Electrochemical Relaxation Techniques

Abstract

ITHIN the past 20 years a wide W variety of electroanalytical techniques have been newly developed and a number of previously conceived techniques have received r2newed attention as a result of electronic advances which facilitated their practical implementation. The purpose of the present review, rather than considering only the advances of the past tv.oyears, will be t o discuss the relative merits of a selection of these methods. No attempt has been made at completely surveying the extensive literature, and discussion is largely confined t o whttt may be termed relaxation techniques.Relaxation methods in general are those in which a perturbing influence, usually in the form of a controlled current or potential, is tipplied to a cell initially a t equilibrium, and the transient relaxation of the system in response to this disturbance (a c u e n t or voltage) is observed. The distinction between small and large amplitude techniques arises naturally on theoretical grounds. In the mathematical treatment of the expected responses, 1 he current and concentrations of electroactive species are normally related through linear algebraic or differential equations, but the relation between concentration and potential (the Nernst equation) or current and potential (absolute rate expression) is an exponential one [cf. (66) for a recent revi1.w of theoretical methods].When the excursions of potential from initial equilibrium are no more than a few millivolts, the exponentials can be expanded t o produce linear equations. Thus, small amplitudl: techniques can be made to yield useful theoretical predictions even for relatively complicated postulated read ion mechanisms. In contrast, the large amplitude techniques, those in which potential excursions are too large to allow expansion of the exponentials with retention of only a few terms, tend t o yield intractable theories for all but the simplest cases. SMALL AMPI.ITUDE RELAXATION TECHNIQUESThe present discussion is limited to qualitative aspects of these techniques. The reader desiring more quantitative and extensive treatment is urged to consult the recent review by Delahay ($5).Although the perturbance from equilibrium in a relaxation technique could in principle be nonelectrical and in fact supersonic waves (84) and ultraviolet and visible radiation (1 1 ) have been employed, most workers have used electrical disturbances in the form of controlled current or voltage waveforms.One of the simplest of the techniques is the voltostatic one studied in detail by Vielstich and Delahay (81). In this case the perturbing influence is a step change in voltage applied t o the cell, and the current is observed, normally oscilloscopically. The technique is useful for studying the kinetics of chargetransfer reactions, but only relatively slow ones. The reason is that the step-change in voltage does not appear instantaneously across the solution-electrode interface but rather a t a rate limited by the R-C time constant associated with the charging of the capacitance of the electrical do...

Some Investigations on Instrumental Compensation of Nonfaradaic Effects in Voltammetric Techniques.

“…In one way or another these nonfaradaic influences limit the scope of voltammetric methods in kinetic and mechanistic studies of electrode reactions as well as in analytical applications. Particularly profound are their effects in modern electrochemical relaxation methods such as cyclic voltammetry (16,20,25,42), a.c. polarography (55,57,64,70,71), and high-speed potentiostatic measurements (10,11,87,55). The contributions of iR drop and double-layer charging current in these latter techniques can be made minimal in measurements involving time scales (the period of the alternating potential in a.c. polarography and cyclic voltammetry, the measurement time in chrono-amperometry, etc.)…”

Applications of Modern Electroanalytical Techniques to Pharmaceutical Chemistry

“…There is, in addition, a whole segment of methods in which current, rather than potential, is controlled (chronopotentiometric or galvanostatic techniques). For a thorough treatment of these the reader is referred to several monographs (2,3,6) and the reviews on "relaxation methods" by Reinmuth (24), and "electroanalysis and coulometric methods" by Bard (25). In addition, amperometric methods, while usually carried out in stirred solution, embody the principles of voltammetry discussed in the previous sections.…”

Stand der polarographischen Methoden und ihrer Instrumentation. Teil II: Wechselspannungsverfahren

“…Obwohl z. Zt. no& keine speziellen Polarographen kommerziell fur diesen Kreis jungst entwickelter Verfahren [6], [8], [61] erhaltlich sind, sei abschlieRend auf sie eingegangen, zumal man klar voraussehen kann, daR ihre Anwendung in naher Zukunft auaerordentliche Bedeutung gewinnen wird. Diese Verfahren stellen einen ganz entscheidenden methodischen Fortschritt dar, der nicht nur die Moglichkeiten der elektrochemischen Spurenanalyse betrachtlich erweitert, sondern vor allem auch einen ganz neuen Abschnitt fur die elektrochemische Kinetik eingeleitet hat.…”

“…Hingegen hat der Kapazitatsstrom wegen des weitgehend linearen Charakters der Doppelschichtkapazitat nur einen sehr geringfugigen Oberwellengehalt. Registriert man daher nach Ausfilterung mit einem HochpaBfilter nur die hoheren Harmonischen, so ergibt sich eine weitere sehr wirkungsvolle Moglichkeit zur Ausschaltung des Kapazitatsstromes[6],[8],[61],[?3] -[??I. Da bei fundamentalen Studien zur Elektrodenkinetik Messungen bei verschiedenen Frequenzen in einem weiten Bereich moglich sein mussen, wird aber der apparative Aufwand sehr groB im Vergleich zu anderen Nichtlinearitatstechniken, wie der faradayschen Gleichrichtung; denn das HochpaRfilter muB eine enge aber in weitem Bereich variable Bandbreite haben.…”