Instrumentation for charging current compensation in d.c. polarography

It is shown that the non-linear charging current in d.c. polarography can be fairly well compensated by a method (suggested by Barker), which consists in applying a small a.c. voltage to the polarographic cell, rectifying the a.c. response, integrating it and subtracting a suitable fraction from the total d.c. response. An instrument incorporating such an automatic compensation for the nonlinear charging current is described. Results obtained with this instrument are presented to show that the limit of measureable concentrations is about 10^?7M for reversible waves and the precision is ±2.5 to ±3.0%. It is possible to obtain, with the instrument described here, well defined polarograms even when the supporting electrolyte concentration is low.     

Alternating current polarography of some complexes in presence of ionic surfactants

Well defined a.c. polarographic waves are obtained in the presence of certain ionic surfactants. The current is brought about by an electron-transfer through the adsorbed layer, probably by the formation of a bridge which accelerates the electrontransfer between the electrode and the depolarizer. The resulting a.c. wave is accompanied by a d.c. polarographic step and the height of the wave increases proportionally to the bulk concentration of the depolarizer. In contrast to ordwary a.c, waves, the a.c. current of these waves increases linearly with increasing height of the mercury column above the capillary.     

Contribution à l'électrochimie des thiols et disulfures : Partie IV. Polarographies d.c., a.c. et impulsionnelle différentielle de la cystéamine et de la cystamine

The electrochemical behaviour of the cysteamine—cystamine system was investigated by the d.c.a.c. and d.p.p. techniques, as functions of concentration and pH. Adsorption phenomens were observed, and the cystéamine anodic and the cystamine cathodic waves were found to be diffusion-controlled. Separation of these two sulfur amino acids was achieved; 6 mg l^-1 of cysteamine HCl can be determined in the presence of 11 mg l^-1 of cystamine dihydrochloride. Detection limits are 5 ·1O^-7 M for cysteamine and 1 · 10^-6M for cystamine; as little as 0.012 mg of the thiol and 0.23 mg of the disulfide per litre can be determined. The polarographic behaviour of the cystamine—cysteamine system resembles that described previously for the cysteine— cystine system.     

On the shape of the reversible d.c. polarogram complicated by reactant and/or reaction product adsorption at the surface of the dropping mercury electrode

An elaboration is made of the consequences of reactant or product adsorption for the shape and situation of d.c. polarographic waves at the dropping mercury electrode. The treatment is based on the diffusion-layer model. As an example the Langmuir isotherm is adopted, but the results will be representative in a more qualitative way also for other isotherms. Detailed features of the polarograms under various circumstances are given. It is concluded that the polarograms can be analyzed precisely if only one component is adsorbed. If both components are adsorbed, it may occur that the information concerning the adsorption cannot be obtained from the polarogram. The adsorption may even seem to be absent.     

Controlled potential electroanalysis

Some fundamental principles involved in controlled potential electroanalysis are discussed. A potentiostat is described which automatically maintains the potential of a working electrode constant during an electrolysis. Cells for different applications of controlled potential electrolysis are described. Some typical applications are reviewed, including electrogravimetric determinations of metals, electrolytic separation of metals with mercury and platinum cathodes prior to polarographic analysis, coulometric analysis, identification of the oxidation states that correspond to polarographic waves, and electrolytic preparation of organic and inorganic compounds.     

The development of a fully computerized system for sampled d.c. polarography with standard interfacing

A complete system, based on the online PDP-11 computer (Digital Equipment Corporation) was developed for computerized sampled d.c. polarography with direct digital control. The system includes compensation of ohmic cell resistance and processing of the polarographic data. The accuracy of the system in the determination of the various polarographic parameters is: diffusion current ± 2 %, half-wave potential ± 2 mV, and slope of the log plot ± 2 mV.     

Admittance measurements of adsorption properties of 2,2′-bipyridine complexes: Part II. Iron(II), cobalt(II) and nickel(II) complexes inacetonitrile

Whereas trisbipyridine complexes of iron(II), cobalt(II) and nickel(II) yield successiveone-electron and reversible waves in d.c. polarographic measurements in acetonitrile, the admittance measurements indicate clearly that the charge transfer reaction is coupled with adsorption. Various tests for the detection of adsorption were used and the extent of adsorption of reactant or products in relation with charge transfer kinetics is discussed.     

