Copper-Selective Electrode in the Analysis of Dye-Containing Solutions

The effect of dye concentrations in solutions on the potential of a copper-selective electrode with a chalcogenide membrane was studied. It was found that the electrode potential depends on the dye concentration in solutions containing no copper ions. Conventional selectivity coefficients of the electrode with respect to dye anions were determined. The selectivity coefficients ambiguously changed with the dye nature and their concentration in solutions. The use of the calibration graph method without taking into account the composition of the test solution gave rise to great errors in the results of determinations.     

A copper ion-selective electrode with high selectivity prepared by sol-gel and coated wire techniques

A sol-gel electrode and a coated wire ion-selective poly(vinyl chloride) membrane, based on thiosemicarbazone as a neutral carrier, were successfully developed for the detection of Cu (II) in aqueous solutions. The sol-gel electrode and coated electrode exhibited linear response with Nernstian slopes of 29.2 and 28.1 mV per decade respectively, within the copper ion concentration ranges 1.0×10^–5–1.0×10^–1 M and 6.0×10^–6–1.0×10^–1 M for coated and sol-gel sensors. The coated and sol-gel electrodes show detection limits of 3.0×10^–6 and 6.0×10^–6 M respectively. The electrodes exhibited good selectivities for a number of alkali, alkaline earth, transition and heavy metal ions. The proposed electrodes have response times ranging from 10–50 s to achieve a 95% steady potential for Cu²⁺ concentration. The electrodes are suitable for use in aqueous solutions over a wide pH range (4–7.5). Applications of these electrodes for the determination of copper in real samples, and as an indicator electrode for potentiometric titration of Cu²⁺ ion using EDTA, are reported. The lifetimes of the electrodes were tested over a period of six months to investigate their stability. No significant change in the performance of the sol-gel electrode was observed over this period, but after two months the coated wire copper-selective electrode exhibited a gradual decrease in the slope. The selectivity of the sol-gel electrode was found to be better than that of the coated wire copper-selective electrode. Based on these results, a novel sol-gel copper-selective electrode is proposed for the determination of copper, and applied to real sample assays.     

Determination of Citrate Ions by Potentiometric Titration with a Copper-Selective Electrode in Monitoring the Production of Citric Acid

A simple and rapid procedure was developed for the potentiometric titration of citrate ions in fermented solutions from citric acid production with the use of a copper-selective electrode.     

Needle-type ultra micro silver/silver chloride reference electrode for use in micro-electrochemistry.

A needle-type ultra micro silver/silver chloride reference electrode having a micro capillary with outer and inner diameters of 1.0 microm and 0.5 +/- 0.2 microm, respectively, was constructed. This micro reference electrode can be stuck into a living cell, and is applicable to use in very small environments, such as an electrochemical cell of an electrochemical scanning tunneling microscope or the detection portion of a micro-TAS. Excellent stability and repeatability of the micro reference electrode potential could be obtained by filling the micro capillary with agar gel containing 3.33 mol/L potassium chloride as a salt bridge, by covering the bare part of the silver wire on which silver chloride was deposited, and by electromagnetic shielding of the measurement cell and wire lead from the electromagnetic waves. The electrode showed stable potential for 7 days after its fabrication using 3.3 mol/L potassium chloride aqueous solution containing silver chloride as an internal electrolyte solution. The electrode exhibited constant electrode potential and excellent stability in test solutions of pH 5-9. The electrode potential of a commercial pH glass electrode measured against the micro reference electrode in standard pH buffer solutions was in good accordance with the Nernst equation.     

Nafion/tetraruthenated porphyrin glassy carbon-modified electrode: characterization and voltammetric studies of sulfite oxidation in water–ethanol solutions

In this work, the modification of a glassy carbon electrode with tetraruthenated porphyrins electrostatically assembled onto a Nafion film, previously adsorbed on the electrode surface, is reported. This modified electrode was characterized by scanning electron microscopy–energy-dispersive X-ray, Raman spectroscopy, UV-Vis spectroelectrochemistry, and cyclic voltammetry. The Nafion film onto the glassy carbon electrode shows a smooth disposition; when the tetraruthenated porphyrin is incorporated on the Nafion film, the complex is adsorbed in a homogeneous way. The modified electrode catalyzes HSO₃⁻ oxidation in water–ethanol solutions and shows an enhanced stability compared with the electrode modified with the dip coating method. Rotating disk electrode experiments showed a kinetic limitation to the electron transfer controlled by charge propagation in the film. I/E curves show a Tafel slope of 120 mV/decade corresponding to a first electron-transfer reaction, depending on the potential, as the determining step. Spectroelectrochemical experiments demonstrated that Ru(II) is the active site for the electrocatalysis.     

