Formation of active species in spark discharge and their possible use

The chemical effects of self-sustained spark discharge with an energy per pulse of 5.9 × 10⁻² J, a pulse repetition frequency of 10 Hz, and a spark-gap breakdown voltage of 6 kV have been studied. The discharge generated a UV photon flux of (2.5 ± 0.3) × 10¹⁵ cm⁻² s⁻¹ with an energy density of (2 ± 0.3) × 10⁻³ J cm⁻¹ s⁻¹ and the emission spectrum maximum at 220 nm. The action of the discharge on water samples leads to a decrease in pH and to buildup of oxidizing and reducing species. The formation of HO₂^· radicals with an initial yield of (1.2 ± 0.3) × 10⁻⁶ mol L⁻¹ s⁻¹ has been detected in the liquid. The initial yields of acid residues (increment in [H⁺]), oxidants, and reducing agents are (5.8 ± 1.6) × 10⁻⁷, (3.3 ± 1) × 10⁻⁶, and (4.2 ± 1) × 10⁻⁷ mol equiv L⁻¹s⁻¹, respectively. The formation of NO₃⁻ and NO₄⁺ ions, nitrosamines, and organic compounds has been established.     

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N^•₃ radicals at pH 6.5 and 10.5 were essentially the same with λ_max at 420 nm and rate constant varying marginally (k = (5−6.5) × 10⁹ dm³ mol⁻¹ s⁻¹). The nature of the transients formed by NO^•₂, NO^• radical reaction at pH 10.5 was the same as that with N^•₃, due to the similarity in decay rates and the absorption maximum. Reaction of ^•OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 10⁹ dm³ mol⁻¹ s⁻¹, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 10⁸ dm³ mol⁻¹ s⁻¹). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 10⁷ and 1.5 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Its reduction potential was also measured by cyclic voltammetry.     

Outer-Sphere electron-transfer and the effect of linkage isomerism in the titanium(III) reduction of nitro-pentacyanocobaltate(III) and nitro-, and nitrito-pentaamminecobalt(III) complexes in aqueous acidic media

The reductions of [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺, by Ti^III in aqueous acidic solution have been studied spectrophotometrically. Kinetic studies were carried out using conventional techniques at an ionic strength of 1.0 mol dm⁻³ (LiCl/HCl) at 25.0 ± 0.1 °C and acid concentrations between 0.015 and 0.100 mol dm⁻³. The second-order rate constant is inverse—acid dependent and is described by the limiting rate law:- k₂ ≈ k₀ + k[H⁺]⁻¹,where k=k′K_a and K_a is the hydrolytic equilibrium constant for [Ti(H₂O)₆]³⁺. Values of k₀ obtained for [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺ are (1.31 ± 0.05) × 10⁻² dm³ mol⁻¹ s⁻¹, (4.53 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ and (1.7 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ respectively, while the corresponding k′ values from reductions by TiOH²⁺ are 10.27 ± 0.45 dm³ mol⁻¹ s⁻¹, 14.99 ± 0.70 dm³ mol⁻¹ s⁻¹ and 17.93 ± 0.78 dm³ mol⁻¹ s⁻¹ respectively. Values of K _a obtained for the three complexes lie in the range (1–2) × 10⁻³ mol dm⁻³ which suggest an outer-sphere mechanism.     

Preparation of silver nanoparticles in aqueous solutions in the presence of carbonate ions as stabilizers

Silver nanoparticles are prepared by reducing Ag⁺ ions with sodium borohydride in aqueous solutions containing carbonate ions (5 × 10⁻⁵−1 × 10⁻² mol l⁻¹). It is established that carbonate ions represent an efficient stabilizer that provides nanoparticles with electrostatic protection via the formation of an electrical double layer. The maximum stability of a silver dispersion is observed at a carbonate ion concentration of 1 × 10⁻³ mol l⁻¹. The average size of silver nanoparticles is 10.0 ± 2.5 nm. The formation kinetics of silver nanoparticles is described by an equation for a first-order reaction with a rate constant of 2.3 × 10⁻³ s⁻¹ (±20%).     

