Effects of alkyltrimethylammonium bromide surfactants on the kinetics of the oxidation of tris(1,10-phenanthroline)iron(II) by azido-pentacyanocobaltate(III) complex

The redox reaction between tris(1,10-phenanthroline)iron(II), [Fe(phen)₃]²⁺, and azido-pentacyanocobaltate(III), [Co(CN)₅N₃]^3? was investigated in three cationic surfactants: dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB) in the presence of 0.1?M NaCl at 35°C. Second-order rate constant in the absence and presence of surfactant, k_w and k_ψ, respectively, were obtained in the concentration ranges DTAB?=?0???4.667?×?10^?4?mol?dm^?3, TTAB?=?0–9.364?×?10^?5?mol?dm^?3, CTAB?=?0???6.220?×?10^?5?mol?dm^?3. Electron transfer rate was inhibited by the surfactants with premicelllar activity. Inhibition factors, k_w/k_ψ followed the trend CTAB?>?TTAB?>?DTAB with respect to the surfactant concentrations used. The magnitudes of the binding constants obtained suggest significant electrostatic and hydrophobic interactions. Activation parameters ΔH^≠, ΔS^≠, and E_a have larger positive values in the presence of surfactants than in surfactant-free medium. The electron transfer is proposed to proceed via outer-sphere mechanism in the presence of the surfactants.     

Characterization of Anilinepentacyanoferrate (II) and (III)

The anilinepentacyanoferrate (II) complex has been characterized in aqueous solution. The complex exhibits a predominant ligand field transition at λ_max = 415 nm with ?_max = 494 M^?1 cm^?1. The corresponding Fe(III) complex displays a strong absorption at λ_max = 700nm(?_max = 1.61×10⁴ M^?1 sec^?1) which can be assigned as a ligand to metal charge transfer transition. The rate constants of formation and dissociation for the Fc(II) complex are (3.14±0.18)×10² M^?1W^?1 and 0.985±0.005 sec^?1, respectively, at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The cyclic voltammetry of the complex shows that a reversible redox process is observed with E_1/2 value of 0.51±0.01 V vs. NHE at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The kinetic study of the oxidation of the Fe(II) complex by ferricyanide ion yielded the rate constant of the reaction k_et = (1.43±0.04)x10 M sec^?1 at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C.     

Ozone reduction at Nafion modified Au electrode and the measurement of ozone concentration in highly resistive solutions

The mass transport and charge transfer kinetics of ozone reduction at Nafion coated Au electrodes were studied in 0.5 mol/L H2SO4 and highly resistive solutions such as distilled water and tap water. The diffusion coefficient and partition coefficient of ozone in Nafion coating are 1.78×10-6 cm2·s-1 and 2.75 at 25℃ (based on dry state thickness), respectively. The heterogeneous rate constants and Tafel slopes for ozone reduction at bare Au are 4.1×10-6 cm·s-1, 1.0×10-6 cm·s-1 and 181 mV, 207 mV in 0.5 mol/L H2SO4 and distilled water respectively and the corresponding values for Nafion coated Au are 5.5×10-6 cm·s-1, 1.1×10-6 cm·s-1 and 182 mV, 168 mV respectively. The Au microelectrode with 3 μm Nafion coating shows good linearity over the range 0-10 mmol/L ozone in distilled water with sensitivity 61 μA·ppm-1 ·cm-2, detection limit 10 ppb and 95% response time below 5 s at 25℃. The temperature coefficient in range of 11-30℃ is 1.3%.     

Electrode kinetics of electroactive electropolymerized polymers deposited on graphite electrode surfaces

A kinetic examination of the charge-transport processes (i.e. (i) heterogeneous electron-transfer process of electrode/film interfaces and (ii) homogeneous charge-transport process within films) at electroactive electropolymerized film-coated electrodes was conducted by normal pulse voltammetry. The films employed were of poly(o-phenylenediamine), Poly(N-methylaniline) and poly(N-ethylaniline), which were prepared on electrodes as coating films by electrooxidative polymerization of the corresponding monomers in an acidic solution. It was found that process (i) obeys the conventional Butler-Volmer equation and that process (ii) can be treated as a Fickian diffusion process. In addition, the kinetic parameters characterizing processes (i) and (ii) (i.e. the standard rate constant (k°) and transfer coefficient (α) for process (i), and the apparent diffusion coefficient (D_app for process (ii)) were estimated: D_app = ca (1–4)×10⁻⁸ cm² s⁻¹ s⁻¹, k° = ca. (4–6)×10⁻⁴ cm s⁻¹, α_a (for anodic process) = 0.83–0.86 and α_c (for cathodic process)=0.13–0.23. The are compared with the data reported previously for other electroactive polymer films.     

