Zero current line

With a low constant current -1 (see Fig. 3.71) one obtains the same type of curve but its position is slightly higher and the potential falls just beyond the equivalence point (see Fig. 3.72, anodic curve -1). In order to minimize the aforementioned deviations from the equivalence point, I should be taken as low as possible. Now, it will be clear that the zero current line (abscissa) in Fig. 3.71 yields the well known non-faradaic potentiometric titration curve (B B in Fig. 2.22) with the correct equivalence point at 1.107 V this means that, when two electroactive redox systems are involved, there is no real need for constant-current potentiometry, whereas this technique becomes of major advantage... 

Current-voltage Curve B is even more nonlinear than Curve A. It intercepts the zero current line, or open cell potential line, at a single point, at which i = 0. That point is the equilibrium potential Eeq of the above redox couple. 

Let s now increase the sensitivity on the current axis such that Curve A appears to cross the zero current line at only one point. Now we are looking specifically at the round insert area of Fig. 5.1. We label the two curves recorded at this magnification as A and B, but otherwise everything is the same as in the first experiment. 

Curve B is the voltammogram of an equimolar mixture of iron(II) and iron(IIl). The portion of the curve below the zero-current line corresponds to the oxidation of the iron(II) this reaction ceases at an applied potential equal to the half-wave potential. The upper portion of the curve is due to the reduction of iron(III). 

Figure 7.1.4 Polarogram for 1 mM CrO in deaerated 0.1 M NaOH, recorded at a DME. The Ilkovic equation describes current flow in the plateau region, at potentials more negative than about -1.3 V. The lower curve is the residual current observed in the absence of CrOl. The recorder was fast enough to follow the current oscillations through most of each drop s life, but not at the moment of drop fall, as one can see by the fact that the trace does not reach the zero-current line before starting a fresh rise with the new drop.

The analysis and methodology for the extraction of characteristic parameters obtained from cyclic voltammograms is shown in Fig. II. 1.9b. A zero current line for the forward scan data has to be chosen (dashed line) as baseline for the determination of the anodic peak current. For the reverse sweep data the extended forward scan (dashed line with Cottrell decay) is folded backwards (additionally accounting for capacitive current components) to serve as the baseline for the determination of the... 

The analysis and methodology for the extraction of characteristic parameters obtained from cyclic voltammograms is shown in Fig. II. 1.9b. A zero current line for the forward scan data has to be chosen (dashed line) as baseline for the determination of the anodic peak current. For the reverse sweep data the extended forward scan (dashed line with Cottrell decay) is folded backwards (additionally accounting for capacitive current components) to serve as the baseline for the determination of the cathodic peak current. This procedure can be difficult and an approximate expression for analysis based on the peak currents and the current at the switching potential has been proposed as an alternative [46]. If the blank current before the anodic peak starts cannot be neglected, this current has to be extrapolated into the range where the peak occurs, or, if possible, has to be subtracted from the sample voltammogram. Also, when the sample solution does not contain only the reduced form (as supposed... 

FIGURE 2.3 Potential distribution in galvanic cells functioning as a battery (a) and as an electrolyzer (b) the dashed lines are for the zero-current situation. 

FIG. 5. SK currents in UBSM cells. (A) Original records of whole-cell currents recorded from a guinea-pig UBSM cell during a 100 ms depolarization from — 60 mV to +10 mV. Current is shown under baseline conditions (control), and after addition of the SK channel blocker apamin (100 nM). (B) The apamin-sensitive portion of the current in A is shown (SK current). Dotted lines indicated zero current. (From G.M. Herrera M.T. Nelson, unpublished observations.)... 

Fig. 10.9. Controlled potential cathodic scans of Tl+ at an amalgamated gold disk electrode. All A scans are RDE curves, all B scans are in-phase HMRDE curves. (A, B) 0.01 mol dm" HC104 (A2, B2) 2.0 x 10-7 mol dm- TT in 0.01 mol dm- HCI04. The current sensitivities are indicated by the markers, zero current in all cases is the dashed line. For all curves ft = 3600 rpm, for HMRDE (B) Aft = 6 rpm, frequency = 3 Hz, averaging time constant is 3 s and the scan rate is 2 mV s"1 (after Reference [27]).

Figure 5.13 A precision and accuracy plot of the atomic electron affinities determined before 1967 versus the current best values. The deviations from the unit slope and zero intercept line result from random and systematic errors.

Fig. 9.10. Examples of pulsed-current charge techniques. Dotted lines are for rests (zero current), low-current discharges or discharge-rest combinations.

Suppose that a current electrode is placed in a uniform conducting medium so that the distribution of currents possesses the spherical symmetry (Fig. 1.32a). It is then a simple matter to realize that the magnetic field is zero everywhere in the medium. This follows directly from Biot-Savart law and the symmetry of the model. In other words, one can always find two current elements which are located symmetrically with respect to the observation point and of which the magnetic field differ by sign only. Let us notice that Ampere s law does not apply here because the current lines are not closed. 

Let us notice that current lines corresponding to oscillations of the magnetic type are located in horizontal planes perpendicular to the borehole axis. Current lines of zero harmonic of the secondary field are circles with centers located on the borehole axis, and therefore they do not intersect the boundary between the borehole and the formation. Current lines of oscillations of the electrical type have a much more complicated form but their distribution has such a character that the field component Hz is equal to zero everywhere. 

The maximum current max shown in Fig. 4.12 is conditioned by the nature of the electrode metals. This value imax is related to the electrode potential difference of the oxide metal pairs (Fig. 4.13, Table 4.5). It is evident (Fig. 4.13) that extrapolation of line / to the ordinate gives zero current at AUok = 0- This is a qualitative confirmation of the insignificant current in the M-P-M system with electrodes of similar metals. There is, however, a certain deviation of experimental points from the linear dependence /, which is explained by the formation of numerous oxides by such metals as, e.g. Ag, Cu, etc., while the A17ox values presented in [50] are a result of averaging of the potentials of these oxides. 

Unlike the cathodic portion of the polarization curve, the anodic portion of the curve in Fig. 3(b) does not exhibit clear Tafel-type behavior. The mechanism for Fe dissolution in acids is quite complex. A line can be drawn in the region just above the corrosion potential, giving a Tafel slope of 34 mV decade k Extrapolation of this line intersects the zero-current potential at 7 X 10 A cm , a considerably different value than the extrapolation of the cathodic portion of the curve. This is not uncommon in practice. When this happens, it is usually considered that the anodic portion of the curve is affected by changes on the electrode surface, that is, surface roughening or film formation. The corrosion rate is typically determined from the extrapolated cathodic Tafel region. 

Figure 6.17 illustrates reciprocal current injection by a small contact area PU electrode into an infinite volume of homogeneous and isotropic material with uniform current density [,(,. From the reciprocally excited PU electrode, the current spreads out in a hemispheric symmetrical geometry in accordance with Figure 6.1. The reciprocal current density near the electrode is very high, and one may be led to believe that the sensitivity is high in this zone. The voxels lying in the vertical line from the PU electrode have zero sensitivity because the reciprocal current hue is perpendicular to the horizontal current lines. Near the surface, the dot product is very high near the PU electrode.

Figure 6.23 Applying current and measuring potential difference between interleaved electrodes in a homogenous medium (a) Current density lines between electrodes 1 and 3 and reciprocal current lines between electrodes 6 and 8 (b) corresponding sensitivity field distribution — white lines showing zero sensitivity (where the current density lines are orthogonal) — positive sensitivity (light grays) between these lines and negative sensitivity (dark grays) outside the lines.

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