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:
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
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, 1980. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications Second Edition; John Wiley and Sons: New York, 2001. Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, pp 267 -353. * Bond, A. M. Broadening Electrochemical Horizons: Principles and Illustration of Voltammetric and Related Techniques. Oxford University Press, 2002. * Oldham, K. B.; Myland, J. C. Fundamentals of Electrochemical Science. Academic Press, San Diego, 1994. Saveant, J-M. Relevant papers.
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
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
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.
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
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?
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:
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
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?
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 )