Application of SQUAD to the refinement of formal potentials from coulometric steady-state and spectrophotometric measurements

The application of SQUAD to the refinement of formal potentials from potentiometric or coulometric steady-state and spectrophotometric measurements was tested. The formal potential thus obtained for the Fe(CN) (3-)(6)Fe(CN) (4-)(6) couple in 0.5 M H(2)SO(4) agreed with published data. This method was found to be satisfactory for the determination of typical standard potentials and it is suggested that it may be advantageous when dealing with multi-component systems.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part III. Potential dependent current

The potential dependence of the steady-state current at a channel electrode has been calculated approximately. Six regions of behavior can be described from a consideration of the characteristic times of diffusion, fluid flow and electrochemical reaction. Wave shapes and half-wave potential shifts are as expected for amperometric cells under all values of the heterogeneous reaction rate constant, and for reversible electrochemical reactions in a coulometric cell. The wave adopts a peculiar shape for irreversible reactions at a coulometric electrode. This is caused by the inapplicability of the concept of mass transfer control in these systems.     

Determination of Nitrates in Water Samples by In‐Electrode Coulometric Titration in Copper Coated Porous Vitreous Carbon Electrode

Nitrate in water samples was determined by in‐electrode coulometric titration in porous electrode made of vitreous carbon particles coated with copper. The sample was mixed with diluted sulfuric acid containing 1 mmol/L hydrochloric acid, the solution was filled into the cell and electrode and the nitrate ions were directly reduced by constant current to ammonium ions. The stoichiometry of the electrode reaction was found by coulometric and photometric measurements. The detection limit and precision were found to be 0.2 mg/L and 1.7 %, respectively. The interfering effect of high chloride contents was eliminated by precipitating chlorides with silver sulfate. The method was applied for the analysis of various water samples and beverages. The results were in good agreement with data from isotachophoretic and photometric measurements.     

Analysis of some electrochemical processes related to coulometric titrations at constant current

By considering the equilibrium potentials for the various electrode reactions which may occur in a coulometric titration, it is possible in certain cases to shorten the experimental exploratory work involved in the development of a new titration. The titration of trivalent arsenic by generated bromine and of dichromate by generated monovalent copper are treated as examples. Possible consequences of the irreversibility of electrode processes are examined. An analysis of the stability of the various chloro-complexes of mono- and di-valent copper is made in connection with the study of coulometric titrations involving generated monovalent copper     

Controlled potential coulometry: the application of a secondary reaction to the determination of plutonium and uranium at a solid electrode

A method is described for the simultaneous determination of plutonium and uranium in mixed oxides by controlled potential coulometry at a gold working electrode in two stages: first a coulometric oxidation, at 0.73 V vs. a silver/silver chloride electrode, of Pu(III) and U(IV) to Pu(IV) and U(VI) by a combination of a direct electrode reaction and a secondary chemical reaction proceeding concurrently, and secondly, a coulometric reduction at 0.33 V of Pu(IV) to Pu(III), leaving uranium as U(VI). The determination is carried out in a mixture of sulphuric and nitric acids, and Ti(III) is used to reduce plutonium and uranium to Pu(III) and U(IV) before electrolysis. The precision (3sigma) of Pu:U ratio results obtained from mixtures containing about 30% and 2% plutonium was 0.5% and 1-5% respectively. The effect of experimental variables on the time taken to complete the coulometric determination is discussed.     

Background current elimination in thin layer ion-selective membrane coulometry

A promising method for the elimination of undesired capacitive currents in view of realizing a potentially calibration free coulometric ion detection system is presented. The coulometric cell is composed of a porous polypropylene tube doped with a liquid calcium-selective membrane and a silver/silver chloride wire as an inner electrode, forming a thin layer sample between wire and tubing. The total charge passed through the system during potential controlled electrolysis of the thin layer sample is indeed found to be proportional to the amount of calcium present, but non-Faradaic processes do contribute to the obtained signal. We introduce here a multi-pulse procedure that allows one to perform a second excitation pulse at the same excitation potential after exhaustive ion transfer voltammetry of calcium has taken place. The intercept of the calibration curve after background subtraction is found as 20.6 ± 0.6 μC, significantly lower than the value of 54.1 ± 0.8 μC for the uncorrected curve. Changes in sample temperature (from 23 °C to 38 °C) did equally not affect the background corrected coulometric readings, supporting that the procedure renders the readout principle more robust.     

Pulse Enhanced Electrosorptive Detection of Ions at Silver in Ion Chromatography

Abstract

Differential double-layer capacitance measurements applied to the silver-aqueous interface may serve as the basis of detection for surface-active ions in ion chromatography. There is a caveat to this approach, though, and that is that the electrode is subject to gradual fouling due to the accumulation of adsorbed matter. We report here on the incorporation of potential pulsing sequences to counter this damaging effect. Two approaches have been taken: 1) inclusion of a periodic potential cleaning pulse in the conventional, fixed-potential double-layer capacitance measurement scheme, and 2) utilization of a potential-pulse/coulometric measurement technique. the potential-pulse/coulometric procedure entails measurement of the change in charge required to charge the electrical double layer in response to specific adsorption of analyte. Its response-concentration behavior is similar to that observed for differential capacitance detection. the use of a combined, dual-potential (sequential) capacitive/amperometric detection scheme that provides unique complementation to either amperometric or capacitance detection is also discussed.     

