酸性介质中碘离子在铂电极上的电化学氧化行为

采用循环伏安法研究了酸性介质中碘离子在铂电极上不同电位区间, 不同酸度下的电化学反应行为. 结果表明, 当极化电位较低(小于0.6 V(vs Hg/Hg2SO4))时, 碘离子在铂电极上发生2I--2e→I2电氧化反应, 反应产物通过I2+I-=I-3被进一步溶解, 整个反应属于E-C(electrochemical-chemical)模式. 电氧化过程中可以形成碘膜, 其也可以被碘离子溶解. 当极化电位升高至0.6 V(vs Hg/Hg2SO4)或以上时, 碘离子会直接电氧化为高价态碘化合物, I-+3H2O→IO-3+6H++6e, 而析出的碘膜并不发生再氧化反应; 在电化学还原过程中, 出现了两个还原峰, 分别对应于I2、I-3的还原反应; 在无碘膜时, 碘离子电氧化过程受溶液中碘离子的液相扩散步骤控制; 碘膜形成后, 主要受碘膜中碘离子的固相扩散控制; 酸度对于碘离子的电化学氧化过程有很大的影响, 其线性极化曲线的起峰电位及电流峰值电位均随酸浓度升高而负移.     

Structure of iodide ion arrays in iodinated nylon 6 and the chain orientation induced by iodine in nylon 6 films

Two species of iodide ions (I₃^? and I₅^?) are found in iodine—nylon 6 complexes. Orientation of I₅^? arrays (most likely I₂/I₃^? complex) along the polymer chain and I₃^? ions perpendicular to the chain axis in uniaxially drawn films and in films with planar orientation suggests that there is and intrinsic relation between the direction of iodide ion arrays and nylon 6 chains. When an unoriented film of nylon 6 in the amorphous or the α crystalline form is treated with an aqueous solution of iodine—potassium iodide, the I₃^? species in the resulting iodine—nylon complex lie in planes parallel to the surface of the film, and I₂/I₃^? units are oriented normal to the surface of the film. The γ form obtained by desorbing the iodine from this complex shows considerable uniaxial rientation with the nylon chains oriented perpendicular to the plane of the film; this orientation is maintained during the γ to α transition. It is proposed that the iodine-induced orientation of the nylon 6 chains is due to the nucleating effects of the iodide ion species as the iodine diffuses unidirectionally into the film.     

Ring-disk electrode studies of the open-circuit dissolution of iodine films formed during the anodic oxidation of iodide on platinum

The open-circuit behavior of iodine films formed on platinum by electrooxidation of iodide was studied at rotating disk and rotating ring-disk electrodes. The potential transient and ring current transient at open circuit for c_I^?>0.012 M can be explained by assuming: (1) convective-diffusion controlled dissolution of the film; (2) establishment of the I₂+I^?→ I₃^? equilibrium; (3) establishment of the I₂ (solid) →I₂ (solution) equilibrium. The behavior at lower concentrations of c_I^? suggests that convective-diffusion control is absent.     

Electrochemistry of iodine and iodide in chloroaluminate melts

The electrochemical behavior of iodine and iodide has been studied in AlCl₃+NaCl mixtures with compositions ranging from NaCl saturated melts to AlCl₃+NaCl (63+37 mol %) at platinum and tungsten electrodes. Iodide is oxidized in two steps to iodine and I(I); a reduction wave to iodide and an oxidation wave to I(I) are obtained in iodine solutions. The equilibrium constant for the reaction, I^?+I(I)=I₂, is 6×10⁸ l mol^?1 in molten chloroaluminate melts at 175°C.     

