1 / 6

Thus far we have been interested in: - MT (diffusion) - ET (kinetics)

Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry. 8.1. Thus far we have been interested in: - MT (diffusion) - ET (kinetics). How do we measure each one? → chemically reversible? → MT by itself. E applied. i c only O + n e R. +.

nysa
Download Presentation

Thus far we have been interested in: - MT (diffusion) - ET (kinetics)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry 8.1 Thus far we have been interested in: - MT (diffusion) - ET (kinetics) How do we measure each one? → chemically reversible? → MT by itself Eapplied iconly O + ne R + - E vs. Ref ia • ET kinetics by itself • MT + ET mixed! • Do we need wave shape? NO! How can we do it? (the above?)

  2. Response i(t) ? time (sec) Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry 8.2 Consider an E-step experiment in unstirred soln: Excitation Efinal Eapplied Einitial time (sec) t = 0 The response is affected by: - Efinal - k0 - Do - Co* Keeping Do and Co* out of the picture, we need To worry about Efinal and k0. Let’s assume two cases (for the moment): Case I: k0 is fast (rev. ET) Case II:k0 is slow (irrev. ET) Looking at Case I, we can be at various potentials, Eapplied. Let us choose two situations: situation A: Eapp > EMT (but not too +) situation B: Eapp≤EMT

  3. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry 8.3 Case I A: If stirred and scan E: ic EF Ei -E vs.Ref Ei EF ia time (sec) t = 0 MT limited, even if no stirring. Thus, but rather MT. Must solve FSL with necessary boundary and initial conditions: Potential Step Boundary Controls (Describes E step conditions) at t > 0 Semi-Infinite Diffusion Boundary Conditions (No at sufficient x) at t > 0 (Before Eapp, the soln is homogenous) at t = 0 Initial Conditions

  4. 1.0 0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 x, cm x 103 Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry 8.4 We now can do the math (see Appendix A): “l“ Get: Recall: and and Do this for: DO = 10-5 cm2 s-1 Basically: 0.001s 0.01s 0.1s 1s So, the current decays with time.

  5. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry 8.5 Recall FFL: Sometimes we call the Cottrell Current the Diffusion Current, id(t) Substituting Cottrell Equation Characteristics of Cottrell Equation i vs. t-1/2 will be linear for diff. controlled O + ne R reaction and no “pre-kinetics” of O: +ne-

  6. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry 8.6 Cottrell Limitations with which to be concerned: • Double Layer concerns/Rs concerns- • At early t we have bothif and iDL flowing. • Thus, imeas > if DL imeas ideal Convection! o o t-1/2 (sec-1/2) • Convection concerns (long t)- • imeas is larger than expected. • Instrument Concerns- • Potentiostat current meas. limits • (voltage compliance) • Recording Device (op ramps)

More Related