Determination of salbutamol in human plasma and urine by high-performance liquid chromatography with a coulometric electrode array system

A method is developed to determine salbutamol in human plasma and urine using high-performance liquid chromatography (HPLC) with a coulometric electrode array system, based on the electrochemical behavior of salbutamol at graphite electrode. The mobile phase component A is 30 mM sodium dihydroxy phosphate-30 mM triethylamine and is adjusted to pH 6.0 with 20% phosphate acid. The mobile phase component B is methanol. The optimized mobile phase composition was A and B in the proportion of 90:10 (v/v). Paracetamol is selected as the external standard. The human plasma and urine samples are pretreated using solid-phase extraction cartridges (Sep-Pak Silica), and the eluting solution is monitored by the coulometric electrode array system. The electrode potentials are set at 300, 400, 550, and 650 mV, respectively. Calibration curves show good linearity, and the recovery of salbutamol proves to be constant and unaffected by the concentration of the drug. This method, developed using HPLC-electrochemical detection, is reproducible and sensitive enough for the determination of salbutamol in human plasma and urine.     

Polarographic behaviour and determination of spironolactone

The polarographic behaviour of spironolactone in Britton-Robinson buffers containing 40% methanol as a solubilizer was studied. Over the pH range 3-12 a cathodic wave was produced. The wave was characterized as irreversible, diffusion-controlled and partially affected by adsorption phenomena. The number of electrons involved in the reduction was found by coulometric measurements at a controlled potential. A method was developed for the determination of spironolactone in tablets and the results obtained were in agreement with those obtained by the B.P. 1988 method. The mechanism of the electrode reaction is discussed.AvH scholar: 1989–1991     

Successful preparation of metabolite of troglitazone by in-flow electrochemical reaction on coulometric electrode

A simple, rapid and efficient system utilizing a coulometric electrode was developed for the preparation of drug metabolites. Trace amounts of reactants are usually generated in electrochemical reactions, which are not suitable for the sufficient preparation of products to obtain NMR and other spectral data for chemical structure confirmation or to obtain data from pharmacological activity screening tests of products. In the developed system, called the "in-flow electrochemical reaction system," a drug, troglitazone, was dissolved in a volatile flow solvent, and pumped into a coulometric electrode under optimized conditions, and the effluent was evaporated. Without any further purification, milligram amounts of a pure oxidation product of troglitazone could be obtained within several hours. The amount obtained was enough for (1)H- and (13)C-NMR analysis by which the structure could be confirmed and was found to be identical to one of the metabolites of troglitazone detected in human plasma. This system will be useful to prepare standard compounds of the required amount for pharmacokinetic study and for toxicokinetic study.     

Flow-injection potentiometric determination of residual chlorine in water

Residual chlorine is measured in water by using a potentiometric system composed of an iodide-selective electrode and a platinum electrode sensing the iodine-iodide ratio. When the sample is added to acidified iodide solution, the cell response is in a logarithmic relation to the iodine concentration which in turn depends on the concentration of residual chlorine. In the flow injection system evolved, 0.1–5.0 mg l^-1 residual chlorine can be determined at a rate of 40–60 samples per hour. The results of potentiometric determinations of residual chlorine in tap water compared to spectrophotometric results suggest that the presence of various organic substances is responsible for discrepancies between these measurements.     

Determination of pipecuronium bromide and its impurities in pharmaceutical preparation by high-performance liquid chromatography with coulometric electrode array detection

A sensitive and selective HPLC method with coulometric electrode array detection for the determination of pipecuronium bromide and its four impurities has been developed. The coulometric electrode array detection at increasing potentials from +300 to +900mV of the porous graphite electrode versus the palladium reference electrode was used. The limit of detection and quantitation for pipecuronium bromide was 8 and 25ngml(-1), respectively. This elaborate method for the simultaneous analysis of pipecuronium bromide and its impurities proved to be fast, precise, accurate, sensitive, and could be applied to analysis in substances and in pharmaceutical preparations.     

