Solvation of Substituted Phenoxide Ions in T-Butyl Alcohol-Water Mixtures,from Kinetic Studies of Proton Exchange

Rate constants (k_2W or k_2A) for proton exchange between a phenol, its conjugate base, and a water or t -butyl alcohol molecule have been measured in a t -butyl alcohol-water mixture by nuclear magnetic resonance. Since k_2W and k_2A are both high (k_2W > 10⁸ sec^?1 M^?1), the ratio k_2W/k_2A measures the selective solvation of the O^?-site (of the phenoxide ions) by water and t -butyl alcohol. In 11.47 mole % t -butyl alcohol — 88.53 mole % water, k_2W/k_2A is found to be 14 ± 3 when the ionic reactant is ortho -bromophenoxide ion, and 32 ± 8 when it is meta -nitrophenoxide ion. The results suggest that substituents have a marked effect on the selective solvation of the O^?-site.     

Singlet Molecular Oxygen and Superoxide Radical Anion

Singlet molecular oxygen generated photochemically by rubrene sensitization or by direct excitation in a microwave-discharged stream of oxygen at low pressure is physically quenched by superoxide radical ion in aprotic media, with a quenching rate k_q = 3.6 ± 0.1 × 10⁷ M^?1 s^?1.     

Kinetic Study of the Photoexcited Triplet State of Free Base Porphyrins by EPR

An electron paramagnetic resonance (EPR) study of the photoexcited triplet state of four free base porphyrins is presented. The zero field splitting parameters (ZFS) |D| and |E| were calculated from the EPR spectra of the porphyrins dissolved in n-octane matrices at 80°K. |D| = 0.0359 cm^?1, |E| = 0.0079 cm^?1 for tetra phenyl porphyrin (H₂ TPP), |D| = 0.0432 cm^?1, |E| = 0.0037 cm^?1 for tetra (per-fluoro) phenyl porphyrin H₂T (per-F) PP, |D| = 0.0366 cm^?1, |E| = 0.0078 cm^?1 for tetra (para-chloro) phenyl porphyrin H₂T(P-Cl)PP, |D| = 0.0369 cm^?1, |E| = 0.0076 cm^?1 for tetra (para-methyl) phenyl porphyrin H₂T(P-Me)PP. The transient behavior of the EPR signal intensities in the last two porphyrins is discussed. The depopulation rate constants of the triplet sublevels k_p, the ratio between the population rate constants A_p (at zero field, p = x,y,z), and the spin lattice relaxation rate W within the triplet manifold, were calculated. k_x = (12 ± 2) × 10² sec^?1, k_y = (0.5 ± 0.1) × 10² sec^?1, k_z = (1.2 ± 0.4) × 10² sec^?1, A_x:A_y:A_z ? 0.63:0.01:0.33, W = (0.4 ± 0.1) × 10⁴ sec^?1 for H₂T(P-Cl)PP, k_x = (7 ± 2) × 10² sec^?1, k_y = (4 ± 1) × 10² sec^?1, k_z = (1.5 ± 0.5) × 10² sec^?1, A_x:A_y:A_z ? 0.56:0.31:0.13, W = (1.7 ± 0.4) × 10³ sec^?1 for H₂T(P-Me)PP.     