Estimation of tungstate ion by tensammetry

It is shown that tungstate ion can be estimated quantitatively by the same experimental technique as that used for alternating current polarography. The underlying process differs, however, fundamentally from polarography in that no electron transfer takes place at the electrode and thus no d.c. polarographic steps arc obtained. The alternating current is produced solely by adsorption/ desorption processes at the electrode/solution interface, a phenomenon to which the name “tensammetry” has been given.The current-voltage curve for sodium tungstatc at pH 1.8 shows two -well defined tensammetric waves which can be used for purposes of quantitative estimation. No waves are obtained at pH values higher than 6, showing that the compound responsible for the production of the waves is not the simple WO₄^-4ion.     

Contribution a l'electrochimie des thiols et disulfures : Partie III. Polarographie d.c., a.c., et polarographie differentielle a impulsions de l'acide thiobarbiturique et du pentothal sodique

D.c., a.c. and differential pulse polarography were used to elucidate the electrochemical behaviour of thiobarbituric acid and sodium pentothal.Important adsorption phenomena were shown, particularly for thiobarbituric acid. Considerable differences were found in the polarographic curves of the two compounds. The number of waves obtained depends on the concentration and the pH for both compounds.     

Electrochemical study of triphenyltin piperidyl dithiocarbamate

The reduction of triphenyltin piperidyl dithiocarbamate in acetone has been investigated using d.c. polarography and cyclic voltammetry. Polarographic reduction yielded four well-defined waves, two of which were diffusion-controlled and two of which appear to represent adsorption processes. The cyclic voltammetric study yielded results in close agreement with the polarographic data. Results appear to indicate the release of the dithiocarbamate moiety, followed by reduction to form the triphenyltin radical, which undergoes dimerization, as well as reduction to the triphenyltin anion. For comparison, the polarographic reduction of triphenyltinchloride was investigated. A mechanism similar to that proposed by earlier authors for the polarographic behaviour of tributyltin oxide was found. For convenience, the more commonly used terminology of piperidyl dithiocarbamate is used throughout the paper, in place of (piperidine-1-carbodithioato) triphenyltin (IV).     

Electrochemical Reduction of Metronidazole and Its Determination in Pharmaceutical Dosage Forms by D.C. Polarography

Abstract

A simple d.c. polarographic method has been developed for the determination of metronidazole in dosages forms. In Robinson - Britton buffer (pH 4.38) and in presence of 1.60 × 10^?3 % Triton X-100, the drug produced a well defined 4-electron polarographic wave followed by another wave of about half the height of the first wave. The current is proportional to the concentration and permits the drug to be determined by d.c. polarography in the concentration range 5.0 × 10^?5 -7.0 × 10^?4 M. Results obtained by the proposed method are in excellent agreement with that provided by the USP-XX method. A rapid, sensitive and accurate polarographic method for the determination of metronidazole in the tablets which are produced locally is proposed.     

Influence of anion-induced adsorption on D.C. polarography of metal ions

Some adsorption isotherms for anion-induced adsorption of metal ions are proposed. The influences of adsorption on d.c polarograms are examined theoretically. The consequences for the determination of stability constants from polarographic data are discussed.     

A polarographic study of Cd(II)-NTA complexes in alkaline solution

Up to four d.c. polarographic waves were observed on voltammetric curves of Cd(II)-nitrilotriacetate complexes on the DME in the pH range 7.0–11.5. These polarograms are analyzed using a non-linear least squares method and the mechanism of the electrode reactions and prior homogeneous reaction are discussed. The kinetic parameters of the separate reactions are calculated and compared with literature values. The accelerating influence of OH^? ions on the dissociation of the complexes is established.     

The Use of Differential Pulse and D.C. Polarography in the Analysis of Solutions Containing Surfactants

Abstract

Peak currents in differential pulse polarography (DPP) are often used for quantitative analysis. In the presence of surfactants, peak currents depend not only on the concentration of the electroactive species, but also on that of the surfactant. In the presence of surfactants of biological origin, limiting currents obtained by d.c. polarography are often less sensitive to the effects of surfactants and are more suitable for quantitative analysis than DPP peak currents. Effects of surfactants on DPP curves were demonstrated for 4-nitrobenzoic acid in the presence of oligomers resulting from washing lignin preparations with water. In the study of adsorption of nitro compounds on lignin, d.c. polarographic curves yielded reliable adsorption isotherms whereas DPP peak currents indicated erroneously much stronger adsorption of nitro compounds on lignin.     

