Linear-sweep polarographic determination of active chlorine in aqueous solutions containing phenylthiourea

The linear-sweep polarographic determination of active chlorine is based on its reaction with phenylthiourea in acidic phosphate buffer (pH 2.5) containing potassium chloride. The product, C,C-diphenyldithiodiformamidine, is strongly adsorbed and then reduced at a mercury electrode with two peaks at about ?0.35 V and ?0.87 V (vs. SCE). In the presence of 0.05 M potassium chloride, the potential of the first peak shifts positively to ?0.31 V. This peak provides high sensitivity and selectivity for the determination of traces of active chlorine. The linear range is 1×10^?7?2.5×10^?5 M and the detection limit is 5×10^?8 M (3.6 μg l^?1). The method is used for the direct determination of active chlorine in tap water. The mechanism of the reaction was studied by cyclic voltammetry, electrolysis and potentiometric titration. The first peak (?0.35 V) is ascribed to the reduction of a mercury (II) sulfide film produced by reaction of the adsorbed dithio product with mercury. In the presence of 0.05 M chloride, the formation of a mixed HgS·xHg₂Cl2 film shifts the peak to ?0.31 V.     

The kinetics of chlorine evolution and reduction on glassy carbon in molten tetrachloroaluminate at 175°C

The chlorine electrode reaction on glassy carbon in sodium tetrachloroaluminate melt (AlCl₃+NaCl) with near equimolar compositions was investigated at 175°C with voltammetric techniques. The kinetic parameters (Tafel slope and exchange current density) measured as functions of chloride ion activity and partial pressure of chlorine, and the reaction orders with respect to Cl^? and Cl₂ have been collected extensively, being compared with the theoretical kinetic derivatives deduced from the rate equations solved under three different kinds of adsorption isotherms: Langmuir, non-activated Temkin and activated Temkin isotherms. All the evidence collected in this study indicates that the reaction mechanism for both evolution and dissolution of chlorine consists of a fast electron transfer (Cl^?→Cl_ad+e) followed (or preceded) by a slow Heyrovsky-type reaction (Cl^?+Cl_ad→Cl₂+e) on glassy carbon surfaces where the adsorbed intermediate obeys the activated Temkin isotherm. The exchange current density was found as 8.6±0.8 μA cm^?2 at 175°C in the melt of pCl=1.1 under an atmospheric pressure of Cl₂, and its electrode potential (E°_CΓ/Cl2) was determined as 2.182±0.005 V vs. Al.     

Potentiometric flow-injection determination of trace chlorine based on its redox reaction with an iron(III)/iron(II) buffer

A very sensitive and rapid potentiometric determination of trace chlorine in water is described. The method is based on the transient potential changes which appears during the reduction of dissolved chlorin with an iron(III)/iron(II) potential buffer containing chloride and sulfuric acid. The sample is injected into a water carrier stream and merged with a stream of this potential buffer solution; chlorine is reduced during passage through a short reaction coil. The potential change from the baseline is measured with a flow-through ORP (oxidation-reduction potential) electrode. Potential changes (peak heights) are proportional to chlorine concentrations from 10^?7 M to 10^?5 M. The detection limit is 5 × 10^?8 M (3.5 μgl^?1 as Cl₂). The sample throughput is 45 h^?1. Reproducibility is in the range 2.5–1.1%. Results for potable water agree with those obtained by the o-tolidine method.     

Direct electrochemistry of single-stranded DNA on an ionic liquid modified carbon paste electrode

Electrochemical oxidation of thermally denatured single-stranded DNA (ssDNA) was studied on a room temperature ionic liquid N-butylpyridinium hexafluorophosphate (BPPF₆) modified carbon paste electrode (IL-CPE). A distinct oxidation peak appeared at +0.772 V (vs. SCE) on the IL-CPE after preconcentration of ssDNA at +0.35 V for 160 s in pH 7.0 phosphate buffer solution (PBS), which was attributed to the oxidation of guanine residue on the ssDNA molecular structure. The results showed an apparent negative shift of the oxidation peak potential and a great enhancement of the oxidation peak current on the IL-CPE compared with that of CPE. The electrochemical parameters of ssDNA on the IL-CPE were further calculated. Under the selected conditions, a linear calibration curve for ssDNA detection was obtained in the concentration range from 10.0 to 110.0 μg mL⁻¹ with the detection limit of 1.5 μg mL⁻¹(3σ).     