Heavy Metal Removal and Neutralization of Acid Mine Waste Water ‐ Kinetic Study

The influence of the apatite on the efficiency of neutralization and on heavy metal removal of acid mine waste water has been studied. The analysis of the treated waste water samples with apatite has shown an advanced purification, the concentration of the heavy metals after the treatment of the waste water with apatite being 25 to 1000 times less than the Maximum Concentration Limits admitted by European Norms (NTPA 001/2005). In order to establish the macro‐kinetic mechanism in the neutralization process, the activation energy, Ea, and the kinetic parameters, rate coefficient of reaction, k_r, and k_t were determined from the experimental results obtained in “ceramic ball‐mill” reactor. The obtained values of the activation energy Ea >> 42 kJ mol^?1 (e.g. Ea = 115.50 ± 7.50 kJ mol^?1 for a conversion of sulphuric acid η_H2SO4 = 0.05, Ea = 60.90 ± 9.50 kJ mol^?1 for η ^H2SO4 = 0.10 and Ea = 55.75 ± 10.45 kJ mol^‐1 for η ^H2SO4 = 0.15) suggest that up to a conversion of H2SO4 equal 0.15 the global process is controlled by the transformation process, adsorption followed by reaction, which means surface‐controlled reactions. At a conversion of sulphuric acid η ^H2SO4 > 0.15, the obtained values of activation energy Ea < 42 kJ mol^‐1 (e.g. Ea = 37.55 ± 4.05 kJ mol^‐1 for η ^H2SO4 = 0.2, Ea = 37.54 ± 2.54 kJ mol^‐1 for η ^H2SO4 = 0.3 and Ea = 37.44 ± 2.90 kJ mol^‐1 for η ^H2SO4 = 0.4) indicate diffusion‐controlled processes. This means a combined process model, which involves the transfer in the liquid phase followed by the chemical reaction at the surface of the solid. Kinetic parameters as rate coefficient of reaction, kr with values ranging from (5.02 ± 1.62) 10‐4 to (8.00 ± 1.55) 10‐4 (s^‐1) and transfer coefficient, kt, ranging from (8.40 ± 0.50) 10‐5 to (10.42 ± 0.65) 10‐5 (m s^‐1) were determined.     

Dynamic adsorption behaviors between Cu2+ ion and water‐insoluble amphoteric starch in aqueous solutions

Dynamic adsorption behavior between Cu²⁺ ion and water‐insoluble amphoteric starch was investigated. The sorption process occurs in two stages: external mass transport occurs in the early stage and intraparticle diffusion occurs in the long‐term stage. The diffusion rate of Cu²⁺ ion in both stages is concentration dependent. In the external mass‐transport process, the diffusion coefficient (D₁) increases with increasing initial concentration in the low‐ (1 × 10^?3‐4 × 10^?3M) and high‐concentration regions (6 × 10^?3‐10 × 10^?3M). The values of adsorption activation energy (k_d1) in the low‐ and high‐concentration regions are 15.46–24.67 and ?1.80 to ?11.57 kJ/mol, respectively. In the intraparticle diffusion process, the diffusion coefficient (D₂) increases with increasing initial concentration in the low‐concentration region (1 × 10^?3‐2 × 10^?3M) and decreases with increasing initial concentration in the high‐concentration region (4 × 10^?3‐10 × 10^?3M). The k_d2 values in the low‐ and high‐concentration regions are 9.96–15.30 and ?15.53 to ?10.71 kJ/mol, respectively. These results indicate that the diffusion process is endothermic in the low‐concentration region and is exothermic in the high‐concentration region for both stages. The external mass‐transport process is more concentration dependent than the intraparticle diffusion process in the high‐concentration region, and the dependence of concentration for both processes is about equal in the low‐concentration region. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2849–2855, 2001     

The Kinetics of Complexed and Free Ho2 Radical in the Reaction of Ce+4 and H2O2

The intermediates formed in the reaction of Ce⁺⁴ with H₂O₂ in 0.5 M perchloric acid were studied, spectrophotometrically and through the quenching method, using a stopped-flow system. The spectrum in the UV range and the kinetics of the generated radicals and their dependence on Ce⁺³ concentration were investigated. On adding Ce⁺³ to the mixture, a change in the spectrum as well as in the recombination rate constant was observed. This behaviour was attributed to the formation of a complex between the cerous ion and the HO₂ radical. The values of the recombination rate constant of the complexed radical is 4.0 ± 0.4·10⁶ F^?1 sec^?1 while that of the free HO₂ radical is 0.9 ± 0.1·10⁶ F^?1 sec^?1. The stabilization constant of the complex was found to be 60 ± 18 F^?1.     

