The electrochemical behavior of PtRu- and Pt-modified zeolite X in alkaline solution

Zeolite NaX was modified by Pt and Pt/Ru nanodispersed metallic clusters. The procedure of impregnation with acetylacetonate salt/acetone solution was applied. Scanning electron microscope analysis confirmed partial zeolite framework destruction. According to energy dispersive X-ray analysis, Pt/Ru ratio in sample was about 1. Electrochemical behavior of PtRu- and Pt-modified zeolites was investigated in alkaline solutions, 5 mM NaOH?+?1 M Na₂SO₄ and 0.1 M NaOH. The shape of cyclic voltammograms of 13XPtRu electrode, recorded in slightly alkaline solution, was greatly affected by the presence of hydrogen that remained in the sample after synthetic procedure. Oxygen reduction reaction (ORR) was investigated in an O₂-saturated aqueous 0.1-M NaOH solution. The obtained Tafel slopes indicated ORR mechanism that involves one-electron discharge-determining step. According to Koutecky–Levich slope, the oxygen reduction reaction followed 4e^? mechanism on both 13XPtRu and 13XPt electrode. The onset of ORR on 13XPtRu electrode was shifted toward more positive potentials in comparison to 13XPt electrode.     

Determination of boron at low abundance levels in geological materials with a tetrafluoroborate-selective electrode

A modified procedure based on the tetrafluroborate-selective electrode is presented for the determination of low amounts of boron in geological materials. The main modifications are the use of hexamethylenetetramine as the buffer solution and a reduction of the temperature during measurement to 1–2°C to prevent hydrolysis; the latter precludes application of the usual method to solutions with low boron levels at very high total ionic strength. Satisfactory results can be obtained in the 10–200 μg g^?1 range (by using up to 250 mg of sample) without much sample handling. The method was tested for the analysis of terrestrial rhyolites (from Lipari/Italy, and Iceland) and tektites (thailandite, and Muong Nong-type tektites).     

Development of highly sensitive cadmium iondashselective electrodes by titration method and its application to cadmium ion determination in industrial waste water

Characteristics of cadmium iondashselective electrode made cadmium sulphide (CdS)-silver sulphide (Ag(2)S) mixture were studied. CdS-Ag(2)S mixtures were obtained by gas/solid-phase reaction between silver-cadmium mixed powder and hydrogen sulphide gas (dry method) and by ionic reaction between cadmium-silver mixed ions and sulphide ion (wet method). As a result, it was found that the CdS-Ag(2)S mixture had to be made in the condition of excess existence of sulfur and had better regulate the excess sulfur quantity minimum, for the CdS-Ag(2)S pressed membrane gave a good Nernstian response against the cadmium ion concentration change. As the best way, CdS-Ag(2)S mixture was obtained by adding sulphide ion solution to 5 mol% cadmium ion and 95 mol% silver ion mixed solution while measuring silver sulphide (Ag(2)S) electrode potential as an indicator electrode. According to the reaction was stopped when the potential variation from the initial potential in the sulphide ion solution reached at 87-116 mV which the sulphide ion concentration became 10(-3) - 10(-4) of the initial concentration, the cadmium ion membrane pressed diameter of 8 mm and thickness of 2 mm showed a Nernstian response from 10(-8) to 10(-1) M of cadmium ion concentration. Furthermore, aiming to its application for industrial waste water, masking buffer for interfering metal ions such as lead ion (Pb(2+)) and copper ion (Cu(2+)), which were possibly coexisted and to adjust total ionic strength and pH of sample was developed. The present Cd(2+) iondashselective electrode was applied to the determination of Cd(2+) in the industrial waste water. The good regression line with correlation factor of 0.984 was obtained compared with the conventional atomic absorption spectroscopy.     

