VOLTAMMETRY

1. 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 appliedpotential - 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)

2. VOLTAMMETRY Instrumentation – Three electrodes in solution containing analyte

3. 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

4. 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 (aR)r(aS)s … 0.0592 Eelectrode =E0 - log n (aP)p(aQ)q … Apply Potential

5. 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

6. 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 Pt working electrode at -1.0 V vs SCE Ag counter electrode at 0.0 V AgCl Ag + Cl- Pb2+ + 2e- Pb EO = -0.13 V vs. NHE K+ + e- K EO = -2.93 V vs. NHE SCE X M of PbCl2 0.1M KCl

7. Pb2+ + 2e- Pb -1.0 V vs SCE Concentration gradient created between the surrounding of the electrode and the bulk solution K+ K+ Pb2+ Pb2+ Pb2+ K+ Pb2+ Pb2+ K+ K+ K+ K+ K+ Pb2+ K+ Pb2+ K+ Pb2+ Pb2+ K+ Pb2+ K+ K+ K+ Pb2+ K+ K+ K+ Pb2+ Pb2+ Pb2+ K+ Pb2+ migrate to the electrode via diffusion K+ Pb2+ K+ Pb2+ Pb2+ K+ Pb2+ Pb2+ K+ Pb2+ Pb2+ K+ K+ K+ K+ Layers of K+ build up around the electrode stop the migration of Pb2+ via coulombic attraction

8. C) Types of Voltammetry • 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

9. 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: • Zn2+ and Cd2+ can be reduced in acidic solutions even though E0 vs NHE = -0.403 (Cd2+/Cd) and -0.763 (Zn2+/Zn) • new electrode surface is continuously generated • Independent of past samples or absorbed impurities • reproducible currents quickly produced 2H+ + 2e- H2 (g) (0V vs NHE)

10. Disadvantages of Hg Drop Electrode • Hg oxidation • Around +0.25 V vs. SCE • Can not be used above a potential of +0.25 V • Hg undergoes anodic dissolution ~ +0.25 V vs. SCE and is oxidized to insoluble Hg2Cl2 in presence of Cl- at zero V vs. SCE. • It cannot be used for anodic oxidation above +0.25 V vs. SCE. • Non-Faradaic (charging/capacitance) current • limits the sensitivity to ~ 10-5 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

11. Voltammetry (solid working electrode) • Note: • Reference Electrode: SCE (saturated calomel electrode) • SCE = + 0.24 V vs NHE • Thus, the Eo of Pb2+ = -0.37 V vs SCE Pt working electrode Ag counter Pb2+ + 2e- PbEO = -0.13 V vs. NHE K+ + e- K EO = -2.93 V vs. NHE AgCl Ag+ + Cl- SCE X M of PbCl2 0.1MKCl

12. Mox + e-»Mred At Electrodes Surface: Eappl = Eo - log [Mred]s 0.0592 at surface of electrode n [Mox]s Applied potential If Eappl = Eo: 0 = log ˆ [Mox]s = [Mred]s [Mred]s 0.0592 n If Eappl << Eo: Eappl = E0 - log \[Mred]s >> [Mox]s [Mox]s [Mred]s 0.0592 n [Mox]s

13. 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) = nFADA( ) where: n = number of electrons in ½ cell reaction F = Faraday’s constant A = electrode area (cm2) D = diffusion coefficient (cm2/s) of A (oxidant) = slope of curve between CMox,bulk and CMox,s dCA dx dCA dx dCA dx

14. 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.

15. Thickness of Diffusion Layer (d): i = (cox, bulk – cox,s) - largest slope (highest current) will occur if: Eappl << Eo (cox,s.0) then i = (cox, bulk – 0) where: k = so: i = kcox,bulk therefore: current is proportional to bulk concentration - also, as solution is stirred,ddecreasesand iincreases nFADox d nFADox d nFADox d

16. Electrode become more and more reducing and capable of reducing Cd2+ Cd2+ + 2e- Cd Current starts to be registered at the electrode All Cd2+ around the electrode has already been reduced. Current at the electrode becomes limited by the diffusion rate of Cd2+ from the bulk solution to the electrode. Thus, current stops rising and levels off at a plateau Working electrode is no yet capable of reducing Cd2+ only small residual current flow through the electrode Current at the working electrode continue to rise as the electrode become more reducing and more Cd2+ around the electrode are being reduced. Diffusion of Cd2+ does not limit the current yet Potential applied on the working electrode is usually swept over (i.e. scan) a pre-defined range of applied potential 0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte i (A) E½ id Base line of residual current -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 V vs SCE

17. Combining Potential and Current Together Limiting current Related to concentration E½ at ½ i Half-wave potential : E1/2 = -0.5 »E0 - Eref E0 = -0.5 + SCE for Mn+ + me-↔ M(n-m)+

18. Voltammograms for Mixtures of Reactants [Fe3+]=1x10-4M [Fe2+]=0.5x10-4M [Fe3+]=0.5x10-4M 0.2V 0.1V [Fe2+]=1x10-4M Two or more species are observed in voltammogram if difference in separate half-wave potentials are sufficient Different concentrations result in different currents, but same potential

19. AmperometricTitrations • 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 endpoint endpoint endpoint Only analyte is reduced Only reagent is reduced Both analyte and reagent are reduced

20. Anodic Stripping Voltammetry (ASV) • 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 limitamong all voltammetric techniques

21. Pulse Voltammetry a) Instead of linear change in Eappl with time use step changes (pulses in Eappl) with time b) Measure two currents at each cycle - S1 before pulse & S2 at end of pulse - plot Di vs. E (Di = ES2 – ES1) - 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 100-1000x compared to normal voltammetry <10-7 to 10-8 M E0 concentration

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

23. Mox + ne- Mred - in forward scan, as E approaches E0’ , current flow due to Mox + ne- Mred - governed by Nernst equation • concentrations made to meet Nernst equation at surface - eventually reach i max - solution not stirred, so d 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 - Epc – cathodic peak potential - Epa – anodic peak potential -ipc – cathodic peak current - ipa – anodic peak current ipc ~ipa (or 1pc/1pa ~ 1) Δ Ep = (Epa – Epc) = 0.0592 V / n n = number of electrons involved in the reaction Fe3+ + e- Fe2+ Formal reduction potential Eo’ (E1/2) = = (Epa + Epc ) / 2

24. Important Quantitative Information • <ipc.ipa • DEp = (Epa – Epc) = 0.0592/n, • where n = number of electrons in reaction • <E0 = midpoint of EpaEpc • < ip= 2.686x105n3/2AcD1/2v1/2 (Randles-Sevcikeqn) • - ip: peak current (A) • - n: number of electrons • - A: electrode area (cm2) • - c:concentration (mol/cm3) • - v: scan rate (V/s) • - D: diffusion coefficient (cm2/s) Thus, • can calculate standard potential for half-reaction • number of electrons involved in half-reaction • diffusion coefficients • if reaction is reversible

25. Cyclic Voltammogram is a good way to determine diffusion coefficient Laser Dye (PM 567) Reduction D = 1.55 x 10-5 cm2/s ip = 7.422 x 10-6 A n = 1 A = 0.0314 cm2 C = 1 x 10-6 mol/cm3 D = diffusion coefficient (cm2/s) v = 0.05 V/s Oxidation Lai and Bard, J. Phys. Chem. B, 2003, 107, 5036-5042.