VOLTAMMETRY A.) Comparison of Voltammetry to Other Electrochemical Methods 1.) Voltammetry: electrochemical method in which information about an analyte is obtained by measuring current (i) as a function of applied potential - only a small amount of sample (analyte) is used Instrumentation – Three electrodes in solution containing analyte Working electrode: microelectrode whose potential is varied with time Reference electrode: potential remains constant (Ag/AgCl electrode or calomel) Counter electrode: electrode (Hg, Pt, C) that completes circuit, conducts e - from signal source through solution to the working electrode Supporting electrolyte: excess of nonreactive electrolyte to conduct current −NaCl or KCl in aqueous solvent −Tetrabutylammonium hexafluorophosphate (TBAPF6) in non-aqueous solvent (e.g., MeCN, DMSO, DMF)

VOLTAMMETRY Instrumentation – Three electrodes in solution containing analyte

Apply Linear Potential with Time Observe Current Changes with Applied Potential 2.) Differences from Other Electrochemical Methods a) Potentiometry: measure potential of sample or system at or near zero current. voltammetry – measure current as a change in potential b) Coulometry: use up all of analyte in process of measurement at fixed current or potential voltammetry – use only small amount of analyte while vary potential

3.) Voltammetry first reported in 1922 by Czech Chemist Jaroslav Heyrovsky (The Father of Polarography) Later given Nobel Prize (1959) for method. B.) Theory of Voltammetry 1.) Excitation Source: potential set by instrument (working electrode) - establishes concentration of Reduced and Oxidized Species at electrode based on Nernst Equation: - reaction at the surface of the electrode E electrode = E 0 - log n (a R ) r (a S ) s … (a P ) p (a Q ) q … Apply Potential

Current is just measure of rate at which species can be brought to electrode surface Two methods: Stirred - hydrodynamic voltammetry Unstirred - polarography (dropping Hg electrode) Three transport mechanisms: (i) migration – movement of ions through solution by electrostatic attraction to charged electrode (ii) convection – mechanical motion of the solution as a result of stirring or flow (iii) diffusion – motion of a species caused by a concentration gradient

Voltammetric analysis   Analyte selectivity is provided by the applied potential on the working electrode.   Electroactive species in the sample solution are drawn towards the working electrode where a half-cell redox reaction takes place.  Another corresponding half-cell redox reaction will also take place at the counter electrode to complete the electron flow.  The resultant current flowing through the electrochemical cell reflects the activity (i.e.  concentration) of the electroactive species involved Pb e - Pb E O = V vs. NHE K + + e - K E O = V vs. NHE Pt working electrode at -1.0 V vs SCE SCE Ag counter electrode at 0.0 V X M of PbCl 2 0.1M KCl AgCl Ag + Cl -

Pb 2+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K V vs SCE Pb e - Pb K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Layers of K + build up around the electrode stop the migration of Pb 2+ via coulombic attraction Concentration gradient created between the surrounding of the electrode and the bulk solution Pb 2+ migrate to the electrode via diffusion

C) Types of Voltammetry 1.Polarography −first type of Voltammetry −controlled by diffusion, eliminates convection −uses dropping Hg electrode (DME) as working electrode; current varies as drop grows then falls off

a.Advantages of Hg Drop Electrode −High overpotential for reduction of H + Allows use of Hg electrode at lower potentials than indicated from thermodynamic potentials Example: Zn 2+ and Cd 2+ can be reduced in acidic solutions even though E 0 vs NHE = (Cd 2+ /Cd) and (Zn 2+ /Zn) −new electrode surface is continuously generated Independent of past samples or absorbed impurities −reproducible currents quickly produced 2H + + 2e - H 2 (g)(0V vs NHE)

b.Disadvantages of Hg Drop Electrode −Hg oxidation Around V vs. SCE Can not be used above a potential of V  Hg undergoes anodic dissolution ~ V vs. SCE and is oxidized to insoluble Hg 2 C l2 in presence of Cl - at zero V vs. SCE.  It cannot be used for anodic oxidation above V vs. SCE. −Non-Faradaic (charging/capacitance) current limits the sensitivity to ~ M residual current is > diffusion current at lower concentrations −cumbersome to use tends to clog, causing malfunction −Hg disposal problems mercury vapors are also very poisonous

2.Voltammetry (solid working electrode) Pb e - Pb E O = V vs. NHE K + + e - K E O = V vs. NHENote: Reference Electrode: SCE (saturated calomel electrode)Reference Electrode: SCE (saturated calomel electrode) SCE = V vs NHESCE = V vs NHE Thus, the E o of Pb 2+ = V vs SCEThus, the E o of Pb 2+ = V vs SCE Pt working electrode SCE Ag counter X M of PbCl 2 0.1M KCl AgCl Ag + + Cl -

At Electrodes Surface: E appl = E o - log n [M red ] s [M ox ] s at surface of electrode Applied potential If E appl = E o : 0 = log ˆ [M ox ] s = [M red ] s n [M red ] s [M ox ] s M ox + e - » M red If E appl << E o : E appl = E 0 - log  [M red ] s >> [M ox ] s n [M red ] s [M ox ] s

Current generated at electrode by this process is proportional to concentration at surface, which in turn is equal to the bulk concentration For a planar electrode: measured current (i) = nFAD A ( ) where: n = number of electrons in ½ cell reaction F = Faraday’s constant A = electrode area (cm 2 ) D = diffusion coefficient (cm 2 /s) of A (oxidant) = slope of curve between C Mox,bulk and C Mox,s  CA xx xx xx

As time increases, push banding further and further out. Results in a decrease in current with time until reach point where convection of analyte takes over and diffusion no longer a rate-limiting process.