Development of pH measurement system for legal traceability of pH standard solutions

The best reproducible technology of pH measurement for precise pH buffer solutions regulated by Japanese Industrial Standards (JIS) was studied. A pH meter was devised with a high resolution of +/- 0.0001 pH. An 18-bit analog-to-digital converter is used, one-bit resolution corresponding to 0.0019 mV (ca. 0.000032 pH) against an input electrode potential +/- 500 mV. Digital data were treated smoothly for some types of noise, a reproducibility of +/- 0.0002 pH being obtained with a potentiometer. A flow cell was devised to attain temperature control within +/- 0.03 degrees C and air-tight measurement prevented contamination with carbon dioxide. Also, the flow cell has a structure such that potassium chloride (KCl) inner solution effused from a ceramic junction of the reference electrode designed so as not to touch the glass membrane. A combination pH electrode (a glass electrode and a reference electrode) was assembled to minimize the dead volume of sample solution. This highly sensitive pH measuring system, consisting of a pH meter, a flow cell, a combination pH electrode, a circulating water thermostat and a peristaltic pump, was used for the certification of pH standard solutions in Japanese metrological law. The performance of this system was within +/- 0.0006 pH reproducibility and 20-30 min response time (5 min within +/- 0.0002 pH) at a sample flow rate of 3 ml min (-1).     

Square-wave anodic stripping voltammetric determination of thallium(I) at a Nafion/mercury film modified electrode

A Nafion/mercury film electrode (NMFE) was used for the determination of trace thallium(I) in aqueous solutions. Thallium(I) was preconcentrated onto the NMFE from the sample solution containing 0.01 M ethylenediaminetetraacetate (EDTA), and determined by square-wave anodic stripping voltammetry (SWASV). Various factors influencing the determination of thallium(I) were thoroughly investigated. This modified electrode exhibits good resistance to interferences from surface-active compounds. The presence of EDTA effectively eliminated the interferences from metal ions, such as lead(II) and cadmium(II), which are generally considered as the major interferents in the determination of thallium at a mercury electrode. With 2-min preconcentration, linear calibration graphs were obtained over the range 0.05-100 ppb of thallium(I). An even lower detection limit, 0.01 ppb, were achieved with 5-min accumulation. The electrode is easy to prepare and can be readily renewed after each stripping experiment. Applicability of this procedure to various water samples is illustrated.     

The behavior of a poly(aniline) solid contact pH selective electrode based on N,N,N',N'-tetrabenzylethanediamine ionophore.

A hydrogen ion-selective solid-contact electrode based on N,N,N',N'-tetrabenzylethanediamine has shown the best Nernstian slope and selectivity and the widest response range in a Tris buffered pH sample solution. Its linear dynamic range was pH 3.50-11.94, and the Nernstian slope showed 52.1 mV/pH (at 20 +/- 0.2 degrees C). When it was directly applied to human whole blood (in pH range 6.0-8.5) we could obtain the same satisfying results. This electrode continuously contacted a Tris 7.47 buffered solution, human whole blood and a hydrofluoric acid solution for one month without any loss of performance. Also, hydrofluoric acid did not influence the surface of this electrode, and thus it was maintained without showing any changes in potentials after being used in a hydrofluoric acid solution. The standard deviation in the determined e.m.f. differences was 1.5 mV (N = 5) for Tris buffer solutions of pH 6.5 and 1.1 mV at a Tris buffer solutions of pH 8.5. The 90% response time of the electrodes obtained by injecting of hydrochloric acid into the Tris buffer sample solution was less than 10 s. Especially, in the this paper, with these potential response characteristics of hydrogen ion selective poly(aniline) solid contact electrode, we have also presented the pH response mechanism of this electrode and the role of poly(aniline) and a doped anion in a poly(aniline) layer.     

电化学改性石墨电极在不同溶液中的准电容及电催化性能

通过循环电流阶跃方法制备电化学改性石墨电极(MGE), 用循环伏安(CV)法评价了MGE在酸性和中性溶液中的准电容特性, 并研究了其在HCl和HNO₃溶液中的电催化作用. 结果表明: 在酸性条件下, MGE在H₂SO₄溶液中具有佷好的准电容特性, 比电容高达1.730 F·cm^-2; 在HCl溶液中也具有很好的准电容特性, 且MGE对析氯反应具有一定的电催化活性(起始析氯电位负移238 mV), 使其电位窗变窄; 在HNO₃溶液中没有准电容特性, 而MGE对NO₃^-离子的还原反应具有极强的电催化能力. 当电介质由酸性变为中性时, 虽然其峰电流密度有所降低, 但电位窗大幅度增宽, 意味着MGE在中性体系中的能量密度比酸性体系会有较大的提高.     