Electrode kinetic studies of single electron charge transfer redox couples using the faradaic rectification method

Three single electron charge transfer redox reactions have been studied using the faradaic rectification method. The kinetic parameters obtained for the ferricyanide-ferrocyanide redox couple are α=0.49, k_a⁰=12×10^?2 cm s^?1; for the chromic-chromous system α=0.47, k_a⁰=2×10^?3 cm s^?1 and for the titanic-titanous reaction α=0.49 and k_a^o=6×10^?4 cm s^?1 at 27°C.     

Kinetic study of the reaction of meso-tetrakis (p-trimethylammoniumphenyl)porphinatodiaquorhodate(II) with chloride,bromide, iodide,hexacyanocobaltate(III) and thiocyanate ions in aqueous solution

The reaction of Cl^?, Br^?, I^?, Co(CN)₆^3? and NCS^? with meso-tetrakis (p-trimethylammoniumphenyl)porphinatodiaquorhodate(III), [RhTAPP(H₂O)₂]⁵⁺, has been studied at 15, 25 and 35°C in 0.1 M [H⁺] with μ = 1.00 M (NaNO₃). The value of the acidity constant, K_al, at 25°C is 4.39 × 10^?9 M. The reactions are first order in anion concentration up to 0.9 M. The values of the stability constants, K₁, and the second order rate constants, k₁, for the reaction with Cl^?, Br^?, I^?, Co(CN)₆^3? and NCS^? are respectively 0.23 M^?1 and 2.5 × 10^?3 M^?1 s^?1, 1.1 M^?1 and 6.92 × 10^?3 M^?1 s^?1, 40.0 M^?1 and 17.0 × 10^?3 M^?1 s^?1, 550 M^?1 and 20.0 × 10^?3 M^?1 s^?1, 3400 M^?1 and 20.9 × 10^?3 M^?1 s^?1. The porphine greatly labilizes the Rh(III). There has been about a 500-fold increase in the rate constant for substitution compared to that of [Rh(NH₃)₅H₂O]³⁺. The substitution rates are however about the same as for [Rh(TPPS)(H₂O)₂]^3?, indicating that the overall charge on the complex plays only a minor role. The kinetic results indicate that dissociative activation is occurring in these reactions.     

The effect of pH on some outer-sphere electrode reactions at carbon electrodes

The apparent heterogeneous rate constant, k°_app, for three couples known to undergo outer-sphere electron transfer reactions has been investigated at carbon, gold, and platinum electrodes as a function of solution pH. The rates are found to be pH dependent at two different types of glassy carbon and at carbon paste but not at metallic electrodes. The rate is found to decrease with increasing pH for Fe(CN)^{$\text{3?}\text{4?}$}₆ and IrCl^{$\text{2?}\text{3?}$}₆, and the opposite effect is observed for the Ru(NH₃)^{$\text{3+}\text{2+}$}₆ couple. For the three compounds examined, the magnitude of the pH dependent rate changes can be associated with a change in the magnitude of the potential of the reaction layer induced by a change in surface charge of ～ 2 × 10^?7 C cm^?2. The pH dependence of the observed rate constant at glassy carbon can be attenuated by heat treatment, chemical reduction, or esterification of the surface. This behaviour is consistent with the theory that the pH dependence of k°_app arises from the presence of functional groups on the carbon surface.     

