Photocurrent kinetics at electron emission from metal to electrolyte solution: Part V. Ionization rate constants of hydrogen and deuterium atoms adsorbed on mercury

Absolute ionization rate constant values of hydrogen and deuterium atoms adsorbed on mercury were measured using the method of pulse photoelectronic emission from metal into solution. In accordance with the Tafel law, these constants decrease from 2.5×10⁷s^?1 to 9×10⁵s^?1 (Table 1) in a 1 M solution of KCl in the range ?0.25 to ?0.5 V SCE. The transfer coefficient is 0.33±0.03 and the isotope ratio about 2.5. Owing to specific anion adsorption, rate constants increase as their concentration increases and KBr is added to the solution. In 0.05 M solutions of HCl and H₂SO₄, transfer coefficients are 0.30±0.05. From a comparison of measured values, with the hydrogen ion discharge rate constants found by extrapolation of experimental values into the potential range mentioned (taking into account the transfer coefficient change), the change of the Gibbs free energy in the reaction H₃O⁺+e_Me^?→H_ads+H₂O was calculated and found to be 0.87–0.99 eV at the potential of the normal hydrogen electrode. Adsorption energy of the hydrogen atom from the gas phase on a mercury electrode is 1.55±0.10 eV.The volt-ampere dependence of the hydrogen ion discharge current in the range ?0.25 to ?2.25 V corresponding to the current change by 18 orders of magnitude, agrees with the theoretically determined values (maximum deviation in the current is less than a factor of 3) for the medium reorganization energy E_r=1.75 eV. Despite constancy of the transfer coefficients of the elementary stages, in the range ?0.5 V (SCE), the effective transfer coefficient of the total hydrogen evolution processes increases from 0.5 to 1.0, as the ionization rate of the adsorbed hydrogen atoms becomes greater than their electrochemical desorption rate.     

Redox reactions of transient species formed during pulse radiolysis of 2-pyridine carboxaldehyde and 2-pyridine methanol in aqueous solutions

Rate constants for the reactions of e _aq ^? , H and OH radicals with 2-pyridine carboxaldehyde and 2-pyridine methanol have been determined by pulse radiolysis technique. Reactions of reducing radicals such as acetone ketyl radicals and CO₂ ^? with these compounds were also evaluated at various pHs. The species produced by the reaction of reducing radicals with these solutes was a strong reductant itself. While pyridinyl were produced in the case of 2-pyridine methanol, one-electron reduction of 2-pyridine carboxaldehyde led to the formation of PyCHOH radical. The one-electron reduction potential of PyCHOH radicals was estimated by establishing an equilibrium with MV⁺ radical cations to be ?0.6V vs NHE. OH radical reaction with 2-pyridine carboxaldehyde gave an OH adduct, while in the case of 2-pyridine methanol, OH radicals reacted partly by H-abstraction from the ?CH₂OH group. SO₄ ^? radical reaction with 2-pyridine carboxaldehyde produced a species which was reducing in nature. The rate constants for the reaction of e _aq ^? and OH radicals are compared with similar values obtained in the case of other 2-pyridine derivatives to see if there is any electron-inductive effect.     

Solvated positron chemistry. II. The reaction of hydrated positrons with Cl−, Br− and I− ions

The reaction of the hydrated positron, e_aq⁺ with Cl^?, Br^?, and I^? ions in aqueous solutions was studied by means of positron The measured angular correlation curves for [Cl^?, e⁺], [Br^?, e⁺, and [I^?, e⁺] bound states were in good agreement with th Because of this agreement and the fact that the calculated positron wavefunctions penetrate far outside the X^? ions in the [X^?, e⁺] sta propose that a bubble is formed around the [X^?, e⁺] state, similar to the Ps bubble found in nearly all liquids. F^?ions did not react w Preliminary results showed that CN^? ions react with e_aq⁺ while OH^?ions are non reactive. The rate constants were 3.9 × 10¹⁰ M^?1 s^?1, 4.4 × 10¹⁰ M^?1 s^?1, and 6.3 × 10¹⁰ M^?1 s^?1 for Cl^?, Br^?, and I^?, respectively, at low (? 0.03 M) X^? concentrations. A 25% decrease in the rate constant caused by the addition of 1 M ethanol to the I^? solutions was i The influence of halide ions on the positronium (Ps) yields in pure water was studied by use of lifetime measurements. The Cl^?, Br^?, and I^? ions reduced the Ps yields at low concentrations (? 0.03 M), while F^? ions only reduced the Ps-yield However, the Ps yields saturated (e.g. at ≈ 21% ortho-Ps yield in the Cl^? case) at higher concentrations. This saturation and the high-concentration effects-in the angular correlation results were interpreted as caused by rather complicated spur effects, wh It is proposed that spur electrons may pick off the positron from the [X^?, e⁺ states with an efficiency which depends on the structure of the     

