Dual microband electrodes: current distributions and diffusion layer ‘titrations’. Implications for electroanalytical measurements

The simulation of transport to double microband electrodes in generator–collector mode is reported focusing especially on the ‘titration curve’ approach to electroanalysis in which a titrant is electrogenerated from a redox active precursor on the generator electrode and reacts homogeneously with the target analyte. The current on the detector electrode reflects the amount of titrant ‘surviving’ passage between the two electrodes. The form of the titration curve – plots of detector current as a function of generator current – is shown to be highly sensitive to the electrode kinetics of the redox couple driven at the generator electrode. Accordingly the naïve use of such methodology for analysis without accompanying simulation and kinetic analysis is fraught with danger. Use of the conformal mapping approach in combination with the ADI method for investigation of the ‘titration’ current distributions at the double band system gives fast and precise simulation of this and similar problems. Convergence analysis is described which allows for the automatic selection of the simulation grid size so as to obtain a chosen accuracy (for example 1%) of the current for all experimentally meaningful values of the geometrical and physico-chemical parameters of the system to be investigated.     

Electrochemical titrations of thiosulfate, sulfite, dichromate and permanganate using dual microband electrodes

Applications of titration using dual gold microband electrodes in generator-collector mode are presented. The main advantage is that the method dispenses with the need for accurate volume measurements and reagent preparation: rather than balancing of molar amounts, the fluxes of analyte and titrant are balanced instead. The titrant is generated electrochemically and the end-point is detected amperometrically by the appearance of a current due to the presence of the titrant at the other band. The aim is to show the versatility and capability of the method that employs disposable mass-produced electrodes and uses pulsed motion of the electrode before a measurement to renew the boundary conditions. The titration of ascorbic acid (vitamin C) with ferricyanide, which has been a model system, was extended to the determination of thiosulfate and sulfite with iodine and the determination of dichromate and permanganate with iron(II). The accuracy, limited by the present fabrication reproducibility of the disposable electrodes, is +/- 10%. The results demonstrate that the method is effective even with a very simple set-up, and the actual time needed for common titrations is significantly decreased. These examples demonstrate a comprehensive basis for further development and applications, including standard titrations in industry.     

Potentiometric titrations using dual microband electrodes.

We present a novel method of analysis using potentiometric end-point detection and dual microband electrodes in generator-collector mode. The titrant is electrogenerated using either a constant current or a current that increases linearly with time, and the fluxes of reactive material rather than molar amounts are balanced. The advantage of the ramp current system over the constant current method is that all of the information needed for a full titration curve and a proper analysis can be obtained during a single scan. The method was applied to the determination of vitamin C with ferricyanide and to the determination of thiosulfate and sulfite with iodine using gold microband electrodes. As a new type of analysis, a potentiometric titration that uses dissolving silver microband electrodes in order to generate the titrant is demonstrated. The system was applied to the detection of chloride, iodide, thiosulfate and cyanide. The accuracy of the analysis is +/-10%, limited mainly by the present screen-printing process, and the method is well adapted for measurements on the millimolar scale.     

多元校正流动注射电位滴定法同时测定苯甲酸和山梨酸

将流动注射应用于酸碱电位滴定分析,建立了一种可同时测定混合有机酸的电位滴定新方法。在该方法中,用氢氧化钠与氯化钾的混合溶液作为滴定剂,在流通池中同时插入pH指示电极和氯离子指示电极,在滴定过程中的任一滴定点,流出液的pH值和酸碱的混合比例可由两个电极的电位测定值同时获得,从而可应用多元校正法由相应的滴定曲线求得混合酸中每一种组分的含量。该方法不仅免去了体积和时间读数,而且减少了试剂和样品的消耗量,分析速度快。应用该方法对混合样品中的苯甲酸和水杨酸进行同时测定,其相对标准偏差分别为0．19％～0．37％,回收率分别为97．3％～102．6％。     

