1	Voltammetry Methods based on an electrolytic cell Apply potential or current to electrochemical cell & concentrations change at electrode surface due to oxidation & reduction reactions Can have 2 or 3 electrodes Stirred or unstirred solution Measure current or voltage
2	"In all electrochemical methods," In all electrochemical methods, the rate of oxidation & reduction depend on: 1) rate & means by which soluble species reach electrode surface (mass transport) 2) kinetics of the electron transfer process at electrode surface (electrode kinetics), which depend on: a) nature of the reaction b) nature of electrode surface c) temperature (we don’t have much control over #2)
3	Mass Transport or Mass Transfer Migration – movement of a charged particle in a potential field – generally bad (important for conductance & electrophoresis) In most cases migration is undesirable and can be eliminated by adding a 100 fold excess of an inert electrolyte (i.e., electrochemically inert – not oxidized or reduced) Inert electrolyte does the migrating, not the analyte
4	Mass Transport or Mass Transfer 2) Diffusion – movement due to a concentration gradient. If electrochemical reaction depletes (or produces) some species at the electrode surface, then a concentration gradient develops and the electroactive species will tend to diffuse from the bulk solution to the electrode (or from the electrode out into the bulk solution)
5	Concentration polarization Reaction is A + e^- à P Diagram for diffusion
6	Fick’s Laws describe diffusion 1^st Law Where J = flux of material i.e., moles passing a 1 cm² plane at point x & time t (mol/cm²/sec) D = diffusion coefficient (cm²/sec) C = concentration t = time (sec) from when power is turned on x = distance from electrode surface (cm)
7	"Using an expression for Conservation..." Using an expression for Conservation of Mass And combining with Fick’s First law gives Fick’s Second Law
8	"Solving Fick’s Laws for particular..." Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions
9	Skipping to the Electrochemical Solution
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11	Quantities & Units
12	Mass Transport or Mass Transfer Convection – mass transfer due to stirring. Achieved by some form of mechanical movement of the solution or the electrode i.e., stir solution, rotate or vibrate electrode Difficult to get perfect reproducibility with stirring, better to move the electrode Convection is considerably more efficient than diffusion or migration = higher currents for a given concentration = greater analytical sensitivity
13	Nernst Diffusion Layer Concept for stirred solution & stationary electrode
14	Convective Mass Transport Electrode converts A + e^- à P at surface
15	Fick’s first law applied to stagnant layer
16	Mass Transport vs Electrode Kinetics Experimentally rate of electron transfer is fast for many processes so can assume: - current depends only on mass transfer - surface concentrations are in equilibrium with applied potential as expressed by the Nernst equation Processes which satisfy these assumptions are known as electrochemically reversible
17	"A process may be reversible..." A process may be reversible under one set of conditions and irreversible under other conditions. Process is more likely to be irreversible if - it involves a high current - employs a rapid potential scan If a process is irreversible, then the rate of reaction at the electrode surface (i.e., current) will be slower than predicted from mass transfer considerations alone