Presentation on theme: "Introduction to Electroanalytical Chemistry"— Presentation transcript:
1 Introduction to Electroanalytical Chemistry Potentiometry, Voltammetry, Amperometry, Biosensors
2 Applications Study Redox Chemistry Electrochemical analysis electron transfer reactions, oxidation, reduction, organics & inorganics, proteinsAdsorption of species at interfacesElectrochemical analysisMeasure the Potential of reaction or processE = const + k log C (potentiometry)Measure the Rate of a redox reaction; Current (I) = k C (voltammetry)• Electrochemical SynthesisOrganics, inorganics, materials, polymers
3 Electrochemical Cells Galvanic Cells and Electrolytic Cells• Galvanic Cells – power output; batteries• Potentiometric cells (I=0) read Chapter 2– measure potential for analyte to reactcurrent = 0 (reaction is not allowed to occur)Equil. Voltage is measured (Eeq)Electrolytic cells, power applied, output meas.The Nernst EquationFor a reversible process: Ox + ne- → RedE = Eo – (2.303RT/nF) Log (ared/aox)a (activity), related directly to concentration
4 Voltammetry is a dynamic method Related to rate of reaction at an electrodeO + ne = R, Eo in VoltsI = kA[O] k = const. A = areaFaradaic current, caused by electron transferAlso a non-faradaic current formspart of background current
5 Electrical Double layer at Electrode Heterogeneous system: electrode/solution interfaceThe Electrical Double Layer, e’s in electrode; ions in solution – important for voltammetry:Compact inner layer: do to d1, E decreases linearly.Diffuse layer: d1 to d2, E decreases exponentially.
6 Electrolysis: Faradaic and Non-Faradaic Currents Two types of processes at electrode/solution interface that produce currentDirect transfer of electrons, oxidation or reductionFaradaic Processes. Chemical reaction rate at electrode proportional to the Faradaic current.Nonfaradaic current: due to change in double layer when E is changed; not useful for analysisMass Transport: continuously brings reactant from the bulk of solution to electrode surface to be oxidized or reduced (Faradaic)Convection: stirring or flowing solutionMigration: electrostatic attraction of ion to electrodeDiffusion: due to concentration gradient.
7 Typical 3-electrode Voltammetry cell O O e- R R Reference electrode CounterelectrodeWorking electrodeOReduction at electrodeCauses current flow inExternal circuitOe-Mass transportRREnd of Working electrodeBulk solution
8 Analytical Electrolytic Cells Use external potential (voltage) to drive reactionApplied potential controls electron energyAs Eo gets more negative, need more energetic electrons in order to cause reduction. For a reversible reaction: Eapplied is more negative than Eo, reduction will occurif Eapplied is more positive than Eo, oxidation will occurO + ne- = R Eo,V electrode reaction
9 Current Flows in electrolytic cells Due to Oxidation or reductionElectrons transferredMeasured current (proportional to reaction rate, concentration)Where does the reaction take place?On electrode surface, soln. interfaceNOT in bulk solution
10 Analytical Applications of Electrolytic Cells AmperometrySet Eapplied so that desired reaction occursStir solutionMeasure CurrentVoltammetryQuiet or stirred solutionVary (“scan”) EappliedIndicates reaction rateReaction at electrode surface produces concentration gradient with bulk solutionMass transport brings unreacted species to electrode surface
11 Cell for voltammetry, measures I vs. E wirepotentiostatinsulatorelectrodematerialreferenceN2inletcounterworking electrodeElectrochemical cellOutput, I vs. E, quiet solutionInput: E-t waveformreductionE, VtimeFigure1
13 Possible STEPS in electron transfer processes Charge-transfer may be rate limitingRate limiting step may be mass transferRate limiting step may be chemical reactionAdsorption, desorption or crystallization polarization
14 Overvoltage or Overpotential η η = E – Eeq; can be zero or finiteE < Eeq η < 0Amt. of potential in excess of Eeq needed to makea non-reversible reaction happen, for examplereductionEeq
15 NERNST Equation: Fundamental Equation for reversible electron transfer at electrodes O + ne- = R, Eo in VoltsE.g., Fe e- = Fe2+If in a cell, I = 0, then E = EeqAll equilibrium electrochemical reactions obey theNernst EquationReversibility means that O and R are at equilibrium at all times, not allElectrochemical reactions are reversibleE = Eo - [RT/nF] ln (aR/aO) ; a = activityaR = fRCR ao = foCo f = activity coefficient, depends on ionic strengthThen E = Eo - [RT/nF] ln (fR/fO) - [RT/nF] ln (CR/CO)F = Faraday const., 96,500 coul/e, R = gas const.T = absolute temperature
16 Ionic strength I = Σ zi2mi, Z = charge on ion, m = concentration of ionDebye Huckel theory says log fR = 0.5 zi2 I1/2So fR/fOwill be constant at constant I.And so, below are more usable forms of Nernst Eqn.E = Eo - const. - [RT/nF] ln (CR/CO)OrE = Eo’ - [RT/nF] ln (CR/CO); Eo’ = formal potential of O/RAt 25 oC using base 10 logsE = Eo’ - [0.0592/n] log (CR/CO); equil. systems