PVC膜修饰电极上零电流示波电位滴定法测定硼的研究

以氯化十六烷基吡啶（ＣＰＣ）和氟硼酸钠形成的离子缔合物为活性物质，制成ＰＶＣ膜涂层石墨电极，提出了一个快速测定硼的零电流示波电位滴定法。以ＣＰＣ为滴定剂，用ＰＶＣ膜电极为指示电极，甘汞电极为参比电极，利用阴极射线示波器荧光屏上荧光点的突然位移来指示ＣＰＣ滴定硼的终点，具有灵敏、准确、快速等特点，用该法测定玻璃中的硼，结果良好。     

Titrimetric analysis with adding the titrant by means of electrodialysis

A titration method is developed that titrant is added continuously to the analyte by electromigration through an ion exchange membrane. A constant rate of the titrant addition is achieved by keeping the electric current constant. Analyte amount then is proportional to the titration time. The method is applied to the most widely used titrations with a visual end point indication.     

Theory of the steady-state current of a redox couple at interdigitated array electrodes of which pairs are insulated electrically by steps

The possibility of constructing an interdigitated array electrode (IDA) with a submicrometre gap is proposed in which adjacent microband electrodes are separated from each other by an insulated step. Then the IDA is an assembly of protrusive and hollow microband electrodes. The unit model of the IDA consisted of half of the lower (hollow) microband electrode, an insulated step and half of the upper (protrusive) microband electrode with a finite thickness on the step. The boundary value problem involving the two-dimensional Laplace equation is presented for redox cycling at the IDA under diffusion control and is solved numerically by a boundary element method. The steady-state current was computed as a function of the height of the step and the thickness of the upper electrode. It was larger than the current at the ordinary IDA, partly because the true electrode area was larger than the area of the ordinary IDA. The current varied linearly with the logarithm of the step height. It was expressed by a simple approximate equation in order to facilitate prediction of its numerical value.     

Microband Sensor for As(III) Analysis: Reduced Matrix Interference

A portable sensor based on a microband design for arsenic detection in drinking water is presented. The work was focused to minimize interference encountered with a standard screen‐printed electrodes featuring an onboard gold working electrode, carbon counter and silver−silver chloride pseudo‐reference electrodes as composite coatings on plastic surface. The interference effect was identified as chloride ions interacting with the silver surface of the reference electrode and formation of soluble silver chloride complexes such as AgCl₄³⁻. By modification of the reference electrodes with Nafion membrane (5 % in alcohols), the interference was entirely eliminated. However, membrane coverage and uniformity can impact the electrodes reproducibility and performance. Hence, the sensor design was further considered and a microband format was produced lending favorable diffusive to capacitive current characteristics. Using the microband electrodes allowed As(III) detection with limit of detection of 0.8 ppb (in 4 M HCl electrolyte), inherently avoiding the problems of electrode fouling and maximizing analyte signal in river water samples. This is below the World Health Organization limit of 10 μg L⁻¹ (ppb). The electrolyte system was chosen so as to avoid problems from other common metal ions, most notably Cu(II). The presented electrode system is cost effective and offers a viable alternative to the colorimetric test kits presently employed for arsenic analysis in drinking water.     

A cell for high-precision constant-current coulometry with external generation of titrant

A cell has been designed for the high-precision coulometric titration, with externally generated titrant, of materials which otherwise undergo undesirable reactions at the working electrodes. With this cell potassium dichromate has been titrated, via its hydrolysis reaction, with hydroxyl ion generated at the cathode, cathodic reduction of the chromium(VI) being circumvented. In this cell 99.9% of the titrant required is generated in one chamber and transferred to another for reaction; the titration is then completed with titrant generated at a second, drip-type electrode working at much lower current. By means of commercially available Leeds and Northrup coulometric titration electrical equipment, titration of NBS 136b Potassium Dichromate gave a purity of 99.976%, standard deviation 0.005%, and of NBS 84d Potassium Acid Phthalate (done as a check) 99.991%, standard deviation 0.005%, both values being in excellent agreement with other work.     

