Lutetium bis(tetra-tert-butylphthalocyaninato): a superior redox probe to study the transfer of anions and cations across the water/nitrobenzene interface by means of square-wave voltammetry at the three-phase electrode

The redox properties of lutetium bis(tetra-tert-butylphthalocyaninato) (LBPC) have been studied in nitrobenzene that is deposited as a microfilm on the surface of highly oriented pyrolytic graphite electrodes. The behavior of the modified electrode, which is immersed in an aqueous electrolyte solution, is typical for the three-phase electrode (Scholz, F.; Komorsky-Lovri?, S.; Lovri?, M. Electrochem. Comm. 2000, 2, 112-118). LBPC can be both oxidized and reduced in one electron reversible processes. The oxidation and the reduction of LBPC at the graphite/nitrobenzene interface is accompanied by the transfer of anion or cation, respectively, from the aqueous phase into the organic layer. Thus, using LBPC as a redox probe for the three-phase electrode, the transfer of both anions and cations across the water/nitrobenzene interface can be studied in a single experiment. The hydrophobicity of LBPC is so high that it enables inspection of cations and anions with Delta (nb)(w) (G)(theta)(Cat+) < or = 43 kJ/mol and Delta (nb)(w) (G)(theta)(X-) < or = 50 kJ/mol, respectively. The direct transfer of Na(+) and Li(+) from water to nitrobenzene, mutually saturated, is achieved for the first time at a macroscopic water/nitrobenzene interface.     

Electron transfer – ion insertion electrochemistry at an immobilised droplet: probing the three-phase electrode-reaction zone with a Pt disk microelectrode

A four-electrode setup was used to study the following system: a nitrobenzene (NB) drop containing no added supporting electrolyte and either ferrocene or decamethylferrocene was placed on a glassy carbon disk and surrounded by an aqueous solution of supporting electrolyte. The glassy carbon electrode was polarised appropriately to generate either the ferrocenium or the decamethylferrocenium cations. A very thin conical-body Pt microdisk electrode was placed inside the NB drop to detect the product cations. The relation between the time of appearance of the reduction current at the microelectrode and the distance from the three-phase junction indicates that the electrode reaction starts at this junction where both the electron transfer and the ion transfer can take place.     

Distribution of three ions in the thin film experiment

In the theoretical model it is assumed that a graphite disk electrode is covered by a thin film of the solution of decamethylferrocene and some supporting electrolyte in nitrobenzene and immersed in an aqueous solution of the same electrolyte. Oxidation of decamethylferrocene is accompanied by the transfer of anions of the electrolyte from water into nitrobenzene. The flux of decamethylferrocenium cations at the electrode surface and the flux of anions at the liquid/liquid interface are separated by the thickness of the film, but the electroneutrality is ensured by the migration of ionic species through the film. Theoretical concentration profiles of ionic species in the film are reported for cyclic voltammetry.     

Chromazurol S ion-selective electrode for chelatometric titration

A liquid membrane type Chromazurol S ion-selective electrode has been constructed and applied to chelatometric titrations. In aqueous medium, zephiramine (benzyldimethyltetradecylammonium chloride) forms with the anion of Chromazurol S an ion-pair which is easily extracted into nitrobenzene. The extract is used as the liquid ion-exchanger for the Chromazurol S ion-selective electrode. The most suitable ion-pair concentration in the liquid ion-exchanger membrane is 10(-4)M. The linear Nernstian-response range of the Chromazurol S anion in aqueous solution is 10(-2)-10(-6)M. The electrode has been successfully applied to the chelatometric titration of Cu(II), Ni(II), Fe(III) and Pb(II) with EDTA.     

硝基苯在铜电极上的电还原特性研究

应用循环伏安法和线性扫描伏安法研究水溶液中硝基苯在铜电极上的电还原特性及其电还原为苯胺的中间步骤和反应机理.结果表明:铜电极上硝基苯的还原电位在-0.58V和-1.32V左右(vs.SCE),溶液的酸性和碱性均有益于该还原反应的发生;还原过程伴有反应物吸附现象,当硝基苯浓度较大时,还原过程受传质过程控制;随着厌氧程度的提高,还原速率加快.     

