Disk current

Solution From the Levich equation, (4-5), one can calculate first the disk current under the new conditions ... 

Figure 6 Steady state rotating ring-disk voltammograms of (A) compound (42) (B) compound (43) and (C) a Ru-bridged polymer of (43) each adsorbed to a graphite working electrode. Disk current shows reduction of 02 while ring current reveals the presence of H202 simultaneously reoxidised at the ring anode poised at +1.0 V (reproduced with permission of the American Chemical Society from Acc. Chem. Res., 1997, 30, 437-444).

Fig. 5 Oxygen reduction at a rotating ring(Pt)-disk(Pt) electrode in 0.1 N H2SO4. PO2 = 1 atm 10 = 120jr disk current, x ring...

Fig. 8 Catalysis of O2 reduction at a ring-disk electrode in contact with an aqueous 0.5 M H2SO4 solution, the graphite disk (EPGE) being modified by adsorption of a biscobalt diporphyrin, C02ETE4. (a) Disk current, solution saturated with oxygen, p02 = 1 atm rotation rate 100 r min scan rate ...

Rotating disk electrode (cont.) diffusion coefficient, 1141 diffusion layer in, 1234 disk current in, 1141 ECE reactions determination by. 1144 electrooxidation of methanol, 1139 kinematic viscosity, 1141,1234 intermediate radicals, determination of. 1139. 

Frumkin and Nekrassow then applied Levich s equation to an analysis of intermediate production when the intermediate could leave the electrode surface, with the possibility of reacting again at the ring or leaving for the bulk. Damjanovic et aL developed the Ivanov and Levich equation to include a term, x, the ratio of the velocity of the two parallel reactions (A) and (B), thus increasing the helpful information obtained by using the equation. Damjanovic et al. s equation for the ratio of disk current to ring current is... 

Figure 2. Ring current and disk current as a function of potential of the ZnO disk electrode. Potential of Pt ring 0 V vs. SCE rotation rate 1000 rpm solution 10 2MKIin0.5MK ,SOl.

Figure 3 shows (X ) (X = I, Br) as a function of the concentration of electrolytes for two different disk currents. According to this figure, a large (X ) was obtained for a high concentration of KI, as easily expected from general electrode kinetics. However, it is noticed that the concentration dependence of (X ) is different even for the same kind of halide ions if the ig chosen is different. In order to investigate this point in detail, (X ) was obtained as a function of ig for different concentration of halide ions. 

Figure 3. Percentage of oxidation of /" and Br as a function of the concentration of halide ions for two different disk currents (-O-, -0-) 10s A ...

Figure 4. Percentage of oxidation of as a function of disk current for a...

Figure 8 (a) Rotating disk current-potential curve for a 1 mmol/1 solution of [(salen)-... 

With a concentric pair of electrodes the product from the disk electrode, which is produced at a given potential, is conveyed centrifically to the ring electrode (Figure 3.12). The latter usually is controlled at a different potential such that the product can be monitored. Because the relationship between the ring current and the disk current has been quantitatively established, the ring-disk electrode provides a means of measuring the kinetics of post-electron-transfer reactions of electrode products. 

The relationship between the disk current (iD) and the ring current (iR) depends on the rate of movement (velocity) of the product species from the disk to the ring electrode. However, only a fraction of the disk-electrode products will reach the ring-electrode surface. Thus, each ring-disk electrode must be calibrated with a well-behaved reversible couple to determine its collection efficiency (N) ... 

Example 4.1 A rotating ring-disk electrode (1600 rpm) yields a disk current of 12.3 pA for the oxidation of a 2 x 10 3M potassium ferrocyanide solution. Calculate the reduction current observed at the surrounding ring using a 6 x 10 3M potassium ferrocyanide solution and a rotation speed of 2500 rpm (N = 0.33). 

Koutecky-Levich plot — The diffusion-limited current fiim> diff at a -> rotating disk electrode is given by the -> Levich equation based totally on mass-transfer-limited conditions. The disk current in the absence of diffusion control, i.e., in case of electron transfer control, would be... 

The initial mass and lifetime of gas in circumstellar disks affect both the formation of giant planets as well as the formation of terrestrial planets. According to the widely accepted scenario of giant-planet formation, rocky cores need to reach several M before being able to accumulate a substantial amount of gas from the protoplanetary disk. Current models require from a few to 10 million years to form Jupiter-like planets at 5AU (see e.g. Lissauer Stevenson 2007), meaning that primordial... 

Figure 8. RRDE experiment illustrating the disk current and emission intensity as a function of the disk potential in a cell containing TMPD and DPA. The ring current is fixed at +0.35 V to generate TMPD+ and the disk potential is scanned. From Maloy and Bard, J. Am. Chem. Soc , 1971, 93, 5968, with permission from the American Chemical Society.

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By inspection of Eqs. (87)-(89) it is seen that measurement of the rotating disk current may provide information concerning the value of n, C%, Da, vy., or ks,f if the remaining parameters are known. Accuracy in determination of n values depends mainly on how accurately the value of Da for the compound in question is known. For the majority of organic compounds, diffusion coefficients in the common organic solvents have not been measured, but comparison with a suitable compound of similar structure for which both n and D are known can often solve the problem (assuming the same value of D for the two substrates). Since Da appears only to the power of in Eq. (88), the resulting n value is only moderately dependent on variations in the Da estimate. The measurement of diffusion coefficients by means of the RDE technique has been the subject of a number of papers [248,253] and need not be treated further at this place. 

In the transient regime, a fraction of the disk current I disk ° ay be stored at the disk surface as the charge q, implied in the Faradaic process through the formation of adsorbed intermediate species (2-D) or of films (3-D). From the conservation of electrical charge,... 

Fig. 5 Oxygen reduction at a rotating ring(Pt)-disk(Pt) electrode in 0.1 N H2SO4. PO2 = 1 atm to = 120jr s o disk current, x ring current the 0.1 N H2SO4 solution has been purified by electrolysis (5 to 10 h) at 0.3 V/SHE, on a Pt gauze (Reproduced with permission from Ref. 57).

Diffusion coefficient of the substrate (Dg) and diffusion coefficient of the electron-exchange (D ) were calculated from cyclic and disk current voltammograms by using the Koutecky-Levich equation and Fick s first law (14, 15) (Table II). Dg in the polymer domains was estimated as 10 - 10 cm /sec, much smaller than in solution (10 cm /sec). Dg is affected by charge density of the polymer domain, e.g., the diffusion of cations is suppressed in the positively charged domain composed of cationic polyelectrolyte, while anions moves faster. A larger Dg value was observed, of course, for the porous film and not for the film with high density. On the other hand, Dg in the polymer domain was also very small, i.e. 10" - 10" cm /sec. This may be explained as follows. An electron-transfer reaction always alters the... 

This can be written in terms of the disk current (9.3.30), which would be observed under identical conditions for a disk of radius rj to yield... 

This problem can be solved in terms of dimensionless variables using the Laplace transform method and results have been given in terms of Airy functions (18, 19). It turns out that the ring current is related to the disk current by a quantity A, the collection efficiency,... 

If the disk current is changed to a finite value, /o, the flux of O to the ring will be decreased. The extent of this decrease will be the same as the flux of stable product R to the ring in a collection experiment—NId- Hence the limiting ring current /r / is given by... 

Thus, when the disk current is at its limiting value, the ring current is decreased by the factor (1 - This factor, always less than unity, is called the shielding... 

Figure 9.5.1 Disk-current transient for potential step at the disk Curve is simulated points are from theoretical equations given by Bruckenstein and Prager (O) and Siver ( ). [From K. B. Prater and A. J. Bard,...

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