Voltammetric characteristics of diazinon on carbon paste electrode modified with tris(ethylenediamine) cobalt(II) iodide

In this study, cyclic and differential pulse voltammetric methods were used to investigate the electrochemical behavior of diazinon at carbon paste electrode modified with tris(ethylenediamine) cobalt(II) iodide ([Co(en)₃]I₂). Cobalt complex showed an anodic peak at 620 mV (vs. Ag/AgCl, in KNO₃ 0.1 M as supporting electrolyte) at carbon paste modified electrode. In the presence of diazinon, anodic peak intensity increased with increasing concentration of diazinon that confirmed electrocatalytic activity of cobalt complex for oxidation of diazinon (EC’ mechanism). Under optimized conditions, a linear calibration curve for diazinon was obtained in the range from 0.05 to 27 mg/L with detection limit 0.0075 mg/L (3S _b /m). Applications of the modified electrode to the determination of diazinon in different water samples were also tested. The results showed a very good precision (RSD < 0.04%) and very stable voltammetric response towards diazinon.     

Application of surface-enhanced Raman spectroscopy to mechanistic electrochemistry: Oxidation of iodide at gold electrodes

The electrooxidation of dilute (1 mM) iodide at the gold-aqueous interface has been examined by rotating disk voltammetry combined with surface-enhanced Raman spectroscopy (SERS) in order to identify the surface species formed and hence to shed light on the electrooxidation mechanism. Marked changes in the SER spectra occur upon shifting the electrode potential through the region where faradaic current flows, the characteristic 123 and 158 cm⁻¹ bands associated with adsorbed iodide being supplemented and eventually supplanted by bands at 110, 145 and 160–175 cm⁻¹, the latter two being especially intense. The new bands are assigned to higher polyiodides and molecular iodine. The latter species appears to be the major interfacial product associated with faradaic current flow. Iodide forms an irreversibly adsorbed and electroinactive layer at gold in the absence of solution iodide, as evidenced by the survival of the 123 and 158 cm⁻¹ SERS bands even at far positive potentials under these conditions. The results obtained for dilute iodide solutions are compared and contrasted with those obtained at higher iodide concentrations. For the latter conditions, the observed “surface” Raman spectra arise from resonance enhancement of the thick insoluble iodine films and solution triiodide formed in the convective diffusion layer rather than from SERS of species present in the double layer. Criteria for distinguishing between these two possibilities for systems involving such electrogenerated species are described.     

Catalytic Detection of Iodide at Cationic Kaolinite Modified Gold Electrode in Presence of Thiosulfate

A gold electrode modified by a thin film of cationic kaolinite was used for the electrochemical detection of iodide in aqueous solution in the presence of thiosulfate. At gold electrode, iodide showed two electrochemical systems in the potential range explored (0.10 V to 0.85 V). The pH‐independent system was assigned to the redox couple I₂/I^? and the pH‐dependent one assigned to the redox couple HIO/ . For increased amount of thiosulfate the oxidation peak intensity of the first system increases sharply followed by the gradual decrease of the reduction peak, due to the chemical reaction between thiosulfate and oxidized iodide. The calibration curve in the presence of excess thiosulfate resulted in an increase of the sensitivity by a factor of 7. To improve this sensitivity, the bare gold electrode was coated by a thin film of an anionic exchanger kaolinite, obtained by grafting the ionic liquid (1‐(2‐hydroxyethyl)‐4‐(tert‐butyl) pyridinium chloride). Accumulation‐detection method yielded a spectacular increase of the oxidation peak current of iodide in the presence of thiosulfate ions. At optimized experimental conditions, a sensitivity of 2.45 μA.μM^?1 and a detection limit of 65 nM were obtained. The method was successfully applied for total iodine determination in povidone?iodine formulation.     

Spectrophotometric determination of carbon tetrachloride via ultrasonic oxidation of iodide accelerated by dissolved carbon tetrachloride

Ultrasonic oxidation of iodide was investigated in the presence of carbon tetrachloride (CCl₄). The ultrasonic oxidation of potassium iodide led to formation of iodine and then I₃⁻ in the presence of excess iodide, and the generated I₃⁻ shows strong UV absorption with a molar absorptivity of 2.31 × 10⁴ L mol⁻¹ cm⁻¹ at the maximum absorption wavelength of 351 nm. The ultrasonic oxidation of iodide was found to be significantly promoted by a small addition of CCl₄, and it was further found that the generation rate was increased with the amount of CCl₄ added. This can be used to analyze the level of CCl₄ dissolved in aqueous solutions. Under optimum conditions, the concentration of generated iodine (or its absorption at 351 nm) was found to correlated linearly with the concentration of CCl₄ in the range of 0.2-50 mg L⁻¹ (detection limit = 0.09 mg L⁻¹, R² = 0.999). As an alternative indirect spectrophotometric method of CCl₄ determination, the proposed method was successfully applied to determine the concentrations of CCl₄ in several practical samples, showing merits of being sensitive and simple of operation.     

