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Introduction to Electroanalytical Chemistry Potentiometry, Voltammetry, Amperometry, Biosensors.

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Presentation on theme: "Introduction to Electroanalytical Chemistry Potentiometry, Voltammetry, Amperometry, Biosensors."— Presentation transcript:

1 Introduction to Electroanalytical Chemistry Potentiometry, Voltammetry, Amperometry, Biosensors

2 Applications Study Redox Chemistry –electron transfer reactions, oxidation, reduction, organics & inorganics, proteins –Adsorption of species at interfaces Electrochemical analysis –Measure the Potential of reaction or process E = const + k log C (potentiometry) –Measure the Rate of a redox reaction; Current (I) = k C (voltammetry) Electrochemical Synthesis Organics, 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 react –current = 0 (reaction is not allowed to occur) –Equil. Voltage is measured (E eq ) Electrolytic cells, power applied, output meas. –The Nernst Equation For a reversible process: Ox + ne- → Red E = E o – (2.303RT/nF) Log (a red /a ox ) a (activity), related directly to concentration

4 Voltammetry is a dynamic method Related to rate of reaction at an electrode O + ne = R, E o in Volts I = kA[O] k = const. A = area Faradaic current, caused by electron transfer Also a non-faradaic current forms part of background current

5 Electrical Double layer at Electrode Heterogeneous system: electrode/solution interface The Electrical Double Layer, e’s in electrode; ions in solution – important for voltammetry: –Compact inner layer: d o to d 1, E decreases linearly. –Diffuse layer: d 1 to d 2, E decreases exponentially.

6 Electrolysis: Faradaic and Non-Faradaic Currents Two types of processes at electrode/solution interface that produce current –Direct transfer of electrons, oxidation or reduction Faradaic 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 analysis Mass Transport: continuously brings reactant from the bulk of solution to electrode surface to be oxidized or reduced (Faradaic) –Convection: stirring or flowing solution –Migration: electrostatic attraction of ion to electrode –Diffusion: due to concentration gradient.

7 Typical 3-electrode Voltammetry cell Counter electrode Reference electrode Working electrode End of Working electrode O R O R e-e- Bulk solution Mass transport Reduction at electrode Causes current flow in External circuit

8 Analytical Electrolytic Cells Use external potential (voltage) to drive reaction Applied potential controls electron energy As E o gets more negative, need more energetic electrons in order to cause reduction. For a reversible reaction: –  E applied is more negative than E o, reduction will occur –  if E applied is more positive than E o, oxidation will occur O + ne- = R E o,V electrode reaction

9 Current Flows in electrolytic cells –Due to Oxidation or reduction –Electrons transferred –Measured current (proportional to reaction rate, concentration) Where does the reaction take place? –On electrode surface, soln. interface –NOT in bulk solution

10 Analytical Applications of Electrolytic Cells Amperometry –Set E applied so that desired reaction occurs –Stir solution –Measure Current Voltammetry –Quiet or stirred solution –Vary (“scan”) E applied –Measure Current Indicates reaction rate Reaction at electrode surface produces concentration gradient with bulk solution Mass transport brings unreacted species to electrode surface

11 E, V time Input: E-t waveform potentiostat Electrochemical cell counter working electrode N 2 inlet Figure1 reference insulator electrode material Cell for voltammetry, measures I vs. E wire Output, I vs. E, quiet solution reduction

12 Polarization - theoretical Ideally Polarized Electrode Ideal Non-Polarized Electrode No oxidation or reduction reduction oxidation

13 Possible STEPS in electron transfer processes Rate limiting step may be mass transfer Rate limiting step may be chemical reaction Adsorption, desorption or crystallization polarization Charge-transfer may be rate limiting

14 Overvoltage or Overpotential η η = E – E eq ; can be zero or finite –E < E eq  η < 0 –Amt. of potential in excess of E eq needed to make a non-reversible reaction happen, for example reduction E eq

15 NERNST Equation: Fundamental Equation for reversible electron transfer at electrodes O + ne - = R, E o in Volts E.g., Fe 3+ + e- = Fe 2+ If in a cell, I = 0, then E = E eq All equilibrium electrochemical reactions obey the Nernst Equation Reversibility means that O and R are at equilibrium at all times, not all Electrochemical reactions are reversible E = E o - [RT/nF] ln (a R /a O ) ; a = activity a R = f R C R a o = f o C o f = activity coefficient, depends on ionic strength Then E = E o - [RT/nF] ln (f R /f O ) - [RT/nF] ln (C R /C O ) F = Faraday const., 96,500 coul/e, R = gas const. T = absolute temperature

16 Ionic strength I = Σ z i 2 m i, Z = charge on ion, m = concentration of ion Debye Huckel theory says log f R = 0.5 z i 2 I 1/2 So f R /f O will be constant at constant I. And so, below are more usable forms of Nernst Eqn. E = E o - const. - [RT/nF] ln (C R /C O ) Or E = E o’ - [RT/nF] ln (C R /C O ); E o’ = formal potential of O/R At 25 o C using base 10 logs E = E o’ - [0.0592/n] log (C R /C O ); equil. systems


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