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ABSTRACT: A surface-controlled dissolution of cylindrical solid particles model is applied to potassium carbonate, sodium bicarbonate and sodium carbonate in dimethylformamide at elevated temperatures. Previously published data for the dissolution of potassium carbonate is interpreted assuming a cylindrical rather than a spherical shape of the particles, the former representing a closer approximation to the true shape of the particles as revealed by scanning electron microscopy. The dissolution kinetics of sodium carbonate and sodium bicarbonate in dimethylformamide at 100 degrees C were investigated via monitoring of the deprotonation of 2-cyanophenol with dissolved solid to form the 2-cyanophenolate anion that was detected with UV-visible spectroscopy. From fitting of experimental results to theory, the dissolution rate constant, k, for the dissolutions of potassium carbonate, sodium bicarbonate and sodium carbonate in dimethylformamide at 100 degrees C were found to have the values of (1.0 +/- 0.1) x 10(-7) mol cm(-2) s(-1), (5.5 +/- 0.3) x 10(-9) mol cm(-2) s(-1) and (9.7 +/- 0.8) x 10(-9) mol cm(-2) s(-1), respectively.
Physical Chemistry Chemical Physics 03/2006; 8(5):633-41. · 3.57 Impact Factor
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ABSTRACT: In this paper we present a mathematical model for the surface-controlled dissolution of cylindrical solid particles. This is employed to interpret experimental data published previously for the dissolution of potassium bicarbonate in dimethylformamide at elevated temperatures. Significant kinetic differences in assuming cylindrical rather than spherical shapes are reported with the former representing a closer approximation to the true shape of the particles as revealed by scanning electron microscopy. From the fits of experimental data to the cylindrical model for the surface-controlled dissolution, the dissolution rate constant, k, for the dissolution of KHCO(3) in DMF was found to be (9.6 +/- 1.6) x 10(-9) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 34.5 kJ mol(-1) over the temperature range of 60-100 degrees C. Comparison between cylindrical and spherical dissolution theory highlights the importance of considering the particle shapes for realistic modeling of surface-controlled dissolution kinetics.
The Journal of Physical Chemistry B 12/2005; 109(44):20786-93. · 3.70 Impact Factor
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ABSTRACT: Understanding the mechanisms of solid-liquid systems is fundamental to the development and operation of processes for the production of agrochemicals and pharmaceuticals. The use of a strong inorganic base in an organic solvent, typically, potassium carbonate in dimethylformamide, is often used to facilitate the formation of a required anionic organic nucleophile. In this paper, the dissolution kinetics of potassium carbonate in dimethylformamide at elevated temperatures is studied in the presence of ultrasound, as revealed via monitoring of the deprotonation of 2-cyanophenol by dissolved K2CO3. Two independent experimental methods were employed; the loss of 2-cyanophenol was detected electrochemically at a platinum microdisk working electrode, and the formation of the 2-cyanophenolate anion was monitored via UV/visible spectroscopic analysis. The results were modeled by fitting the experimental data to a theoretical model for the surface-controlled dissolution of solid particles. The dissolution rate constant, k, for the dissolution of K2CO3 in DMF was found to have a value of (1.3 +/- 0.2) x 10(-7) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 44.2 +/- 0.4 kJ mol(-1) over the temperature range of 70-100 degrees C studied.
The Journal of Physical Chemistry B 05/2005; 109(16):8263-9. · 3.70 Impact Factor
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ABSTRACT: Measurements on the diffusion coefficient of the neutral molecule N,N,N',N'-tetramethyl-para-phenylenediamine and the radical cation and dication generated by its one- and two-electron oxidation, respectively, are reported over the range 298-348 K in both acetonitrile and four room temperature ionic liquids (RTILs). Data were collected using single and double potential step chronoamperometry at a gold disk electrode of micrometer dimension, and analysed via fitting to the appropriate analytical expression or, where necessary, to simulation. The variation of diffusion coefficient with temperature was found to occur in an Arrhenius-type manner for all combinations of solute and solvent. For a given ionic liquid, the diffusional activation energies of each species were not only closely equivalent to each other, but also to the RTIL's activation energy of viscous flow. In acetonitrile supported with 0.1 M tetrabutylammonium perchlorate, the ratio in diffusion coefficients of the radical cation and dication to the neutral molecule were calculated as 0.89 +/- 0.05 and 0.51 +/- 0.03, respectively. In contrast, amongst the ionic liquids the same ratios were determined to be on average 0.53 +/- 0.04 and 0.33 +/- 0.03. The consequences of this dissimilarity are considered in terms of the modelling of voltammetric data gathered within ionic liquid solvents.
