Rotating disk electrode study of the kinetics of formation of palladium(II) ethylenediaminechloride complexes from palladium(II) bis-ethylenediamine complexes

The kinetics of substitution of chloride ions for ethylendiamine in a bis-ethylenediamine complex of palladium(II) is studied in solutions with pH 0.3–2.0 and the ionic strength of 0.11–1.0 M by measuring the transients of limiting diffusion currents of the electroreduction of palladium(II) ethylenediamine complexes on a rotating disk electrode. The first reaction orders in hydrogen and chloride ions are found at the ionic strength of 1 M (NaClO₄). The activation energy of the homogeneous reaction under study is determined from the temperature dependence of its rate constant. The mechanism of substitution of chloride ions for ethylenediamine is discussed.     

Hydrodynamic voltammetry at channel electrodes: Part IV. Theory of chronopotentiometry for reversible electrode reactions

The theory of chronopotentiometric measurements at channel electrodes is developed for reversible electrode reactions, by rigorously solving the corresponding time-dependent boundary value problem, where the non-uniform accessibility of the channel electrode surface is taken into consideration. The theoretical equations of the transition time and the potential-time curve are derived as functions of (I_d/I), where I denotes the applied current intensity and I_d the limiting diffusion current obtained at steady-state. Finally, the time variation of the current density distribution at the electrode surface is given.     

Diffusion of sodium alginate in aqueous solutions at T = 298.15 K

Taylor dispersion technique was used for measuring mutual diffusion coefficients of sodium alginate aqueous solutions at T = 298.15 K, by using as carrier stream solution both pure water and solutions of this polyelectrolyte at a slightly different concentration. The limiting values found at infinitesimal ionic strength, D⁰, were determined by extrapolating to c → 0. These studies were complemented by molecular mechanics calculations. From the experimental data, it was possible to estimate both the limiting conductivity and the tracer diffusion coefficient values for the alginate anion, and the hydrodynamic radius of the sodium alginate (NaC₆H₇O₆), as well as to discuss the influence of the kinetic, thermodynamic and viscosity factors on the diffusion of sodium alginate in aqueous solutions at finite concentrations. Thus, the aim of our innovative research is to contribute to a better understanding of the structure and the thermodynamic behavior of these polymeric systems in solution and supplying the scientific and technological communities with data on these important parameters in solution transport processes.     

Single-ion activities and Mn(Hg)/Mn(II) reactions in aqueous solutions of alkaline-earth chlorides

Combined thermodynamic and kinetic studies have yielded convenient single-ion activity coefficients for manganese(II), alkaline-earth, and chloride ions and standard exchange currents for the two steps of the Mn(Hg)/Mn(II) electrode in 0.005 M MnCl₂+0.495 M MeCl₂ (for Me=Mg, Ca, Sr, Ba, and Mn) at 25°C. The results indicate that the fall in mean ionic activity coefficient for the alkaline-earth chlorides along the sequence from magnesium to barium is carried to a larger extent by the cation than by the anion, that also the activity coefficient for the minority cation Mn(II) falls along this sequence, and that other than activity-coefficient effects on the Mn(Hg)/Mn(II) reactions appear only with barium ions, which retard the reactions additionally. The results are discussed with emphasis on ionic interactions and double-layer effects.     

Analytical formulae for chronoamperometry of a charge neutralisation process under conditions of linear migration and diffusion

The electrochemical reaction of the charge neutralisation of a binary electrolyte ion, at a planar electrode, is a rare example for which analytical formulae for potential step chronoamperometry can be obtained, under conditions of migration-diffusion transport in semi-infinite, one-dimensional space domain. This has been previously shown by Myland and Oldham [Electrochem. Commun. 1 (1999) 467], assuming equal diffusion coefficients. In the present work, a more general analytical solution is obtained, for the case of unequal diffusion coefficients. The effect of the diffusion coefficient ratio D_R/D_C, of the electroactive ion R and the counter-ion C, on the limiting chronoamperometric current, is examined. The effect increases with the growing ratio |z_R/z_C| of the electric charges of these ions, and becomes nonnegligible for |z_R| greater than or equal about 2, and for typical departures of D_R/D_C from unity. Among other things, the formulae obtained can be useful for testing digital simulation methods for the Nernst–Planck equations.     

