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Rotated Disk Electrode Voltammetry RDEV

Rotated Disk Electrode Voltammetry RDEV. insulator. r 1. w in s -1 , so f in rps revolutions per second. “dead” or diffusion layer. Conductor electrode. Laminar flow occurs up to a point, at too high w , we find that turbulent flow occurs. This is when the value

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Rotated Disk Electrode Voltammetry RDEV

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  1. Rotated Disk Electrode Voltammetry RDEV insulator r1 w in s-1, so f in rps revolutions per second “dead” or diffusion layer Conductor electrode Laminar flow occurs up to a point, at too high w, we find that turbulent flow occurs. This is when the value exceeds the Reynold’s number for that particular fluid with a given kinematic viscosity, n, in cm2 s-1. ~ Re=2x105 (Poise) Look in Table 9.2.1 So, w should be ~ 2 ×105n/r2, but other limitations actually mean w < 1000 s-1 or f 10,000 rpm. On the low w side, must rotate fast enough to establish constant, homogeneous supply of material to electrode surface. w > 10 s-1

  2. If one applies a potential which is that needed to obtain mass-transport limited conditions, then what is i? consider: - hydrodynamics - diffusion Why? O + n e R only O present C 0 x d Real profile Must scan E slowly, n < 100 mV s-1 Recall that d is F(1/w). So, How solve? As we did before except incorporate hydrodynamics. Also: Two Cases: 1. Reversible use q expression (Nernst) 2. Before reach MT limit and - irrev. - quasi-rev. ET rxns.

  3. no iDL effects. Case 1:Levich Equation Know: Know: Levich Layer Levich plot If reaction is D–C, then ilim vs. w1/2 is linear with zero intercept. Also if ET reversible: No dependence of wave shape on w! E1/2 Then plot of Eapp vs. will be straight with slope Case 2A: Totally irreversible; O only But, kf is F(E) , so we denote this kf(E). We call this current iK and it is: This is the Kinetic current. So, at high enough -h, we should get kf ?? NO. Zero

  4. We have no ET effects at -h, Irrev. Rev. for i, so we merely get ilim B-V / No MT ic + - ia MT effects E vs Ref ET effects So, if we could vary E and measure iK, we could get ?? Yes! How? Turns out we have: Koutecký – Levich or Inverse Levich plot So make plot of vs. at a given Eapp. E1 E2 E3 E4(on i lim) Same slopes In each. E1 turbulence Slope is vary Eapp intercept is w-1/2 w-1/2 i iK Levich line E1>E2>E3>E4 (for E>E4) More - w1/2

  5. Case 2B: Quasi – Reversible for O and R MT? Do this on your own. Fnc(E) ET Now we have both kfand kb a function of n. Thus, the Koutecký – Levich plots do not have same slope for various Potentials (h). Problems! Minimize errors by using small potential range near the foot of the wave where i is not changing so drastically.

  6. RRDE dR r1 (R) Ring (D) Disk r3 dD r2 dR = dD r1 disk radius r2 – r1 gap r3 – r2 width of ring The collection Efficiency, N, is defined as It is a Function of electrode geometry but is independent of etc. if R is stable. kchem If R Z occurs, then Nexptl < NtheoandN = F(w).

  7. For RRDE Collection Experiments: 1. ERing is held positive enough so as to oxidize any R. 2. No bulk 3. EDisk is scanned. 4. iDisk is measured. 5. iRing is measured. iD,C O + ne R ERing iD,lim + - EDisk vs. Ref iR,lim R O + ne iR,a Review:

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