Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Working, curves potential

The conditions in which ipT/ipr assumes values between 0.45 and 0.95 are the best for calculating the rate constant of the following reaction. In fact, from the working curve reported in Figure 16, based on the current ratio ipT/ipf, one can determine the value of /Cf x. 9 Since x is easily calculated, being the time necessary to move from E° (calculated under the conditions where ipr/ipt =1) to the inversion potential E(, k is determined. [Pg.78]

However, if the redox couples Ox/Red and Ox /Red have sufficiently different standard potentials, can be also calculated using the working curve reported in Figure 16. In fact, considering the process simply as a reversible electron transfer followed by an irreversible first-order chemical reaction (see Section 1.4.2.2), one measures only the current ratio /pr//pf of the first couple Ox/Red. Obviously, the return peak must be recorded before the second process begins to appear this means that the direction of the potential scan must be reversed immediately after having traversed the first forward peak. [Pg.90]

The simulation of the ECE mechanism may also employ the double-potential-step technique, but a working curve can be constructed from single-potential-step data also. This is because some of the current that passes, as A is converted to B, is due to the electrolysis of C, the decomposition product of B. The greater the decomposition rate of B, the more current flows, approaching the rate of... [Pg.603]

Thus one may obtain kt by multiplying the quantity previously referred to as the dimensionless time by k,tf, the dimensionless rate constant. This is particularly useful in constructing the single-potential-step working curve for the ECE mechanism mentioned earlier. This parametric substitution allows the experimental time to be rendered dimensionless by the inverse of the rate constant instead of by some known time tk. [Pg.607]

Kinetic studies of ECE processes (sometimes called a DISP mechanism when the second electron transfer occurs in bulk solution) [3] are often best performed using a constant-potential technique such as chronoamperometry. The advantages of this method include (1) relative freedom from double-layer and uncompensated iR effects, and (2) a new value of the rate constant each time the current is sampled. However, unlike certain large-amplitude relaxation techniques, an accurately known, diffusion-controlled value of it1/2/CA is required of each solution before a determination of the rate constant can be made. In the present case, diffusion-controlled values of it1/2/CA corresponding to n = 2 and n = 4 are obtained in strongly acidic media (i.e., when kt can be made small) and in solutions of intermediate pH (i.e., when kt can be made large), respectively. The experimental rate constant is then determined from a dimensionless working curve for the proposed reaction scheme in which the apparent value of n (napp) is plotted as a function of kt. [Pg.632]

D.. .. to here, where the oxidation of I occurs at a diffusion-controlled rate. After a prescribed electrolysis time t, the potential is stepped back to the initial potential, C. Only the reduction of the unreacted carbonium ion II occurs at this potential. The value of the pseudo-first-order rate constant, k[, is then determined from a dimensionless working curve that relates the ratio of the cathodic and anodic currents to k,t. Details for the construction of the working curves (each ratio of tr/t requires a different working curve) and their subsequent use may be found in the literature [8]. [Pg.636]

Figure 28.13 ECL generation by the step technique. The Ag/AgCl, KC1 (satd.) reference is -0.045 V vs. SCE. (a) Cyclic voltammetric curve (0.593 mAf rubrene in benzo-nitrile with 0.1 M TBAP) (b) working electrode potential program (c) emission intensity versus time. Figure 28.13 ECL generation by the step technique. The Ag/AgCl, KC1 (satd.) reference is -0.045 V vs. SCE. (a) Cyclic voltammetric curve (0.593 mAf rubrene in benzo-nitrile with 0.1 M TBAP) (b) working electrode potential program (c) emission intensity versus time.
Fig. 6.7 The double potential step chronoamperometry working curve for the eCei, mechanism (full line) and experimental data for the protonation of the anthracene radical anion by phenol (points). The scale at the top corresponds to the working curve and the scale at the bottom to the experimental data. (The parameter 6 in the figure corresponds to ff in the text.) Note that the data for the variation of ff and [PhOH] have been plotted on the same working curve. Reprinted with permission [35]. Fig. 6.7 The double potential step chronoamperometry working curve for the eCei, mechanism (full line) and experimental data for the protonation of the anthracene radical anion by phenol (points). The scale at the top corresponds to the working curve and the scale at the bottom to the experimental data. (The parameter 6 in the figure corresponds to ff in the text.) Note that the data for the variation of ff and [PhOH] have been plotted on the same working curve. Reprinted with permission [35].
Table 4.1 Heterogeneous rate constant (k°), electron transfer coefficient (a), and formal potential ) corresponding to the best fit of theoretical working curves (Eq. 4.120) to the RPV experimental results [48]... [Pg.271]

A stirred solution is used also during the stripping step to facilitate the transport of the oxidant. Alternately, the oxidation can be carried out by passing a constant anodic current through the electrode. During the oxidation step, the variation of the working electrode potential is recorded, and a stripping curve,... [Pg.89]

With the substrate biased at a potential slightly more positive than E° of A/B couple, B is oxidized to form A for both DISP1 and ECE mechanisms. However, in the latter case the reduction of C also occurs at the substrate. The numerical solution of corresponding diffusion problems (see Ref. [85] for problem formulations) yielded several families of working curves shown in Fig. 12 (DISP1 pathway) and Fig. 13 (ECE pathway). In both cases, the tip and the substrate currents are functions of the dimensionless kinetic parameter, K = ka2/D. [Pg.205]