Anodic reactions of the halides in dimethyl sulfoxide at the pyrolytic graphite electrode

The anodic reactions of the halide ions in dimethyl sulfoxide at the pyrolytic graphite electrode have been studied. The iodide ion demonstrates a 3-step oxidation; the bromide, a 2-step oxidation and chloride, a 1-step oxidation. The electrode reaction (X^-→$\text{1}\text{2}$ X₂ + e^-) is complicated by a catalytic reaction occurring after the electrode reaction. The catalytic reaction is important for only bromide and chloride causing a considerable diffusion current enhancement. The αn_a value for all 3 primary reactions is of the order of 0.5.     

A biofuel cell based on pyrroloquinoline quinone and microperoxidase-11 monolayer-functionalized electrodes

A pyrroloquinoline quinone (PQQ) monolayer-functionalized-Au-electrode and a microperoxidase-11 (MP-11)-modified Au-electrode are used as catalytic anode and cathode in a biofuel cell element, respectively. The cathodic oxidizer is H₂O₂ whereas the anodic fuel-substrate is 1,4-dihydronicotinamide adenine dinucleotide, NADH. The PQQ-monolayer electrode catalyzes the oxidation of NADH in the presence of Ca²⁺ ions. The MP-11-functionalized electrode catalyzes the reduction of H₂O₂. The biofuel cell generates an open-circuit voltage, V_oc, of ca. 320 mV and a short-circuit current density, I_sc, of ca. 30 μA·cm⁻². The maximum electrical power, W_max, extracted from the cell is 8 μW at an external load of 3 kΩ. The fill factor of the biofuel cell, f=W_max·I_sc⁻¹·V_oc⁻¹, is ca. 25%.     

The effects of isotopic substitution and competitive adsorption on the oxidation of aldehydes and alcohols at gold electrodes

The nature of the mechanism of the anodic oxidation of aldehydes in aqueous base on gold electrodes has been probed using isotopic substitution and competitive adsorption studies. A primary kinetic isotope effect of k_H/k_D=3?4 was observed upon substitution of deuterium for protium on the formyl group of benzaldehyde and cyclohexanecarboxaldehyde on gold in aqueous base using the techniques of cyclic voltammetry, rotating disc electrode voltammetry and chronoamperometry. Similar results are reported for the same aldehydes on a silver electrode, and also for the anodic oxidation of 2-propanol and 2-propanol-2-d on gold under similar conditions. Inhibition of the anodic cyclic voltammetric peak for benzaldehyde on gold by a variety of adsorbed species including CN^?, I^?, Br^?, (C₂H₅)₄N⁺ and diethylenetriamine is also described. These observations are used to propose a mechanism for the low potential oxidations of aldehydes and alcohols on gold involving a rate limiting dissociative adsorption step with cleavage of the α-carbon-hydrogen bond.     

Radiolysis of aqueous cesium iodide

In hypothetical accident scenarios for Light Water reactors, the extent of release of iodine upon irradiation needs to be assessed for the purpose of evaluation of the applicable source term. In this context, an understanding of the behaviour of aqueous cesium iodide solutions subjected to high gamma-ray fluxes acquires significant importance. In the present work, gamma radiolysis of a cesium iodide solution (10^–2M I^–) with and without boron additive is investigated by irradiating with⁶⁰Co source at ambient temperature. Upon irradiation of the CsI solution, iodine is liberated, and the concentration of iodide in the KOH trap present in the radiolysis vessel increases with dose. The radiolytic products I ₃ ^– , IO ₃ ^– and H₂O₂ formed in the irradiated solution are also estimated and G values obtained are reported. G(I ₃ ^– ) and G(IO ₃ ^– ) are of the order of 10^–3 and 10^–4, respectively. G(H₂O₂) decreases with increase in dose. Addition of boron up to 200 ppm, does not appear to alter significantly the release fraction of iodine.     

