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Okay, let’s back up and think about what happens at

Okay, let’s back up and think about what happens at Electrode/Solution interface for O + ne - R:. K ads. M.T. O soln. O ads. O bulk. n e -. K ads. R bulk. R ads. R sdn. Rate of ET = F( E ), ignoring other “stuff”. (also). i is measure of flux.

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Okay, let’s back up and think about what happens at

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  1. Okay, let’s back up and think about what happens at Electrode/Solution interface for O + ne- R: Kads M.T. Osoln Oads Obulk ne- Kads Rbulk Rads Rsdn Rate of ET = F(E), ignoring other “stuff” (also) i is measure of flux Sometimes better to normalize for A: (current density) Okay, if we are MT controlled, 1.) We can allow diffusion 2.) We can stir (hydrodynamic) 3.) We can allow migration

  2. But, if we use S.E. (~100x [analyte]) - Reduce Rs (tdl and iRs) - Reduce migration So, we are left with 1) Diffusion + 2) Convection 1 Occurs only when a quiet solution is inspected 2 + 1 Occur when we stir, why? We will rotate electrode: motor Soln Flow Get Laminar flow solution flows parallel to electrode surface. See page 337 of B&F.

  3. But we have a stagnant layer due to frictional losses. No stirring motion Di This layer is the “Nernst Diffusion Layer” and is also called the “diffusion layer.” Its thickness is denoted as d, in cm or mm, usually. Look at Fe3+ concentration gradient at +1.0V vs. SCE if Fe3+ + e- Fe2+ and no Fe2+ in solution initially. C(x) C0* Eapp = + 1.0V 0 X (mm) 0

  4. Recall: CO* C(x) CR* = 0 at w2 “old” delta at w1 + 0.2V H+ ½ H2 at Pt ic +1.0 ~ + 0.528V E vs. Ref (SCE) ia Now at 0.2 V, at ilim, so flux is max at this w: 0 0 x For any geometry of electrodes etc. Rotate faster! CO* C(x) CR*= 0 w2 > w1 0 x 0

  5. At , C0* C(x) Be able to draw all cases! CR* = 0 0 x 0 ic ilim E vs. Ref E at ½ilim is E½ ia or See why later so Mass-Transfer Coefficient (cm/s) or

  6. Note: Flux of R away = 0 What is general form of i equation when we are anywhere on the i – E curve? or Solving for Conc of O at Electrode: Now for R:

  7. at substituting Canceling common terms Recall I said before:

  8. So what is the deal? Well: ic Reversible Fe3+/2+ - + Eapp ? ia iff But they often do not! So, * * Slope is for So: Eapp

  9. Also, if irreversible, ic Rev. Irrev. + - Eapp ia < ; it takes more to get reaction going at same rate of E. T. ! For any O/R couple: Check out the other cases: 1. O+R present (Do i-E plot for them) 2. R insoluble (metal deposits) Test Material

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