The microporous structure of silver/silver chloride electrodes and the implications for Harned cell operation

The stability of silver/silver chloride reference electrodes when moved between different electrolyte solutions is limited by the small amount of solution transferred. The time required for the Ag/AgCl electrode potential to stabilize has been shown to increase with electrode size in an empirical fashion. The results presented suggest the presence of a microporous structure in the electrodes that limits the rate at which traces of any previous solutions are diluted by any new solution environment. Operational implications for the stability and accuracy of the Harned Cell used as the primary standard for pH measurements and the certification of primary reference buffers are discussed. Presented to the CCQM Electrochemical Analysis Working Group, Paris, April 2004.     

钽在无水乙醇中的腐蚀

采用动电位极化、循环伏安、交流阻抗和扫描电镜等技术研究了钽在四乙基氯化铵（TEA）乙醇溶液中的腐蚀行为．在循环伏安曲线的扫描初期,电极表面因存在一薄层氧化物膜而使得电流密度缓慢增加．后来钝化膜因受到氯离子的攻击而被击穿,即点蚀．扫描电镜图很好地显示出蚀孔的生长过程．点蚀电位随着TEA浓度的增加而下降,随着水含量的增加而上升．在所研究的温度范围内,电化学反应的活化能为36kJ／mol．所有电极电位下的交流阻抗图谱都包含两个时间常数,钝化膜电阻和电荷传递电阻均随电极电位的增加而下降．     

Ion-selective electrodes in potentiometric titrations; a new method for processing and evaluating titration data

A new method to convert the potential of an ion-selective electrode to concentration or activity in potentiometric titration is proposed. The advantage of this method is that the electrode standard potential and the slope of the calibration curve do not have to be known. Instead two activities on the titration curve have to be estimated e.g. the starting activity before the titration begins and the activity at the end of the titration in the presence of large excess of titrant. This new method is beneficial when the analyte is in a complexed matrix or in a harsh environment which affects the properties of the electrode and the traditional calibration procedure with standard solutions cannot be used. The new method was implemented both in a method of linearization based on the Grans's plot and in determination of the stability constant of a complex and the concentration of the complexing ligand in the sample. The new method gave accurate results when using titrations data from experiments with samples of known composition and with real industrial harsh black liquor sample. A complexometric titration model was also developed.     

Stability of ternary copper—nitrilotriacetic acid complexes

A copper-selective electrode was used to investigate ternary copper—nitrilotriacetic acid complexes by means of combined pH and pM measurements. The theory presented for the evaluation of the stability of mixed-ligand complexes is based on the conditional constants. The emphasis is on the formation of 1:1:1 complexes of the type copper(II)—NTA—(hydroxide, amino acid, ammonia, triphosphate).     

Effect of chloride ions on the behaviour of the orion copper(II) ion-selective electrode

The normal shiny surface of a copper(II) ion-selective electrode tarnishes when exposed to chloride ions. Polishing with fine emery cloth easily removes this dull surface layer and fully restores the proper potential response characteristics of the electrode. No such loss of character is evident with a non-chloride based reference electrode except in the presence of added chloride ions. The electrode also seems less affected in premixed solutions of copper(II) and chloride. the chloride ions then being largely bound as copper(II) chloro-complexes.     

Micelle-Counterion Interaction,I. Critical Micelle Concentrations OF SDS Under the Influence of Copper Counterion

We investigated the effect of counter ion (Cu²⁺) adsorption on Sodium Dodecylsulfate and Sodium Tetradecylsulfate micelle by three different techniques: surface tension decrease, copper-selective electrode and electric conductivity. The effect of added copper ion on critical micelle concentration (CMC) was determined. The effectiveness of these three techniques was critically evaluated. It is concluded that copper ions are able to strongly displace Sodium ions from the micellar surface.     

