A.M. Bond

Influence of anion-induced adsorption on half-wave potentials and other polarographic characteristics

Summary Previously observed differences between accepted and apparent polarographic stability constants for some systems containing adsorbable ligands have been confirmed by d.c. and a.c. polarography. The same discrepancy is not observed with amalgam potentiometry. This behaviour is believed to result from anion-induced adsorption of the depolarizer and at least partial subsequent reduction in the adsorbed state. Both the d.c. and a.c. polarographic behaviour of such systems has been shown to be characterized by subtle departures from normal Nernstian behaviour although not in the sense usually attributed to kinetic irreversibility. Detailed study of the lead(II)-bromide and-iodide and some thallium(I) systems are reported. Maxima, depletion and other unusual effects also attributable to specific adsorption of the depolarizer are evident.

The inequivalence of apparent polarographic and potentiometric stability constants for cadmium(II) bromide and iodide systems

Investigations of the “classical” cadmium(II) bromide and iodide complex ion systems show conclusively that in the presence of moderately strong anion-induced adsorption, apparent polarographic stability constants are larger than those determined by other methods such as potentiometry. Such systems can exhibit seemingly normal d.c. polarographic behaviour, but adsorption is readily detected by a.c. methods. The authors therefore strongly recommend that in systems in which adsorption is detected, polarography should not be used to determine stability constants. Theoretical approaches to explain the observed phenomena are discussed and it is concluded that apparently differing approaches that have appeared in the literature are in fact equivalent.

On-line FFT faradaic admittance measurements application to A.C. cyclic voltammetry

A measurement approach is described and data are presented which demonstrate the ability to effect a.c. cyclic voltammetric measurements with the on-line digital FFT approach to faradaic admittance data acquisition. The equipment utilized enables complete faradaic admittance spectra to be obtained at an effective spectrum acquisition rate of 10 s−1, so that the d.c. potential range encompassed by a typical cyclic wave can be encompassed with adequate resolution in the Edc dimension in ≥6 s, approximately. The instrument features dynamic, computerized measurement and compensation of the non-faradaic ohmic resistance and double-layer capacitance contributions to the acquired total cell admittance. Measurements with quasi-reversible systems yield the expected faradaic admittance and phase angle responses over a quite generous bandwidth. Applications to mercury and platinum electrodes are illustrated.

Cyclic differential pulse voltammetry: A versatile instrumental approach using a computerized system

Abstract An extremely versatile instrumental approach in differential pulse voltammetry using computerized instrumentation is described. Scan rates in excess of 1 V s−1 can be used and each of the d.c. (staircase), pulse, and differential pulse components of the experiment can be measured simultaneously. Measurement of the individual components of this dual time domain technique rather than just the differential readout is shown to provide important additional information concerning both the nature of the technique and the electrode process under consideration. The use of cyclic differential pulse voltammetry is also described and shown to be a valuable extension to the currently available methodology in differential pulse voltammetry. An upper useful limit in the scan rate of about 1 V s−1 is imposed on the technique because of the d.c. faradaic distortion terms arising from the d.c. (staircase) component of the experiment and for other reasons.

The validity of the polarographic determination of stability constants of metal ions with adsorbable ligands

Summary The validity of the widely used polarographic method for the determination of stability constants is considered for adsorbable ligands. A polarographic study of the stability constants of the lead(II)-halides indicates that results become less reliable down the group. Agreement with accepted values for the fluoride and chloride systems is good. However, the bromide and iodide stability constants are significantly higher than values obtained by other techniques. This is attributed to complexation with adsorbed ligand in the double layer. It is concluded that polarographically determined stability constants of metal ions with adsorbable ligands are liable to be higher than “true” values. Such systems would be better evaluated by other techniques.

A study of the fluoride complexes of cadmium by a.c. and d.c. polarography

Summary A.c. polarographic measurements on aqueous solutions containing cadmium and fluoride ions have shown the existence of the species, CdF + and CdF 2 , with stability constants (at 30° and an ionic strength of 1.0) of 5.8 and 4, respectively. Reproducibility studies of a.c. techniques as a means for measuring changes in half-wave potentials (upon which the polarographic method for the determination of stabilities is based) have shown that the results obtained from a.c. polarographic measurements should be more accurate than those obtained by conventional d.c. polarography.

