the formation of one mole of inosose. The reaction is catalyzed both by sulfuric and perchloric acid, but the rate is faster in sulfuric acid than in perchloric acid. In 1M–6M perchloric acid solutions the reaction has shown a variable order in H⁺, but in solutions of 2M–5M sulfuric and perchloric acid of constant ionic strength, the rate has a linear dependence on [H⁺]². There is also a linear correlation between the rate and bisulfate ions in sulfuric acid at constant hydrogen ion concentration. The energy of activation is found to be 19 kcal/mole and a negative entropy value of ? 14 e.u. A suitable mechanism, consistent with the kinetics in 2M–5M acid solutions, is suggested and the values of various rate constants are evaluated.     

硫酸溶液中的铋对铅酸蓄电池负极性能的影响

电解液;硫酸溶液中的铋对铅酸蓄电池负极性能的影响     

Structure of butanol and hexanol at aqueous, ammonium bisulfate, and sulfuric acid solution surfaces investigated by vibrational sum frequency generation spectroscopy

The organization of 1-butanol and 1-hexanol at the air-liquid interface of aqueous, aqueous ammonium bisulfate, and sulfuric acid solutions was investigated using vibrational broad bandwidth sum frequency generation spectroscopy. There is spectroscopic evidence supporting the formation of centrosymmetric structures at the surface of pure butanol and pure hexanol. At aqueous, ammonium bisulfate, and at most sulfuric acid solution surfaces, butanol molecules organize in all-trans conformations. This suggests that butanol self-aggregates. The spectrum for the 0.052 M butanol in 59.5 wt % sulfuric acid solution is different from the other butanol solution spectra, that is, the surface butanol molecules are observed to possess a significant number of gauche defects. Relative to surface butanol, surface hexanol chains are more disordered at the surface of their respective solutions. Statistically, an increase in the number of gauche defects is expected for hexanol relative to butanol, a six carbon chain vs a four carbon chain. Yet, self-aggregation of hexanol at its aqueous solution surfaces is not ruled out because the methylene spectral contribution is relatively small. The surface spectra for butanol and hexanol also show evidence for salting out from the ammonium bisulfate solutions.     

Influence of Two-step Mechanism of Dissociation of H2SO4 on the Limiting Cathodic Current of Copper Deposition from Sulfate Solutions in Conditions of Natural Convection

The limiting cathodic current of copper deposition at a flat vertical electrode from solutions containing a mixture of copper sulfate and sulfuric acid (in excess) in conditions of natural convection is calculated. The possibility of the two-step dissociation of sulfuric acid is accounted for. The obtained formulas yield the limiting cathodic current at any ratio of bisulfate and sulfate anions in solution.     

The limiting current density of copper electrodeposition on vertical electrode under the conditions of electrolyte natural convection

The problem of determining the limiting current density of copper electrodeposition on the vertical electrode under the conditions of electrolyte natural convection is solved numerically. The incomplete dissociation of copper sulfate and sulfuric acid by the second stage and the concentration dependences of physicochemical properties of solution are taken into consideration. The effect of incomplete dissociation of copper sulfate and sulfuric acid by the second stage on the ionic transport in the copper electrodeposition and on the limiting current density is estimated by comparing the results, which are calculated taking into account and ignoring the concentration dependences of dissociation constants, and also for the limiting cases of virtually complete dissociation of copper sulfate and sulfuric acid. It is shown that, in excess of supporting electrolyte, incomplete dissociation of copper sulfate and sulfuric acid only slightly affects the limiting current density: the limiting current density changes not more than by 10% with the variation of the dissociation constants by several orders of magnitude. At the moderate and small concentrations of supporting electrolyte, incomplete dissociation should be taken into account, because the error of limiting current density can exceed 50%.     

Electroreduction of Oxalate Complexes of Ruthenium(III) on a Dropping Mercury Electrode: Effect of Alkali Metal Cations and Solution Acidity

It is established that the reaction of recharging trioxalate complexes of ruthenium(III) occurs in the case of solutions with excess supporting oxalate salts of alkali metals K⁺, Na⁺, and Cs⁺ in reversible conditions, and limiting recharge currents are caused by diffusion. At the same time, values of diffusion coefficients for complex anion [Ru(C₂O₄)₃]^–3 decrease by almost two times upon going from potassium to sodium and cesium electrolytes. Substantial differences in the limiting currents in solutions containing excess amounts of the above salts are explained by the formation, at least in the case of cesium and sodium electrolytes, of ionic associates whose reduction rate at a fixed potential is lower than that of nonassociated anion [Ru(C₂O₄)₃]^–3. With solution dilution by supporting salts, transition is observed from reversible recharge conditions to absolutely irreversible conditions and a change in the above sequence of the effect of supporting cations on the recharge rate; at a fixed potential, the process decelerates in the series Cs⁺ > K⁺ > Na⁺. The reduction wave of the ruthenium(II) oxalate complexes in solutions with excess supporting electrolyte happens to depend on pH and, probably, is determined by simultaneous formation of adsorbed atoms of hydrogen (or ruthenium hydride) on atoms of ruthenium(0).     