Decay of the HO2 and O−2 Radicals Catalyzed by Superoxide Dismutase. A Pulse Radiolytic Investigation

The pulse radiolysis technique has been employed in the investigation of the dismutation of superoxide radicals, O^?₂ and HO₂, in the presence of superoxide dismutase in aqueous solutions. The decay of superoxide radicals in the presence of the enzyme was found to be first order in both enzyme and superoxide concentrations. An apparent second order reaction rate constant was found to be about 2 × 10⁹ M^?1 sec^?1, decreasing slightly as the pH is increased from 5 to 9.5. A mechanism which accounts for all our observations is proposed. It includes two steps: (1) formation of a product (EO^?₂ or E^?) from one enzyme (E) molecule and one O^?₂ radical ion; (2) regeneration of E by a reaction of this product with an additional O^?₂ ion radical. The reaction rate constants k = (1.4 ± 0.2) × 10⁹ and k = (1.9 ± 0.6) × 10⁹ M^?1 sec^?1 were measured at pH = 7 in an oxygenated 0.16 M sodium formate solution.     

Kinetics of reactions between chlorine or bromine and the herbicides diuron and isoproturon

The chemical oxidation of two herbicide derivatives of the phenylurea group—diuron and isoproturon—has been carried out by means of chlorine, in the absence and in the presence of bromide ion. Apparent second‐order rate constants for the reactions between chlorine and the herbicides were determined to be below 0.45 L mol^?1 s^?1. Hypobromous acid reacts faster with the investigated herbicides, especially with isoproturon (k_app = 24.8 L mol^?1 s^?1 at pH 7). While pH exerts a negative effect on the bromination rate, the maximum chlorination rate was found to be at circumneutral pH. In a second stage, the oxidation of each compound was conducted in different natural waters, in order to simulate the processes which take place in water purification plants. Again, chlorine was used as an oxidant, and bromide ion was added in some experiments with the aim of producing the more reactive HOBr oxidant. The herbicide oxidation rate was inversely proportional to the organic matter content of the natural water. However, the formation of trihalomethanes (THMs) was directly proportional to the organic matter content and constitutes a limitation for the application of chlorine during drinking water treatment. Finally, the evolution of herbicide concentration was modeled and predicted by applying a kinetics approach based on the rate constants for the reactions between the herbicides and the active oxidants. Copyright © 2007 Society of Chemical Industry     

Kinetic study of ytterbium(III) extraction from sulfate medium with Cyanex 923

The kinetics of ytterbium(III) extraction from sulfate medium with Cyanex 923 in heptane has been investigated with a constant interfacial cell with laminar flow, which aimed to identify the extraction regime, reaction zone and rate equations. It was found that the extraction rate of ytterbium(III) increased linearly with stirring speed and specific interfacial area. The activation energy E_a (9.56 kJ mol^?1), activation enthalpy ΔH^± (7.05 kJ mol^?1), activation entropy ΔS^±₂₉₈ (?0.31 kJ mol^?1) and Gibbs free energy of activation ΔG^±₂₉₈ (98.3 kJ mol^?1) were calculated from the dependence of extraction rate on temperature. The experiential rate equations were obtained by investigating the influence of the concentration of various species on the extraction rate. A diffusion regime has been deduced from evidence of the linear dependence of extraction rate on stirring speed and the low value of the activation energy. The liquid–liquid interface is most probably the reaction zone in view of the linear dependence of extraction rate on specific interfacial area, the high interfacial activity and low water‐solubility of extractant. Thus the mass transfer rate is controlled by interfacial film diffusion of species. Copyright © 2007 Society of Chemical Industry     

Alkaline hydrolysis of homopolymers and copolymers of 2-hydroxypropyl methacrylate