Use of Linear Regression for the Processing of Curves of Differential Potentiometric Titration of a Binary Mixture of Heterovalent Ions Using Precipitation Reactions

A method is proposed for the computer processing of curves of differential potentiometric titration of a binary mixture of heterovalent ions using precipitation reactions. The method is based on the transformation of the titration curve to piecewise linear characteristics, whose parameters (within the accuracy of the least-squares method) determine the sequence of the equivalence points and solubility products of the resulting precipitates. The computational algorithm is implemented as a Turbo-Pascal program DIFTITR. The method is applied to the titration of Ag(I)–Cd(II), Hg(II)–Te(IV), and Cd(II)–Te(IV) mixtures by a sodium diethyldithiocarbamate solution with membrane sulfide and glassy carbon indicator electrodes. For 4 to 11 mg of the analyte in 50 mL of the solution, RSD varies from 1 to 9%.     

Potentiometric aspects of mercury(II)-thiosemicarbazonates : II. Titration by the use of iodide ion-selective and amalgamated gold electrodes. Determination of apparent formation constants in a water-acetone mixture

The technique for titration of benzaldehyde (BAT), salicylaldehyde (SAT), and p-hydroxybenzaldehyde (PHBT) thiosemicarbazones carried out by the using iodide ion-selective and amalgamated gold electrodes is described. Mercury(II) is used as the titrant reagent. The apparent formation constants for these chelates are determined by the Ringbom and Harju method. The advantages and disadvantages of the use of these electrodes in the proposed determinations are discussed.     

络合体系中的相对滴定法及其应用于铝的测定

本文提出了络合体系中的相对滴定分析法，它以滴定到一定的状态时，滴定剂体积与被测物的基之间的线性关系为定量基础，用仪器分析的定量方法进行定量，适应于所有类型的络合体系，即使对滴定突跃很小或根本无突跃的体系也同样适用。用氟离子滴定铝离子的实验结果表明本方法的精密度很高。一般相对误差小于0．1％。即使对于铝离子的量小到10-7mol／L时，在最佳实验条件下的相对误差也不大于0．2％。该方法既具有滴定分析法的高精密度，又具有仪器分析法可以测量微量物质的优点，并且设备和方法简单。     

Coulometric titration with air transported sample

Air transported samples of a mixture containing the analyte and a supporting electrolyte are consecutively propelled into a microconduit system through a titration cell, a coil and a detector cell. Different amounts of electrochemically generated titrant are obtained in the separate samples while they are passing through the titration cell. Various quantities of the titrant are achieved by changing the duration of the titrant generating process that is done by various time for the sample solution staying in the titration cell (different sample movement rate). The titrated sample is then homogenised in the coil and detected in an electrochemical detector cell that supplies necessary data for the analyte determination. The declared method is experimentally verified on coulometric titration of hydrochloric acid (HCl) with electrogenerated sodium hydroxide and a titration of aniline with bromine generated in a bromide solution.     

Fabrication of Miniaturised Screen‐printed Glucose Biosensors,Using a Water‐based Ink,and the Evaluation of their Electrochemical Behaviour

This paper describes a simple, convenient approach to the fabrication of microband electrodes and microband biosensors based on screen printing technology. These devices were printed in a three‐electrode configuration on one strip; a silver/silver chloride electrode and carbon counter electrode served as reference and counter electrodes respectively. The working electrodes were fabricated by screen‐printing a water‐based carbon ink containing cobalt phthalocyanine for hydrogen peroxide detection. These were converted into a glucose microband biosensor by the addition of glucose oxidase into the carbon ink. In this paper, we discuss the fabrication and application of glucose microband electrodes for the determination of glucose in cell media. The dimensions (100–400 microns) of the microband electrodes result in radial diffusion, which results in steady state responses in the absence of stirring. The microband biosensors were investigated in cell media containing different concentrations of glucose using chronoamperometry. The device shows linearity for glucose determination in the range 0.5 mM to 2.5 mM in cell media. The screen‐printed microband biosensor design holds promise as a generic platform for future applications in cell toxicity studies.     