The Reductions of Bis(terpyridyl) and Ethylenediaminetetraacetato Complexes of Cobalt(III) by Ascorbic Acid

The reductions of Co(terpy)₂³⁺ and Co(edta)^? complexes by ascorbic acid have been subjected to a detailed kinetic study in the range of pH =1–10.9. For each complex the rate law of the reaction is interpreted as a rate determining reaction between Co(III) complex and the ascorbic acid in the form of HA^? (k₁) and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Co(III) complex. With given Ka₁ and Ka₂, the rate constants are k₁ = 0.25 and 9.87 × 10^?5 M^?1s^?1, k₂ = 1.28 × 10⁶ and 18.7 M^?1s^?1 for Co(terpy₎2³⁺and Co(edta)^? complexes, respectively, at T = 25 °C and μ = 0.50M (terpy)and 1.0 M (edta) HClO₄/LiClO₄. The mechanism of the reaction is discussed on the basis of Marcus theory for outer sphere electron transfer process. Spin change and charge effect, duly considered, account for the non‐adiabatic behavior in the reduction of Co(edta)^? complex.     

Spectrocoulometric study of the hydrolysis of the tris (1,10-phenanthroline)iron(III) complex ion

The spectrocoulometric technique reported earlier is applied to verify the mechanism and to evaluate the contributions k_Bi of the individual bases to the total rate constant k of the hydrolysis of the tris (1,10-phenanthroline) iron(III) complex, Fe (phen)³⁺₃. Both normal and “open-circuit” spectrocoulometric experiments are used. Partial rate constants for four bases in the acetate-buffered solutions are k_H2O=(3.4±1.2) × 10^?4s^?1 (k_H2O includes the H₂O concentration), k_OH=(1.20±0.06)×10⁷ mol^?1dm³s^?1, k_phen=(1.4±0.2) mol^?1dm³s^?1, k_Ac=(3.8±0.3)×10^?2 mol^?1dm³s^?1, at 25°C and ionic strength 0.5 mol dm^?3. The Fe(phen)³⁺³ hydrolysis, with (phen)₂ (H₂O) Fe-O-Fe (H₂O) (phen)⁴⁺² formation, is first order with respect to Fe (phen)³⁺³ and the bases present in the solution. The rate-determining step in the hydrolysis is the entry of a base to the coordinating sphere of the complex, as in the hydrolysis of the analogous 2,2′-bipyridyl complex.     

Base hydrolysis of (αβ S)-(o-methoxy benzoato) (tetraethylenepentamine) cobalt(III) ion: A comparative study of the role of ion pairs and micelles

The kinetics of base hydrolysis of (αβ S)-(o -methoxy benzoato) (tetraethylenepentamine)cobalt(III) obeyed the rate law: k_obs = k_OH[OH^?], in the range 0.05 ? [OH^?]_T, mol dm^?3 ? 1.0, I = 1.0 mol dm^?3, and 20.0–40.0°C. At 25°C, k_OH = 13.4 ± 0.4 dm³ mol^?1 s^?1, ΔH^≠ = 93 ± 2 kJ mol^?1 and ΔS^≠ = 90 ± 5 JK^?1 mol^?1. Several anions of varying charge and basicity, CH₃CO₂^?, SO₃^2?, SO₄^2?, CO₃^2?, C₂O₄^2?, CH₂(CO₂)₂^2?, PO₄^3?, and citrate^3? had no effect on the rate while phthalate^2?, NTA^3?, EDTA^4?, and DTPA^5? accelerated the process via formation of the reactive ion pairs. The anionic (SDS), cationic (CTAB), and neutral (Triton X-100) micelles, however, retarded the reaction, the effect being in the order SDS> CTAB > Triton X-100. The importance of electrostatic and hydrophobic effects of the micelles on the selective partitioning of the reactants between the micellar and bulk aqueous pseudo-phases which control the rate are discussed. © 1994 John Wiley & Sons, Inc.     

Accelerating the electron transfer of choline oxidase using ionic-liquid/NH2-MWCNTs nano-composite

A nano-composite consisting of amine functionalized multi-walled carbon nanotubes and a room temperature ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate) was prepared and used for modification of glassy carbon electrode. By immobilizing choline oxidase (ChOx) on the modified electrode, the enzyme direct electron transfer has been achieved. The modified electrode exhibited a pair of well-defined cyclic voltammetric peaks at a formal potential of ?0.395?V versus Ag/AgCl in 0.2?M phosphate buffer solution at pH 7.0. This peak was characteristic of ChOx-FAD/FADH₂ redox couple. The electrochemical parameters such as charge transfer coefficient (??) and apparent heterogeneous electron transfer rate constant (k _s) were estimated to be 0.36 and 2.74?s^?1, respectively. When the enzyme electrode was examined for the detection of choline, a relatively high sensitivity (2.59???A?mM^?1) was obtained. Under the optimized experimental conditions, choline was detected in the concentration range from 6.9?×?10^?3 to 6.7?×?10^?1?mM with a detection limit of 2.7???M. The peak currents of ChOx were reasonably stable and retained 90% of its initial current after a period of 2?months.     

