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Generation and Interpretation of Cyclic Voltammetric Responses: What you see may be telling you what you’ve got; and maybe not. Stephen W. Feldberg Guest.

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Presentation on theme: "Generation and Interpretation of Cyclic Voltammetric Responses: What you see may be telling you what you’ve got; and maybe not. Stephen W. Feldberg Guest."— Presentation transcript:

1 Generation and Interpretation of Cyclic Voltammetric Responses: What you see may be telling you what you’ve got; and maybe not. Stephen W. Feldberg Guest Scientist, Brookhaven National Laboratory Upton, NY 11973

2 Objectives: I. Optimizing the experiment –Minimizing experimental artifacts –Maximizing the information about the operative chemistry II. Honing interpretive skills. Suggested reading (in whole or in part; *authors at Monash!): Nicholson, R. S.; Shain, I. Analytical Chemistry 1964, 36, 706. *Bond, A. M. Modern Polarographic Methods in Analytical Chemistry. Marcel Dekker: New York, Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications Second Edition; John Wiley and Sons: New York, Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, pp * Bond, A. M. Broadening Electrochemical Horizons: Principles and Illustration of Voltammetric and Related Techniques. Oxford University Press, * Oldham, K. B.; Myland, J. C. Fundamentals of Electrochemical Science. Academic Press, San Diego, Saveant, J-M. Relevant papers.

3 Optimizing the Experiment Minimizing Experimental Artifacts Assume perfect electronics: 3-electrode system with – perfect function generator: v = |dE/dt| = v + = -v - Know thine protocol!! – perfect potentiostatic control of E W vs E ref – perfect current measurement

4 Optimizing the Experiment Minimizing Experimental Artifacts Experimental Design Objectives: Minimal uncompensated resistance (R u ) Minimal electrode capacitance (C el ) Minimal stray capacitance (C stray ) Requisite theoretical tools available for chosen design Maximum relevant electrochemical window; function of –Solvent –Supporting Electrolyte –Working electrode material –Impurities –Analyte –Temperature –v Safety Minimal R u C el, iR u

5 Optimizing the Experiment: Minimizing Experimental Artifacts: Cell Design – Electrodes (Working, Reference, Auxiliary) material geometry (available theory?) size location isolation – physical – capacitive – chemical – Quiescence- no adventitious stirring caused by MHE (Monash Hood Effect) or any other source of vibration gas flow through or over solution density gradients (electrochemically induced) temperature gradients – Temperature Control – Integrity (“air” tight; vacuum tight) Solvent Supporting Electrolyte (excess assumed!) Choose analyte concentration selection and purification; maximize relevant electro- chemical window.

6 Optimizing the Experiment Maximizing the Information about the Operative Chemistry Key Factors: CV Protocol Additional Chemical Information from –Other electrochemical experiments Coulometry Spectroelectrochemistry Chronoamperometry Square wave/a.c. –Nonelectrochemical experiments EPR NMR X-ray; UV-VIS-IR Structural

7 Optimizing the Experiment: CV Protocol The basic CV protocol allows the user to control: scan rate v = |dE/dt| = v + = |v - | E initial =(?) E start, E rev, E end n cycles where – E start, E rev, E end (n cycles = 1) – E start, E rev, E end, E rev, E end (n cycles = 2) – E start, (E rev, E end ) k (n cycles = k) Just what values do you choose?

8 Optimizing the Experiment: Choosing the CV Protocol that will Maximize the Information about the System of Interest So you’ve done the basics; you’ve: Read Nicholson and Shain, Analytical Chem. 36 (1964)706. Found a suitable solvent for your analyte Found a suitable supporting electrolyte (SE) Begged, borrowed or stolen a cell, electrodes and potentiostat Run a background –SE + whatever (e.g., buffer, ligand, acid, base…..) –no analyte Run a simple CV with the analyte –Chosen a value of v so that  expt <  max (e.g, 60 s) Hopefully you will then:

9 Change voltage ranges within the voltage window for the system See what happens when n cycles = 2, 3, 4………50 Run CVs (with and without analyte) over a range of v consistent with working electrode size & geometry Change the concentration of analyte Look at T-dependence Check theory – to confirm/exclude specific mechanistic models – identify artifacts Re-evaluate requirements and consider – optimizing/modifying cell/electrodes – using different solvent, SE, etc. – variations addressing specific interests

10 Honing Interpretive Skills: A Course unto Itself Identify some “basic” mechanisms: E A + e = B EE A + e = B; B + e = C; 2B = A + C EC A 1 + e = B 1 ; B 1 = B 2 EC’ A + e = B; B + P = A + Q EC 2 A + e = B; 2B = B2 CE Y = A; A + e = B ECE A 1 + e = B 1 ; B 1 = B 2 ; B 2 + e = C 2 ; B 1 + B 2 = A 1 + C 2 Sq. Schm. A 1 + e = B 1 ; B 1 = B 2 ; A 2 + e = B 2 ; A 1 = A 2 ; A 1 + B 2 = A 2 + B 1 A 1 + e = B 1 ; B 1 = B 2 ; A 2 + e = B 2 ; A 1  A 2 ; A 1 + B 2 = A 2 + B 1 Use DigiSim or a simulator of choice (?) to explore the behavior of selected basic mechanisms (couple with relevant reading). And do just what?

11 Honing Interpretive Skills For starters focus on the easy ones - the E, EC and EC 2 : Explore the effects on the CV of changing – v – D-values – electrode geometry (planar, spherical, cylindrical, disk, band) & size –stirring Identify the relevant dimensionless parameters required to completely describe the mechanisms. Assume planar geometry and explore dependence of the CV response on: – v, E start, E rev, E end, n cyc, k s ’s, Eo’s, k’s,  ’s, K eq ’s (suggestions: assume fast ET first; for slow ET take a look at Marcusisan kinetics )

12 To Be Continued!


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