Modifiying glassy carbon electrode with ferrocene-bridged polysilsesquioxanes

Ferrocene-bridged polysilsesquioxanes film electrodes were prepared via depositing the sols formed by hydrolysis of 1,1′-bis[(2-triethoxylsilyl)ethyl]ferrocene (BTEF) or co-hydrolysis of poly(vinylalcohol) (PVA) with BTEF or tetraethoxysilane (TEOS) with BTEF onto glassy carbon electrode (GCE) surface. The electrochemical behavior of the modified electrodes were characterized by cyclic voltammogram (CV) in aqueous solution. The BTEF film, BTEF/PVA film and BTEF/TEOS film all exhibit a redox wave at E^0’ are 0.504, 0.326, 0.318 V, with the peak potential separation (ΔE) are 0.132, 0.042, 0.028 V, and the value of i_pa/i_pc are 1.8097, 1.007, 1.064 (vs. SCE), respectively. This suggests that BTEF/PVA film and BTEF/TEOS film have a good reversible redox behavior and the redox peak is corresponded to a one-electron reduction and oxidation process. After successive 50 time’s cyclic voltammetric, there is no peak potential shift, and the peak current only decreased 5.58, 4.95%, respectively. BTEF/PVA film has better hydrophilicity and shows a more perfect electrocatalytic activity in the oxidation of H₂A than that of the BTEF/TEOS film. The catalytic peak current has a linear relationship with the concentration of H₂A in the range of 1.0 × 10⁻⁵~1.0 × 10⁻³ M.     

Surface reactions of chlorine molecules and atoms with water and sulfuric acid at low temperatures

Reactions of chlorine-containing species are of great current interest, particularly in reference to possible stratospheric ozone depletion. We have been investigating the potential role of heterogeneous reactions of these species with stratospheric aerosols. Here we wish to report on the results of a study of the interactions of Cl atoms and Cl₂ with H₂SO₄ and H₂O, the dominant components of stratospheric aerosols, at low temperature using X-ray photoelectron spectroscopy (XPS). It appears that chlorine atoms and molecules form chloride ion and higher oxidation states of chlorine upon reacting with cooled surfuric acid surfaces while only chloride ion forms with ice. To test the validity of the chlorine oxidation assignment, an XPS study was also made of chlorine in its various oxidation states as found in the chlorides, hypochlorites, chlorites, chlorates, and perchlorates of sodium, potassium, and calcium. Possible mechanisms by which chlorine changes its oxidation state on contact with H₂SO₄ glasses are discussed.     

Nonenzymatic electrochemical glucose sensor based on MnO2/MWNTs nanocomposite

In this communication, an amperometric glucose biosensor based on MnO₂/MWNTs electrode was reported. MnO₂ was homogeneously coated on vertically aligned MWNTs by electrodeposition. The MnO₂/MWNTs electrode displayed high electrocatalytic activity towards the oxidation of glucose in alkaline solution, showing about 0.30 V negative shift in peak potential with oxidation starting at ca. −0.20 V (vs. 3 M KCl–Ag/AgCl) as compared with bare MWNTs electrode. At an applied potential of +0.30 V, the MnO₂/MWNTs electrode gives a linear dependence (R = 0.995) in the glucose concentration up to 28 mM with a sensitivity of 33.19 μA mM⁻¹. Meanwhile, the MnO₂/MWNTs electrode is also highly resistant toward poisoning by chloride ions. In addition, interference from the oxidation of common interfering species such as ascorbic acid, dopamine, and uric acid is effectively avoided. The MnO₂/MWNTs electrode allows highly sensitive, low-potential, stable, and fast amperometric sensing of glucose, which is promising for the development of nonenzymatic glucose sensor.     