The relaxation of HCN(011) by V-T,R and V-V energy transfer

Tuned output from an optical parametric oscillator has been used to excite HCN directly to its (011) level. By careful use of a “cold-gas filter”, it has proved possible to distinguish between the time-resolved fluorescence from HCN(011) and that from HCN(001) formed during collisional relaxation. Rate constants for relaxation from both levels have been obtained for the partners: (i) He, Ne, Ar and Kr, and (ii) HCN, CO₂, N₂O, OCS, CS₂, C₂H₂ and C₂D₂. With the rare gases, HCN(011) is relaxed to (001) by V-T,R energy transfer, with rate constants (cm³ molecule^?1 s^?1) at 298 ± 4 K of: k_He⁰¹¹ = (7.9 ± 1.05) × 10^?13; k_Ne⁰¹¹ = (1.56 ± 0.12) × 10^?13; k_Ar⁰¹¹ = (1.20 ± 0.17) × 10^?13; k_Kr⁰¹¹ = (6.7 ± 0.65) × 10^?14. The molecular collision partners also transfer HCN(011) to (001). The rates are much greater and clearly near-resonant V-V energy exchange is important. The results are compared to first-order Sharma-Brau theory, with fair agreement where near-resonant channels exist.     

A study of the cathodic reduction of ozone at the rotating disc electrode in acid electrolyte

The cathodic reduction of ozone according to O₃+2H⁺+2e^?→H₂O+O₂ was studied at a rotating disc electrode consisting of bright platinum in 1 M HClO₄ at 20°C. The current-potential curves obtained potentiostatically are determined by slow charge transfer in the range of low polarization (Tafel region) and diffusion as rate controlling step at high overvoltage. From the exchange current densities (i₀), obtained by extrapolation of Tafel lines to the reversible potentials, the heterogeneous rate constant k was evaluated giving an average value of 2.5×10^?7 cm s^?1. Likewise the diffusion coefficient (D_O3=1.7×10^?5 cm² s^?1) was calculated from the limiting currents measured at various partial pressures of ozone and different values of rotation rate. An equation for the total current-polarization curve was developed. The computed curves are in good agreement with the data which were obtained experimentally.     

Temperature dependence of the reaction rate constants for O(3P) atoms with C2H4, C3H6 and NO(M = N2O), determined by a modulation technique

Rate constants for the reaction of O(³P) atoms with C₃H₄, C₃H₆ and NO(M = N₂O) have been measured over the temperature range 300–392°K using a modulation-phase shift technique. The Arrhenius expressions obtained are:C₂H₄, k₂ = 3.37 × 10⁹ exp[?(1270 ± 200)/RT]liter mole^?1 sec^?1,C₃H₆, k₂ = 2.08 × 10⁹ exp[?(0 ± 300)/RT]liter mole^?1 sec^?1,NO(M = N₂O), k₁ = 9.6 × 10⁹ exp[(900 ± 200/RT]liter² mole^?2 sec^?1.These temperature dependencies of k₂ are in good agreement with recent flash photolysis-resonance flourescence measurements, although lower than previous literature values.     

A Thermodynamic Study of Chromotropico-Titanium(III) Complex from Spectrophotometric Measurements

The formation of 1 : 2 titanium(III) complex with chromotropic acid (4, 5-dihydroxy-2, 7-naphthalene-disulfonic acid) was observed by spectrophotometric measurements at various ionic strengths. An expression, [Ti(III)]/D=1/Δ? + α_H²+/KΔ?[H₂R^2?]², was derived for the determination of the formation constant, K=7.2×10² liter² mol^?2 for the Ti(III).(HR)₂ ion in the pH range of 1.3–1.8 at constant ionic strength, I=0.2 M, at 25°C. The thermodynamic data for the reaction, Ti(III)+2H₃R^2?=Ti(III) (HR)₂+2H⁺, were calculated to be ΔG° = ?16 kJ mol^?1 ΔH° = 18 kJ mol^?1, ΔS° = 110 JK^?1 mol^?1, at 25°C.     