A novel electrochemical method for the determination of trinitrophenol

In pH 5.5, 0.1 mol l⁻¹ HAc-NaAc buffer solution, trinitrophenol has been determined quantitatively with differential pulse voltammetry by detecting its reduction peak currents at the glassy carbon electrode. The detection sensitivity was enhanced significantly by the addition of the surfactant of cetyl pyridinium chloride, and the enhancement mechanism was also studied in detail. The linear calibration range was 8.0 × 10⁻⁷ to 2.0 × 10⁻⁴ mol l⁻¹, and the detection limit was established to be 1.9 × 10⁻⁷ mol l⁻¹. This method has been applied to the determination of trinitrophenol in water sample, and the recovery was from 97.6 to 103.5%.     

Radical scavenging reactions of chlorogenic acid: A pulse radiolysis study

At near neutral pH (approx. 5.5), the OH-adduct of chlorogenic acid (CGA), formed on pulse radiolysis of N₂O-saturated aqueous CGA solutions (λ _max = 400 and 450 nm) with k = 9 × 10⁹ dm³ mol⁻¹ s⁻¹, rapidly eliminates water (k = 1 × 10³ s⁻¹) to give a resonance-stabilized phenoxyl type of radical. Oxygen rapidly adds to the OH-adduct of CGA (pH 5.5) to form a peroxyl type of radical (k = 6 × 10⁷ dm³ mol⁻¹ s⁻¹). At pH 10.5, where both the hydroxyl groups of CGA are deprotonated, the rate of reaction of · OH radicals with CGA was essentially the same as at pH 5.5, although there was a marked shift in the absorption maximum to approx. 500 nm. The CGA phenoxyl radical formed with more specific one-electron oxidants, viz., Br ₂ ^·− and N ₃ ^· radicals show an absorption maximum at 385 and 500 nm, k ranging from 1–5.5 × 10⁹ dm³ mol⁻¹ s⁻¹. Reactions of other one-electron oxidants, viz., NO ₂ ^· , NO^· and CCl₃OO^· radicals, are also discussed. Repair rates of thymidine, cytidine and guanosine radicals generated pulse radiolytically at pH 9.5 by CGA are in the range of (0.7–3) × 10⁹ dm³ mol⁻¹ s⁻¹.     

The electrochemical properties of dopamine,epinephrine and their simultaneous determination at a poly(L-methionine) modified electrode

A poly(L-methionine) modified electrode, fabricated by electrochemical immobilization of the L-methionine on a glassy carbon electrode, was used for simultaneous determination of dopamine and epinephrine through cyclic voltammetry. The electrochemical properties of dopamine and epinephrine have been investigated. This sensor gave two separated cathodic peaks at −0.282 and 0.112 V for EP and DA, respectively. A linear response was obtained in the range of 5.0 × 10⁻⁷ to 1.0 × 10⁻⁴ mol l⁻¹ for epinephrine, and 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol l⁻¹ for dopamine. The detection limits were 3.6 × 10⁻⁷ mol l⁻¹ and 4.2 × 10⁻⁷ mol l⁻¹ for epinephrine and dopamine, respectively. This method was successfully applied for simultaneous determination of dopamine and epinephrine in human urines. The text was submitted by the authors in English.     