Simultaneous Determination of Acetaminophen and Ascorbic Acid at an Unmodified Boron‐Doped Diamond Electrode by Differential Pulse Voltammetry in Buffered Media

A boron‐doped diamond electrode (BDDE) was used for the simultaneous anodic determination of L ‐ascorbic acid (AA) and acetaminophen (AC) in aqueous buffered media by differential pulse voltammetry (DPV). Linear calibration plots of anodic current peaks versus concentration were obtained for both analytes in the concentration range 0.01–0.1 mM with very high correlation coefficients. RSD of 2–3% and high sensitivities were obtained from DPV data in single and dicomponent systems. The potential applicability of the DPV technique associated with standard addition was illustrated by simultaneous determination of AA and AC in real sample solutions made up from pharmaceutical products.     

Natural pyrite as an electrochemical sensor for potentiometric titrations with EDTA, mercury(II) and silver(I)

The results obtained in potentiometric titrations of copper(II), mercury(II) and iron(III) with standard EDTA solutions are presented. The titration of copper(II) at pH values in the range from 8.11 to 10.99 (ammonia buffer) and the titration of mercury(II) and iron(III) at pH values from 3.59 to 5.65 (acetate buffer) were performed. The titration end-point (TEP) was detected with an indicator electrode made from natural crystalline pyrite as an electrochemical sensor. The results obtained in potentiometric titration with the pyrite electrode were compared with those obtained using a platinum electrode (Fe³⁺), a Cu ion selective electrode (Cu²⁺) and a Hg electrode (Hg²⁺). Accurate and reproducible results with good agreement were obtained, but higher potential changes at the TEP were obtained using the pyrite electrode. In the course of the titration the potential was established within less than 1 min, whereas at the TEP it was within about 2–3 min. The potential changes at the TEP were in the range from 60 to 200 mV per 0.1 ml EDTA, according to the stability constant of the complex formed. The highest potential changes, ranging from 160 to 200 mV, were obtained in the titration of iron(III) at pH 3.59. Reverse titration was also performed and accurate and reproducible results were obtained. Moreover, titration of halogenide and thiocyanate with standard mercury(II) solutions, as well as cyanide with silver(I) solution, were performed and accurate and reproducible results were again obtained. Received: 20 February 1998 / Accepted: 19 November 1999     

Potentiometric determination of bromate using an Fe(III)-Fe(II) potential buffer by circulatory flow-injection analysis.

A method for the potentiometric determination of bromate by circulatory flow injection analysis (CFIA) is described. The procedure involves the use of an Fe(III)-Fe(II) potential buffer solution, which is recycled via a reservoir. The analytical method is based on a linear relationship between the concentration of bromate and a very transient potential change in the electrode potential due to the generation of intermediate bromine during the reaction of bromate with the Fe(III)-Fe(II) potential buffer solution, which also contains NaBr, (NH4)6Mo7O24 and H2SO4. An aliquot (5 microl) of a bromate sample solution was injected into the stream of the potential buffer solution, 100 ml of which was circulated at a flow rate of 1 ml/min; the potential buffer solution stream was then returned to the reservoir after passing through a flow-through redox electrode detector. A potential change due to the reaction of the injected sample with the potential buffer in a reaction coil was measured with the detector in the form of a peak signal. The effects of the bromide, sulfuric acid and Fe(III)-Fe(II) concentrations in the potential buffer, and length of the reaction coil on the peak heights were examined in order to optimize the proposed CFIA method. The analytical sensitivities to bromate were 5.6 mV/microM for 1 x 10(-2) M and 30.9 mV/microM for 1 x 10(-3) M in the concentration of Fe(III)-Fe(II) in a potential buffer solution containing 0.35 M NaBr, 0.2% (NH4)6Mo7O24 and 1 M H2SO4. The detection limit of bromate obtained by a 1 x 10(-3) M Fe(III)-Fe(II) potential buffer solution was 0.02 microM (2.5 ppb). The numbers of repetitive determinations in which the relative sensitivities within 5% were regarded as being tolerated were ca. 4000 and 2000 for the use of only 100 ml of 1 x 10(-2) M and 1 x 10(-3) M Fe(III)-Fe(II) potential buffer solution, respectively.     