The Soret effect with chemical reaction in aqueous sulfuric acid solutions

The Soret coefficient of aqueous sulfuric acid has been determined conductimetrically from 0.0005 to 0.2m at 25°C. The derived enthalpies of transport increase sharply as the concentration drops, approaching a limiting value near 35 kJ-mol^–1. The increase, in the enthalpy of transport at low concentrations is due to the large intrinsic enthalpies of transport of the hydrogen and sulfate ions that are produced by the dissociation of bisulfate ions. The enthalpy of dissociation of the bisulfate ions reduces the acid's enthalpy of transport by up to 1.8 kJ-mol^–1.     

Solvation effect of the cation of the supporting electrolyte in hexamethylphosphoramide

Polarographic reductions of sodium and potassium ions in hexamethylphosphoramide (HMPA) have been examined in various supporting electrolytes. The supporting electrolytes, which have much the same solvated radii and much the same electrocapillary curves, sometimes have a significantly different influence on the polarographic reductions of metal ions. The Li⁺ and Hex₄N⁺ ions provide a typical example. Their effective radii are seen to have much the same characteristics. However, the polarographic reduction of the sodium ion shows a difference in shape between that occurring in Li⁺ solution and that in Hex₄N⁺ solution. Another example is found in the case of Et₄N⁺, Me₄N⁺ and 5N6⁺, whose r_eff and the electrocapillary curves are much the same. However, the polarographic reductions of the sodium and potassium ions are different in these solutions. The solvation number of the solvent molecule of the supporting electrolyte cation seems to exert a great influence on these reductions. The electrocapillary curves were also examined with the tetradodecylammonium ion, tetradecylammonium ion and tetraphenylphosphonium ion used as the supporting electrolytes. The inhibition of the reduction of metal ion for these cations is evidence for their lack of solvation. The effects of the solvated asymmetrical tetraalkylammonium ions on the polarographic behaviour were also examined. When some methyl groups cooperate with the tetraalkylammonium ion, the chemical character is between that of the Et₄N⁺ ion and that of the Me₄N⁺ ion.     

The interpretation of solution electrolyte vibrational bands in potential-difference infrared spectroscopy

The influence of ionic migration to and from the surrounding solution reservoir upon potential-difference infrared (PDIR) spectra is examined for some cases involving anionic adsorption in order to elucidate its consequences upon the net potential-induced compositional changes in the thin-layer solution. Representative PDIR spectra for the adsorption of azide anions on gold are compared in the absence and presence of excess alkali perchlorate supporting electrolyte. In the latter, the loss of solution azide in the spectral thin layer upon stepping to a more positive potential, resulting from increased azide adsorption, is accompanied by extensive migration of perchlorate into the thin layer. The form of the spectra induced by potential-dependent azide specific adsorption differs in these two circumstances since in the former the ionic migration between the thin-layer cavity and the solution reservoir necessary for charge compensation is provided by the azide electrolyte itself, whereas in the latter case migration of the supporting electrolyte yields a fixed quantity of azide in the thin layer. The intensities and sign of the PDIR bands arising from solution-phase azide and perchlorate enable the extent of the potential-dependent anionic redistribution in the thin-layer cavity to be quantified. In the absence of added perchlorate, the magnitude of the solution azide band is diminished substantially, inferring that replenishment of the thin-layer solution concentration occurs predominantly via N⁻₃ migration from the surrounding solution reservoir. Similar results were also obtained for cyanate adsorption on gold. The influence of cation as well as anion migration on this thin-layer charge redistribution was examined by employing an infrared-active cation, NH⁺₄, as well as from the addition of H₃O⁺. While the results indicate that cation migration can contribute substantially to this charge redistribution, anion migration typically appears to predominate when specific anion adsorption is encountered. Some general consequences of such ion migration effects to the interpretation of PDIR spectra are noted.     