The scavenging of the precursors of the solvated electron from the positron lifetime spectroscopy and the doppler broadening of annihilation line shape technique

The electron scavenging properties of aqueous solutions of two series of solutes are investigated, using the positron as a probe. For a better interpretation of the data, both lifetime spectroscopy and the Doppler broadening of annihilation line shape technique are used. All solutes inhibit the positronium (Ps) formation, by the scavenging of electrons. The first series consists of the halate ions, that should follow the Hunt linear relation between the rate constant for reaction with the solvated electrons, k_(e^?aq+S), and that for its precursor(s), 1/C₃₇. The Ps inhibition constants, k, are 0.14, 1.44 and 3.45 M^?1 for ClO^?₃, BrO^?₃ and IO^?₃, respectively. This sequence is quantitatively consistent with that of the respective k_(e^?eq+S). The second series includes the SeO⁼₄, Te(OH)₆ and BrO^?₄ species, and the Ps inhibition constants are 5.62, 10.5 and 14.3 M^?1 respectively. These values are much higher than expected from the k_(e^?aq+S) constants, on basis of the Hunt relation, in agreement with previous results from pulse radiolysis experimets.     

eaq− hydration energy and photoemission threshold potentials for mercury in contact with aqueous electrolytes

The meaning of the “red limit” potential in photoemission experiments is discussed. For mercury in contact with aqueous electrolytes, the energy of a photoemitted electron at the “red limit” is 0.6 eV higher than the solvation energy of e_aq^?. This difference is attributed to the solvent reorganization energy contribution to the hydration energy of e_aq^?.     

Polarographic determination of glycine with a dropping copper amalgam electrode

The anodic oxidation of a dropping copper amalgam electrode in presence of dilute solutions of glycine in 0.50 M NaClO₄ has been studied. An anodic wave at ?0.28V (SCE) is observed, yielded by diffusion of glycinate anion in the solution towards the electrode surface. The wave-heights increase with the glycinate concentration (function of glycine concentration and pH value) until the anodic oxidation is controlled by the metal diffusion into mercury. The effect of pH is interpreted by attributing it to the depolarizer effects at glycinate anion even though the zwitterion is present in much larger concentrations. The applicability of anodic oxidation of a dropping copper amalgam electrode in the determination of glycine in the range of concentrations 10^?4–10^?2M with a rigorous control of pH (8.0<pH<10.5) is shown. The standard deviation of the proposed method is 4.1% and the minimum concentration determinable is in the 1×10^?4M range.     

Kinetic and redox characteristics of semireduced species derived from phenosafranine in homogeneous aqueous and sodium dodecyl sulfate micellar media

One‐electron reduction of phenosafranine (PS⁺ 3,7‐diamino‐5‐phenylphenazinium chloride), a phenazinium dye, has been studied in homogeneous aqueous and sodium dodecyl sulfate (SDS) micellar media, using the pulse radiolysis technique. The various reducing radicals employed for the study in homogeneous aqueous medium were e_aq^?, H^˙, CO_2,^˙?, and isopropyl ketyl radicals (CH₃)₂ ^˙COH. Semireduced species generated by these reactions have been characterized by their absorption spectra, decay kinetics, and pK_a. The one‐electron reduction potential of PS⁺ was determined at pH 7 in homogeneous aqueous solution employing nitrobenzene (NB/NB^˙?) as the standard couple. One‐electron reduction in SDS micellar medium and a detailed spectrophotometric investigation of the parent dye in this surfactant system was carried out in order to understand the dye–surfactant interactions in the micellar and premicellar media.© 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 56–66, 2002     

Solvated positron chemistry. The reaction of hydrated positrons with chloride ions

The reaction of hydrated positrons (c_aq⁺ with cloride ions in aqueous solutions has been studied by means of positron annihilation angular correlation measurements. A rate constant of k = (2.5 ± 0.5) × 10¹⁰ M^?1 s^?1 was found. Probably the reacting positrons annihilated from an e⁺ Cl^? bound state resulting in an angular correlation curve 8% narrower than for the hydrated positron. Carbontetrachloride in benzene seems to give similar, but smaller effect.     