Sensitive quantification of iodide by ion-exchange chromatography with electrochemical detection at a modified platinum electrode

A rapid and very sensitive method for the accurate determination of free iodide in real samples is described. The method is based on anion-exchange chromatographic separation coupled with amperometric detection at a modified platinum electrode under constant applied potential (+0.85 V vs. Ag AgCl). An experimental setup with an in-line and very effective method of electrode modification is proposed using an amperometric thin-layer cross-flow detector and a flowing solution 300 mg/L of iodide; the working electrode is polarised to the limiting current for oxidation of iodide to iodine in acidic solutions with the consequent formation of an iodine-based film. The results indicated that the modified electrode exhibits high analytical response for iodide electrooxidation with good stability and long-life. The signal intensity of daily experimental sessions (8 h), during which standards and real samples were repeatedly injected, exhibits a moderate lowering (i.e. <6%). Using a mixture of 25 mM HNO₃ and 50 mM NaNO₃ as an eluent phase in ion-exchange chromatography, the detection limit of iodide was estimated to be 0.5 g/L (S/N=3) with an injection volume of 50 L. This method was applied successfully to quantify the iodide content of milk samples and in wastewaters as well as trace amounts in common vegetables and solutions containing high chloride levels.     

Convective Instability of the Limiting Current of the Triiodide Reduction in a Vertical Channel

The instability of the limiting current of the cathodic reaction is studied in the I^– ₃–I^–/Pt system with excess of potassium iodide at the electrode situated at the bottom of a vertical cylindrical channel. Dependence of parameters of the oscillation process on the concentration of electroactive and supporting ions and solution viscosity is analyzed. The deceleration of the transport processes in solutions containing glycerol is shown to be due to a considerable decrease in the triiodide ion diffusion coefficient.     

Polarography of cadmium(II) in presence of substituted alkanolamines

A study has been made of the effect of an irreversibly adsorbed iodide layer on the anodic oxidation of formic acid at a platinized platinum electrode. It is shown that, in the presence of the preadsorbed iodide layer, the oxidation process obeys the following rate expression, i=nF k^→c^α g(θ_I) exp(α_anFφ_r/RT where α≈0.75 and α_an≈0.5. This is explained in terms of the following rate-determining step, (HCOOH)_ads→C_*OOH+H⁺+e and involves the adsorption of formic acid on the iodide covered surface. A strong catalytic effect of the iodide layer is observed; the function g (θ_I) passes through two maxima at θ_I values of 0.15 and 0.53. It is suggested that these effects arise from a coverage-dependent variation of bond strength between the adsorbed iodide and platinum.     

Selective sorption of iodide onto organo-MnO2 film and its electrochemical desorption and detection

This paper reports an electrochemically grown film consisting of layered MnO₂ intercalated with hexadecylpyridinium cations (HDPy⁺), which can selectively sorb and detect iodide anions in aqueous solution amperometrically. Sorption of iodide by the HDPy/MnO₂ film did not occur via ion exchange, but through hydrophobic interactions between the interlayer organic phase of the film and iodide ions in solution. The sorption rate increased with the deposited amount of MnO₂. During the sorption process, the interlayer spaces expanded, and new diffraction peaks appeared that were attributed to the incorporated species. Anodic polarization of the iodide-sorbed HDPy/MnO₂ film led to electron transfer from the incorporated iodide to the underlying substrate through the MnO₂ sheets. The oxidized iodide was expelled from the film as molecular I₂, while the expanded interlayer spaces were restored to their original state. Thus, the MnO₂ layers and the incorporated HDPy can synergistically sorb/desorb iodide anions, resulting in a unique “self-cleaning” function that can operate electrochemically. This property allowed amperometric detection of iodide at a concentration as low as 0.0186 μM, which was below the detection limits reported for previous iodide sensors.     