Fabrication of TiO2/Ti nanotube electrode and the photoelectrochemical behaviors in NaCl solutions

Titanium oxide nanotube electrodes were successfully prepared by anodic oxidation on pure Ti sheets in 0.5 wt.% NH₄F + 1 wt.% (NH₄)₂SO₄ + 90 wt.% glycerol mixed solutions. Nanotubes with diameter 40–60 nm and length 7.4 μm were observed by field emission scanning electron microscope. The electrochemical and photoelectrochemical characteristics of TiO₂ nanotube electrode were investigated using linear polarization and electrochemical impedance spectroscopy techniques. The open-circuit potential dropped markedly under irradiation and with the increase of Cl⁻ concentrations. A saturated photocurrent of approximately 1.3 mA cm⁻² was observed under 10-W low-mercury lamp irradiation in 0.1 M NaCl solution, which was much higher than film electrode. Meanwhile, the highest photocurrent in NaCl solution implied that the photogenerated holes preferred to combine with Cl⁻. Thus, a significant synergetic effect on active chlorine production was observed in photoelectrocatalytic processes. Furthermore, the generation efficiency for active chlorine was about two times that using TiO₂/Ti film electrode by sol–gel method. Finally, the effects of initial pH and Cl⁻ concentration were also discussed.     

刺状Pd纳米粒子的室温合成及其对乙醇氧化的电催化性能

室温下以氯化胆碱为稳定剂,用化学还原法合成了刺状Pd纳米粒子(Pdtnh0o0r0n).透射电子显微镜和电化学循环伏安研究结果表明Pdtnh0o0r0n具有较高密度的台阶位,与商业Pd黑催化剂相比较,Pdtnh0o0r0n对乙醇氧化的电催化活性显著提高,氧化电流密度是商业Pd黑催化剂的1.2倍(-0.40--0.30V)-1.5倍(-0.65--0.40V),起始氧化电位和峰电位均负移50mV.相同电流密度下,Pdtnh0o0r0n催化剂对乙醇的氧化电位均更低.     

Pulse radiolysis study of one electron oxidation of riboflavin

One electron oxidation of riboflavin (Rf) has been studied using various oxidising species such as Cl^-.₂, SO^-.₄ and OH radicals. The transient species produced by the reaction of SO^-.₄ with riboflavin gave spectra with λ_m at 680 and 640 nm at pHs 4 and 7.1 respectively with a pK_a at ≈ 6. Cl^-.₂ radicals reacted with riboflavin to give a transient spectrum with λ_m at 570 nm. The possibility of two sites viz. C-8 methyl group and the extended π-ring system of the molecule for oxidation reaction are discussed. The reaction of Cl^-.₂ with riboflavin is an equilibrium from which the redox potential for the Rf^+./Rf couple has been evaluated to be 2.28 V vs NHE. OH radicals reacted with riboflavin to give a transient spectrum attributable to a mixture of species produced by addition or abstraction reactions.     

Some aspects of the chlorine evolution reaction at ruthenium dioxide anodes

Cyclic voltammograms for ruthenium dioxide-coated titanium electrodes in acid solutionwere unaffected, below 1.2 V, by the addition of large quantities of chloride ions. Chlorine discharge at such surfaces was assumed to involve participation of surface oxygen species such as O_ads, rather than specific adsorption of Cl^? ions, at the oxide surface. In contrast to the oxygen evolution reaction, the rate of chlorine evolution was independent of both the roughness factor and the oxide loading of these anodes, the discharge apparently occurring at the external surface of these microporous electrodes. In view of the reversibility of the chlorine/chloride electrode system, especially as compared with oxygen, the main factor influencing the rate of chlorine evolution was assumed to be transfer of chlorine gas away from the electrode surface. Deviations from simple Tafel behaviour were also attributed to mass transfer of the product. Cyclic voltammograms recorded at various temperatures indicated that reduction of the oxide layer at potentials below about 0.3 V was considerably enhanced by an increase in temperature.     

Direct and mediated electrochemical oxidation of ammonia on boron-doped diamond electrode

Direct (non-mediated) electrochemical oxidation of ammonia on boron-doped diamond (BDD) electrode proceeds mainly at high pH (> 8) via free ammonia (NH₃) oxidation. To enhance ammonia oxidation on BDD at low pH (< 8), where mainly ammonium (NH₄⁺) is present, oxidation of ammonia was mediated by active free chlorine. In this process, electro-generated in situ active chlorine rapidly reacts with ammonia instead of being further electro-oxidized to chlorate at the electrode surface. Thus, active chlorine effectively removes ammonia from an acidic solution, while the formation of by-products such as chlorate and possibly perchlorate is minimized.     