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.     

微电极研究 XV.微带电极及阵列微带电极上的扩散

本文讨论了微带电极的计时电流、扩散层与时间的关系;研究了微带阵列电极的扩散层交叠情况及其对计时电流的影响;进而对微带阵列电极上电极间的屏蔽效应进行了研究,屏蔽效应与电解时间、电极间距离及电极宽度有关,用已有的实验结果验证了本文的理论分析.     

Titrations using an apparatus for recording the antilogarithm of pH or pM

A titration apparatus plotting either the concentration of sample ion or the concentration of titrant ion has been tested. An antilog apparatus, converting measured e.m.f. values into concentrations is connected to ordinary titration equipment. The instrument has been tested by means of acid—base titrations (titrations of mixtures of weak acids and of a weak and a strong acid), precipitation titrations (determination of the chloride concentration in tap-water, titration of mixtures of halides), titrations with ion-selective electrodes (determination of the fluoride content of toothpaste) and complexometric titrations (determination of copper with EDTA, using mercuric ion as indicator ion and amalgamated silver rod as indicator electrode, or using a copper-selective indicator electrode). The method considerably simplifies the evaluation of the results as compared to conventional potentiometric titrations.     

The boundary element method: chronoamperometric simulations at microelectrodes

The application of the boundary element method (BEM) as an efficient and powerful method for the analysis of the time-dependent electrochemical processes at multiple electrode configurations is presented. The paper describes the theory and numerical details required for developing one-, two- and three-dimensional transient diffusion models for chronoampermetric simulations under diffusion control. The benefits of the BEM approach are discussed, including the reduction in dimensionality brought about by the formulation procedure and complete elimination of the need for domain discretisation with the time-domain convolution approach. The versatility and efficiency of the numerical procedures are examined with respect to a number of electrode geometries. Results are presented for chronoamperometric simulations at microband, microhemishpere, microcylinder and microdisc electrodes compared to appropriate analytical theory. The three-dimensional simulations focus on the modelling of double-electrode configurations (microdisc and microhemisphere) operating in a generator–collector mode. The influence of electrode separation on the transient current response is presented and normalised working curves currently unavailable via analytical methods are provided.     

Acidites et complexes des acides (alkyl- et aminoalkyl-) phosphoniques—II: Affinement multiparametrique applique aux complexations suivies par potentiometrie

The determinations are based on potentiometric titration data (volume of reagent, potential). To avoid any systematic error, the acidity constants are refined together with every other significant parameter (initial protolyte concentration, concentration of impurities, electrode chain characteristics …) with the program MUPROT. Comparison of the multiparametrically adjusted results with those obtained by the usual methods reveals noticeable discrepancies. Particular attention is paid to carbonate as impurity; to take it into account, modified expressions for the titrant volume are established. The current of nitrogen used in the classical methods shifts the liquid-gas equilibrium of carbonic acid; in that case, only the approximate equations are to be considered. Working with a closed system is proposed as a more rigorous procedure.     

Microband electrodes of ideal and nonideal geometries: AC impedance spectroscopy

The AC impedance behavior of microband electrode geometries which deviate from the ideal is derived via numerical modelling of the chronoamperometric response under diffusion-only conditions. Specifically attention is given to four electrode shapes in addition to the ideal microband geometry: elevated microband electrodes (with conducting supporting sides), recessed microband electrodes (with insulating pit walls), platform electrodes (with insulating supporting sides) and, for the purposes of comparison, a hypothetical line electrode without any support which permits diffusional mass transport to both sides of the infinitesimally thin electrode. Simple analytical expressions are established for the frequency dependence of the AC impedance in each case.