Instrumental Lecture 24 Electrochem

Instrumental Lecture 24 Electrochem - In all...

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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)
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Mass Transport or Mass Transfer 1) 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
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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)
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Concentration polarization Reaction is A + e - P Diagram for diffusion Before power is turned on the analyte (A) is at its maximum concentration throughout the solution Product (P) is zero throughout
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Fick’s Laws describe diffusion 1 st Law Where J = flux of material i.e., moles passing a 1 cm 2 plane at point x & time t (mol/cm 2 /sec) D = diffusion coefficient (cm 2 /sec) C = concentration t = time (sec) from when power is turned on x = distance from electrode surface (cm)
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Skipping to the Electrochemical Solution Current is the flux of electrons at the electrode surface Time Diffusion coefficient Concentration Electrode area Faraday’s constant Number of electrons
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