Fast sweep a.c. polarography

Summary The design of a fast sweep a.c. polarograph suitable for recording second harmonic and intermodulation polarograms is described. The instrument was found suitable for determining reducible species down to concentrations of approximately 2×10^–7 M. By integrating the faradaic response a considerable improvement in precision can be obtained.

---

Fast-Sweep-Wechselstrompolarographie

Zusammenfassung Ein Polarograph wird beschrieben, der zur Aufnahme von Polarogrammen mit Hilfe der 2. Harmonischen und der Zwischenmodulation geeignet ist. Reduzierbare Substanzen können bis herab zu 2 · 10^–7 M erfaßt werden. Durch Integration des Faradaystroms kann eine beträchtliche Verbesserung der Genauigkeit erreicht werden.

Lecture presented at Euroanalysis I Conference, 28. 8.–1. 9.1972 in Heidelberg, Germany.     

Anodic a.c. polarography of methylthymol blue

The anodic a.c. polarographic properties of methylthymol blue (MTB) were investigated in the supporting electrolyte of potassium nitrate containing buffer. The difference in the peak potentials of MTB and EDTA is a good measure of the difference in the stabilities of both chelates. The stability constant of mercury(II)-MTB complex was successfully determined on the basis of that reported for the mercury(II)-EDTA complex.     

Analysis of metal alloys by second-harmonic phase-selective a.c. voltammetry

Second-harmonic phase-selective a.c. voltammetry and second-harmonic a.c. anodic stripping voltammetry are shown to be particularly suitable for simultaneous determinations of elements having very close half-wave potentials (differences of <50 mV). The technique are applied to real matrices. Samples of standard materials BCS 207/2 Gunmetal, SRM 631 SPectrographic Zinc Spelter and SRM 899 (Nickel-base High-temperature Alloy) were digested with a sulphuric acid/nitric acid mixture; after dissolution, the samples were taken up in hydrochloric acid. After adjustment to 1 mol l^?1 HCl, the solutions were used directly for voltammetric measurements of Pb(II)/Sn (II), Sb(III)/Bi(III), Pb(II)/Tl(I) and In(III)/Cd(II) as appropriate. The confidence intervals of the experimental data were in agreement with the certified values for each element. Both the accuracy, expressed as percentage error, and precision, expressed as relative standard deviation, were better than 5%. The standard addition technique was found to improve the resolution of the a.c. voltammetric peaks even in the case of severe overlapping.     

Resistance effects in phase sensitive fundamental harmonic a.c. polarography

Equations relating the effect of cell resistance to the background current in phase sensitive fundamental harmonic a.c. polarography have been derived. Under normal experimental conditions the background current was found to be proportional to the specific resistance of the supporting electrolyte and the squares of the a.c. signal frequency and electrode double layer capacity.     

Instrumental advantages obtained with short controlled drop-time a.c. polarography

The performance of short drop-time a.c. polarography has been examined in detail, and found to be better than that for natural drop-times in almost every respect. Uncompensated resistance terms are smaller and potentiostat stability is improved. In addition, faster potential scanrates and more rapid data acquisition are possible. A variation of the drop-time over at least three orders of magnitude is possible and this, coupled with excellent instrumental performance, should offer considerable scope in studies of electrode kinetics.     

Determination of some methylcarbamate insecticides by a.c. polarography and cyclic voltammetry

The study of adsorption-desorption phenomena represents an important extension to the range of organic compounds that can be determined by electrochemical methods. The present work has utilized this approach for the determination of a range of methylcarbamate insecticides. The tensammetric behaviour of this class of compound at the mercury electrode has been investigated by a.c. polarography and the optimum experimental conditions for their analytical determination have been derived. Cyclic voltammetry has also been used to study the electrode process and by using the peak obtained on the cathodic sweep it was possible to extend the limit of detection to the ppm level.     

Determination of saccharin in electroplating baths by differential pulse and a.c. polarography

Saccharin can be determined in palladium, gold and nickel electroplating baths by differential pulse polarography or second-harmonic a.c. polarography after extraction with an ethyl acetate/carbon tetrachloride mixture and after masking the noble metal with cyanide. Detection limits lie between 0.025 and 0.005 g l^-1. Bath components such as chloride, nitrate, sulfate, phosphate, glucose, citrate, Fe³⁺, Sn⁴⁺, Cr³⁺, Ni²⁺ and Pb²⁺ do not interfere.     

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.     

Determination of germanium(IV) in perchloric acid solution by a.c. polarography and differential pulse polarography in the presence of 3,4-dihydroxybenzaldehyde

Germanium(IV) at trace levels can be determined in perchloric acid solution containing 3,4-dihydroxybenzaldehyde by alternating current and differential pulse polarography with dropping mercury and hanging mercury drop electrodes. The results are comparable by the two methods. The possible interferences of some elements are discussed. The serious interference of lead(II) can be prevented by addition of EDTA in hydrochloric acid medium. Interference of selenium can be avoided by precipitation with potassium iodide. The method is applied to silver—germanium alloys.     

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.     

D.c. polarography: Current-potential curves for electrode processes involving a preceding first-order chemical reaction

An explicit equation, which is valid without any limitation in the magnitude of rate constant of the chemical reaction involved, is derived in a rigorous way, for the d.c. polarographic instantaneous (and average) current-potential curves with a c.e. mechanism accompanied by nernstian behaviour. On the basis of these equations, the range of validity of the steady state approximation is discussed.     

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.     

Simultaneous measurement of the in-phase and quadrature components of the signal in a.c. polarography using multiplier circuitry

The use of multiplier circuitry is shown to provide a simple way of simultaneously measuring the in-phase and quadrature (phase-angle) components of the signal in alternating current polarography. When used with previously developed circuitry an extremely versatile instrumental approach is available for detecting the second harmonic and intermodular (frequency difference) components in addition to the in-phase and quadrature components of the fundamental harmanic.     

Automatic polarographic elucidation of electrode mechanisms by means of a knowledge-based system : Part 1. Sampled d.c. polarography

A knowledge-based system has been developed for the automatic elucidation of electrochemical mechanisms. The system is based on sampled direct current (or Tast) polarography at a dropping mercury electrode as a technique for collecting experimental information and consists of a general expert system shell for the reasoning process, the specific set of rules and experimental modules. The set of rules allows the elucidation of eight relatively simple electrode reaction mechanisms fully atomatically. The computer system has been validated with chemical systems the electrochemical behaviour of which is well established. All parts of the program are written in FORTH language for Apple II microcomputers. This expert system has an open character and new rules can be added to extend the set of mechanisms that can be determined.     

Comparison of surface activity of organic compounds by the influence of surfactants on reduction peaks by a.c. polarography

Summary Surface activity of closely related organic compounds are compared by utilising the optimum concentrations corresponding to more negative peaks and concentrations required to remove or to suppress less negative reduction peak due to organic compound and due to metal ion to maximum extent instead of tensammetric peak by a. c. polarography. Surface activity measurements are p_H dependent. The correlation between optimum concentrations of surface-active substance and the amounts of reducible species would be of great significance in suppressing the polarographic maxima in d. c. polarography.

---

Zusammenfassung Es wurden die Oberflächenaktivitäten eng verwandter organischer Verbindungen durch tensammetrische Studien mit der Methode der Wechselstrompolarographie bestimmt.