Oxidations with alkaline permanganate using monovalent thallium for the back-titration: Determination of iodate,iodide and ferrocyanide

Iodate, iodide, iodine and ferrocyanide can be estimated by oxidation with KmnO₄ in alkaline media; the excess is back-titrated with TI⁷. I^- and I₂ are oxidized to IO₄^- in the presence of Ba⁺² ions but only to IO₃^- in absence of such ions. The direct titration of IO₃^-, I^- with KmnO₄ proved valueless.Ferrocyanide is oxidized by KmnO₄ in alkaline solutions and MnO₂ is formed. In the presence of telluric acid and 0.025–0.1 N NaOH satisfactory results are obtained. Reduction of MnO₄^- with ferrocyanide gives MnO₄^-2 and the results are variable, depending on the rate of adding the ferrocyanide.     

Ternary diffusion with charged-complex formation in aqueous I2+NaI and I2+KI solutions

Diaphragm cells have been used to measure ternary diffusion coefficients for I₂+NaI and I₂+KI in aqueous solution at 25°C. Although most of the iodine molecules are bound to iodide ions and are transported as the triiodide species [I₂(aq)+I^–(aq)=I ₃ ^– (aq)], diffusion of the iodide salts produces relatively small countercurrent coupled flows of the iodine component. The ternary diffusivity of the iodine component in the solutions is 10 to 20% larger than the diffusivity of the triiodide species. This behavior can be understood by considering electrostatic coupling of the ionic flows. The diffusion equations for I₂+NaI and I₂+KI components are reformulated in terns of NaI₃+NaI and KI₃+KI mixed electrolyte components.     

Effect of the activation of a graphite-epoxy composite electrode and the buffer capacity of a solution on the reversibility of the hydroquinone electrode reaction

Peak potentials and the kinetics of hydroquinone oxidation at an electrode from a graphite-epoxy composite in activated and passivated states were studied in supporting electrolytes of different buffer capacities and pH varying from 0.1 to 8.8, using methods of direct-current and cyclic voltammetry. The electrode was activated before its polarization by mechanically cutting a 0.2–4-μm surface layer directly in a test solution. The electrode was passivated by storing in air for two or more days. The behavior of hydroquinone in its oxidation at the passivated and activated electrodes was compared using diagnostic criteria for the following functions: I _a?v ^1/2, logI _a?logv, I _a/v ^1/2?v ^1/2, and I _a/c, where v is the rate of the potential sweep and c is the volumetric concentration of hydroquinone. The potential difference of anodic and cathodic peaks in cyclic voltammograms indicated the reversibility of the electrode reaction in all supporting electrolytes.     

Determination of the standard Gibbs energies of transfer of cations across the nitrobenzene|water interface utilizing the reduction of iodine in an immobilized nitrobenzene droplet

When a nitrobenzene (NB) droplet containing iodine is attached to a graphite electrode and immersed into a chloride containing aqueous (AQ) solution, the electrochemical reduction of iodine is accompanied by a transfer of chloride ions from NB to water. These chloride ions enter the NB phase in a preceding partition between the AQ and the NB phases, supported by formation of I₂Cl⁻ ions in NB and accompanied by the transfer of stoichiometric amounts of cations. The overall electrode reaction is of CE_rev type, where C refers to the preceding chemical step forming I₂Cl⁻, and E_rev refers to the reversible reduction of iodine at the graphite|NB interface and the simultaneous transfer of chloride from NB to water. If the chloride concentration in NB is insufficient to compensate by leaving the NB the amount of electrochemically produced iodide, a second voltammetric signal occurs at more negative potentials due to the transfer of iodide from NB to water. The kinetics and thermodynamics of the preceding chemical step C, determine the voltammetric behaviour of the system in such way that the ratio of peak currents of the first and second signals depends linearly on the Gibbs energy of transfer of the co-partitioned cations. The method was validated for cations of known Gibbs energies of transfer, and it was applied to cations of amino acids.