Linear current ramp

As shown above, potentiodynamic generation of limiting currents is more rapid and, therefore, preferable in principle to the galvanodynamic technique. However, during a linear decrease of potential to the limiting-current condition, the current density initially rises more rapidly than when a current ramp is used. Therefore, in the case of copper deposition at the cathode, a linear potential ramp tends to yield a rougher deposit and a less well-defined plateau than a linear current ramp (see Section III,F). 

The constant current may be reversed in direction at, or before, the transition time, as shown in Fig. 19(a), or repeatedly reversed, thus becoming periodic [Fig. 19(b)]. Less frequently employed is a current waveform which varies as a known function of time [66—69], such as the linear current ramp in Fig 19(c). 

Fig. 19. Current waveforms, (a) Double step (b) periodic double step (c) linear current ramp.

Differential-pulse voltammetry is an extremely useful technique for measuring trace levels of organic and inorganic species, hi differential-pulse voltammetry, fixed-magnitude pulses—superimposed on a linear potential ramp—are applied to the working electrode at a time just before the end of the drop (Figure 3-5). The current... 

Experimental results obtained at a rotating-disk electrode by Selman and Tobias (S10) indicate that this order-of-magnitude difference in the time of approach to the limiting current, between linear current increases, on the one hand, and the concentration-step method, on the other, is a general feature of forced-convection mass transfer. In these experiments the limiting current of ferricyanide reduction was generated by current ramps, as well as by potential scans. The apparent limiting current was taken to be the current value at the inflection point in the current-potential curve. 

Sampled DC polarography A form of polarography in which a linear potential ramp of d /df is applied to the working electrode, but where the current is measured only during the last 15% or so of each drop cycle. 

Note that digital instrumentation approximates the linear potential ramp as a staircase waveform [3, 6, 7]. There is a good agreement between the linear and staircase currents for ls/r = 0.25 — 0.30 for reversible processes (with ts being the time between the application of the potential pulse and the current sampling), if the potential step AE is less than 8 mV. 

Figure 3. Differential pulse voltammogram of a mixture of dibenzothiophene and benzothiophene in acetonitrile. Supporting electrolyte 0.1 M tetraethylammonium perchlorate. Indicator electrode glassy carbon disk, rotated at 1800 rpm. Linear potential ramp, 0.002 volt/s. Pulse amplitude, AE = 0.025 V. Pulse duration, 57 ms. Current sampling time, 17 ms.

Differential pulsed voltammetry (DPV) is a technique in which potential pulses of fixed but small amplitudes are superimposed periodically on a linear voltage ramp. The most commonly used working electrode is the SMDE and one pulse is applied for each drop. The mV pulse is applied near the end of the life of the mercury drop. Current is measured once before the pulse and after the pulse. The difference between the currents is plotted against potential (Figure 5.8). The resultant peak-shaped current-voltage signal, which... 

Classical DC polarography uses a linear potential ramp (i.e., a linearly increasing voltage). It is, in fact, one subdivision of a broader class of electrochemical methods called voltammetry. Voltanunetric methods measure current as a function of applied potential where the WE is polarized. This polarization is usually accomplished by using microelectrodes as WEs electrode... 

Figure 17-12 Voltage profiles for voltammetry (a) linear voltage ramp used in vitamin C experiment (b) staircase profile for sampled current polarography. Inset (c) shows how faradaic and charging currents decay after each potential step.

Two recent developments in the measurement technique have resulted in a major reduction in the charging current. Firstly, with the introduction of digitally controlled instruments, the linear voltage ramp applied to the electrodes has been replaced with a stepped (staircase) waveform. The voltage... 

In this technique, a linear voltage ramp is modulated with a sinusoidal alternating voltage of small amplitude (A . = 10-100 mV) and low frequency (/=5-100Hz). The superimposed alternating voltage causes an alternating current, whose size depends on the instantaneous value of... 

Series of rectangular pulses (typical length some 10 ms) with constant amplitude AE are superimposed to linear potential ramp in differential-pulse voltammetry (Fig. 3). The currents are detected twice per pulse, just before the pulse and before the end of the pulse. The differences are plotted versus potential of the linear... 

The principle of the technique is illustrated in Fig. 1.65. From Fig. 1.65 we note that a series of pulses of amplitude A (usually from 5 50 mV) are superimposed on a slow sweep rate (ca. lmVs ) linear potential ramp that acts as a slowly changing baseline. The pulse is repeated after a time T >, and the pulse has a duration To t. The current is measured over a fixed time interval 8t just before and again toward the end of the pulse, as indicated in Fig. 1.65. The differential pulse voltammogram then simply consists of a plot of the difference in these two current measurements A/ = i To) — i r) as a function of the base potential E. 

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