echemintro_chm5336
TRANSCRIPT
-
7/30/2019 EChemIntro_CHM5336
1/16
Introduction to
Electroanalytical Chemistry Potentiometry, Voltammetry,
Amperometry, Biosensors
-
7/30/2019 EChemIntro_CHM5336
2/16
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
-
7/30/2019 EChemIntro_CHM5336
3/16
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
-
7/30/2019 EChemIntro_CHM5336
4/16
Voltammetry is a dynamicmethod
Related to rate of reaction at an electrode
O + ne = R, E o in Volts
I = kA[O] k = const. A = areaFaradaic current, caused by electron transfer
Also a non-faradaic current formspart of background current
-
7/30/2019 EChemIntro_CHM5336
5/16
Electrical Double layer at Electrode
Heterogeneous system: electrode/solution
interface The Electrical Double Layer, es 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.
-
7/30/2019 EChemIntro_CHM5336
6/16
Electrolysis: Faradaic and Non-FaradaicCurrents
Two types of processes at electrode/solutioninterface that produce current Direct transfer of electrons, oxidation or reduction
Faradaic Processes . Chemical reaction rate atelectrode 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 thebulk 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.
-
7/30/2019 EChemIntro_CHM5336
7/16
Typical 3-electrodeVoltammetry cell
Counter electrode
Reference electrode
Working electrode
End of Working electrode
O
R
O
R
e -
Bulk solution
Mass transport
Reduction at electrodeCauses current flow inExternal circuit
-
7/30/2019 EChemIntro_CHM5336
8/16
Analytical Electrolytic Cells
Use external potential (voltage) to drivereaction
Applied potential controls electron energy As E o gets more negative, need more
energetic electrons in order to causereduction. For a reversible reaction: E applied is more negative than E o, reduction
will occur if Eapplied is more positive than E o, oxidation
will occur
O + ne- = R Eo
,V
electrode reaction
-
7/30/2019 EChemIntro_CHM5336
9/16
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
-
7/30/2019 EChemIntro_CHM5336
10/16
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
-
7/30/2019 EChemIntro_CHM5336
11/16
E, V
time
Input: E-t waveform
potentiostat
Electrochemical cell
counter
working electrode
N2inlet
reference
insulator electrodematerial
Cell for voltammetry, measures I vs. E wire
Output, I vs. E, quiet solution
reduction
-
7/30/2019 EChemIntro_CHM5336
12/16
Polarization - theoretical
Ideally Polarized Electrode Ideal Non-Polarized Electrode
No oxidation or reduction
reduction
oxidation
-
7/30/2019 EChemIntro_CHM5336
13/16
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
-
7/30/2019 EChemIntro_CHM5336
14/16
Overvoltage or Overpotential
= E Eeq; can be zero or finite E < E eq < 0 Amt. of potential in excess of E eq needed to makea non-reversible reaction happen, for example
reduction
E eq
-
7/30/2019 EChemIntro_CHM5336
15/16
NERNST Equation : Fundamental Equationfor reversible electron transfer at electrodes
O + ne - = R, E o in VoltsE.g., Fe 3+ + e- = Fe 2+
If in a cell, I = 0, then E = E eq All equilibrium electrochemical reactions obey the
Nernst EquationReversibility means that O and R are at equilibrium at all times, not allElectrochemical reactions are reversible
E = Eo
- [RT/nF] ln (a R/a O) ; a = activitya R = f RCR a o = f oCo 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
-
7/30/2019 EChemIntro_CHM5336
16/16
Ionic strength I = z i2m i, Z = charge on ion, m = concentration of ion
Debye Huckel theory says log f R = 0.5 z i2 I1/2
So f R/f Owill 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 = Eo
- [RT/nF] ln (C R/C O); Eo
= formal potential of O/R
At 25 oC using base 10 logs
E = E o - [0.0592/n] log (C R/CO); equil. systems