Effects of ionic strength and humic acid on 99TcO4 ? sorption and diffusion in Beishan granite

In terms of pre-safety assessment of a potential site for high-level radioactive wastes disposal in China, the geochemical behavior of key radionuclides which tend to be released from the repository must be thoroughly investigated. ⁹⁹Tc is a long-lived fission product with appreciable productivity in nuclear fuel, and Tc (+7) has unlimited solubility in near-field geochemical environments. In this study, the effects of ionic strength and humic acid on ⁹⁹TcO₄ ^? sorption and diffusion in Beishan granite were investigated with through-diffusion and batch sorption experiments. Results indicated that the effective diffusion coefficients (D _e) of ⁹⁹TcO₄ ^? in Beishan granite varied from 1.07?×?10^?12 to 1.28?×?10^?12?m²/s without change with ionic strength, while the distribution coefficients (K _d) negatively correlated with ionic strength of the rock/water system. This study also indicates that there is no evident influence of humic acid concentration on the diffusion behavior of ⁹⁹TcO₄ ^? in Beishan granite, due to the limited interaction between humic acid and ⁹⁹TcO₄ ^?.     

Kinetics of multiple electron transfer reactions at porous electrodes under stationary and flow conditions

The kinetics of consecutive two-electron transfer reactions at porous flooded electrodes are investigated under both stationary and flow conditions, where mass transfer is due respectively to diffusion and forced convection. The current-polarization relations were calculated for both modes of mass transfer as a function of the specific surface area of the electrode, the rates of the respective steps of the electron transfer reaction and the appropriate mass transfer coefficients. The computed solutions degenerate to the known limiting cases of single electron transfer control under conditions of very high or very low polarizations. Thus, at high anodic polarization, the electrochemical reaction is controlled by a single electron transfer step, the other step being too fast. Under conditions of 0.1<i/i_L<1, the overall reaction rate is controlled by both mass transfer and electrochemical activation. For flooded diffusion electrodes, the current-voltage curves follow the Tafel equation with a slope of double the normal value. This is attributed to mass transfer control in agreement with previous work. Experimental results, obtained on the porous flow-through electrode, agreed well with the theoretical predictions. The calculations presented here enable a quantitative evaluation of the relative influence of the rate of any step on the overall behaviour of the electrode under the appropriate experimental conditions.     

Diffusion coefficients of bolaform alkane-1,n-bis(trimethylammonium bromide) surfactants

Mutual diffusion coefficients of alkane-1,n-bis(trimethylammonium bromide), C_nMe₆Br₂ (n = 8, 10, 12), surfactants have been measured using the Taylor dispersion technique, at T = 298.15 K, at concentrations from (0.000 to 0.0380) mol · dm⁻³. The dependence of mutual diffusion coefficients on the concentration has been discussed in the framework of Onsager–Fuoss and Pikal models. On the basis of this discussion, it is suggested that these surfactants behave as associated electrolytes. From limiting mutual diffusion coefficient values, extrapolated from experimental values for c → 0, limiting ionic conductance, tracer diffusion coefficients, and hydration radii of alkane-1,n-bistrymethyl ammonium ions have been estimated. For the case of dodecane-1,12-bis(trimethylammonium bromide), no aggregation has been noticed up to 0.04 mol · dm⁻³.     