ChemPhysChem 04/2005; 6(3):526-33. · 3.41 Impact Factor
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ABSTRACT: We present a mathematical model for the surface-controlled dissolution of solid particles. This is applied to the dissolution of a solid having different particle size distribution functions: those of a monodispersed solid containing particles of all one size, a two-size-particle distribution, and a Gaussian distribution of the particle sizes. The dissolution of potassium bicarbonate in dimethylformamide is experimentally studied indirectly at elevated temperatures. We monitor the dissolution via the homogeneous deprotonation of 2-cyanophenol by dissolved KHCO3. The loss of 2-cyanophenol was detected electrochemically at a platinum microdisk electrode, and separately, the formation of the 2-cyanophenolate anion was monitored via UV-visible spectroscopic analysis. The results presented show that the kinetics of the loss of 2-cyanophenol behaves on one hand as a homogeneous chemical process and on the other hand as a dissolution-rate-controlled process. Initially, predissolved KHCO3 in solution deprotonates the 2-cyanophenol and homogeneous reaction dominates the observed kinetics, and at longer times, the observed kinetics is controlled by the rate of KHCO3 dissolution. Modeling of the experimental results for the surface-controlled dissolution of KHCO3 in dimethylformamide (DMF) yielded a mean value for the dissolution rate constant, k, at elevated temperatures; k was found to have a value of (1.1 +/- 0.3) x 10(-8) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 34.4 +/- 0.4 kJ mol(-1) over the temperature range 60-100 degrees C.
The Journal of Physical Chemistry B 03/2005; 109(7):2862-72. · 3.70 Impact Factor
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ChemPhysChem 09/2004; 5(8):1234-40. · 3.41 Impact Factor
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ABSTRACT: Steady-state voltammetry is used to measure the heterogeneous electron-transfer rates for the reduction of quinones to determine the dependence of k0 on molecular size, according to Marcus theory. This dependence is then used to predict the electron-transfer rate constants of related quinones, and the predictions are compared to experimental measurements.
08/2004;
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ABSTRACT: The electrochemical reduction of oxygen is reported in four room temperature ionic liquids (RTILs) based on quaternary alkyl -onium cations and heavily fluorinated anions in which the central atom is either nitrogen or phosphorus. Data were collected using cyclic voltammetry and potential step chronoamperometry at gold, platinum, and glassy carbon disk electrodes of micrometer dimension under water-free conditions at a controlled temperature. Analysis via fitting to appropriate theoretical equations was then carried out to obtain kinetic and thermodynamic information pertaining to the electrochemical processes observed. In the quaternary ammonium electrolytes, reduction of oxygen was found to occur reversibly to give stable superoxide, in an analogous manner to that seen in conventional aprotic solvents such as dimethyl sufoxide and acetonitrile. The most significant difference is in the relative rate of diffusion; the diffusion coefficients of oxygen in the RTILs are an order of magnitude lower than in common organic solvents, and for superoxide these values are reduced by a further factor of 10. In the quaternary phosphonium ionic liquids, however, more complex voltammetry is observed, akin to that expected for the reduction of oxygen in acidified organic media. This is shown to be consistent with the occurrence of a proton abstraction reaction between the electrogenerated superoxide and quaternary alkyl phosphonium cations following the initial electron transfer.