Radiation chemistry of xylenol orange in aqueous solutions at different acidities

The radiolytic decolourization and peroxide formation have been studied in aqueous solutions of xylenol orange (XO) at different acidities. The G(-XO) increases from 0.78 at pH 11, to 3.70 at pH 3. The peroxide yield also increases from 1.19 at pH 11, to 3.34 in 0.025 mol dm^-3 H₂SO₄. In alkaline solutions only the OH^. decolourizes XO whereas in acidic solutions both the H and OH^. decolourize XO though G(-XO) due to H-atoms is less. The ionization of the phenolic group in XO influences the e^-_aq reaction with it. In alkaline solution, the oxidized and reduced XO formed by OH^. and e^-_aq reactions, respectively, react together regenerating original XO. Near 0.025 mol dm^-3 H₂SO₄, there is an abstraction of H-atom from XO by HO₂ whereas at other acidities, H₂O₂ is formed by disproportionation of peroxides. Reaction schemes have been given to explain the various radiolytic yields.     

Application de la radiopolarographie a l'étude dequelques éléments à l'échelle des indicateurs

Basic equations of classical polarography are transposed and tested withmanganese and europium at tracer scale. Using the previously presented automatic system, standard dispersion for experimental points is about 3% and residual activity before the wave 3±2% compared with diffusion activity A_d. Results with manganese in 10^?3M ≤[LiCl]≤10^?1M and 2≤pH≤4 solutions, give an experimental yield about 100%, E_1/2=?1.47 V/SCE and α?0.85. The last values agree with results achieved at a weighable scale. For potentials <?2.00 V/SCE a screening effect with supporting electrolyte ions is observed. The screening effect influence on the diffusion coefficient is taken into consideration and results for europium (R?100%, E_1/2??1.95 V/ECS, α=1 at pH 2.7) are in good agreement with the literature. Radiopolarograms for Mn and Eu and mixtures of both are similar. Thus the range of polarographic analysis is enlarged.     

A re-examination of the decay kinetics of pulse radiolytically generated Br-2 radicals in aqueous solution

The decay of Br^-₂ in Ar-purged or N₂O-saturated aqueous solutions of KBr (0.01-1.0 M) in the pH range 1–7 has been re-examined using the techniques of pulse radiolysis and computer simulation. The dependence of the rate constant for the intrinsic decay of Br^-₂ on ionic strength (controlled by KBr) has been established; the values of k (Br^-₂ + Br^-₂) are (1.9 ± 0.1) × 10⁹, (2.2 ± 0.3) × 10⁹ and (2.4 ± 0.3) × 10⁹ M^-1 s^-1 in the presence of 0.01, 0.1 and 1.0 M KBr, respectively, independent of pH between 2 and 7. The computer simulation of the decay of Br^-₂ has also generated, for the latter species, ϵ = 10,000 ± 700 M^-1 cm^-1 at λ_max = 360 nm; this value has been calculated without making any assumption concerning G(Br^-₂). For the reduction of Br^-₂ by H atoms, a value of k (H + Br^-₂) = (1.4 ± 0.3) × 10¹⁰ M^-1 s^-1 has been obtained in the presence of 0.01-1.0 M KBr, independent of pH between 1–4. For the reduction of Br^-₂ by e^-_aq at pH 7 (10^-3 M phosphates) and μ = 0.1, a value of k (Br^-₂ + e^-_aq = (1.1 ± 0.2) × 10¹⁰ M^-1 s^-1 has been obtained.     