Thickness of Diffusion Layer (  ): i = (c ox, bulk – c ox,s ) - largest slope (highest current) will occur if: E appl << E o (c ox,s. 0) then i = (c ox, bulk – 0) where: k = so: i = kc ox,bulk therefore: current is proportional to bulk concentration - also, as solution is stirred,  decreases and i increases nFAD ox   

i (  A) Potential applied on the working electrode is usually swept over (i.e. scan) a pre-defined range of applied potential M Cd 2+ in 0.1 M KNO 3 supporting electrolyte V vs SCE Working electrode is no yet capable of reducing Cd 2+  only small residual current flow through the electrode Electrode become more and more reducing and capable of reducing Cd 2+ Cd e - Cd Current starts to be registered at the electrode Current at the working electrode continue to rise as the electrode become more reducing and more Cd 2+ around the electrode are being reduced. Diffusion of Cd 2+ does not limit the current yet All Cd 2+ around the electrode has already been reduced. Current at the electrode becomes limited by the diffusion rate of Cd 2+ from the bulk solution to the electrode. Thus, current stops rising and levels off at a plateau idid E½E½ Base line of residual current

Combining Potential and Current Together Half-wave potential : E 1/2 = -0.5  E 0 - E ref E 0 = SCE for M n+ + me - ↔ M (n-m)+ E ½ at ½ i Limiting current Related to concentration

Voltammograms for Mixtures of Reactants Two or more species are observed in voltammogram if difference in separate half-wave potentials are sufficient 0.1V 0.2V Different concentrations result in different currents, but same potential [Fe 2+ ]=1x10 -4 M [Fe 2+ ]=0.5x10 -4 M [Fe 3+ ]=0.5x10 -4 M [Fe 3+ ]=1x10 -4 M

Amperometric Titrations -Measure equivalence point if analyte or reagent are oxidized or reduced at working electrode - Current is measured at fixed potential as a function of reagent volume endpoint is intersection of both lines Only analyte is reduced Only reagent is reduced Both analyte and reagent are reduced endpoint

- Analyte first deposited (reduced) onto the working electrode from a stirred solution -“deposit” analyte for a known period of time - analyte is redissolved or stripped (oxidized) from the electrode - analyte “preconcentrated” onto electrode, thus ASV yields lowest detection limit among all voltammetric techniques 3.Anodic Stripping Voltammetry (ASV)

a) Instead of linear change in E appl with time use step changes (pulses in E appl ) with time b) Measure two currents at each cycle - S 1 before pulse & S 2 at end of pulse - plot  i vs. E (  i = E S2 – E S1 ) - peak height ~ concentration - for reversible reaction, peak potential  standard potential for ½ reaction c) differential-pulse voltammetry d) Advantages: - can detect peak maxima differing by as little as 0.04 – 0.05 V < 0.2V peak separation for normal voltammetry - decrease limits of detection by x compared to normal voltammetry < to M concentration E0E0 4.Pulse Voltammetry

a.Method used to look at mechanisms of redox reactions in solution. b.Looks at i vs. E response of small, stationary electrode in unstirred solution using triangular waveform for excitation Segment 1 Segment 2 Segment 1 Segment 2 Cyclic voltammogram (solution phase redox species) 5.Cyclic Voltammetry

M ox + ne - M red - in forward scan, as E approaches E 0’, current flow due to M ox + ne - M red - governed by Nernst equation concentrations made to meet Nernst equation at surface - eventually reach i max - solution not stirred, so  grows with time, leads to decrease in i max - in reverse scan - see less current as potential increases until reduction no longer occurs - then reverse reaction takes place (if reaction is reversible) - important parameters - E pc – cathodic peak potential - E pa – anodic peak potential - i pc – cathodic peak current - i pa – anodic peak current i pc ~ i pa (or 1 pc /1 pa ~ 1) Δ E p = (E pa – E pc ) = V / n n = number of electrons involved in the reaction Formal reduction potential E o ’ (E 1/2 ) = = (E pa + E pc ) / 2 Fe 3+ + e - Fe 2+

< i pc. i pa   E p = (E pa – E pc ) = /n, where n = number of electrons in reaction < E 0 = midpoint of E pa  E pc < i p = 2.686x10 5 n 3/2 AcD 1/2 v 1/2 (Randles-Sevcik eqn) - i p : peak current (A) - n: number of electrons - A: electrode area (cm 2 ) - c: concentration (mol/cm 3 ) - v: scan rate (V/s) - D: diffusion coefficient (cm 2 /s) Important Quantitative Information Thus, - can calculate standard potential for half-reaction - number of electrons involved in half-reaction - diffusion coefficients - if reaction is reversible

Cyclic Voltammogram is a good way to determine diffusion coefficient i p = x A n = 1 A = cm 2 C = 1 x mol/cm 3 D = diffusion coefficient (cm 2 /s) v = 0.05 V/s D = 1.55 x cm 2 /s Laser Dye (PM 567) Oxidation Reduction Lai and Bard, J. Phys. Chem. B, 2003, 107,