Kinetics of electrode reaction coupled to ion transfer across the liquid/liquid interface

In the theoretical model it is assumed that a graphite disk electrode is covered by a thin film of solution of decamethylferrocene (dmfc) and some electrolyte CX in nitrobenzene and immersed in an aqueous solution of the electrolyte MX. Oxidation of dmfc is accompanied by the transfer of anion X ⁻ from water into nitrobenzene since it is also assumed that cations dmfc ⁺ and C ⁺ are insoluble in water and cation M ⁺ is insoluble in nitrobenzene. Kinetic parameters of the electrode reaction can be determined if the total potential difference across the nitrobenzene/water interface is maintained constant by adding the electrolytes CX and MX in concentrations which are much higher than the initial concentration of dmfc in nitrobenzene.     

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.     

Charge transfer across the water/nitrobenzene interface by three-electrode system

A droplet of aqueous solution containing a certain molar ratio of redox couple is first attached onto a platinum electrode surface, then the resulting drop electrode is immersed into the organic solution containing very hydrophobic electrolyte. Combined with reference and counter electrodes, a classical three-electrode system has been constructed. Ion transfer (IT) and electron transfer (ET) are investigated systematically using three-electrode voltammetry. Potassium ion transfer and electron transfer between potassium ferricyanide in the aqueous phase and ferrocene in nitrobenzene are observed with potassium ferricyanide/potassium ferrocyanide as the redox couple. Meanwhile, the transfer reactions of lithium, sodium, potassium, proton and ammonium ions are obtained with ferric sulfate/ferrous sulfate as the redox couple. The formal transfer potentials and the standard Gibbs transfer energy of these ions are evaluated and consistent with the results obtained by a four-electrode system and other methods.     

ZrO(2) gel-derived DNA-modified electrode and the effect of lanthanide on its electron transfer behavior

A new method of immobilizing deoxyribonucleic acid (DNA) was developed based on sol-gel technique, the resulting DNA-modified electrode was characterized with the cyclic voltammetry. The electrode was used to study the electron transfer of DNA in 1.0 mM potassium ferricyanide system in different concentrations of lanthanum(III), europium(III), and calcium(II). The heterogeneous rate constants of the reduction of Fe(CN)(6)(3-) with and without the above cations were calculated by Tafel equation. The results show that lanthanide ions can increase the electron transfer rate much more than calcium ion.     

Electroactive ceramic carbon electrode impregnated with organic liquid

The carbon ceramic electrodes impregnated with hydrophobic organic solvent (toluene, hexadecane, nitrobenzene) containing redox probe (decamethylferrocene) were prepared. The electrode material was obtained by sol–gel process. It consists of graphite powder homogeneously dispersed in hydrophobic silica matrix. After gelation and drying it was filled with organic liquid. The electrochemical properties of the electrode were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. Approximately symmetric cyclic voltammograms were obtained with these electrodes immersed in aqueous electrolyte solution. Their shape and current magnitude and position on the potential scale depends on the organic solvent and the salt present in aqueous phase. It has been concluded that the mechanism of the electrode process involves electron transfer between graphite particle and the redox probe in organic phase, followed by anion transfer from the aqueous phase.     

血红蛋白在碳纳米管修饰碳糊电极上的直接电化学行为

利用吸附法将血红蛋白(Hb)固定在碳纳米管修饰碳糊电极表面,制成稳定的固载Hb碳纳米管修饰电极,研究了Hb在碳纳米管修饰电极上的直接电化学行为.固载Hb的碳纳米管修饰电极在pH=7.0的PBS（磷酸盐缓冲溶液）中有一对相当可逆的循环伏安氧化还原峰,为Hb血红素辅基Fe(Ⅲ)/Fe(Ⅱ)电对的特征峰.式电位为－0.160 V(vs SCE),随扫描速度变化很小.电子转移数为1.021,近似为一个辅基发生电子转移.Hb在碳纳米管修饰电极表面的电子转移常数为0.0816 s－1,远大于亚甲蓝作媒介体时Hb的电子转移反应速率常数.应用于过氧化氢、三氯乙酸和硝基苯等的电催化还原,固定在碳纳米管修饰碳糊电极的血红蛋白表现出稳定且较高的催化活性.     