Pulse radiolysis study of dithio-oxamide in aqueous solutions

The reactions of e⁻ _aq, H-atoms, OH radicals and some one electron oxidants and reductants were studied with dithio-oxamide (DTO) in aqueous solutions using pulse radiolysis technique. The transient species formed by the reaction of e⁻ _aq with DTO at pH 6.8 has an absorption band with λ _max at 380 nm and is reducing in nature. H-atom reaction with DTO at pH 6.8 also produced the same transient species. The semi-reduced species was found to be neutral indicating that the electron adduct gets protonated quickly. However at pH 1, the species produced by H-atom reaction had a different spectrum with λ _max at 360 and 520 nm. Reaction of acetone ketyl radicals and CO₂ ⁻ radicals with DTO at pH 6.8 gave transient spectra which were identical to that obtained by e⁻ _aq reaction. However at pH 1, the spectrum obtained by the reaction of acetone ketyl radicals with DTO was similar to that obtained by H-atom reaction at that pH. The transient species formed by OH radical reaction with DTO in the pH range 1–9.2 also has two absorption maxima at 360 and 520 nm. This spectrum was identical with the spectrum obtained by H-atom reaction at pH 1. This means that all these radicals viz. OH, H-atom and (CH₃)₂COH radicals react with DTO at pH 1 by H-abstraction mechanism. The transient species produced was found to be sensitive to the presence of oxygen. One-electron oxidizing radicals such as Br₂ ^−· and SO₄ ^−· radicals reacted with DTO at neutral pH to give the same species as produced by OH radical reaction having absorption maxima at 360 to 520 nm. At acidic pHs, only Br₂ ^−· and Cl₂ ^−· radicals were able to oxidize DTO to give the same species as produced by OH radical reaction. The semioxidized species is a resonance stabilized species with the electron delocalized over the-N-C-S bond. This species was found to be neutral and non-oxidizing in nature.     

Electrode kinetics of the metal complexes with aminopolycarboxylic acids: Part II. Cadmium-ethylenediamine-N,N,N′,N′-tetraacetate (EDTA) complexes

The d.c. polarographic current-potential curves of Cd(II)-EDTA complexes were examined in the pH range 0.5–10.0, to elucidate the mechanism of their electrode processes and to determine the relevant electrochemical kinetic parameters. It was shown that the first wave observed below pH 3 at ?0.58 to ?0.65 V vs. SCE is the reversible reduction wave of Cd(II) aquo-ion with kinetically-controlled limiting current, and the second wave observed above pH 1.5 at ?0.75 to ?1.21 V vs. SCE corresponds to the simultaneous irreversible reduction of four complex species, CdH₃L⁺, CdH₂L, CdHL^? and CdL^2?, where CdH_pL^(p?2)+ and L^4? denote the protonated complex species with p protons and the unprotonated EDTA ion, respectively. Analysis of the dependence of limiting current on the hydrogen ion concentration led to the conclusion that the preceding reaction determining the behaviour of limiting current is CdH₃L⁺?Cd²⁺+H₃L^? with k₃^d=6.3×10² s^?1 and k₃^f=3.3×10⁶ s^?1M^?1, where k₃^d and k₃^f are the dissociation and formation rate constants, respectively. On the other hand, from analysis of the dependence of half-wave potentials of the second wave on the hydrogen ion concentration, the kinetic parameters of the four complex species were evaluated, and are given in Table 1. Further, it was shown that the cathodic rate constants of these four charge transfer processes at some reference potential together with those of Cd(II)-HEDTA complexes fulfil the linear free energy relationship.     

Radical-induced destruction of diethyl phthalate in aqueous solution: kinetics, spectral properties, and degradation efficiencies studies

In this work, the reactions of ^?OH, e _aq ^? , and SO ₄ ^?? with diethyl phthalate (DEP) were investigated in aqueous solution by pulse radiolysis, and degradation efficiencies of DEP with ^?OH and e _aq ^? were evaluated in water using steady-state radiolysis as well. The absolute rate constants of ^?OH, e _aq ^? , and SO ₄ ^?? with DEP were determined as 2.3 × 10⁹, 1.0 × 10¹⁰, and 1.0 × 10⁸ M^?1 s^?1, respectively. The degradation efficiencies for the ^?OH and e _aq ^? reactions were 81 and 33 %, respectively. Transient absorption spectra were observed for the intermediate radicals produced by ^?OH, e _aq ^? , and SO ₄ ^?? reactions. The results suggested that e _aq ^? transferred to the ester group, resulting in the formation of DEP radical anions. In contrast, ^?OH and SO ₄ ^?? added predominantly to the aromatic ring of DEP, forming the corresponding ^?OH adducts. The fundamental mechanistic parameters and degradation efficiencies derived from these results were significant for evaluations and applications of advanced oxidation processes.     