Measurement of mutual diffusion coefficients,densities, viscosities,and osmotic coefficients for the system KSCN-H2O at 25°C

Mutual diffusion coefficients measured on the volume-fixed frame of reference are reported for KSCN-H₂O at 25°C over the concentration range 0.0 to 10.26 mol-dm^–3. The diffusion coefficient at infinite dilution was obtained from limiting ionic equivalent conductances of K⁺ and SCN^–. Low concentration conductances of KSCN-H₂O at 25°C used to obtain the limiting ionic equivalent conductance of SCN^– are reported. Values of density and viscosity for this system are reported from 0.0 to 10.30 mol-dm^–3. Osmotic coefficienss of KSCN-H₂O at 25°C were measured by the isopiestic method. These are reported over the concentration range of 0.30 to 24.94 molal (saturation). Values of thermodynamic diffusion coefficients for the concentration range 0.0 to 10.26 mol-dm^–3 are tabulated. Results are compared to other potassium salts with monovalent anions at 25°C.     

Ternary diffusion coefficients of diethanolamine and N-methyldiethanolamine in aqueous solutions containing diethanolamine and N-methyldiethanolamine

Ternary diffusion coefficients of diethanolamine (DEA) and N-methyldiethanolamine (MDEA) in aqueous solutions containing DEA and MDEA using the Taylor dispersion technique have been measured for temperatures (303.2, 313.2, and 323.2 K). The systems studied were aqueous solutions containing total amine concentrations of 2.5 and 4.0 kmol m⁻³, each having four different amine molar ratios. The density and viscosity of the blended amine solutions were also measured. The mutual diffusion coefficients of aqueous DEA and aqueous MDEA solutions were also reported. The main diffusion coefficients (D₁₁ and D₂₂) and the cross-diffusion coefficients (D₁₂ and D₂₁) were reported as function of temperature and concentration of alkanolamines. The limiting conditions for the main diffusion coefficients and the cross-diffusion coefficients were discussed at first, and a comparison between the ratios of the cross-diffusion coefficients to the main diffusion coefficients for DEA and MDEA was made. The dependence of the main diffusion coefficients on the viscosity of solutions was also investigated.     

Thermodynamic Properties of {NaCl-CsCl-LiCl}(aq) at T=298.15 K: Water Activities, Osmotic and Activity Coefficients

A hygrometric method has been used to measure water activities for the quaternary aqueous system NaCl-CsCl-LiCl-H₂O as a function of the solutes concentrations at T=298.15 K. The measurements were performed by measuring the diameter of solution droplets as a function of the surrounding relative humidity. The total ionic strength range covered in this study was about 0.6 to 6 mol?kg^?1 for different ionic strength fractions y of one of the three solutes ranging from y=1/3 to 1/2, whereas the constant ratio r of molalities of the two other solutes was fixed at unity. The osmotic coefficients of these aqueous mixtures were calculated over this ionic strength range. The obtained data were treated by the ECA I and ECA II rules, and the Pitzer and Kim, and Pitzer-Dinane models. The solute activity coefficients of components in the studied mixtures were also determined for different ionic strength fractions of the different solutes.     

Correlation of the Extraction Constant Values of Zn2+ and Cd2+ from Phosphoric Acid Media by Bis(2,4,4-trimethylpentyl)dithiophosphinic Acid Dissolved in Toluene

Values of the extraction constants of Zn²⁺ and Cd²⁺ from aqueous phosphoric acid solutions (0.36 to 7.31 mol?L^?1) by Cyanex 301 in toluene, involving formation of the complexes ZnR₂ and CdR₂ with R being bis(2,4,4-trimethylpentyl)dithiophosphinate, have been correlated at T=298 K as a function of the ionic strength. For this purpose the activity coefficients of all of the aqueous species have been calculated taking into account both the protolytic equilibria of concentrated phosphoric acid and complexation reactions between the cations and the phosphoric acid species. Good correlations have been obtained for the extraction constant values with the ionic strength, provided the release of water molecules during the extraction processes is considered. Finally, extraction constant values are reported at infinite dilution.     