In voltammetric experiments a normal type of calibration is the recording of voltammetric curves for a known system, constructing plots such as variation of limiting current with the transport parameter, or of current with concentration. In potentiometric experiments the equivalent would be the variation of potential with concentration. These curves are especially important in electroanalytical experiments working curves permit the immediate conversion of a measured current or potential into a concentration. [Pg.142]

Usually, in LS AAS, the most sensitive analytical line is used for the determination of an element, because AAS is mostly applied for trace and ultra-trace analysis, which obviously requires the highest sensitivity. Another reason for using the most sensitive line is that it makes it possible to apply higher dilution in case of complex sample matrices, and hence avoid potential interferences. On occasions, however, the most sensitive line is not recommended in LS AAS, as it does not provide the best SNR, as in the case with the 217.001 nm Pb line. Another reason might be a strongly nonlinear working curve due to the presence of other lines in the lamp spectrum that cannot be excluded even with a 0.2 nm bandwidth [3]. [Pg.94]

The manner in which kinetic data are treated in arriving at an electrode mechanism depends primarily upon whether the technique gives a direct measure of the response of the intermediate or an indirect measure, usually the effect of the chemical reaction on the electrode response of the substrate. In the former case, the conventional way of handling the data is to compare the experimental response with theoretical data in the form of a working curve and determine the mechanism from the best fit with theoretical data. The latter case usually involves the calculation of the electrode response to a particular mechanism and then comparing some measurable quantity, for example the sweep rate dependence of the peak potential, with the theoretical value. Which type of analysis is appropriate, direct or indirect, depends upon the... [Pg.162]

Alternatively, the determination of k may be based on measurements of Ep/4 — Ep mam where the quarter-peak potential Ep/4 is the value of E at i = ip,mam/4 [123] [see Fig. 17(a)]. The advantage of this approach is that measurements can be made even in cases where the prepeak appears only as a shoulder on the main peak, and where Ep.pre therefore cannot be determined. The working curve for the eCen mechanism is shown in Fig. 17(b) together with experimental data obtained for the protonation of the anthracene radical anion by benzoic acid. The rate constant resulting from these data is 2.7 X 10 M s [123]. [Pg.123]

Figure 27. Double potential-step chronoamperometry working curve (top scale) for the eCeh mechanism (solid line) and experimental data (bottom scale) for the protonation of the anthracene radical anion by phenol in DMF (O.IM BU4NBF4). CX = lmM and CphOH/tnM = 9.95(-f-), 18.2 ( ), 40.0 (o), 78.8 (x), 100 ( ), and 200 ( ). R in the figure corresponds to Ri in this chapter and A. = ktfCphOH- (From Ref. 213.)... Figure 27. Double potential-step chronoamperometry working curve (top scale) for the eCeh mechanism (solid line) and experimental data (bottom scale) for the protonation of the anthracene radical anion by phenol in DMF (O.IM BU4NBF4). CX = lmM and CphOH/tnM = 9.95(-f-), 18.2 ( ), 40.0 (o), 78.8 (x), 100 ( ), and 200 ( ). R in the figure corresponds to Ri in this chapter and A. = ktfCphOH- (From Ref. 213.)...
Double potential-step chronocoulometry [1,2,221] may be used similarly to DPSCA. The working curves now include the charge ratio —Qb/Qf, which takes the value 0.414 for a simple electron transfer reaction. The reductive cyclization of ethyl cinnamate (see Chapter 21) illustrates the use of the technique [226,227]. [Pg.142]

By analogy to the extension of CA to DPSCA (Sec. IV), chronoabsoi ptometry has also been applied in a double potential-step version [350,351], and working curves for a large number of mechanisms have been calculated [352]. [Pg.165]

Connect the ion-selective electrode and the second reference electrode to the pH meter as shown in Figure 21F-1. Prepare a series of standard solutions of the ion of interest, measure the cell potential for each concentration, plot a working curve of eii versus log c, and perform a least-squares analysis on the data (see Chapter 8). Compare the slope of the line with the theoretical slope of (0.0592 V)/n. Measure the potential for an unknown solution of the ion and calculate the concentration from the least-squares parameters. [Pg.606]


See other pages where Working, curves potential is mentioned: [Pg.79]    [Pg.32]    [Pg.200]    [Pg.382]    [Pg.84]    [Pg.58]    [Pg.174]    [Pg.104]    [Pg.633]    [Pg.638]    [Pg.705]    [Pg.709]    [Pg.935]    [Pg.236]    [Pg.249]    [Pg.321]    [Pg.134]    [Pg.351]    [Pg.275]    [Pg.58]    [Pg.871]    [Pg.139]    [Pg.252]    [Pg.25]    [Pg.138]    [Pg.199]    [Pg.200]    [Pg.177]    [Pg.178]    [Pg.104]    [Pg.100]   
See also in sourсe #XX -- [ Pg.120 ]




SEARCH



Potential curves

Work potential

Working, curves

© 2024 chempedia.info