Electrochemical Methods - Chapter 11 PDF

# Electrochemical Methods - Chapter 11 ## Signals: current, potential, charge ## Interfacial Electrochemistry: - Current, potential, or charge depends on species at electrode interface - Measure concentrations: - Electro- - Analytical - Chemistry - Characterize reactivity ## Bulk Electr...

# Electrochemical Methods - Chapter 11 ## Signals: current, potential, charge ## Interfacial Electrochemistry: - Current, potential, or charge depends on species at electrode interface - Measure concentrations: - Electro- - Analytical - Chemistry - Characterize reactivity ## Bulk Electrochemistry: properties of solution (e.g. conductivity) ### First: Oxidation and Reduction - loss of e - OIL - gain of e - RIG - occurs at an anode - occurs at a cathode ## Units & Definitions | Symbol | Name | Units | Notes | |---|---|---|---| | q | charge | Coulombs (c) | 1e- = 1.602 x 10^-19 C/e- | | i | current | Amperes (A) | 1A = 1 C/s, 1 coulomb per second, also: q = ∫idt (current) x (time) = charge | | E | potential| Volts (V) | 1V = 1 J/C, potential energy/charge | | R | resistance | Ohms (Ω) | Ohm's Law: E = iR | ## F= Faraday's constant = charge of 1 mole of e- = 96485 C/mol. ## Faraday's Law: q = nFN - q = charge - n = # moles of molecules - F = Faradays constant - Calculate moles ox or red from measured charge # 5 Important Concepts ## 1) Potential determines analyte's form - Change in electrode potential drives oxidation or reduction - See Slide 2 ## E° = Standard Reduction Potential (standard state conditions) - Unit activity (1M, 298K, 1 atm) - At potentials: - E > E° analyte is in oxidized form - E < E° analyte is in reduced form ## Ex: - E°Fe3+/Fe2+ = +0.771V at +0.500V - reduced form (Fe2+) - E°Sn4+/Sn2+ = +0.154V at +0.500V - oxidized form (Sn4+) ## See Ladder diagram on Slide 2 ## 2) Interfacial conc. may not equal bulk concentrations - Consider Nernst Eqn - E = E°Fe3+/Fe2+ - (0.05916/n) * log([Fe2+]/[Fe3+]) - E = electrode's potential - E° = standard reduction potential for Fe3+ + e- → Fe2+ - R = gas const. - T = temp in K - F = Faraday const. - n = # e- (1 for Fe3+/Fe2+) - Potential of electrode controls [Fe2+]/[Fe3+] - See Slide 3 - At E = +1.00 V, Fe3+ is not reduced. [Fe3+interface] = [Fe3+bulk] - At E = + 0.500V, Fe3+ is reduced. [Fe3+interface] < [Fe3+bulk] - [Fe2+interface] > [Fe2+bulk] ## Concentration gradients arise in the diffusion layer. ## 3) The analyte may participate in other reactions. - Oxidation + reduction + other reactions influence concentrations - Ex: Fe3+ adsorbs and sticks to electrode interface. - Fe(OH)2+ forms → reduces [Fe3+](aq) - (Fe2+)aq + (OH-)aq → Fe(OH)(s) ## 4) Current is a measure of redox reaction rate - Fe3+ + e- = Fe2+ - Reduction of Fe3+ requires 1e- from the electrode - Oxidation reaction happens at a 2nd electrode - X = X+ + e- - Some species X - Flow of e- between the electrodes is a measure of the rate of Fe3+ reduction. - At equilibrium current flow = 0. ## 5) Cannot control current and potential simultaneously. - Start at equilibrium → change potential E - [Fe2+]/[Fe3+] will change to satisfy Nernst Eqn - Current will flow as redox rxn proceed until equilibrium is reached. - Start at equilibrium → pass current forcing Fe3+ + e- → Fe2+ - E will change to satisfy Nernst Eqn because [Fe2+]/[Fe3+] changes - E will stabilize when equilibrium is reached # Electrodes & Control / Measure Current and Potential ## Simplest EC cell: 2 electrodes - Working (indicator) electrode: sensitive to analyte's concentration - Counter electrode: completes circuit & provides reference potential - Ideally, reference potential remains constant, but not always possible. - If not possible: - Working (indicator) electrode: same as above - Reference electrode: potential remains constant (no current flows through) - Auxiliary electrode: completes the circuit ## 3-types of measurements: 1. Measure E when i = 0 - Potentiometer | Circuit diagrams p.641-643 2. Measure E and control i - Galvanostat 3. Measure i and control E - Potentiostat ## See slide 4 - Potentiostat # Interfacial Electrochemical Techniques - Many different techniques! - Differ in what is controlled and what is measured. - Differ in how things are controlled (linear, pulsed, cyclic) ## See Slides