Free radical reactions of bromoform (BF) in air free and air saturated aqueous solutions. Effect of pH

G(Br̄) and G(H₂O₂) have been reported in Ar, N₂O and air saturated bromoform, containing 0.1 M t-butanol at various pHs. The results demonstrate an increase in G(Br̄) with increasing bromoform concentration and increasing pH. e^-_aq and Ḣ atoms react with bromoform to give Br̄, whereas ȮH radicals produce the t-butanol radical which interacts with bromoform and generates Br̄. This reaction is catalysed at acidic and alkaline pH. The results suggest that the reactivity of the primary radicals and t-butanol radical appear to be in the order e^-_aq > ḢȮH > ĊH₂(CH₃)₂COH. Ḃr radicals formed as a result of reactions of dibromomethyl radical with bromoform, further react with bromoform to give Br̄.G(Br̄) values at high pH are attributed to the formation of a reactive intermediate BrOḢ^- (Ḃr + OH^-), which decays to Br̄ and ȮH at higher pH. G(Br̄) values are considerably higher in the presence of oxygen. This is because of the formation of peroxy radicals CHBr₂Ȯ and CBr₃Ȯ which decompose to Br̄ and CO, and Br̄ and CO₂, respectively. High G(Br̄) values are found in the presence of isopropanol, ethanol and methanol suggesting a chain reaction. Rate constants for the reaction of ċHBr₂ radical with isopropanol, ethanol and methanol have been measured as 553, 136 and 56 mol^-1dm³s^-1 respectively. G(H₂O₂) ∼ 0.6 in Ar and N₂O saturated solution, and G(H₂O₂) ∼ 1.0 in air saturated solutions.     

Pulsed radiolysis of perrhenate in neutral aqueous solution: Comparison with pertechnetate

Reaction of perrhenate with the aquated electron in neutral aqueous solution yields ReO₄^2? (k_f 1.3 × 10¹⁰ M^?1 s^?1), with an absorption maximum at 290 nm (ε 1700). This decays by a second-order path (k_d 1.5 × 10⁹ M^?1 s^?1) at a rate ～ 100-fold faster than the decay of TcO₄^2? under similar conditions.     

Electrochemical approach to detect the presence of peroxynitrite in aerobic neutral solution

This work describes the conditions of use of bare gold electrode to detect electrochemically the presence of peroxynitrite ONOO^? in phosphate buffer solution at pH 7.1. As ONOO^? is extremely unstable in neutral solution, current–potential curve was reconstructed between ?0.5 and 0.7 V vs SCE by amperometry experiments at rotating disk electrode at different potentials. Comparison of this reconstructed curve with voltammograms of the common interfering species (dopamine, hydrogen peroxide, nitrite, ascorbic acid and glutamate) shows that the presence of ONOO^? can be selectively determined at ?0.1 V vs SCE. This detection occurs through the electrochemical reduction of peroxynitrous acid ONOOH, the conjugated acid of ONOO^?. Detection of ONOO^? produced in situ by the reaction of nitric oxide with superoxide was also achieved.     

Kinetic and spectral properties of the intermediates formed in the pulse radiolysis of 2-mercaptobenzimidazole

The reactions of primary species of water radiolysis such as e^- _aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λ_max) at 330 and 590 nm. The transient species (pK_a = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e^{- aq} at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λ_max at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.     

A faradaic impedance study of the reduction of oxygen from aqueous alkaline solution at the dropping mercury electrode

The reduction of oxygen to hydrogen peroxide at a dropping mercury electrode in an aqueous solution of 1 M KNO₃+0.04 M KOH (pH=12.35) has been studied by means of impedance measurements as a function of frequency and d.c. potential. The reaction appears to be nearly reversible in the dc sense, but quasi-reversible in the ac sense. The impedance data obey the Randles' equivalent circuit with the following apparent values for the kinetic parameters: standard heterogeneous rate constant k_sh^a=0.035 cm s^?1 and cathodic transfer coefficient α^a_c=0.22. The results are interpreted in terms of a two-step charge transfer mechanism with the step O₂+eO₂^? being rate-determining.     

Determination of electrochemical kinetic parameters by square-wave polarography: Polarographic reduction of Zn(II) in 1 M KCl,KBr, and KNCS solutions

A new method of determining electrochemical kinetic parameters by square-wave polarography was presented, in which the faradaic current at θ/2, θ being the half-period of superimposed square-wave voltage, was used for the analysis. The method gave the following kinetic parameters for the electrode reaction, Zn(II) + 2e(Hg), in aqueous solutions at 25° C: k_c^θ=0.0052 cm s^?1 and α_c=0.36 in 1 M KCl, k_c^θ=0.011 cm s^?1 and α_c=0.30 in 1 M KBr, and k_c^θ=0.020 cm s^?1 and α_c=0.52 in 1 M KNCS. Induced adsorption of Zn(II) on the dropping mercury electrode was suggested in solutions containing thiocyanate ions.