Voltammetric determination of 2,4-dinitrophenol and 2,5-dinitrophenol using a poly-aspartic acid modified electrode1

In pH 5.0, 0.1 mol l⁻¹ NaAc-HAc buffer solution, 2,4-dinitrophenol and 2,5-dinitrophenol exhibited sensitive and distinguishable voltammetric responses at the glassy carbon electrode modified with poly-aspartic acid. By measuring the reduction peak currents of nitro groups in different positions, dinitrophenol isomers have been determined simultaneously and quantitatively. The linear calibration ranges were 1.1 × 10⁻⁶–6.0 × 10⁻⁴ mol l⁻¹ for 2,4-dinitrophenol and 7.0 × 10⁻⁷–6.0 × 10⁻⁴ mol l⁻¹ for 2,5-dinitrophenol, with detection limits of 2.7 × 10⁻⁷ and 1.1 × 10⁻⁷ mol l-1 respectively. This method has been applied to the detection of dinitrophenols in simulation water sample, and the recovery was from 96.7 to 102.5%.     

Determination of levofloxacin hydrochloride with multiwalled carbon nanotubes-polymeric alizarin film modified electrode

Multiwalled carbon nanotubes-polymeric alizarin film modified electrode was made. The electrochemical behavior of levofloxacin hydrochloride on modified electrode was studied with cyclic voltammetry, linear sweep voltammetry and chronopotentiometry. The results indicated that the electrical oxidation of levofloxacin hydrochloride on MWNT-PAR electrode, in HAc-NaAc buffer solution at pH 4.2 was irreversible and was controlled by diffusion. Some important parameters m, n, D, E _D, ΔS _rc and ΔH _rc of the electrochemical process were evaluated. Good linearity relationship between peak current and its concentration in the range of 5.0 × 10⁻⁶–1.0 × 10⁻⁴ mol l⁻¹ was found, of which the equation was I _p(A) = −5.456 × 10⁻⁶ 0.2667c, the correlative coefficient r = −0.9976 and detect limitation was 4.0 × 10⁻⁷ mol l⁻¹. The recovery of levofloxacin hydrochloride in levofloxacin hydrochloride injection was between 94.6 and 104.7%.     

Tetracyanoquinodimethanide adsorbed on a silica gel modified with titanium oxide for electrocatalytic oxidation of hydrazine

The electrocatalytical oxidation of hydrazine at low potential using tetracyanoquinodimethanide adsorbed on silica modified with titanium oxide was investigated by cyclic voltammetry and amperometry. The modified electrode was prepared modifying a carbon paste electrode employing lithium tetracyanoquinodimethanide adsorbed onto silica gel modified with titanium oxide. This electrode showed an excellent catalytic activity and stability for hydrazine oxidation. With this modified electrode, the oxidation potential of hydrazine was shifted toward less positive value, presenting a peak current much higher than those observed on a bare GC electrode. The linear response range, sensitivity and detection limit were, respectively, 2 up to 100 μmol l⁻¹, 0.36 μA l μmol⁻¹, and 0.60 μmol l⁻¹. The repeatability of the modified electrode evaluated in term of relative standard deviation was 4.2% for 10 measurements of 100 μmol l⁻¹ hydrazine solution. The number of electrons involved in hydrazine oxidation (4), the heterogenous electron transfer rate constant (1.08 × 10³ mol⁻¹ l s⁻¹), and diffusion coefficient (5.9 × 10⁻⁶ cm² s⁻¹) were evaluated with a rotating disk electrode.     

Reactions of hydroxyl radical (·OH) and oxide radical anion (O·−) with 2-aminopurine in aqueous medium