Environmental Coordination Chemistry: Binary Systems Comprising Some Bivalent Cations and Monocarboxylates in Aqueous Solution. Ionic Medium Effects on Equilibrium Constants

Abstract

The molar single activity coefficients associated with propionate ion (Pr) have been determined at 25°C and ionic strengths comprised between 0.300 and 3.00 M, adjusted with NaClO₄, as background electrolyte. The investigation was carried out potentiometrically by using a second class Hg/Hg₂Pr₂ electrode. It was found that the dependence of propionate activity coefficients as a function of ionic strength (I) can be assessed through the following empirical equation: log y_pr = –0.185 J^3/2 + 0.104 I². Next, simple equations relating stoichiometric protonation constants of several monocarboxylates and formation constants associated with 1:1 complexes involving some bivalent cations and selected monocarboxylates, in aqueous solution, at 25°C, as a function of ionic strength were derived, allowing the interconversion of parameters from one ionic strength to another, up to I = 3.00 M. In addition, thermodynamic formation constants as well as parameters associated with activity coefficients of the complex species in the equilibria are estimated. The body of results shows that the proposed calculation procedure is very consistent with critically selected experimental data.     

Quinary Mutual Diffusion Coefficients of Aqueous Mannitol + Glycine + Urea + KCl and Aqueous Tetrabutylammonium Chloride + LiCl + KCl + HCl Solutions Measured by Taylor Dispersion

Taylor dispersion is widely used to measure binary mutual diffusion. Studies of three- and four-component solutions show that the dispersion method is also well suited for multicomponent diffusion measurements, including cross-coefficients for coupled diffusion. Numerical procedures are reported here to calculate mutual diffusion coefficients from dispersion profiles measured for solutions of any number of components. The proposed analysis is used to measure the sixteen quinary mutual diffusion coefficients of five-component aqueous mannitol + glycine + urea + KCl solutions and aqueous NBu₄Cl + LiCl + KCl + HCl solutions. Mannitol, glycine, urea and KCl interact weakly at the low solute concentrations used (0.010 mol·dm^?3). The diffusion coefficients of this system are compared with pseudo-binary predictions. Strong coupling of the NBu₄Cl, LiCl, KCl and HCl fluxes is interpreted by using ionic conductivities and Nernst equations to calculate limiting quinary diffusion coefficients for mixed electrolytes that interact by the electric field generated by ion concentration gradients.     

Medium effects: The adsorption of ions on a mercury electrode from mixed H2O-organic solvent systems

The problems met in the investigations of ionic adsorption at the mercury electrode from mixed solvent systems are discussed. An attempt is made to give such an interpretation of non-monotonic changes of the anion adsorption as function of solvent composition which could be equally well applied for interpreting the non-monotonic changes of the rate constants of electrode reactions. The practical difficulties of evaluating the surface excesses in the case of mixed solvents are discussed. Some experimental results for the adsorption of SCN^? ions from the system 0.2 M (NaSCN+NaF) in H₂O+acetone are presented. A suggestion for checking the consistency of the evaluated surface excesses is given.     

Thermodynamic study of the mixed system (CsCl + CaCl2 + H2O) by EMF Measurements at T = 298.15 K

This work reports the results of a thermodynamic investigation of the ternary mixed-electrolyte system (CsCl + CaCl₂ + H₂O). The activity coefficients of this mixed aqueous electrolyte system have been studied with the electromotive force measurement (EMF) of the cell: Cs ion-selective electrode (ISE)|CsCl(m_A), CaCl₂(m_B), H₂O|Ag/AgCl at T = 298.15 K and over total ionic strengths from (0.01 to 1.50) mol · kg^?1 for different ionic strength fractions y_B of CaCl₂ with y_B = (0, 0.2, 0.4, 0.6, and 0.8). The cesium ion-selective electrode (Cs-ISE) and the Ag/AgCl electrode used in this work were made in our laboratory and had a good Nernst response. The experimental results obey the Harned rule, and the Pitzer model can be used to describe this ternary system satisfactorily. The osmotic coefficients, excess Gibbs free energies and activities of water of the mixtures were also calculated.     

