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

Abstract

tion with positive feedback and doublepulse instrumentation. Of course, the use of better amplifiers will improve the effectiveness of all the methods for interpretation of the data taken at short times.Even with advanced instrumentation, full advantage of the potentiostatic technique will be realized only by application of correction methods.

High Speed Non-Faradaic Resistance Compensation in Potentiostatic Techniques

“…The positive feedback technique has long been employed by electrical engineers in amplifier design (10, I1), but has only recently been used in electrochemical studies. Several workers have given circuits devised to achieve compensation (i.e., reduce the effect of Re) in some cases (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23), but frequency limitations and phase instabilities have been the major causes of the lack of generality of these techniques. Further, there appears to be a rather major misconception about what the oscillations (whether damped or undamped) caused by too much positive feedback mean in terms of the degree of compensation.…”

“…Using Eq. [14] to [16] the critically damped behavior allowable in terms of [13] is then rc Zu + --+ re = ~/4 ~c re [17] K+I Equation [17] then allows the experimentally useful minimum time constant ~mi. of the system at maximum stable positive feedback to be established Train : X~TcTe [18] Consideration of [18] indicates that the larger the cell time constant or the longer the closed loop potentiostat rise time, the larger the minimum time constant of the system.…”

“…To understand the significance of this, it is sufficient to examine how *min is defined in terms of both the cell and potentiostat parameters. Using [17] and [18] rmin may be defined as Tc--4Te (K + 1) 2 [21] which means that the time constant of the cell has to be considerably larger than that of the closed loop potentiostat in which case the minimum time constant contains only electrochemical parameters. Further examination of [20] indicates that, provided [21] is satisfied, it is possible for ru to become negative.…”

Instrumentation for Digital Data Acquisition in Voltammetric Techniques. D. C. And A. C. Polarography.

“…An all solid-state operational amplifier instrument which has been described in detail in a separate communication (9) was employed in this work. The following main features of this instrumentation are of interest in the present discussion.…”

“…Second, the source of a voltage pulse from the timing circuit of the electrochemical instrument which signals the end of each mercury drop life [the output of Trigger 2 of Figure 3, Ref. (9)] was routed to the coil of the relay whose closure initiates a reading cycle of the DDAS.…”