Microcomputer-controlled polarographic instrumentation and its use in the determination of stability constants of crown ether complexes

A computer-controlled polarographic system is described, based on a commercially available polarograph interfaced to a microcomputer. Experiments are controlled and monitored entirely from software, including automatic evaluation of the Tast polarograms and addition of solutions to the polarographic cell from a motor burette. The program was written in FORTH, a computer language especially apt for laboratory automation. The system is used in the determination of stability constants of crown ether complexes.     

Polarographic determination of hydroxylamines: application to analysis of photographic processing solutions

Procedures for the determination of hydroxylamine and N,N-diethylhydroxyIamine (DEHA), based on anodic polarographic waves, are described. The importance of using a strongly alkaline supporting electrolyte and of complete removal of dissolved oxygen is illustrated. With rapid alternating current (a.c.) polarography, 3 × 10^-6 M hydroxylamine and 4 × 10^-5 M DEHA can be detected. Detection limits with the differential pulse technique are approximately tenfold lower. In a practical application, rapid a.c. polarography is shown to be suitable for the direct determination of hydroxylamine and DEHA in photographic processing solutions. The only pretreatment of samples is dilution with a strongly alkaline supporting electrolyte. Possible interferences from other constituents of the processing solutions are avoided by using the standard addition method.     

Simultaneous polarographic analysis of pyrazine and its methyl derivatives by iterative target transformation factor analysis

The polarographic waves of pyrazine and its methyl derivatives are seriously overlapping, so they cannot be determined individually by polarographic methods without a prior separation. In this paper, a chemometric approach, iterative target transformation factor analysis (ITTFA), is developed and applied to the determination of mixtures of pyrazines at trace level (2.0–9.0 × 10⁻⁶moll⁻¹) by using differential pulse polarography (DPP) and a static mercury drop electrode (SMDE). Different from the general ITTFA method, only one-dimensional measurement data of n − 1 standards and an unknown were used in this work. It produced acceptable results with average recoveries in the 96–108% range and relative standard errors in the 3.4–9.5% range.     

Polarographic behaviour and determination of paracetamol and salicylamide after treatment with nitrous acid

A polarographic procedure is described for the determination of paracetamol (acetaminophen) and salicylamide after treatment with nitrous acid. The different experimental parameters affecting the derivatization process and the polarographic analysis were studied. The derivatization products were found to be reduced at the dropping mercury electrode over the whole pH range in Britton-Robinson buffers. At pH 7.0, well defined diffusion-controlled cathodic waves were produced for both compounds. Plots of limiting current vs. concentration were linear over the ranges 0.05–0.75 and 0.25–1.5mM for paracetamol and salicylamide, respectively, in the d.c. mode, with minimum detectability of 2.5 × 10^–6 and 1.25 × 10^–5 M, respectively. The procedure was applied to the analysis of some pharmaceutical dosage forms and the results were in good agreement with those obtained by official and compendial methods.     

Kalman filtering for the evaluation of the current-time function in d.c. polarography

Kalman filtering was applied to the current vs. time data obtained at the growing mercury drop of a DME under d.c. polarographic conditions, to separate the faradaic and capacitive components of the electrode current. Polarograms consisting of the pure faradaic current vs. applied d.c. potential were subjected to a four-parameter curve-fitting procedure to obtain the polarographic characteristics, viz. half-wave potential, limiting current and slope of the log plot together with the baseline current. The method was tested with cadmium and zinc in the 10(-6)-10(-5)M range. The standard deviations of the half-wave potentials and the limiting current/concentration ratios were found to be 1.0 mV and 0.04 respectively.     

Desorption Preceding Polarographic Reduction of Some Acetophenone Derivatives in Alkaline Media

Abstract

Change in d.c. polarographic capacity currents, in a.c. polarographic curves, in electrocapillary curves, and in oscillographic dE/dt = f(E) curves indicate that ω-benzyl and ω-phenyl acetophenones are reduced at potentials at which their oxidized form is already desorbed.