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NAU CHM 320 - Electroanalytical Techniques

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CHM 320 Lecture 27 Chapt. 17Chapter 17 – Electroanalytical TechniquesHomework – Due Wednesday, April 12Problems 17-4, 17-15, 17-19, 17-22, 17-28CHM 320 Lecture 27 Chapt. 17Potentiometry: No current, measure potential differences and relate to concentration of analyte (Chapter 15)Techniques in Chapter 17 based on electrolysis: For example: apply a potential, force a redox reaction to go, measure the current.What can you do with electrolysis-based techniques?• Determine concentration of analytes• Identify analytes• Characterize redox behavior of analytes (how much voltage does it take to drive the reaction)CHM 320 Lecture 27 Chapt. 17In theory, when no current is flowing, the potential should be stable. Ohm’s Law - E = IRHowever in reality, when no current is flowing the following issues affect the potential:• Over potential – the voltage needed to overcome the activation energy for a redox reaction to occur at the electrode. If you want the reaction to go fast (i.e. high current), then you applyhigh voltages.• Ohmic potential – the voltage needed to overcome the resistance of the solution (high resistance solutions do provideeasy migration of the ions).• Concentration polarization – the concentration of ions at the surface of the electrode are less than they are in bulk solution.CHM 320 Lecture 27 Chapt. 17Concentration Polarization• Arises because of differing analyte concentrations in solution when a reaction is initiated• An electrical layer is created at the electrode surface• This layer resists the flow of electrical charge unless fresh ions are brought to the electrode– Stirring – DiffusionCHM 320 Lecture 27 Chapt. 17How do you overcome or compensate for these issues?Overpotential – choose an electrode material that minimizes the overpotential (like the evolution of H2at Pt has a smaller overpotential compared to evolution of H2at Hg).Ohmic potential – add an electrolyte to increase the ionic strength of the solution which improves the conductivity of the solution (decreases the resistance). Higher conductivity means the ions are more mobile.Concentration polarization – use methods to minimize the depletion of analyte concentration at the electrode surface (convection, flow cells, etc.).CHM 320 Lecture 27 Chapt. 17Three-Electrode System• When current is flowing, the potential is changing. • We want to affect the current/potential a the working electrode (where our redox reaction of interest is occurring). • However, we want the reference electrode to stay stable. • With only 2 electrodes (working and reference), there will be current flowing making the reference unstable. Overcome instability in reference electrode under electrolysis conditions by introducing a 3rdelectrode.• Auxiliary (or counter) electrode• Purpose of 3rdelectrode is to allow current to flow between working and auxiliary electrodes but measure potential between working and reference electrodes.CHM 320 Lecture 27 Chapt. 17CHM 320 Lecture 27 Chapt. 17Electroanalytical techniques are categorized by:• the excitation waveform - 2 kinds:– Variation in Applied Potential (E)•Step•Ramp– Variation in Applied Current (I) (we will NOT cover these types of techniques)• the response waveform– Voltammetry (I vs. E)CHM 320 Lecture 27 Chapt. 17Potential Step MethodsApply voltage then measure current or charge before and after voltage applied• Chronoamperometry(CA)– Response: i vs. t• Chronocoulometry (CC)– Response: Q vs. tAll in unstirred solution.Time, msE, mVE1E2All use potential step excitationCHM 320 Lecture 27 Chapt. 17ExcitationResponsetimetimeEQtotoτQdlExcitationResponsetimetimeEItotoτcharge vs. timechronocoulometrycurrent vs. timechronoamperometryCHM 320 Lecture 27 Chapt. 17Chronoamperometry (CA)• Solution: Cottrell equation•E1- no redox activity•E2: |E| > E0•n = # of e-, • A = area of electrode•Do= diffusion of ions•Co= initial concentration• t = time()tCDoonFAti2/12/1*2/1π=ExcitationResponsetimetimeEItotoτCHM 320 Lecture 27 Chapt. 17Chronocoulometry (CC)• Anson Plots– Q vs. t1/2– Intercept: Qdl– Slope: 2nFADo1/2Co*/π1/2(Qdl= charging of double layerQads= charging associated with adsorption of analyte)ResponseQt1/2QdlQQtCDadsdloonFAQ ++=2/12/1*2/12πCHM 320 Lecture 27 Chapt. 17Why would you use chronoamperometryor chronocoulometry????• Determination of:– n (# of electrons)– A (surface area of electrode)–Do(diffusion coefficient of analyte)• Kinetics/reaction mechanism• Double potential step– Generate species, probe fateCHM 320 Lecture 27 Chapt. 17Linear Sweep Voltammetry (LSV)• Excitation: potential ramp/sweep at constant rate– scan rate, ν=dE/dt– 5 mV - 10 V/s• Response: voltammogram, I vs. ETime, sEapp, VExcitationExcitationE1E2Eapp, VI, AEpEcE1E2Response Response EoXEoXCHM 320 Lecture 27 Chapt. 17Applications of Linear Sweep Voltammetry• Determination of:– n, A, (Do, Co*)• Study of kinetics• Study of adsorption• Characterization of new materialsCHM 320 Lecture 27 Chapt. 17Potential Sweep Methods• Cyclic Voltammetry (CV)– Excitation: E1to E2and back to E1– Response: I vs. E– Linear sweep voltammetry (LSV)• Excitation: E1to E2All in unstirred solutionTime, sEapp, VExcitationExcitationE1E2Eapp, VI, AEaEcE1E2Response Response ratescandtdE_,ν=CHM 320 Lecture 27 Chapt. 17Bioelectrochemistry: The ExacTechGlucose Electrode• Amperometric Biosensor: Mediated electron transferelectrodesurfaceFc+FcFcGlu OxGlu OxGlu Oxglucoseglucolactone+ 2H+2e-+0.5 V-0.1 Ve-Idea: measure current which is correlated with [Glucose]CHM 320 Lecture 27 Chapt. 17Enzyme Electrodes• Amperometric ethanol sensorBDD ElectrodeNADH NAD+ADHEthanolAcetaldehydeRao, T.N.; Yagi, I.; Miwa, T.; Tryk, D.A.; Fujishima Anal. Chem. 1999, 71, 2506-11.CHM 320 Lecture 27 Chapt. 17Electroanalytical Methods:So much to choose


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