Homopolymers of 2-hydroxypropyl methacrylate (HPMA) and copolymers with acrylic acid (AA) were prepared in 1,4-dioxane. The reactivity ratios were determined to be r_AA = 0.27 ± 0.04 and r_HPMA = 2.2 ± 0.2. The alkaline hydrolysis by sodium hydroxide of the HPMA monomer and polymers showed that while the HPMA monomer hydrolyzed readily as expected for a low-molecular-weight carboxylic ester the HPMA homopolymer and water-soluble sodium acrylate (NaA) copolymers were extremely resistant to alkaline hydrolysis. The saponification reaction followed a second-order rate equation, being first order with respect to both HPMA and hydroxide ion concentration. The Arrhenius parameters, activation energy E and frequency factor A, for the alkaline hydrolysis of HPMA monomer in water were found to be E = 10.3 Kcal/mol and A = 1.5 × 10⁸ L/mol min, and those for the NaA–HPMA copolymers in water were found to be E = 24 kcal/mol and A = 4 × 10¹² L/mol min. The NaA–HPMA copolymers had a limiting extent of hydrolysis, ranging from 9–90% ester conversion. A sharp rate decrease at low conversion was noted during the HPMA homopolymer hydrolysis in 58/42 dimethyl sulfoxide/water, allowing the calculation of two distinct reaction rates. © 1992 John Wiley & Sons, Inc.     

Kinetics of the metal exchange reaction of a Cu(II)-poly(vinyl alcohol) complex with Zn(II)-ethylenediamine-N,N,N′,N′-tetraacetic acid in aqueous solution

The kinetics of the metal exchange reaction between Cu(II)-poly(vinyl alcohol) [Cu(II)-PVA] and Zn(II)-ethylenediamine-N,N,N′,N′-tetraacetic acid [Zn(II)-EDTA] has been studied by mixing both solutions in a spectrophotometer at pH 10.0 to 11.0, ionic strength μ=0.10(KNO₃), and 15 to 35°C. The reaction is initiated by the formation of unstable Cu(II)-H-PVA through attack of H⁺ ion on the Cu(II)-PVA complex, and both reactions, ligand exchange and metal exchange, proceed simultaneously. The metal exchange step may be rate determining. The rate equation and rate constants of this reaction were determined as follows: ?d[Cu(II)-PVA]/dt=k _0(H)[PVA^?][Cu(II)-PVA] [Zn(II)-EDTA], wherek _0(H)=k ₁+(k′₂+k′₃)[H⁺],k ₁=5.98±1.64M ^?1 s^?1, andk ₂+k ₃=k′₂ K _Cu(II)-H-PVA ^?H +k′₃ K _Zn(II)-EDTA ^H =(5.91±0.89)×10⁷ M ^?2 s^?1.     

Kinetics measurements on a stationary disk electrode in a uniformly rotating fluid (SDERF). II. The electron transfer reaction in the Fe(CN)4−6/Fe(CN)3−6-system

The operation of the SDERF-cell in the study of the electron transfer kinetics of the Fe(CN)^4?₆/Fe(CN)^3?₆-system in 1 M KCl and 1 M KNO₃-solutions at a stationary Pt-disk electrode is reported. The experimental current—overpotential curves are recorded by linear sweep voltammetry and analysed by two different methods using the theoretical relationship derived for a stationary disk electrode placed in a free rotating fluid. Both methods give the same value for the experimental rate constant k*. The effects of the temperature (0° to 40°C) and of the ratio of the rotor radius (r_r) to the electrode radius (r_e)(r_r/r_e = 0.50 to 0.81) have been studied. The activation energy for the redox process in 1 M KCl and 1 M KNO₃ are: E_a = 3.4 ± 0.6 kcal/mol and E_a = 3.7 ± 0.7 kcal/mol respectively, while the k*-values at 25°C are: k* = (5.67 ± 0.41) × 10^?3 cm.s^?1 and k* = (4.53 ± 0.29) × 10^?3 cm.s^?1 respectively. The difference from the standard rate constant k₀ ? 0.100 cm.s^?1 is explained by the effect of the cell-geometry characterized by the G-factor, so that k° = Gk*, where G ? 19 for our cell.     