Aromatic sulphonate ion-selective electrode membrane with crystal violet as ion-exchange site

Four triphenylmethane derivatives (cations) and a high molecular-weight quaternary ammonium ion were used as the ion-exchange site in the liquid membranes of electrodes responsive to aromatic sulphonate ions, such as benzenesulphonate and alpha-naphthalenesulphonate. The nitrobenzene or 1,2-dichloroethane membrane containing the Crystal Violet-aromatic sulphonate pair had good sensitivity, showing an approximately Nernstian response down to 10(-4)M sulphonate. The potential of the Crystal Violet membrane was independent of pH variation from 2.5 to 12. Chloride and sulphate ions in the aqueous sample solution did not affect the electrode potential. 1,3,6-Naphthalenetrisulphonate exerted essentially no influence on the potential of the alpha-naphthalenesulphonate electrode. The interference of the nitrate ion was relatively large. The conductivity and association of the solute species in the membrane were estimated.     

薄层循环伏安法研究硝基苯/水界面上离子诱导的电子转移反应

应用薄层循环伏安法研究了硝基苯/水两相界面间,且有共同离子四丁基铵TBA+存在于两相中,在有机相中的四氰化二甲基苯醌(TCNQ)与水相中的K4Fe(CN)6之间发生的反向电子转移反应。在直径为0.64cm的裂解石墨电极上用2μL硝基苯溶液使之自然扩散在电极表面形成薄层的有机相,并以此作为工作电极。对电极为铂丝(0.5mm),参比电极为Ag/AgCl电极,均置于总体积为2mL的水相中。由于共同离子TBA+的诱导,在硝基苯/水界面间,在已氧化的TCNQ+阳离子(在有机相中)与[Fe(CN)6]4-阴离子(在水相中)之间发生了反向电子转移反应。试验证明:在一定条件下,通过改变两相中共同离子的浓度,可使一些不能发生的两相界面的电子转移反应得以发生;这类电子转移反应系受界面电位差所控制。此外,还测得了在恒定的共同离子浓度比值的条件下,此两相界面电子转移反应的表观速率常数(k)为0.135cm.s-1.mol-1。     

Extraction studies on iron

The extraction of Fe(III) and Fe(II) from various aqueous acidic solutions, with nitrobenzene, Amberlite LA-2, TBP and HDEHP is described. Conditions are given for the separation of Fe(III) from Fe(II). The extraction and separation of Fe(III) and Fe(II) is most adequate from HCl solutions, using the four solvents. The extraction of iron halides from H₂SO₄ solutions has been studied. The effect of water-miscible alcohols on the distribution of Fe(III) and Fe(II) was also studied. Extraction equilibria and mechanisms were proposed on the basis of the obtained results.     

A nitrate-selective electrode based on bis(triphenylphosphine)iminium salts

A nitrate-sensitive electrode based on the ionic associates between bis(triphenylphosphine)iminium and nitrate ions in nitrobenzene solution is proposed. The electrode can be used for determinations of 10(-5) 0.1M nitrate in aqueous solutions.     

Liquid-state membrane electrode sensitive to bismuth(III)

A liquid ion-exchange electrode containing a tetrachloraethane solution of the complex of bismuth(III) with 5-mercapto-3-(naphthyl-1)-1,3,4-thiadiazol-2-thione is described. The electrode is sensitive to Bi³⁺. The slope of the calibration graph (electrode potential vs. concentration) is 18.7 mV/pBi in the pBi range 6.5–9.5 in ammonium acetate buffer (pH = 4.0). Bivalent cations and Al(III), Fe(III) and Th(IV) do not interfere (K_{Bi³⁺+,M²⁺}<10⁻⁵). The dissociation constant of bismuth acetate has been determined with the aid of the electrode.     