05/2004;
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ABSTRACT: The validity of Marcus theory for outer-sphere heterogeneous electron transfer for the electro-oxidation of a range of anthracene derivatives in alkyl cyanide solvents is investigated. The precision measurement of these fast electron transfers (k(0) >or= 1 cm s(-1)) is achieved by use of the high-speed channel electrode and, where necessary, fast-scan cyclic voltammetry. First, the solvent effect on the rate of electron transfer is studied by considering the first oxidation wave of 9,10-diphenylanthracene in the alkyl cyanide solvents: acetonitrile, propionitrile, butyronitrile, and valeronitrile. Second, the variation of k(0) for a series of substituted anthracenes is investigated by analyzing the voltammetric response of the one-electron oxidations of 9-phenylanthracene, 9,10-dichloroanthracene, 9-chloroanthracene, 9,10-dicyanoanthracene, 9-cyanoanthracene, 9-nitroanthracene, 9,10-diphenylanthracene, and anthracene in acetonitrile. It is shown that the rate of electron transfer of a single compound in different alkyl cyanides is determined by the longitudinal dielectric relaxation properties of the solvent, while differences in rate between the substituted anthracenes in acetonitrile can be quantitatively rationalized by considering their relative hydrodynamic radii. This makes possible the accurate prediction of electron-transfer rates for a molecule by interpolation of rate constants known for related molecules.
Journal of the American Chemical Society 05/2004; 126(19):6185-92. · 9.91 Impact Factor
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ABSTRACT: The reduction of oxygen in the presence of carbon dioxide has been investigated by cyclic voltammetry at a gold microdisk electrode in the two room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N6222][N(Tf)2]). With increasing levels of CO2, cyclic voltammetry shows an increase in the reductive wave and diminishing of the oxidative wave, indicating that the generated superoxide readily reacts with carbon dioxide. The kinetics of this reaction are investigated in both ionic liquids. The reaction was found to proceed via a DISP1 type mechanism in [EMIM][N(Tf)2], with an overall second-order rate constant of 1.4 ± 0.4 × 103 M-1 s-1. An ECE or DISP1 mechanism was determined to be the most likely pathway for the reaction in [N6222][N(Tf)2], with an overall second-order rate constant of 1.72 ± 0.45 × 103 M-1 s-1.
02/2004;
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ABSTRACT: The application of steady-state and fast-scan linear sweep voltammetry to a high-speed wall-tube electrode (HWTE) is reported in different solvents to investigate the response of the HWTE over a wide range of Reynolds' numbers (Re). Experiments are reported for the oxidation of N,N,N‘,N‘-tetramethyl-p-phenylenediamine (TMPD) in propylene carbonate (PC), water, butyronitrile (BN), acetonitrile (AN), and acetonitrile−water mixture solutions containing 0.10 M supporting electrolyte for a 24 μm radius platinum microdisk electrode housed within the HWTE using a range of average flow jet velocities from 0.03 to 19.8 m s-1 (corresponding to volume flow rates of 0.003−0.25 cm3 s-1 and center-line jet velocities from 0.05 to 39.5 m s-1). Fast scan linear sweep voltammetry is presented for the oxidation of TMPD in PC and of 9,10-diphenylanthracene (DPA) in AN. Theoretical results are derived using finite element methods for both one- and two-dimensional mass transport models. It is found that, for solvents with a kinematic viscosity above ca. 7.5 × 10-3 cm2 s-1, the hydrodynamic behavior for Re < 2000 is as expected with current responses in accordance with those predicted for a laminar, parabolic inlet flow profile. In low viscosity solvents, where Re < 2000, currents are lower than expected, indicating a departure from laminar flow in practical cells even at low Re. The HWTE is compared to the channel electrode in the light of the experimental results, theoretical limits of electron-transfer rate detectable, and conclusions drawn that the channel electrode is more reliable for kinetic measurements.