The Free-Radical Cleavage of the Disulfide Bond (Studied by Pulse Radiolysis)

Reaction of H atoms with glutathione leads rapidly to H + RSSR → RS · + RSH. The first observed product is RS, the spectrum of which is obtained. The spectrum of the RS?SR radical was obtained by direct attack of e^?_aq on glutathione. The rate constants of these processes were also measured. k_e?aq + RSSR = (2.7 ± 0.3) × 10⁹ M^?1 sec^?1 k_{H + RSSR} = (1.0 ± 0.2) × 10¹⁰ M^?1 sec^?1 When the OD of RS?SR is plotted vs pH a titration curve is obtained. This is due to the protonation of RS?SR with a rate constant of 2.6 × 10¹⁰ M^?1 sec^?1 which is probably followed by a cleavage to RS and RSH. In both cases the RSSHR radical cannot be detected. The spectrum attributed to the RSSHR radical is more likely to be that of RS.     

Anionic polymerization of styrene in triglyme-benzene mixtures

Living anionic polymerization of styrene was kinetically investigated in triglyme-benzene mixtures. At low concentrations of triglyme the overall propagation rate constant, k_p, was much larger than at the same concentration of monoglyme (DME) in DME-benzene mixtures. The Szwarc-Schulz plot did not have negative slopes for lithium and sodium salts at triglyme contents of 5～20vol%, and no contribution of free anions to the propagation was observed for the sodium salt. The sodium ion pair was more highly reactive than the lithium ion pair; thus at 25°C, the ion pair rate constant, k′_p, for the lithium salt was 43, 102, 135 and 165 M^?1sec^?1 at triglyme concentrations of 5, 10, 15, and 20%, respectively, while that for the sodium salt was 410, 920, and 1460 M^?1sec^?1 in 5, 10, and 15% triglyme, respectively. The dissociation constant, K, for the lithium salt was 2·4×10^?11, 1·9×10^?10 and 1·3×10^?9 M in 10, 15, and 20% triglyme, respectively and the free ion rate constant, k″_p, was 2～2·5×10⁴ M^?1sec^?1 for the lithium salt.     

Potentiometric and polarographic studies of cadmium(II) complexes with tetraethylenepentamineheptaacetic acid

Stability constants for cadmium(II) complexes with tetraethylenepentamineheptaacetic acid (TPHA, H₇L) were determined by the pH titration method. In an aqueous solution (μ = 0.1), three complex species, CdH₂L, CdHL and CdL are confirmed. The structure of uninuclear complexes are discussed. The formation constants of the complexes stated above have been calculated as follows (at 25 ± 0.1°C): log K_CdL = 15.35, log K_CdHL = 13.33 and log K_CdH2L = 7.89. The polarographic behaviour of the cadmium(II) in the presence of TPHA was studied over the pH range 3–5. Mechanisms of the electrode processes were elucidated and electrochemical kinetic parameters were evaluated from dependence of the half-wave potentials on the hydrogen ion and TPHA concentration. In the presence of an excess of TPHA, the wave B is assigned to the irreversible reductions of the complex, CdH₂L^3? (pH range 3–4) or CdHL^4? (pH range 4–5). The electrode reaction can be written:

and

Where k_e (the rate constant) = 2.3 × 10^?2 cm s^?1 and k′_e = 1.59 × 10^?4 cm s^?1. The other polarographic methods were also used in the elucidation of the electrode process.     