Diffusion in dilute aqueous acetic acid solutions at 25°C

A conductimetric technique has been used to measure diffusion coefficients for aqueous solutions of acetic acid at concentrations from 0.002 to 0.02 mol-dm^–3 at 25°C. The acetic acid component diffuses more rapidly at lower concentrations where a higher proportion of the slower acid molecules are converted by dissociation to acetate ions and highly mobile hydrogen ions. The observed concentration dependence of the diffusion coefficient verifies the limiting law for weak electrolyte diffusion. A new type of conductimetric diffusion cell with several practical advantages over earlier designs is described together with an improved procedure for the conductimetric determination of accurate diffusion coefficients for weak electrolyte systems.     

Kinetics of Neutralization of Weak Electrolyte Ion-Exchange Resins

The adsorption of acids and alkalis by weak electrolyte resins is described by diffusion of the ions through the resin bead, with an accompanying neutralization reaction. A model proposed by Helfferich has been extended to account for the situation where the solution concentration of the reagent is changing during the rate experiment. Two mechanisms are necessary, depending on the concentration of the reagent. The rate expression for systems where the reagent concentration is greater than 0.01 M is given. It predicts that the adsorption rate is dependent on the solution hydrogen ion concentration in the case of weakly basic resins, and that it varies inversely with the square of the particle radius. The rate should also be unchanged by the resin of added salt.

Experimental evidence in the form of rate data for the uptake of hydrochloric acid by a commercial weakly basic resin support the derived equation, which can be modified further to account for the diffusional resistance of the liquid film around the resin particle.     

Differential Pulse Polarography of Nickel(II) with Imidazole and Related Compounds

Abstract

Electrochemical reductions of nickel(II) complexes with imidazole, histamine, histidine, and pilocarpine have been studied using differential pulse polarography in the presence of 1.0 mol dm^?3 sodium acetate as supporting electrolyte. The peak potential for the complexes appeared at more positive potentials than for the nickel aquo ion. The positive shift ceases and then reverses to the negative direction for high ligand concentrations. Both histidine and pilocarpine have shown another wave which may be ascribed to the catalytic reduction of hydrogen ions in the solution. The peak height of the differential pulse pre-wave increases with the concentration of the ligands only when shift in potential is in the positive direction with the nickel ion concentration in excess of the organic ligand. The linearity of variation of the peak height with the concentration for the above mentioned compounds has been investigated. A mixture of histamine and histidine showed two separate peaks, permitting possible simultaneous determination when the compounds are in admixture.     

Exact steady state solutions in symmetrical Nernst–Planck–Poisson electrodiffusive models

We compare three one-dimensional Nernst–Planck–Poisson systems that describe ion distribution near the electrode surfaces with planar, cylindrical and spherical symmetry respectively. These three models take into account ion diffusion and migration. In particular they describe the diffusive layers formed by Li+ ions in the vicinity of the graphite electrode particles. The three types of symmetry appear due to three different ways of particle ordering inside the electrode. In this paper we construct the exact steady state solutions to these systems and approximate solutions in form of power series. Then we solve the systems numerically and compare the results. We discuss the influence of symmetry in electrode particle ordering on the steady state distribution of ions in the diffusive layer.     

Convective Instability of the Limiting Current of the Triiodide Reduction in a Vertical Channel

The instability of the limiting current of the cathodic reaction is studied in the I^– ₃–I^–/Pt system with excess of potassium iodide at the electrode situated at the bottom of a vertical cylindrical channel. Dependence of parameters of the oscillation process on the concentration of electroactive and supporting ions and solution viscosity is analyzed. The deceleration of the transport processes in solutions containing glycerol is shown to be due to a considerable decrease in the triiodide ion diffusion coefficient.     

Influence of Analyte Flow Rate on Signal and Response Time of the Amperometric Gas Sensor with Nafion Membrane

This paper presents results of an investigation on influence of volumetric flow rate on the signal and response time of a prototype of sulfur dioxide gas sensor with Nafion membrane. The sensors differing in type of working electrode and composition of internal electrolyte were compared. We used Au and Pt working electrodes obtained via vacuum sublimation deposition on a Nafion membrane surface. The electrolytes were aqueous solutions of sulfuric acid of the summary concentration 5 mol dm^?3 (electrolyte A). The electrolyte B contained an addition of dimethylsulfoxide (DMSO); the water/DMSO molar ratio was 2 : 1. Based on a proposed equation, which takes diffusion resistance into account, the obtained sensor signals were analyzed for the flow rate within a range of 0–100 cm³ min^?1. The sensor response time was also determined for the above flow rate range.