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OKAY – How detect an EC reaction? k chem O + n e R Z

Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry. OKAY – How detect an EC reaction? k chem O + n e R Z we assume k rev electroinactive at E i

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OKAY – How detect an EC reaction? k chem O + n e R Z

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  1. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry OKAY – How detect an EC reaction? kchem O + n e R Z we assume krev electroinactive at Ei If kchem krev or krev < kchem, then will not have a current for R O + ne So, a reversal technique is in order. ic Ei stirred E vs. Ref Ef - + ia unstirred tr – t = tf ic tr and tf are at the same point wrt the step time. tf tr DPSCA t ia t = 2t t = 0

  2. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry recalling we see The maximum ratio, if R is stable. If , R unstable at this value. decreasing stability of R 1. larger kchem 2. larger 1.0 0.293 morestable 1 2 So, a chemically irreversible reaction can be made reversible and D–C if t is small enough. But, if t too small, then tf is small enough such that reaction looks ET limited.

  3. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry Measuring instantaneous currents is not always easy. Chronocoulometry (CC) charge Ef E Ei i Q t 0 t t For O + n e R, plot Q vs. t1/2 If plot linear, then reaction is D-Controlled Q QDL QDL t1/2

  4. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry What if redox species (O) is adsorbed on electrode surface? Q Qads QDL(blank) only S.E. t1/2 is 2 – D conc. in mol/cm2 We assume that Not so, due to inherent Capacitance of adsorbed species on surface. How deal with this situation? Double Potential Step Chronocoulometry Q t

  5. Potential Step Methods – From Chronoamperometry to Double-potential Step Chronocoulometry Qf vs. t For adsorption: Qads + QDL sec1/2 QDL iff R not adsorbed! Now get Qads by subtraction. What about stability of R? (What else is DPSCC good for?) As kchem becomes faster 1.0 Completely irreversible 0 0 1.0 2.0

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