Reactivation of passive titanium: the enhancement of O2 evolution after potentiodynamic cyclings

Titanium oxide films were grown potentiodynamically at 50 mV s⁻¹ from −1.1 up to 10.0 V, at room temperature (23±1 °C) in H₃PO₄/NaH₂PO₄ aqueous solutions of ionic strength 0.5 mol L⁻¹ and pH 3.0. After the oxide growth, the passive electrode was subjected to different repetitive potentiodynamic cycles at 50 mV s⁻¹ between pre-set cathodic (E_s,c) and anodic (E_s,a) switching potentials. The changes in the electrochemical behaviour of the passive electrode, specially that of the O₂ evolution reaction, were followed as a function of the number of cycles and of the limiting negative potential value used, E_s,c. The enhancement of the oxygen reaction rate occurring with the repetitive potential sweeps might be due to an increase of both the oxide conductivity and the porous-oxide surface area.     

Transport behaviour of the lanthanide 152Eu(III), 153Gd(III) and 170Tm(III) and transplutonium element 254Es(III), 244Cm(III), 241Am(III), 249Cf(III) and 249Bk(III) ions in aqueous solutions at 298 K

Ionic self-diffusion coefficients (D) for trivalent radiotracers, lanthanide and actinide ions have been determined in concentrated aqueous solutions of supporting electrolytes of Gd(NO₃)₃–HNO₃ or Nd(ClO₃)₄–HClO₄ up to 1.5 mol L^?1 at 298.15 K and pH 2.50 by the open-end capillary method. The data obtained in large range of concentrations, allow to derive the limiting value D°, the validity of the Onsager limiting law and a more extended law. This study contributes to demonstrate similarities in transport and structure properties between 4f and 5f trivalent ions explained by a similar electronic configuration, ionic radius and hydration number. An empirical equation is suggested for predicting ionic hydration number with a good precision.     

Osmotic and Activity Coefficients of {yKCl+(1−y)MgCl2}(aq) at T=298.15 K

Isopiestic vapor-pressure measurements were made at the temperature 298.15 K for aqueous KCl + MgCl₂ solutions using KCl(aq) as the reference standard. The measurements for these ternary solutions were made at KCl ionic strength fractions of y=0.0, 0.1989, 0.3996, 0.5993, 0.7925 and 1.0 (with two additional sets at y=0.0, 0.2021, 0.3998, 0.6125, 0.8209 and 1.0) for the ionic strength range from 0.4014 to 6.2790 mol?kg^?1. Our results, and those from two previous isopiestic studies, were combined and used with previously determined parameters for KCl(aq) and those determined here for MgCl₂(aq) to evaluate mixing parameters for the Clegg-Pitzer-Brimblecombe model. These combined data were also used to determine the mixing parameters of the Scatchard model. Both sets of model parameters are valid for ionic strengths of I≤12.8 mol?kg^?1, where higher-order electrostatic effects have been included in the Clegg-Pitzer-Brimblecombe mixture model. The activity coefficients for KCl and MgCl₂ were calculated from these models and the results for KCl were compared to experimental data from Emf measurements. The Scatchard model interaction parameters were used for calculation of the excess Gibbs energy as a function of the ionic strength and ionic strength fraction of KCl. The Zdanovskii-Robinson-Stokes rule of linearity for mixing of isopiestic solutions was tested.     

Parameters of electrolysis in 3D flow electrode in limiting diffusion current mode

The analytical expressions were derived to calculate thickness of a three-dimensional flow electrode (TFE) working in limiting diffusion current mode in the cases of unidirectional and multidirectional electric field and solution flow for the given degree of metal extraction. The algorithm was suggested on the basis on the derived formulas and earlier published mathematical models of nonstationary electrolysis at TFE to calculate the whole electrode working conditions at the limiting diffusion current with parallel electrochemical reactions. The calculations and experimental studies were carried out on copper electrodeposition from sulfate electrolyte on TFE working in the limiting diffusion current mode. The effect of major electrochemical reactions to the distribution of potential and partial current densities by the electrode thickness was demonstrated. The process potential distribution was given for various electrode conductivities. A good compliance was shown between the results of calculations and experimental studies.