Pseudo-derivative d.c. and pulse polarographic methods at short drop times

Abstract Short drop time, fast scan rate d.c. polarographic techniques, whilst being superior to conventional methods in many respects, have the disadvantage of a higher charging to faradaic current ratio. Recent theoretical studies have suggested similar considerations, as well as additional problems from faradaic distortion, would apply in normal pulse polarography. In this work derivative readout is examined as a means for providing improvement in the short drop time d.c. and pulse methods. The validity of the Ilkovic (d.c.), Cottrell (pulse) and other polarographic equations are examined at drop times down to 50 ms. At this drop time the limiting current in pulse polarography can be less than in d.c. polarography, contrary to normal expectations. In the 50 ms drop time region, maxima, inhibition and other phenomena present under conventional conditions can be eliminated. With derivative methods, the noise level at a given scan rate decreases, the shorter the drop time. Since the charging current problem is also decreased, short drop time pseudo-derivative methods coupled with fast scan rates provide an attractive analytical technique. The technique of differential pulse polarography at short drop time is also examined and results are compared with the derivative methods. For the reduction of cadmium in 1 M HCl, a limit of detection of between 5×10−7 and 10−6 M was found with both the pseudo-derivative and differential pulse methods at a drop time of 0.10 s. This represents an order of magnitude improvement over the d.c. method.

A proposed simple method of calculation of stability constants for some non-reversible electrode reactions at the dropping mercury electrode

Summary Polarographic methods using half-wave potentials are frequently used to evaluate complex ion systems. When the electrode reaction at the DME is reversible, calculation methods are simple. However, for non-reversible electrode reactions, calculations are usually relatively difficult and consequently polarographic methods are not commonly used to study systems involving non-reversible electrode reactions. In this work, the possibility of using the simple calculation method of plotting Δ E 1/2 vs . the logarithm of the concentration of the ligand, for the class of the electrode reaction in which the non-reversibility of the electrode reaction is due to the charge transfer step and is independent of complex formation, has been investigated. The zinc-fluoride complex ion system has been studied by d.c., a.c. and rapid polarographic methods and the results have been used to assess the reliability and extent of applicability of the simple method of calculation proposed.

Evaluation of stability constants of metal ion-fluoride complexes by the specific fluoride ion electrode

Summary A method is given for calculation of the stability constants of some fluoridecomplexes using the fluoride specific ion electrode. Results are given for the fluoride complexes of Cd 2+ , Mg 2+ , Zn 2+ , Ni 2+ , Ag + and Tl + , and they are found to be in good agreement with those obtained by other methods. Methods and experimental techniques of measurement of stability constants of fluoride complexes using this fluoride electrode are very simple and should allow a considerable amount of thermodynamic data on fluoride complexes in solution to be obtained which is not available or easily accessible experimentally at present.

Some data for the electroanalytical use of fundamental second and third harmonic alternating current polarography

Summary Analytical applications of the various harmonics of alternating current polarography are more satisfactory the higher the degree of reversibility of the a.c. electrode process. At present very few reports on the development of a.c. polarographic methods of analysis attempt to assess the reversibility of the a.c. electrode process because most methods for doing this are rather complex and require the measurement of parameters that are of little interest to the analytical chemist. In d.c. polarography the Heyrovskyllkovic equation allows the ready assessment of reversibility and in analytical papers on d.c. polarography this equation is frequently used. With a.c. polarography, where it is even more important to establish the reversibility or otherwise of the a.c. electrode process, the establishment of easily used equations based on parameters of interest to analytical chemists seems desirable. This project has been undertaken in this work. Equations and data are given which allow the reversibility of fundamental, second and third harmonic a.c. electrode processes to be determined directly from the recorded polarogram. The equations and data are shown to be applicable to virtually all a.c. polarographic techniques and conditions. They apply to phase sensitive and non-phase sensitive a.c. polarography, a.c. voltammetry at hanging mercury drop and mercury pool electrodes, rapidly dropping and other varieties of a.c. methodology. Contrary to expectation, the equations apply to much larger amplitudes of a.c. potential than would be anticipated from the literature.