A time-resolved lif study of the kinetics of OH(ν = 0) and OH(ν = 1) with HCl and HBr

The kinetics of OH(ν = 0) and OH(ν = 1) have been followed using pulsed photolysis of H₂O or HNO₃ to generate hydroxyl radicals, and time-resolved, laser-induced fluorescence to observe the rates of their subsequent removal in the presence of HCl or HBr. The experiments yield the following rate constants (cm³ molecule^?1 s^?1) at 298 ± 4 K: OH(ν = 0) + HCl: k_o = (6.8 ± 0.25) × 10^?13; OH(ν = 0) + HBr: k_o = (11.2 ± 0.45) × 10^?12; OH(ν = 1) + HCl: k₁ = (9.7 ± 1.0) × 10^?13; OH(gn = 1) + HBr; k₁ = (8.1 ± 1.05) × 10^?12 For OH(ν = 1), the measurements do not distinguish between loss by reaction and relaxation, and the fact that k₁ > k_o for HCl is tentatively attributed to relaxation, probably by near-resonant vibrational—vibrational energy transfer. Clearly, neither of these exothermic, low-activation-energy reactions is enhanced to any great extent, if at all, by vibrational excitation of the OH radical.ft]*|Present address: Battelle/Pacific Northwest Laboratories, P.O. Box 999, Richland, Washington 99352, USA.     

Absolute rate constants for the reaction of O(3P) aroms with n-butane and NO(M  Ar) over the temperature range 298–439 K

Absolute rate constants for the reaction of O(³P) atoms with n-butane (k₂) and NO(M  Ar)(k₃) have been determined over the temperature range 298–439 K using a flash photolysis-NO₂ chemiluminescence technique. The Arrhenius expressions obtained were k₂ = 2.5 × 10^?11exp[-(4170 ± 300)/RT] cm³ molecule^?1 s^?1, k₃ = 1.46 × 10^?32 exp[940 ± 200)/ RT] cm⁶ molecule^?2 s^?1, with rate constants at room temperature of k₂ = (2.2 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 and k₃ = (7.04 ± 0.70)×10^?32 cm⁶ molecule^?2 s^?1. These rate constants are compared and discussed with literature values.     

Poly(2-amino-5-(4-pyridinyl)-1, 3, 4-thiadiazole) film modified electrode for the simultaneous determinations of dopamine, uric acid and nitrite

Poly(2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole) (PAPT) modified glassy carbon electrode (GCE) was fabricated and used for the simultaneous determinations of dopamine (DA), uric acid (UA) and nitrite (NO₂ ^?) in 0.1 mol?L^?1 phosphate buffer solution (PBS, pH 5.0) by using cyclic voltammetry and differential pulse voltammetry (DPV) techniques. The results showed that the PAPT modified GCE (PAPT/GCE) not only exhibited electrocatalytic activities towards the oxidation of DA, UA and NO₂ ^? but also could resolve the overlapped voltammetric signals of DA, UA and NO₂ ^? at bare GCE into three strong and well-defined oxidation peaks with enhanced current responses. The peak potential separations are 130 mV for DA–UA and 380 mV for UA–NO₂ ^? using DPV, which are large enough for the simultaneous determinations of DA, UA and NO₂ ^?. Under the optimal conditions, the anodic peak currents were correspondent linearly to the concentrations of DA, UA and NO₂ ^? in the ranges of 0.95–380 μmol?L^?1, 2.0–1,000 μmol?L^?1 and 2.0–1,200 μmol?L^?1 for DA, UA and NO₂ ^?, respectively. The correlation coefficients were 0.9989, 0.9970 and 0.9968, and the detection limits were 0.2, 0.35 and 0.6 μmol?L^?1 for DA, UA and NO₂ ^?, respectively. In 0.1 mol?L^?1 PBS pH 5.0, the PAPT film exhibited good electrochemical activity, showing a surface-controlled electrode process with the apparent heterogeneous electron transfer rate constant (k _s) of 25.9 s^?1 and the charge–transfer coefficient (α) of 0.49, and thus displayed the features of an electrocatalyst. Due to its high sensitivity, good selectivity and stability, the modified electrode had been successfully applied to the determination of analytes in serum and urine samples.