The electrooxidation of methanol and related compounds at ruthenium dioxide-coated electrodes

The underlying metal was observed to corrode when a ruthenium dioxide-coated titanium electrode was anodized in an aqueous methanol solution. With a similarly coated platinum electrode peaks were observed on the voltammogram below 1.0 V which were attributed to methanol oxidation on the underlying metal. This effect was more pronounced when the electrode was subjected during cycling to potentials close to 0 V. Rapid oxidation of methanol on RuO₂ was observed at potentials above 1.0 V, the rate at a given potential increasing in an approximately linear manner with increasing alcohol concentration. The rate of reaction also increased with increasing temperature and increasing surface roughness. Tafel slope values were rather high (>100 mV decade^?1) and a mechanism involving anodically generated species such as OH_ads was proposed to account for these results. The variation of activity with pH was similar to that reported earlier for oxygen evolution at these anodes and this was again explained in terms of partial deactivation of the surface due to a combination of proton loss and phosphate ion adsorption at intermediate pH values. The release of carbon dioxide from aqueous solutions of higher alcohols at 25°C confirmed the high oxidizing power of RuO₂ anodes.     

The electrodeposition of magnesium using solutions of organomagnesium halides,amidomagnesium halides and magnesium organoborates

The electrochemical behaviour of three groups of electrolyte systems having the ability of electrochemical magnesium deposition, were investigated against the background of galvanotechnical or battery application. Solutions of organomagnesium halides, amidomagnesium halides and magnesium organoborates dissolved in tetrahydrofuran were characterized in terms of conductivity, reversibility of the magnesium deposition process and stability of the electrolytes versus irreversible oxidation. Furthermore the open circuit potential of a magnesium electrode in contact with these electrolytes was measured versus a Ag/Ag⁺ reference electrode. A high reoxidation efficiency was found for magnesium electrodeposited from solutions of organomagnesium halides and amidomagnesium halides. The solutions of magnesium organoborates were superior in terms of electrical conductivity and oxidation stability, the process of magnesium electrodeposition and reoxidation, however turned out to be poor.     

复合型扫描微pH电极及其在局部腐蚀中的应用

研制了一种新型的微米级复合型扫描pH探针.该探针具有体积小、响应速度快、稳定性好和制作方便等优点，能很好地满足微区pH快速测定的要求.应用复合型扫描微pH电极技术研究了Al/Cu合金在0.01 mol•L－1 NaCl溶液中开路状态下局部腐蚀发生的早期过程.     

Complexation of copper ions with histidine-containing tripeptides immobilized on solid surfaces

Short oligopeptides that complex with metal ions with high affinity and high specificity are of interest to the design of chemical sensors. In this study, we compare the complexation properties of two copper-selective tripeptides, Gly-Gly-His and His-Gly-Gly, either in aqueous solutions or immobilized on solid surfaces. Our results show that the copper complex formed by Gly-Gly-His is more stable than the complex formed by His-Gly-Gly in aqueous solutions, because the position of histidine (His) in the Gly-Gly-His permits the formation of a tetragonal copper complex with a high stability. However, when the tripeptides are immobilized on aldehyde-decorated silicon wafer surfaces under a reaction condition that gives rise to near maximum surface densities of tripeptides, both immobilized Gly-Gly-His and His-Gly-Gly experience strong steric hindrance on the over-crowded surfaces. The surface crowding effect causes less complexation with copper ions than that in aqueous solutions. To ensure a proper surface density on the surface for complexation with copper ions, a so-called two-dimensional (2D) metal-ion imprinting technique is employed to avoid the surface crowdedness. By immobilizing Gly-Gly-His in the presence of copper ions, we create a tripeptide-functionalized surface that exhibits high complexation capability for copper ions. We attribute the higher copper complexation capability to the proper intermolecular distances obtained from the ion-imprinting procedure that gives the copper-tripeptide complex a preferential tetragonal geometry. Our results show that the amounts of copper complexed to a copper-imprinted surface functionalized with Gly-Gly-His are 62% higher than those of a nonimprinted surface.     