Electrochemical Investigation of Metal-Free and Nickel-Containing Porphyrazines Carrying Eight Tosylaminoethylthia Groups

 In this study, the electrochemical properties of metal-free and nickel-containing porphyrazines with eight tosylaminoethylthia groups were investigated using cyclic voltammetry, double potential step chronoamperometry, double potential step chronocoulometry, and controlled potential coulometry. Cyclic voltammetry measurements showed that both compounds exhibit two quasi-reversible and an irreversible reduction waves. The first electron transfer reaction was followed by an irreversible chemical reaction, the second one by a reversible chemical reaction for both species. The electrode processes of metal-free and nickel-containing porphyrazines are diffusion controlled, but the double potential step chronocoulometry measurements indicated that the nickel porphyrazine is adsorbed at the electrode. However, a small adsorption current has no significant effect on the mass transport mechanism of the system. Diffusion coefficients of both compounds were determined by both cyclic voltammetric and chronocoulometric measurements. The diffusion coefficients of the reduced forms of the porphyrazines were found to be smaller than those of the neutral forms.     

以多孔碳毡为工作电极的流动注射库仑检测器的研究及应用

本文设计了一个以多孔碳毡为工作电极的库化检测器并将其于用流动注射分析,以抗坏血酸、氢醌,没食子酸及二氧化硫为电活性物质检验该检测器的电化学特性。详细地研究了该检测器的库仑转换效率,选择性,线性范围,检测限及质量转移系数,并将该检测器作为双检测器流动注射分析系统中的“清除”电解池,用于酒中二氧化硫的测定,其结果与常规的吹气－氧化法非常吻合。     

The coulometric titration of bromate with cuprous copper

A coulometric procedure has been developed by which 18 to 240 μg quantities of bromate may be determined to within 0.3 μg. The bromate is allowed to react with bromide in an acid solution and electrolytically generated cuprous copper is used for the titration. A dual platinum electrode indicator system is used in an amperometric end-point procedure     

Coulometric-acidimetric titrations in anhydrous acetic acid

A technique for the coulometric generation of hydrogen ions in an acetic acid-acetic anhydride-sodium perchlorate electrolyte by anodically oxidizing a mercury electrode is described. Accurate coulometric titrations of sodium acetate and potassium hydrogen phthalate have been carried out. The technique should be applicable to many of the basic substances which can be titrated volumet-rically in acetic acid.     

The electrochemistry of proteins and related substances part III. Bovine serum albumin

The electroreduction of bovine serum albumin (BSA) in pH 7.4 solution was studied at the hanging mercury drop electrode by double potential step chronocoulometry and cyclic voltammetry, and at a mercury pool electrode by controlled potential coulometry. The proposed mechanism involves reduction of an adsorbed monolayer of BSA (maximum coverage 3.3 μC^?2); at short times 3 or 4 disulfide bonds are reduced, the product remains adsorbed and may be reoxidized. On the coulometric time scale (hours) 8 or 9 of the total 17 disulfide bonds are broken and an insoluble product which cannot be oxidized is formed.     

Use of glassy carbon as a working electrode in controlled potential coulometry

Glassy carbon has been used as the working electrode in controlled potential coulometry. The results of coulometric investigations of chromium, copper, iron, uranium and neptunium are compared with results obtained with platinum or mercury working electrodes. The accuracy of results with the glassy carbon electrode compares favourably with the results obtainable with the other electrodes, but the precision is poorer.     

Electrochemical Investigation of Metal-Free and Nickel-Containing Porphyrazines Carrying Eight Tosylaminoethylthia Groups

Summary.  In this study, the electrochemical properties of metal-free and nickel-containing porphyrazines with eight tosylaminoethylthia groups were investigated using cyclic voltammetry, double potential step chronoamperometry, double potential step chronocoulometry, and controlled potential coulometry. Cyclic voltammetry measurements showed that both compounds exhibit two quasi-reversible and an irreversible reduction waves. The first electron transfer reaction was followed by an irreversible chemical reaction, the second one by a reversible chemical reaction for both species. The electrode processes of metal-free and nickel-containing porphyrazines are diffusion controlled, but the double potential step chronocoulometry measurements indicated that the nickel porphyrazine is adsorbed at the electrode. However, a small adsorption current has no significant effect on the mass transport mechanism of the system. Diffusion coefficients of both compounds were determined by both cyclic voltammetric and chronocoulometric measurements. The diffusion coefficients of the reduced forms of the porphyrazines were found to be smaller than those of the neutral forms. Received October 29, 2001. Accepted (revised) December 17, 2001     

Coulometric sensors,the application of a sensor-actuator system for long-term stability in chemical sensing

We present a new type of chemical transducer, the coulometric sensor. This sensor is in fact an integrated sensor-actuator system that is able to measure the concentration of acids and bases by means of coulometric titration. An ISFET is used as the sensor to monitor the pH changes induced by the actuator, which is a gold electrode that fits closely around the ISFET's gate area. Coulometry is an absolute method and therefore the output of the new sensor is only determined by its dimensions and is not subject to changes in offset and sensitivity of the indicator electrode. It is thus expected that the operation of the sensor will be stable for a long time, so that only a one-time calibration is needed. As a first example of this new class of chemical transducers, a carbon dioxide sensor is presented. It is shown that the stability is some orders of magnitude better than that for a ‘classical” potentiometric sensor.