Determination of NH(3) in pyrolysis gases by ammonia selective electrode

The suitability of ion-selective electrode for the determination of ammonia in pyrolysis gases of fossil fuels was studied. The ammonia was absorbed into acidic solution and two kinds of determination methods were carried out. The ammonia was either measured directly from the acid solution, or ammonia was first released into the gas phase and then determined (air gap method) by the ammonia selective electrode. The electrode functioned well in both cases, but the linear calibration range was rather narrow, slightly more than one tenfold. The quantitative detection limit in the water phase was 5 x 10(-6)M (0.085 ppm) NH(3) and in gas phase operation solutions above 5 x 10(-4)M (8.5 ppm) NH(3) it was possible to measure quantitatively. The applications were carried out with Finnish energy peat samples and a coal sample.     

On-line coupling of sequential injection lab-on-valve to differential pulse anodic stripping voltammetry for determination of Pb in water samples

Sequential injection lab-on-valve (LOV) was first proposed for analyzing ultra-trace amounts of Pb using differential pulse anodic stripping voltammetry (DPASV) with a miniaturized electrochemical flow cell fabricated in the LOV unit. Deposition and stripping processes took place between the renewable mercury film carbon paste electrode and sample solution, the peak current was employed as the basis of quantification. The mercury film displayed a long-term stability and reproducibility for at least 50 cycles before next renewal, the properties of integrated miniature LOV unit not only enhanced the automation of the analysis procedure but also declined sample/reagent consumption. Potential factors that affect the present procedure were investigated in detail, i.e., deposition potential, deposition time, electrode renewable procedure and the volume of sample solution. The practical applicability of the present procedure was demonstrated by determination of Pb in environmental water samples.     

Application of the superposition principle to the study of multistep electrode processes and systems with several components in chronopotentiometry with programmed current. Part I

Summary In this paper we present the solution to the differential equations system corresponding to the problem of multistep processes and systems with several components when a programmed current-time function is applied to planar and spherical electrodes. The analytical solutions have been obtained in both cases, by a recurrent procedure, which facilitates the numerical determination of results. This solution has been found for an expanding spherical electrode, since by using simple substitutions it is transformed into that corresponding to a stationary spherical electrode, to an expanding plane electrode or to a stationary plane electrode.     

Response Characteristics of a Glucose Electrode with a Sensing Membrane Prepared by Plasma Polymerization

A glucose electrode was composed of a dissolved oxygen electrode and an immobilized glucose oxidase membrane prepared by plasma polymerization of propargyl alcohol as a monomer. Fairly good precision of the electrode response to sample solutions was obtained by measurements using the steady-state method or the reaction rate method. Activity of the glucose oxidase immobilized within the membrane and mounted on the electrode lasted for 50 consecutive measurements over 5 days, and, if the membrane was stored in a buffer solution of pH 7.0 at a temperature of 0°C, the activity was preserved for more than 2 months. Such immobilization of the glucose oxidase with the plasma polymer effectively suppressed interference from Cu²⁺ions, which would seriously interrupt oxidation of the glucose in homogeneous solutions, in the sample solutions.     

Determination of proteins by amperometric titration with 12-phosphotungstic acid at rotating gold electrodes

Amperometric titration of various protein fractions and human serum with 12-phosphotungstic acid at the rotating gold electrode is described. The titrant is electroreducible and its concentration may be monitored by poising an amperometric indicator electrode at a potential of -0.25 V vs. SCE. In 1.0 M hydrochloric acid, well defined titration curves of proteins may be obtained in the presence of airsaturated solutions. The relative standard deviation of replicate samples is 0.2–0.3% in the range 2–4 g of protein per 1 in the sample solution. The technique is principally applicable to homogeneous mixtures of proteins.     