Pulse radiolysis has been used to investigate the reaction of hydroxyl radical (^·OH) and oxide radical anion (O^·−) with 2-aminopurine (2AP), a fluorescent analogue of adenine, in aqueous medium. The second-order rate constant for the reaction of ^·OH with 2AP was determined to be 3 × 10⁹ dm³ mol⁻¹s⁻¹ and for the reaction of O^·− it was 7.1 × 10⁸ dm³ mol⁻¹s⁻¹. The transient absorption spectrum obtained in the reaction of ^·OH at pH 7 has absorption maxima at 370 and 470 nm. The spectrum undergoes a time-dependent transformation at higher time-scale. The intermediate species was found to react with N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD). The yield of TMPD^·+ was calculated in terms of G(TMPD^·+) to be 3.3 × 10⁻⁷ mol J⁻¹ at pH 7. The ^·OH reactions were also carried out at pH 10 and the transient absorption spectra have λ _max at 400 and 480 nm. The transient spectra obtained in the reaction of O^·− at pH ≈14 have maxima at 400 and 480 nm. The transient intermediate species at pH 7 are assigned to the formation of 2AP-4-OH^· (54%), 2AP-5-OH^· (7%) and 2AP-8-OH^· (39%) based on the spectral evidence and TMPD^·+ build-up. Both 2AP-4-OH^· and 2AP-5-OH^· undergo OH⁻ elimination to form a radical cation. At higher pH (pH 10), the dehydration reaction of these OH-adducts leads to a N-centered radical (2AP-N(9)^·, 71%). Formation of 2AP-8-OH^· (29%) is also proposed at this pH. In the reaction of O^·− with 2AP, it is proposed that a similar nitrogen centered 2AP-N(9)^· radical is formed by an electron-transfer reaction at N(9).     

Electrochemistry at cobalt(II)tetrasulfophthalocyanine-multi-walled carbon nanotubes modified glassy carbon electrode: a sensing platform for efficient suppression of ascorbic acid in the presence of epinephrine

Electrochemistry of water-soluble cobalt(II) tetrasulfophthalocyanine (CoTSPc) electrodeposited on glassy carbon nanotube pre-modified with acid-functionalized multi-walled carbon nanotubes (MWCNT) is described. Both charge transfer resistances toward [Fe(CN)₆]^3−/4− redox probe and electrocatalytic responses toward epinephrine (EP) detection follow the trend: bare GCE < GCE-MWCNT < GCE-CoTSPc < GCE-MWCNT-CoTSPc. EP analysis was then carried out in details using GCE-MWCNT-CoTSPc. The catalytic rate constant value k _ch = 2.2 × 10⁷ (mol cm⁻³)⁻¹ s⁻¹ was obtained from rotating disk electrode experiment. Interestingly, GCE-MWCNT-CoTSPc efficiently suppressed the detection of ascorbic acid (the natural interference of neurotransmitters in physiological conditions) showing good sensitivity (0.132 ± 0.003 A l mol⁻¹), limit of detection (4.517 × 10⁻⁷ mol l⁻¹), and quantification (15.056 × 10⁻⁷ mol l⁻¹). In addition, GCE-MWCNT-CoTSPc was conveniently used to determine EP in epinephrine hydrochloric acid injection with recovery of 101.1 ± 2.2%.     

Kinetics of the ?OH-radical initiated reactions of acetic acid and its deuterated isomers

Kinetics of the ^•OH-initiated reactions of acetic acid and its deuterated isomers have been investigated performing simulation chamber experiments at T = 300 ± 2 K. The following rate constant values have been obtained (± 1σ, in cm³ molecule⁻¹ s⁻¹): k ₁(CH₃C(O)OH + ^•OH) = (6.3 ± 0.9) × 10⁻¹³, k ₂(CH₃C(O)OD + ^•OH) = (1.5 ± 0.3) × 10⁻¹³, k ₃(CD₃C(O)OH + ^•OH) = (6.3 ± 0.9) × 10⁻¹³, and k ₄(CD₃C(O)OD + ^•OH) = (0.90 ± 0.1) × 10⁻¹³. This study presents the first data on k ₂(CH₃C(O)OD + ^•OH). Glyoxylic acid has been detected among the products confirming the fate of the ^•CH₂C(O)OH radical as suggested by recent theoretical studies.     