γ-Induced single-step synthesis of ethylene glycol from methanol-formaldehyde solutions

A mechanism is developed for the initiated nonbranched-chain formation of ethylene glycol in methanol-formaldehyde solutions at formaldehyde concentrations of 0.1–3.1 mol dm^?3 and temperatures of 373–473 K. At a formaldehyde concentration of 1.4 mol dm^?3 and T = 473 K, the radiation-chemical yield of ethylene glycol is 139 molecules per 100 eV. The effective activation energy of ethylene glycol formation is 25 ± 3 kJ mol^?1. The quasi-steady-state treatment of the reaction network suggested here led to a rate equation accounting for the nonmonotonic dependence of the ethylene glycol formation rate on the concentration of the free (unsolvated) form of dissolved formaldehyde. It is demonstrated that the peak in this dependence is due to the competition between methanol and CH₂=O for reacting with the adduct radical HOCH₂CH₂O^?.     

Comments on the reactivity of methoxy pentadecyl phenyl isocyanate isomers

The reactions of 2-methoxy-4-pentadecyl phenyl isocyanate and 4-methoxy-2-pentadecyl phenyl isocyanate with excess 2-ethyl hexanol originally reported by Ghatge and co-workers to follow zero order kinetics have been re-examined on the basis of their data and shown to follow more realistically the product catalyzed pseudo first order kinetics. The new rate constant, k_s (sec^?1) for the spontaneous reaction and k_p (li. mole^?1 sec^?1) for the product catalyzed reaction are found to be: k_s = 0.57 × 10^?6 and k_p = 34 × 10^?6 for 2-methoxy-4-pentadecyl phenyl isocyanate and k_s = 1.2 × 10^?6 and k_p = 82 × 10^?6 for 4-methoxy-2-pentadecyl phenyl isocyanate.     

Effect of diantimony trioxide on direct esterification between terephthalic acid and ethylene glycol

Experiments at various Sb₂O₃ concentrations were made in a pilot plant and the effect of Sb₂O₃ on continuous esterification between terephthalic acid (TPA) and ethylene glycol (EG) was obtained. Reaction rate constants of the previously reported reaction scheme were determined to fit with the experimental data obtained. It was found that the effect of Sb₂O₃ on reaction rate constant (k_i) can be expressed as follows.

• k₁ = (3.75 × 10^?4Sb + 1.0) × 1.5657 × 10⁹exp(?19,640/RT)
• k₂ = (4.75 × 10^?4Sb + 1.0) × 1.5515 × 10⁸exp(?18,140/RT)
• k₃ = (6.25 × 10^?4Sb + 1.0) × 3.5165 × 10⁹exp(?22,310/RT)
• k₄ = (4.50 × 10^?4Sb + 1.0) × 6.7640 × 10⁷exp(?18,380/RT)
• k₅ = (3.50 × 10^?4Sb + 1.0) × 7.7069 × exp(?2810/RT)
• k₆ = (1.75 × 10^?4Sb + 1.0) × 6.2595 × 10⁶exp(?14.960/RT)
• k₇ = (3.75 × 10^?4Sb + 1.0) × 2.0583 × 10¹⁵exp(?42,520/RT)

Simulation of esterification with these reaction rate constants at various Sb₂O₃ concentrations was made and the following results were obtained.

• 1 Sb₂O₃ accelerates the esterification reaction between TPA and EG.
• 2 Sb₂O₃ accelerates the main reaction and its effects on side reactions are minor. The higher the addition rate of Sb₂O₃, the lower the carboxyl end-group concentration (AV) and diethylene glycol content (DEG).
• 3 The comparison between simulation with potassium titanium oxyoxalate (PTO) in the previous report and with Sb₂O₃ in the present report shows that the acceleration of polycondensation reaction by Sb₂O₃ is higher. DEG formation rate is lower with PTO than Sb₂O₃.

     

Electrolyte effects on oxygen reduction kinetics at platinum: A rotating ring-disc electrode analysis

A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H₂SO₄ ～ CF₃SO₃H > H₃PO₄ > HClO₄. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH^? ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.