Sorption and Abiotic Redox Transformation of Nitrobenzene at the Smectite-Water Interface

The effect of the redox state of structural Fe on the surface reactivity of iron-bearing phyllosilicates in aqueous suspension was investigated using a molecular probe. For this purpose the structural Fe in montmorillonite and ferruginous smectite was chemically reduced by sodium dithionite in citrate-bicarbonate (CB) buffer solution under N(2) at 70 degrees C, with the excess reactants removed by washing and centrifugation. Nitrobenzene was chosen as an electron acceptor probe to react with unaltered and chemically reduced smectites. Nitrobenzene was transformed into aniline only in the presence of the reduced smectites. This abiotic reductive transformation depended on the concentration of the electron acceptor in solution and the total accessible structural Fe(II) in smectites. As much as 40% of the crystal layer structural Fe(II) of the reduced smectites was oxidized to Fe(III); these electrons migrated to the surfaces/edges and then were transported into the adjacent aqueous layer. No significant effect of the oxidation state of structural Fe on the sorption of nitrobenzene on smectites was observed, but the reduced smectites sorbed less aniline than the unaltered smectites. The electronic structure and molecular geometry of the probe were modified within the smectite-water interface. Reducing structural Fe in smectites perturbed the surface reactivity. Copyright 2001 Academic Press.     

Iron (III) Ion Selective Electrode Based on Dithia 12-Crown-4

An iron (III) ion selective PVC membrane electrode based on 1,7-dithia 12-crown-4 as a neutral carrier was prepared. Monovalent responses with a Nerstian slope of 56+1 mV/decade was observed for the Fe (III) ion-selective electrode within the concentration range 10^?3–10^?5 M Fe (NO₃)₃. The monovalent responses may be attributed to the formation of Fe (OH)₂⁺ or Fe (OH)₂(H₂O)⁺₄ species in aqueous solutions and the absorption of these ions into the PVC electrode membrane. The electrode exhibited good selectivity for Fe (III) in comparison with various alkali, alkali-earth and some heavy metal ions. The effects of the composition of the membrame, addition of STPB (sodium tetraphenyl borate), the concentration of internal solution of the electrode and anions in the test solutions were discussed.     

化学修饰电极的研究 V:铁卟啉修饰电极对氧的催化还原

By surface organio synthesis a Fe (III) tetra-o-aminophenyl porphyrin modified electrode was successfully prepared on glassy carbon electrode surface through amidization. The preparation considitions were invesigated in detail. Using cyclie voltametry and cyclie semidifferential polarography in an acidic aqueous solution the behavior of the catalytic reduction of oxygen on iron porphyrin modified electrode was studied. Based on experimental results it was shown that the irreversible process of 2- electronreduction to hydrogen peroxide belongs to EC parallel catalytic process.     

Construction of a new Ho<Superscript>3+</Superscript> PVC-membrane electrochemical sensor based on <Emphasis Type="Italic">N,N</Emphasis>′-dipyridoxyl(1,4-butanediamine)

N,N′-dipyridoxyl(1, 4-butanediamine) (PYBA) (2 wt % ), nitrobenzene (66 wt %), sodium tetra phenyl borate (2 wt ), and poly vinyl chloride (30 wt %) were applied to create a PVC membrane in Ho³⁺ selective electrode (Ho-SE). Over the concentration range of 1.0 × 10^?5 to 1.0 × 10^?2 M in holmium(III) solution a linear response with lower detection of limit equal 4.0 × 10^?6 M was exhibited by new holmium(III) sensor. Ho-SE possessed a Nernstian slope of 20.1 ± 0.8 mV decade^?1 in very short time spans (~9 s). The potential response of electrode remains without any changes over a pH range of 2.77–8.71. The selectivity coefficient data which was considered by matched potential method for variety of common cations indicated no significant interference. An acceptable recovery for Ho³⁺ in the blend of several cations has been achieved. The utility of Ho-SE in potentiometric titration as an end point indicator electrode gave successfully results.