Sensitive and selective determination of mercury by differential pulse stripping voltammetry after accumulation of mercury vapour on a gold plated graphite electrode

A selective and sensitive method is proposed for the determination of mercury by anodic stripping voltammetry after its preconcentration from the gas phase. Mercury from the sample solution is reduced to elemental Hg by SnCl₂ and volatilized by the bubbles of a carrier gas. The gas containing mercury vapour is dried and passed through a capillary onto a gold coated graphite electrode. An anodic stripping voltam-mogram is recorded from 0.1 mol/1 HClO₄ + 3 × 10⁻³ mol/1 HCl solution. The calibration curve is linear from 1 × 10⁻⁹ to 4 × 10⁻⁸ mol/1 Hg(NO₃)₂. The absolute detection limit is 0.46 ng Hg. The relative standard deviations for 4 × 10⁻⁹ mol/1 and 2 × 10⁻⁸ mol/1 Hg(NO₃)₂ are 9.8% and 6.1%, respectively n = 5).     

Platinum electrode coated with a bentonite–carbon composite as an environmental sensor for detection of lead

A Pt wire coated with a bentonite–carbon composite in a poly(vinyl chloride) membrane was used for detection of lead. The sensor has a Nernstian slope of 29.42±0.50 mV per decade over a wide range of concentration, 1.0×10⁻⁷ to 1.0×10⁻³ mol L⁻¹ Pb(NO₃)₂. The detection limit is 5.0×10⁻⁸ mol L⁻¹ Pb(NO₃)₂ and the electrode is applicable in the pH range 3.0–6.7. It has a response time of approximately 10 s and can be used at least for three months. The electrode has good selectivity relative to nineteen other metal ions. The practical analytical utility of the electrode is demonstrated by measurement of Pb(II) in industrial waste and river water samples.     

The amperometric determination of indole-3-acetic acid based on CeCl<Subscript>3</Subscript>-DHP film modified gold electrode

An amperometric sensor for the determination of indole-3-acetic acid (IAA) based on the CeCl₃-DHP film modified gold electrode was developed. CeCl₃ was dissolved into water in the presence of dihexadecyl hydrogen phosphate (DHP). The IAA sensor was prepared via evaporating solvent of the CeCl₃-DHP dispersion on the gold electrode surface. The amperometric response of IAA on the CeCl₃-DHP film modified gold electrode was investigated. The experimental results indicate that the passivation of the electrode due to the adsorption of the oxidation product of IAA decreases significantly at the CeCl₃-DHP film modified gold electrode, in contrast to that at the bare and the DHP modified gold electrode. The experimental parameters were optimized and an electrochemical method for the determination of IAA was established. The oxidation peak current is linearly with the concentration of IAA from 1 × 10⁻⁷ to 2 × 10⁻⁵ mol l⁻¹ and the detection limit is 3 × 10⁻⁸ mol l⁻¹. The relative standard deviation of eight measurements is 3.2% for 5 × 10⁻⁷ mol l⁻¹ IAA. The IAA in plant leaves were extracted and determined by the IAA sensor.     

Voltammetric determination of pentachlorophenol with a silica gel-modified carbon paste electrode

Summary The anodic voltammetric behaviour of pentachlorophenol at a silica gel-modified carbon paste electrode in aqueous solution is reported. Adsorption of the fungicide onto the electrode under open-circuit conditions is followed by application of either differential or square wave voltammetry. The results obtained by differential pulse voltammetry at pH 2.9, with a modifier concentration of 10% and accumulation times of 120 and 300 s, allowed the development of a method to determine pentachlorophenol in the ranges of 1.0×10⁻⁶–1.0×10⁻⁵ and 1.0×10⁻⁷–1.0×10⁻⁶ mol l⁻¹. The relative standard deviation is 2.6% for a concentration of 2.0×10⁻⁷ mol l⁻¹, with a detection limit of 1.8×10⁻⁸ mol l⁻¹ (5 ppb). The effect of other chlorophenols was studied. The results obtained by square wave voltammetry showed a behaviour typical of an irreversible electron transfer. Amplitude, step height and frequency were optimised, taking into account the resolution of the voltammetric response. No improvement in sensitivity was obtained with respect to DPV. Good results were obtained by applying the proposed differential-pulse voltammetric method to the determination of pentachlorophenol in a commercial fungicide.     

