Sweep rate

The scan rate, u = EIAt, plays a very important role in sweep voltannnetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for nonnal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diflfiision and the peak-shaped response. Wlien the scan rate is slow enough to maintain steady-state diflfiision, the concentration profiles with time are linear within the Nemst diflfiision layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diflfiision layer caimot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

In the presence of 6-iodo-l-phenyl-l-hexyne, the current increases in the cathodic (negative potential going) direction because the hexyne catalyticaHy regenerates the nickel(II) complex. The absence of the nickel(I) complex precludes an anodic wave upon reversal of the sweep direction there is nothing to reduce. If the catalytic process were slow enough it would be possible to recover the anodic wave by increasing the sweep rate to a value so fast that the reduced species (the nickel(I) complex) would be reoxidized before it could react with the hexyne. A quantitative treatment of the data, collected at several sweep rates, could then be used to calculate the rate constant for the catalytic reaction at the electrode surface. Such rate constants may be substantially different from those measured in the bulk of the solution. The chemical and electrochemical reactions involved are... 

Fig. 8.21 Current density dilTerences between fast and slow sweep rate polarisation curves and stress corrosion cracking suspectiblity as a function of potential for a C-Mn steel in nitrate, hydroxide and carbonate-bicarbonate solutions...

The time factor in stepwise potentiostatic or potentiodynamic polarisation experiments is very important, because large differences can be caused by changes in the scanning rate. Since the steady state depends on the particular system and conditions of exposure, no set rule exists for the magnitude or frequency of potential changes. Chatfield etal. have studied the Ni/H2S04 system and have shown how becomes more passive with increase in sweep rate. 

All P.M.R. spectra were measured with a Varian HA 100 spectrometer operating in the frequency-sweep mode with tetramethylsilane as the reference for the internal lock. The double and triple resonance experiments were performed using a Hewlett Packard 200 CD audio-oscillator and a modified Hewlett Packard 200 AB audio-oscillator (vide infra). Spectra were measured using whichever sweep width was required to ensure adequate resolution of the multiplets under investigation, generally 250 or 100 Hz, and sweep rates were selected as necessary. Extensive use was made of the Difference 1 and Difference 2 calibration modes of the instrument, both for the decoupling experiments and for the calibration of normal spectra. 

Of these neutron interactions, those that produce prompt-7 rays were evaluated as the most feasible for mine detection. As discussed in the Introduction, we define a prompt 7-ray as one which is produced as a direct result of the primary neutron interaction, usually within nanoseconds of that initial event. Such reactions are obviously attractive because they can best satisfy the desired rapid sweep rate over a minefield. The three specific neutron-prompt gamma reactions that were intensively investigated by the US Army are listed below ... 

FIGURE 1. Voltammetric curves in DMF in the presence of Bu4NBF4 (0.1m), reference electrode Ag/Agl/I (0.1m), mercury stationary microelectrode (A) PhS02Me (10 3m), sweep rate 500mVs". (B) PhS02Ph (103m), sweep rate 500mVs 1. 

The relative importance of the disproportionation process (SET between two anion radicals) depends principally on the thermodynamic constant (K). It can be easily determined more or less accurately from the potential difference existing between the first cathodic peak and the second one. (An exact calculation would be possible from the thermodynamic potentials of the two reversible transfers in the absence of proton sources and at reasonable sweep rates so as to inhibit any undesirable chemical reaction.)... 

In this example24 redox catalysis kinetics is governed partly by chemical reaction, i.e., the scission of C6H5S02CH3. For given concentrations of pyrene and sulphone at sweep rate v one can find values of klk/k2 from published graphs23 in the case of EC processes. 

Thus in order to obtain the value of k it is absolutely necessary to know the standard cathodic redox potential. In the case of MeS02Ph, very fast voltammetric measurements at high sweep rates (1000 Vs 1) permit one to reach the reversible step of the sulphone and give °Meso2ph= — 1.85 V. Hence, the value k =0.9 x 105 s 1 may be estimated for the... 

FIGURE 3. Voltammetric curves at a stationary mercury microelectrode for anthracene in the presence of 17b (a) anthracene alone, 6.5 x 10 3m (b, c, d) previous solution with 2.3, 5.1 and 7.6 x 10"3m of 17b (e) disulphone 17b without anthracene. Medium, DMF-Bu4NC104 0.14m sweep rate, 10mVs 1 (after Reference 25). 

FIGURES. Voltammograms at a microelectrode of mercury of 2,2-diphenylvinyl phenyl sulphone in DMF, sweep rate 600 mV s , substrate concentration 10 3 m broken line curve (a) without proton donor present, full line curves (b) and (c) with phenol at concentration 0.5 and 4 x 10 3m, respectively, reference electrode Ag/Agl/I" 0.1m in DMF (after Reference 37). 

FIGURE 6. Voltammetric curves in DMF-TBAP 0.1m, stationary mercury tnicro-electrode, sweep rate lOmVs-1 (1)without phenol, (2)10 2m phenol added (a) PhSOjCHjPh, (b) PbSO,QEt>(Me)Ph. 

FIGURE 10. Voltammetric curves of fully aliphatic allylic sulphones (c = 3 x 10-3 M) in DMF/TBAP 0.1 m electrolyte, stationary mercury electrode, sweep rate 10 mV s—1 (a) and (b) curves in aprotic DMF (c) response of the sulphone, (b) with phenol 10-2 m (after Reference 26). 

Figure 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

As shown in Fig. 25, an example of the extrapolation of the current transient obtained from the potential sweep yields the critical potential after ascertaining that the data obtained are independent of the sweep rate. Figure 26 exhibits the results of the critical pitting potential measurement for the majority salt of NaCl and the minority ion of Ni2+when the concentration of NaCl is varied under the condition of constant Ni2+ionic concentration. From the plot in Fig. 26, it follows that... 

Figure 25. Diagram for critical potential measurement79 The sweep rate it 4 x 10-3 V s"1. [Nicy = 100 mol nf [NaCl] = 0.1 mol nf3. T= 300 K. (From R. Aogaki, E. Yamamoto, and M. Asanuma, J. Electrochem. Soc. 142, 2964, 1995, Fig. 2. Reproduced by permission of The Electrochemical Society, Inc.)...

Thus, at constant temperature and at a constant sweep rate, the influence of the cathodic overpotential (tjc) on the peak overpotential (t]p) of the voltammogram obtained under conformational relaxation control of the polymeric structure is described by... 

Voltammetry performed at different sweep rates, keeping both the cathodic overpotential and the temperature constant, is predicted to have... 

Figure 61. Experimental voltammograms obtained for thin films of polypyrrole in 0.1 M LiC104-propylene carbonate solutions. The potential sweep was carried out between -2500 and 300 mV vs. SCE at 25°C, and the sweep rate was varied from 10 to 50 mV s-1. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, J. Phys. Chem. 101, 8525, 1997, Figs. 3-11, 13. Copyright 1997. Reproduced with permission from the American Chemical Society.)...

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