Instrumental Lecture 24 Electrochem

Instrumental Lecture 24 Electrochem - In all...

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Unformatted text preview: 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) 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 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) Concentration polarization Reaction is A + eP Diagram for diffusion Before power is turned on the analyte (A) is at its maximum concentration throughout the solution Product (P) is zero throughout Fick's Laws describe diffusion 1st 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) Skipping to the Electrochemical Solution Number of electrons Faraday's constant Electrode area Concentration Diffusion coefficient Time Current is the flux of electrons at the electrode surface Experiment showing how Cottrell equation describes current as a function of time Voltage applied to cell begins at V1 where no reaction occurs and is stepped up to V2 causing electrode process to begin and a current spike results. Current drops off with time according to the Cottrell equation since material must diffuse to the electrode surface in order to react. current (i) i time Mass Transport or Mass Transfer 3) 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 Nernst Diffusion Layer Concept for stirred solution & stationary electrode Electrode Nernst diffusion Layer () (stagnant solution) Turbulent mixing region (bulk solution) Convective Mass Transport Electrode converts A + eP at surface Fick's first law applied to stagnant layer = Cbulk - Csurface Cbulk - Csurface i = nFAD For stirred solutions i = nFAD Cbulk 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 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 Varying potential (E) linearly at a stationary electrode in a stirred solution for Red Ox + e- Ox + e- Red Reversible Process Overpotential or Overvoltage Icathode Irreversible Process Ecathode Overpotential (overvoltage) = potential to achieve the same current as if process was reversible Large overvoltage process more irreversible For reversible processes Eoverpotential = 0 Overvoltage characteristics: 1) increases with current density (current/area) 2) decreases with increasing temp 3) high for reactions producing gases 4) depends on electrode composition 5) difficult to specify exactly electrode surface Other Electrochemistry Fundamentals 1) Kinds of current a) Faradaic current = current due to electron transfer (usually what we are interested in) b) Capacitive current = current that flows as electrode surface charges up like a capacitor positively charged electrode negatively charged electrode attracts negative Ions to surface attracts positive Ions to surface If we change the electrode potential current flows to carry charge to the surface & charge up electrode = capacitive current Capacitances are significant, often 40 60 F/cm2 of electrode area In methods involving potential scan, capacitive current is the major source of background detection limits occur at those concentrations where Faradaic current is too small to be adequately distinguished from Icapacitive 2) Kinds of Potentials a) Junction Potentials already discussed b) Potential Due to IR Drop when current flows, solution has some electrical resistance, therefore there is an IR drop across the solution (i.e., between electrodes) V = IR Three ways to handle this interfering potential: 1) work at low currents so V is small 2) Minimize R using a supporting electrolyte which in turn minimizes V 3) Correct for V by making a single measurement of R and measuring I throughout the experiment (not usually done) 3) Kinds of Electrodes a) Stationary e.g., Pt wire in solution b) Self-Renewing e.g., dropping Hg electrode c) Hydrodynamic rotating, vibrating, wall-jet 4) Electrode Materials Want an electrode material that is not easily oxidized or reduced Most common materials: Hg, Pt, Au, C (graphite) Available potential range max cathodic potential limited by reduction of solvent potential is e.g., for H2O H2(g) pH dependent 2 H+ + 2 e- Electrode Materials (cont.) For Pt, Au, C max anodic potential limited by solvent oxidation e.g., for H2O potential is 2 H2O pH dependent O2(g) + 4 H+ + 4 e- For Hg max anodic potential limited by oxidation of Hg Large usable potential range because of high overvoltage for production of gaseous products Because Hg is a liquid which has a very smooth surface, it has a particularly high overvoltage for H+ reduction, therefore it is possible to go to more cathodic potentials with Hg than for Pt, Au, or C The purer the Hg, the higher the overpotential for H2 evolution For C (graphite) Using graphite electrodes, necessary to impregnate pores with wax or other hydrophobic material to prevent solution from seeping in and changing the electrode surface area with time Carbon paste electrodes can be prepared by mixing carbon powder with Nujol (of Nujol mull fame) Glassy carbon electrodes are the more modern version of carbon electrodes Electrogravimetry Apply potential to cause a soluble species to reduce or deposit on a solid electrode e.g., reduce Cu2+ onto Pt cathode Cu (metal on Pt) Cu2+(aq) + 2 e Change in weight of dried cathode before & after deposition = amount of Cu in sample Assumptions: All Cu is plated out Nothing else plates out Cu2+(aq) + 2 eCu Eo = 0.34 v H2O Eo = 1.23 v O2 + 2 H+ + 2 e------------------------------------------ O2 + 2 H+ + Cu Cu2+ + H2O For zero current Ecell = ECu EO2,H2O Use Nernst Equation with Eo's & concentrations . 1 . 0.059 Ecell = 0.34 log 2+ 2 [Cu ] - 0.059 . 1 . 1.23 log 0.5[H+]2 = - 0.91 v 2 (PO2) Apply potential more negative than 0.91 v to force system to reach an equilibrium where [Cu2+] is small (like 99.9% lower than the approximate starting concentration) Choose cathode potential to reduce equilibrium [Cu2+] to any desired value Must be cautious not to set potential too far negative to make sure nothing else is reduced Normally set conditions so that reduction is complete in a reasonably short period of time ...
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This note was uploaded on 02/13/2012 for the course UML 84.314 taught by Professor Ryan during the Fall '11 term at UMass Lowell.

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