Transcript 18_chp25_slides

1.1 Three electrodes voltammetry
Fig. 25-2 (p.718) A system for
potentiostatic three-electrode linearscan voltammetry
Fig. 25-8 (p.724) A three-electrode
cell for hydrodynamic voltammetry.
1.2 Working electrodes
At +E limit, oxidation of water to generate O2: 2H2O  4H+ + O2(g) + 4eAt -E limit, reduction of water to generate H2: 2H2O + 2e-  H2 + 2OH-
Fig. 25-4 (p.720) Potential ranges for three types of electrodes in
various electrolyte solutions
1.3 Excitation signals
Fig. 25-1 (p.717) Potential excitation signals used in voltammetry
1.4 Voltammograms (voltammetric waves): graphs of current vs applied
voltage
Fig. 25-6 (p.722) Linear-sweep voltammogram for the reduction of a
hypothetical species A to give a product P.
2.1 Concentration profiles at electrode surfaces (stirred solution)
Fig. 25-12 (p.726) Flow patterns
and regions of interest near the
working electrode in hydrodynamic
voltammetry
Fig. 25-13 (p.727) Concentration profile at an electrode-solution interface
during the electrolysis A + ne-  P from a stirred solution of A.
2.2 Application of hydrodynamic voltammetry
-
-
Single voltammogram can quantitatively record many species provided
enough separation between waves (01.~0.2 is required)
Problems with dissolved O2 – must purge solutions
Further reduction of H2O2  water
Reduction of O2 to hydrogen peroxide
Fig. 25-14 (p.729) Voltammograms for
two-components mixtures, with E1/2
differ by 0.1 V
Fig. 25-16 (p.729) Voltammogram for
the reduction of oxygen in an airsaturated 0.1M-KCl solution
On a single electrode apply both anodic and cathodic sweep.
3.1 Fundamental studies
System is reversible if
E = 0.0592/n and
ipc = ipa
Note:
Epc = E0 -1.1RT/nF
3.2 Quantitative analysis (not common)
ip

peak
current (A)
 2.686105 n 3 / 2
A c

1/ 2
D

1/ 2
v
electrode
diffusion
scan
2
2
area (cm ) coefficient (cm /s) rate (V/s)
Switching
potential
Fig. 25-23 (p.737) Cyclic voltammetric excitation signal
3
Fe(CN )6  e  Fe(CN )6
4
3.1 Fundamental studies
System is reversible if: E = 0.0592/n and
and ipc = ipa
Note: Epc = E0 -1.1RT/nF
3.2 Quantitative analysis (not common)
ip

peak
current (A)
 2.686105 n 3 / 2
A c

D1/ 2

1/ 2
v
electrode
diffusion
scan
area (cm2 ) coefficient (cm2 /s) rate (V/s)
Fig. 25-24 (p.738) a) Potential vs. time waveform and b) cyclic
voltaqmmogram for a solution that is 0.6mM k3Fe(CN)6 and 1.0 M in KNO3