Constant-current stripping analysis for iron(III) by adsorptive accumulation of its Solochrome Violet RS complex on a carbon-fibre electrode

The optimum experimental conditions with respect to pH, adsorption potential and Solochrome Violet RS concentration for the constant-current stripping determination of iron(III) have been investigated. The suggested procedure has been used for the determination of iron(III) in tap water and the accuracy investigated both by analysing standard solutions and by comparison with results obtained by atomic-absorption spectrometry. The relative precision was 6-13% and the detection limit for iron(III), set by the reagent blank, was approximately 1 mug/l. The results obtained with a mercury-film coated carbon-fibre flow electrode have been compared with those obtained with a hanging mercury drop electrode. The advantages of measuring electrode potential vs. time instead of vs. current have been evaluated.     

A study of potentiostat noise

This paper deals with a generalized version of the galvanostatic reversed coulostatic pulse method proposed by van Leeuwen and Sluyters. The potential of the working electrode is stepped to an appropriate potential by any kind of d.c. perturbation and then allowed to relax. The relaxing potential is perturbed again at an arbitrary time by a coulostatic pulse with polarity opposite to that of the first perturbation. The charge of the coulostatic pulse is adjusted so that the potential vs. time curve may have a horizontal tangent at an appropriate time after the end of the pulse. The overpotential at that time is known from the potential vs. time curves obtained with and without the coulostatic pulse. Analysis of experimental results is made on the basis of the rigorous solution for the boundary value problem of diffusion. The proposed procedure is tested with success by experiments on the trisoxalatoferrate(III)/(II) electrode and the corresponding equivalent circuit of Randles type. Its applicability to such an electrode reaction as involves an unstable product is suggested.     

The study of PVP/Pd/IrO(2) modified sensor for amperometric determination of sulfur dioxide

A novel electrochemical sensor for the detection of sulfur dioxide in both gas and solution is described. The chemically modified electrode was constructed by polymerizing (4-VP), palladium and iridium oxide (PVP/Pd/IrO(2)) onto a platinum microelectrode which exhibits excellent catalytic activity toward sulfite with an oxidation potential of +0.50 V. The SO(2) gas sensor is based on the PVP/Pd/IrO(2) modified electrode as working electrode, Ag/AgCl electrode as reference electrode, Pt electrode as counter electrode and a porous film, which is in direct contact with the gas-containing atmosphere. The effect of different internal electrolyte solutions of hydrochloric acid, sulfuric acid, phosphates buffer solution, mixed solution of dimethyl sulfoxide and sulfuric acid to the determination of SO(2) was also studied. The sensor was found to have a high current sensitivity, a short response time and a good reproducibility for the detection of SO(2). It has good potential to be used in the field of environmental monitoring and controlling.     

Determination of fluorine and chlorine in geological materials by induction furnace pyrohydrolysis and standard-addition ion-selective electrode measurement

Fluorine and chlorine in geological materials are volatilized by pyrohydrolysis at about 1150 degrees in a stream of oxygen (1000 ml/min) plus steam in an induction furnace. The catalyst is a 7:2:1 mixture of silica gel, tungstic oxide and potassium dihydrogen phosphate. The sample/catalyst mixture is pyrohydrolysed in a re-usable alumina crucible (already containing four drops of 1 + 3 phosphoric acid) inserted in a silica-enclosed graphite crucible. The absorption solution is buffered at pH 6.5 and spiked with 1.6 mug of fluoride and 16 mug of chloride per g of solution, to ensure rapid and linear electrode response during subsequent standard-addition measurement. The simple plastic absorption vessel has 99.5% efficiency. The 3s limits of detection are 5-10 mug/g and 40-100 mug/g for fluorine and chlorine respectively. The procedure is unsuitable for determining chlorine in coal.     

浓度同倍数递减离子选择电极标准加入法

本文报道了试液和标液浓度同倍数递减标准加入法。该法可测定几个体积均为V_x而浓度以C_x/N方式递减的试液(1≤N≤2),每个试液加等体积标液1次,在半对数算图上,各试液加入标液后的电位对原始试液电位差ΔE_N与浓度递减倍数N之间有线性关系,由交点N⁰解C_x=C_sV_s/[(N⁰-1)V_x+N⁰V_s]方程求得分析结果。该法不必测定电极的能斯特斜率。