Anodic behavior of gold in acid thiourea solutions: A cyclic voltammetry and quartz microgravimetry study

It is shown that at potentials E < 0.5 V (NHE) gold undergoes practically no dissolution in thiourea solutions containing no catalytically active species. The dissolution at a perceptible rate (> 100 μA cm^?2) starts at E ≥ 0.65 V, with the primary process being the oxidation of thiourea, which gives rise a current peak at E ? 1.0 V. The thiourea oxidation at E ≥ 1.1 produces the appearance of catalytically active species, which drastically accelerate the gold dissolution process in the potential region extending from a steady-state value to 0.6 V, where the current efficiency for gold approaches 100% and a peak emerges at E ? 0.55 V. The peak’s height is commensurate with the value of the limiting diffusion current associated with the ligand supply. The species in question make no discernible impact on the thiourea oxidation process. Formamidine disulfide, which forms during the anodic oxidation of thiourea or which is added in solution on purpose, exerts no noticeable catalytic influence on the anodic gold dissolution. The catalytically active species is presumably the S^2? ion, product of decomposition and deep oxidation of thiourea and formamidine disulfide. Indeed, adding sulfide ions in solution has a strong catalytic effect on the gold dissolution, whose character is identical to that of the effect exerted by products of anodic oxidation of thiourea at E ≥ 1.1 V μA cm^?2.     

The role of direct oxalate oxidation in electrogenerated chemiluminescence of poly(4-vinylpyridine)-bound Ru(bpy)2Cl/oxalate system on indium tin oxide electrodes

The electrochemical and electrogenerated chemiluminescence (ECL) properties of indium tin oxide (ITO) electrodes modified with poly(4-vinylpyridine) (PVP)-bound Ru(bpy)₂Cl⁺ (where bpy = 2,2′-bipyridine) have been studied. In a sodium oxalate solution, two irreversible oxidation waves as well as two ECL emission waves were observed during the potential scan in the range 0.4-1.4 V (versus Ag/AgCl/saturated KCl reference). The first ECL wave appeared at ca. 0.8 V, which was caused by the excited-state Ru^2+* generated through a bimolecular redox reaction between electrogenerated Ru³⁺ and the strong reducing agent, C⁻O₂. The latter was formed via a Ru³⁺-mediated oxidation of oxalate. Direct oxidation of oxalate was not involved in the first ECL process. The second ECL wave started at ca. 1.1 V, which was also from the excited-state Ru^2+* generated via the redox reaction between Ru³⁺ and C⁻O₂. However, both direct and Ru³⁺-mediated oxidation of oxalate contributed to the formation of C⁻O₂. The important role of the direct oxidation of oxalate in the ECL mechanism of PVP-bound Ru(bpy)₂Cl⁺/oxalate system was demonstrated. The relative contribution of direct oxidation of oxalate to the observed ECL depended upon the surface concentration of PVP-bound Ru²⁺, the concentration of oxalate and the electrode potential applied.     

Travelling wave in the chlorite-thiourea reaction

The oxidation of thiourea by chlorite within the pH range of 2 to 5.5 has been found to produce a single wave of chlorine dioxide in unstirred solutions. The wave has been studied in narrow tubes of varying diameters and in petri dishes. The wave appears after an induction period that depends on the acid concentration, the [ClO₂^?]/[CS(NH₂)₂] ratio, the temperature, and the diameter of the tube. The wave starts from the surface in a tube and from the edges in a petri dish. The rate of wave movement is proportional to the ratio and the acid concentration. Barium chloride and starch were used as indicators. The wave could be initiated electrochemically and by addition of a drop of solution containing chlorine dioxide. The chlorine dioxide is produced by the oxidation of chlorite by hypochlorous acid.     

Separation of two overlapping redox waves by cyclic-voltammo-thermometry: Applications to Cu/CuCl and Ag/AgCl systems in chloride solutions

When two reactions take place at a given potential, other information is necessary, in addition to the relation between the applied potential and electric current, in order to evaluate the individual contribution of the reactions. Information about the heat evolved at the electrode during cyclic voltammetry enables the reaction components to be separated. It is concluded that the Cu/Cu^I redox system in chloride solution (0.1 < [Cl^?] < 0.45 M) giving a simple CV curve, can be divided into two reactions: Cu/CuCl_ad and Cu/CuCl^?₂. A similar overlap of the Ag/AgCl_ad and Ag/AgCl^?₂ reactions is also observed for the Ag/Ag^I redox system in 7 < [Cl^?] < 9 M solutions.     

Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution–oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H₂SO₄ containing Cl⁻. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl⁻ (Cl_ad), and it occurs as a 3.5e⁻ reduction via solution phase peroxide formation. In the hydrogen adsorption (H_upd) potential region, the orr is even more inhibited, and corresponds only to a 2 e⁻ reduction at the negative potential limit where the electrode is covered by one monolayer of H_upd and some (unknown) amount of Cl_ad. On the Pt(111)---Cl_ad surface, the orr is inhibited relatively little (in addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl⁻ free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl⁻, since Cl_ad does not affect platinum sites required for the breaking of the H---H bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Cl_ad is present on the surface even near 0 V.     

Electrochemical behaviors of guanosine on carbon ionic liquid electrode and its determination

The electrochemical behaviors of guanosine on the ionic liquid of N-butylpyridinium hexafluorophosphate (BPPF₆) modified carbon paste electrode (CPE) was studied in this paper and further used for guanosine detection. Guanosine showed an adsorption irreversible oxidation process on the carbon ionic liquid electrode (CILE) with the oxidation peak potential located at 1.12 V (vs. SCE) in a pH 4.5 Britton-Robinson (B-R) buffer solution. Compared with that on the traditional carbon paste electrode, small shift of the oxidation peak potentials appeared but with a great increment of the oxidation peak current on the CILE, which was due to the presence of ionic liquid in the modified electrode adsorbed the guanosine on the surface and promoted the electrochemical response. The electrochemical parameters such as the electron transfer coefficient (α), the electron transfer number (n), and the electrode reaction standard rate constant (k_s) were calculated as 0.74, 1.9 and 1.26 × 10⁻⁴ s⁻¹, respectively. Under the optimal conditions the oxidation peak current showed a good linear relationship with the guanosine concentration in the range from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol/L by cyclic voltammetry with the detection limit of 2.61 × 10⁻⁷ mol/L (3σ). The common coexisting substances showed no interferences to the guanosine oxidation. The CILE showed good ability to distinguish the electrochemical response of guanosine and guanine in the mixture solution. The urine samples were further detected by the proposed method with satisfactory results.     

Interaction between ozone and the chloride ion in sulfuric acid solutions up to 6-M concentration

The effect of sulfuric acid concentration on Cl₂ evolution in the reaction between O₃ and Cl^? has been investigated. The catalytic effects of metal ions in this reaction have been studied as a function of solution acidity. The chlorine evolution rate increases markedly with increasing acid concentration. At acid concentrations below 4 mol/l, the most effective catalyst is Co²⁺. The catalytic activities of Fe³⁺ and Cu²⁺ peak at $C_{H_2 SO_4 } $ = 4.8 mol/l. In passing to highly acidic solutions ( $C_{H_2 SO_4 } $ > 5 mol/l), the catalytic activity of the metal ions decreases, but the chlorine evolution rate remains high owing to the high acidity. Kinetics of VO²⁺ oxidation with ozone in acid media have been studied, and the ozone solubility in aqueous sulfuric acid has been measured.     

Influence of cyanide and sulphide ions on the reduction and reoxidation of cobalt(II) in thiocyanate solution at mercury electrodes

The process of reduction and reoxidation of cobalt(II) in thiocyanate solution at hanging mercury drop electrode has been investigated by cyclic voltammetric, chronoamperometric and anodic stripping methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M cobalt(II), the voltammogram on the first cycle at 0.05 V s^?1 gives a cathodic peak at ?1.06 V with hysteresis on reversal, and an anodic wave with a peak potential of ?0.28 V and with two shoulders near ?0.38 and ?0.45 V, respectively. Multicyclic voltammograms under the same conditions give a cathodic peak at ?0.90 V and an anodic peak at ?0.45 V. The reduction and reoxidation of cobalt(II) in thiocyanate solution is accelerated by the reduction products of thiocyanate ion, cyanide and sulphide ions, which are produced during the electroreduction of cobalt(II).A mechanism of reduction and reoxidation of cobalt(II) which involves a chemical reduction of thiocyanate ion by electroreduced metallic cobalt and takes into account cyanide and sulphide ions is proposed. The hysteresis on the cathodic wave is caused by the difference in reduction potentials of cobalt(II)-thiocyanate and-cyanide complexes. Cyclic voltammetric study of cobalt(II) in perchlorate solution containing trace amounts of cyanide and sulphide ions supports these conclusions.