11/2003;
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ABSTRACT: The electrochemical reduction of oxygen in two different room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis((trifluoromethyl)sulfonyl)imide ([N6222][N(Tf)2]) was investigated by cyclic voltammetry at a gold microdisk electrode. Chronoamperometric measurements were made to determine the diffusion coefficient, D, and concentration, c, of the electroactive oxygen dissolved in the ionic liquid by fitting experimental transients to the Aoki model. [Aoki, K.; et al. J. Electroanal. Chem. 1981, 122, 19]. A theory and simulation designed for cyclic voltammetry at microdisk electrodes was then employed to determine the diffusion coefficient of the electrogenerated superoxide species, O2•-, as well as compute theoretical voltammograms to confirm the values of D and c for neutral oxygen obtained from the transients. As expected, the diffusion coefficient of the superoxide species was found to be smaller than that of the oxygen in both ionic liquids. The diffusion coefficients of O2 and O2•- in [N6222][N(Tf)2], however, differ by more than a factor of 30 (DO2 = 1.48 × 10-10 m2 s-1, DO2•− = 4.66 × 10-12 m2 s-1), whereas they fall within the same order of magnitude in [EMIM][N(Tf)2] (DO2 = 7.3 × 10-10 m2 s-1, DO2•− = 2.7 × 10-10 m2 s-1). This difference in [N6222][N(Tf)2] causes pronounced asymmetry in the concentration distributions of oxygen and superoxide, resulting in significant differences in the heights of the forward and back peaks in the cyclic voltammograms for the reduction of oxygen. This observation is most likely a result of the higher viscosity of [N6222][N(Tf)2] in comparison to [EMIM][N(Tf)2], due to the structural differences in cationic component.
09/2003;
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ABSTRACT: A finite element approach for numerical simulation of linear sweep voltammetry at the wall tube electrode is presented. Working curves and surfaces are computed and reported which permit the analysis of reversible, quasi-reversible and irreversible voltammograms for all voltage scan rates. # 2002 Elsevier Science B.V. All rights reserved.
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ABSTRACT: The five room temperature ionic liquids: 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([CnMIM][N(Tf)2], n=2, 4, 8, 10) and n-hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N6222][N(Tf)2]) were investigated as solvents in which to study the electrochemical oxidation of N,N,N′,N′-tetramethyl-para-phenylenediamine (TMPD) and N,N,N′,N′-tetrabutyl-para-phenylenediamine (TBPD), using 20 μl micro-samples under vacuum conditions. The effect of dissolved atmospheric gases on the accessible electrochemical window was probed and determined to be less significant than seen previously for ionic liquids containing alternative anions. Chronoamperometric transients recorded at a microdisk electrode were analysed via a process of non-linear curve fitting to yield values for the diffusion coefficients of the electroactive species without requiring a knowledge of their initial concentration. Comparison of experimental and simulated cyclic voltammetry was then employed to corroborate these results and allow diffusion coefficients for the electrogenerated species to be estimated. The diffusion coefficients obtained for the neutral compounds in the five ionic liquids via this analysis were, in units of 10−11 m2 s−1, 2.62, 1.87, 1.12, 1.13 and 0.70 for TMPD, and 1.23, 0.80, 0.40, 0.52 and 0.24 for TBPD (listed using the same order for the ionic liquids as stated above). The most significant consequence of changing the cationic component of the ionic liquid was found to be its effect on the solvent viscosity; the diffusion coefficient of each species was found to be approximately inversely proportional to viscosity across the series of ionic liquids, in accordance with Walden's rule.
Journal of Electroanalytical Chemistry. 556:179-188.
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ABSTRACT: The application of the high-speed microband channel electrode to the study of the heterogeneous electron transfer kinetics of the oxidation of some N-substituted phenylenediamines is described. Experiments to investigate the standard electrochemical rate constant, k0, of the oxidation of 1,4-phenylenediamine (PPD), N,N-dimethyl-1,4-phenylenediamine (DMPD), and N,N-diethyl-1,4-phenylene-diamine (DEPD) in acetonitrile solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 2.5 and 5 μm platinum microband electrodes using a range of centre-line velocities from 12 to 25 m s−1. The measured values of k0 for PPD, DMPD, and DEPD are 0.84±0.16, 3.15±0.30 and 1.64±0.25 cm s−1, respectively. The respective formal oxidation potentials are also found to be 0.287±0.002, 0.245±0.001, and 0.208±0.003 V (all measured vs. Ag). Experiments are also presented using “fast scan” cyclic voltammetry to obtain measurements of the heterogeneous rate constants for PPD, DMPD and DEPD to compare between the steady-state channel electrode and the ‘established’ transient methodologies. Scan rates in the range 102–104 V s−1 were used to measure peak separations with the resulting k0 values of 0.51±0.05, 1.89±0.10, and 1.28±0.20 cm s−1, respectively. The use of steady-state voltammetry obviates the need for capacitative corrections, perhaps suggesting a greater reliability in the resulting data.
