11/29/11 1 Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Cyclic Voltammetry 1- Instrumentation 2- Linear Voltammetry -25B, 25C Suggested exercises: 25-2, 25-3, 25-4, 25-10 Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Figure 25-8, Section 25B Three-electrode cell • N2 sparging needed to avoid oxygen waves (Fig 25-16) • Inert supporting electrolyte (~100x higher concentration to reduce migration of reactant) • Working electrode. Vary potential as a function of time. Maintain polarized • Reference electrode. Saturated calomel (SCE) or a silver-silver chloride electrode (Section 23-B) • Counter electrode. Pt wire to conduct electricity from the source Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Working electrodes Figure 25-3, Section 25B Disc Flow Microelectrode Hanging drop Materials: Pt, Carbon, Mercury For useful ranges see Figure 25-4. Positive potential limit: oxidation of water Negative potential limit: reduction of water11/29/11 2 Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Operational amplifier potentiostat Figure 25-2, Section 25B Eo, See equation 25-1 ZRE = 1011 Ω I ≈ 0 VCE = Eo = VRE – VWE ICE ≈ IWE Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Voltammetry: Current versus excitation voltage Figure 25-1, Sections 25A Linear scan Triangular scan Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Figure 25-13, Sections 25C Concentration profiles during electrolysis A + ne- P CAO CPO € Eappl= EAo−59.2nlogCPoCAo −ErefEq. 25-311/29/11 3 Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Figure 25-6, Section 25B Linear-Scan Voltammogram EO = -0.26 V Voltammetric wave E1/2 = half wave potential Il = kA • CA Eq. 25-6 i = kA • (CA –Cao) Eq. 25-5 Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Figure 25-6, Section 25B Current-Voltage Relationships for Reversible Reactions € Eappl= EAo−59.2nlogkAkP −59.2nlogiil−i −ErefEq. 25-11 € Eappl= E1/ 2= EAo−59.2nlogkAkP −Eref≈ EAo−ErefEqs. 25-12, 25-14 € Eappl= E1/ 2−59.2nlogiil−i Eq. 25-13 Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Figure 25-14, Section C Voltammogram of mixtures Difference in E1/2 = 0.1 V Difference in E1/2 = 0.2V For n = 2 electrons, E1/2 differences of 0.1-0.2 are sufficient11/29/11 4 Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Figure 25-15, Section C Anodic and cathodic waves Anodic wave Cathodic wave Mixed wave Lecture 33 – Nov 30 Chem 4101 – Fall 2011 Air-saturated 0.1 M KCl Figure 25-16, Section
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