对再生式光电化学电极的开路电位及其稳定性的探讨

光电化学电极的开路电位及其稳定性与电解质溶液的性质有关。光电极的开路电位与照射在电极上光的强度呈半对数关系。实验表明:对n-型半导体电极,随着溶液中氧化态浓度的增加光电位向正方向移动,而改变溶液中还原态的浓度对光电位影响不大。本文根据电极过程的基本原理解释了这些事实。在某些电解质溶液中还观察到光电极光生电位的滞后现象,文中用费米能级分裂的概念对出现这一现象的原因进行了讨论。     

Comparative electrochemical study of self-assembly of octanethiol from aqueous and aqueous ethanol solutions on a gold electrode

The self-assembly of octanethiol (OT) on the surface of a polycrystalline gold electrode in aqueous and aqueous ethanol thiol-containing (1 × 10^–4 М) 0.1 М NaClO₄ solutions was studied. The blocking properties and electrochemical stability of monolayer OT films were studied by chronopotentiometry during OT adsorption under the open circuit conditions (chronoamperometry at a fixed potential) combined with cyclic voltammetry for modified Au/OT electrodes. It was found from the change in the rate of electrochemical reactions in the range of monolayer stability potentials that in aqueous media, compact insulating OT monolayer films formed at a open circuit potential within ~100 s, and the shift of the adsorption potential toward negative values (to–0.6 V) allowed a considerable decrease in the monolayer self-assembly time. The potential shift toward higher negative values (–0.9 V) leads to a removal of OT from the electrode surface during the reductive desorption, with a multipeak current signal recorded on the voltammograms. A transition from aqueous to aqueous ethanol solutions accelerated the formation of an insulating OT monolayer (≈6 s) and led to a change in the shape of the desorption current peak, whose value was almost independent of the ОТ accumulation time and potential.     

Effect of the platinum surface on the potential of nitrate-selective electrodes without internal solution

The effects of oxidation or reduction of the platinum contact used in nitrate-selective electrodes without internal reference solutions were studied by using voltammetric curves. A correlation was found between the state of the platinum surface and the standard potential of the ion-selective electrode under potentiometric conditions.     

Development of pitting corrosion on 20Kh13 steel

The nucleation and propagation of corrosion pits on 20Kh13 steel in the NaCl solutions with various concentrations, temperatures, and pH values are studied under the potentiostatic conditions and at the free-corrosion potential. In the potentiostatic experiments, the variations of the depth and diameter of pits and their number with the time are determined. It is shown that the state of metal surface in a certain area adjacent to the active pits changes. The dimensions of the area are estimated. The results, which were obtained at the free-corrosion potential, were much less reproducible. At the free-corrosion potential, in contrast to the potentiostatic conditions, the pit depth increased only slightly and the pit width increased to a larger extent. It is shown that the pH value of NaCl solutions has a pronounced effect on the development of pitting corrosion on 20Kh13 steel.     

Electrochemical instabilities due to pitting corrosion of iron

Electrochemical instabilities induced by chlorides and bromides due to pitting corrosion of iron in sulfuric acid solutions are investigated. Analysis of the electrochemical instabilities as a function of the applied potential and the nature and concentration of the aggressive chemical species shows that the system exhibits a transition from aperiodic bursting of large-amplitude to small-amplitude chaotic oscillations at a critical potential (bifurcation potential, E _bif). The E _bif is determined by the halide concentration inside the pits and coincides with the repassivation potential defined in corrosion studies to explain pit repassivation due to changes in pit chemistry. Surface observations show that, at E < E _bif, an active-passive state dissolution (etching) occurs, while at E > E _bif, a polishing state dissolution is reached. Spatial interactions between early initiated pits and the adjacent electrode surface, oxide film alteration, aggressive species accumulation around active pits, and formation of ferrous salt layers in front of the Fe electrode are all considered to be associated with electrochemical instabilities emerging during pitting corrosion of iron under different dissolution states. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 5, pp. 535–550. The text was submitted by the authors in English.