Journal of Electroanalytical Chemistry. 534(2):151-161.
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ABSTRACT: An investigation into the oxidative electrochemistry of 1,3,5-Tris[4-[(3-methylphenyl)phenylamino]phenyl]benzene (TMPB) is described. In particular, the heterogeneous electron-transfer kinetics in N,N-dimethylformamide (DMF) and dichloromethane (DCM) are studied using the high-speed microband channel electrode. Experiments to investigate the standard electrochemical rate constants, k0, transfer coefficients, α, and formal potentials, Ef0, of the observable oxidations of TMPB in both DMF and DCM solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 12.5 μm platinum microband electrodes using a range of linear flow velocities from 12 to 25 m s−1. The measured values of k0 for the two measurable oxidations in DMF are 1.03 ± 0.41 and 1.02 ± 0.20 cm s−1, and for the three oxidations in DCM are 0.44 ± 0.04, 0.17 ± 0.03 and 0.08 ± 0.02 cm s−1, respectively. The values of α for these oxidations in DMF are 0.48 ± 0.06 and 0.52 ± 0.02, and in DCM are 0.52 ± 0.02, 0.62 ± 0.03 and 0.53 ± 0.07, respectively. The respective formal oxidation potentials (all measured vs Ag) are 0.533 ± 0.002 and 0.766 ± 0.003 V in DMF, and 0.161 ± 0.002, 0.495 ± 0.002 and 1.128 ± 0.004 V in DCM. The presence of the monocation radical in DCM is confirmed by ESR measurement.Experiments are also presented to explore the voltammetry of TMPB in microdroplets of toluene and also in the solid-phase, when in contact with aqueous solutions of sodium fluoride, perchlorate, nitrate and sulphate. It was found that, when TMPB is dissolved in a toluene microdroplet, anion insertion accompanies the first oxidation for the case of perchlorate and nitrate, with anion-facilitated dissolution occurring for sulphate and fluoride. More complex reactions occur at more positive potentials. In the solid-phase, however, slow anion-facilitated dissolution still occurs for fluoride and sulphate, and rapid direct dissolution takes place in the case of perchlorate.
Journal of Electroanalytical Chemistry.
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ABSTRACT: The application of fast-scan linear sweep voltammetry methods to a high-speed wall-tube electrode (HWTE) is reported. Experiments are reported for the oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in propylene carbonate solution containing 0.10 M tetrabutylammonium perchlorate for a 24 μm radius platinum microdisk electrode housed within the HWTE using a range of scan rates from 200 to 3000 V s−1 and average flow jet velocities from 0.24 to 9.4 m s−1 (corresponding to volume flow rates of 0.003–0.12 cm3 s−1, and centre-line jet velocities from 0.5 to 18.9 m s−1). Linear sweep voltammograms (LSVs) are analysed for a simple electron transfer under high volume flow rates, by curve fitting. Analysis of the transient LSVs yielded values for k0, α, and Ef0 for TMPD of (5.9±2.4)×10−2 cm s−1, 0.46±0.08 and 0.217±0.019 V (vs. Ag), respectively. This is in good agreement with independent experiments conducted using the high-speed channel electrode which yielded the results: k0=(6.3±0.4)×10−2 cm s−1, α=0.52±0.01, and Ef0=0.234±0.005 V (vs. Ag). The range of applicability of this method for measuring k0 was also investigated and compared with existing channel electrode techniques.
Journal of Electroanalytical Chemistry.
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ABSTRACT: An efficient approach to the simulation of the double potential step chronoamperometry at a microdisk electrode based on an exponentially expanding time grid and conformal mapping of the space is presented. The dimensionless second potential step flux data are included as a function of the first potential step duration and the ratio of the diffusion coefficients of the reacting species allowing instant analysis of the experimental double potential step chronoamperograms without a need for simulation. The values of the diffusion coefficients are determined for several test systems and found to be in good agreement with existing literature data.
Journal of Electroanalytical Chemistry.