Pulse radiolysis study on the free radical scavenging activities of puerarin

Free radical theory of biology and medicine suggests that free radicals are involved in the happening of many chronic diseases including cancers and cardiovascular diseases, and natural antioxidants play positive effect in the prevention of such diseases[1]. This theory leads to much interest in the free radical scavenging activities of the pharmacologically active components in herbal medicines. Puerarin (fig. 1), a compound of isoflavone, is isolated from Chinese herbal medicine radix puera…     

Electrochemical determination of <Emphasis Type="Italic">p</Emphasis>-cresol using mesoporous TiO<Subscript>2</Subscript>-modified carbon paste electrode

A mesoporous TiO₂ (meso-TiO₂) was synthesized, and used to prepare modified carbon paste electrode (CPE). The electrochemical sensing properties were characterized using K₃[Fe(CN)₆], showing that meso-TiO₂ modified CPE possesses larger surface area and higher electron transfer rate. The electrochemical behavior of p-cresol was investigated. At the meso-TiO₂ modified CPE, the oxidation peak current of p-cresol remarkably increases, and the oxidation peak potential shifts negatively, suggesting that meso- TiO₂ exhibits highly efficient catalytic activity to the oxidation of p-cresol. Based on this, a sensitive, rapid and convenient electrochemical method was developed for the detection of p-cresol. The linear range is from 1.5 × 10⁻⁷ and 2.0 × 10⁻⁵ mol l⁻¹, and the limit of detection is as low as 8.0 × 10⁻⁸ mol l⁻¹. Finally, the new method was successfully used to determine p-cresol in water samples.     

Kinetic study of reaction of [ReN(H2O)(CN)4]2− with quinoline-2-carboxylate and pyridine-2,3-dicarboxylate anions

The kinetics of the reaction between [ReN(H₂O)-(CN)₄]²⁻ with different κ² N,O-donor ligands (quin⁻ and 2,3-dipic⁻, respectively) have been studied in the pH 4–12 range in aqueous solution. Two consecutive reaction steps with the formation of the [ReN(η¹-quin)(CN)₄]³⁻ and [ReN(μ²-quin) (CN)₃]²⁻ complexes, respectively, were spectrophotometrically observed and kinetically investigated. The same reaction mechanism is proposed for these two ligands. The first fast reaction (for quin⁻) is attributed to the aqua substitution of [ReN(H₂O)(CN)₄]²⁻ with forward and reverse rate constants of 1.96(5) × 10⁻¹ M⁻¹ s⁻¹ and 5.6(3) × 10⁻² s⁻¹, while a rate of 2.64(3) M⁻¹ s⁻¹ was observed for the reaction between the conjugate base [ReN(OH)(CN)₄]³⁻ and quin⁻ at 40.2 °C. Due to small absorbance changes, it was difficult to obtain any good quality data for the fast reactions for 2,3-dipic⁻. The second, slower reaction is attributed to cyano substitution with rate constants (k ₃ K ₁) of 4.17(4) × 10⁻³ for quin⁻ and 4.68(7) × 10⁻³ M⁻¹ s⁻¹ for 2,3-dipic⁻, at 80.02 °C, respectively. The acid dissociation constant for the aqua complex was spectrophotometrically determined as 11.58(3) and 11.54(2) and kinetically as 11.51(8) and 11.41(1), at 80.4 °C, respectively. Negative values of −83.5(2) and −144.1(2) J K⁻¹ mol⁻¹ as well as the of 71.4(3) and 47.3(3) kJ mol⁻¹, for the slow quin⁻ and 2,3-dipic⁻ reactions, respectively, point to an ordered transition state where bond formation is responsible for the major driving force of the reaction. The and for the fast forward reaction of quin⁻ is indicative of expected associative activation in the transition state. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.