电化学课件-纯英文6An Introduction to Electrochemical Methods (cont’d)

– Form insoluble, oxidized Hg salt of analyte anion – Stir (maximize convection)
• Equilibrate (stop stirring) • Scan potential in opposite sense (cathodic)
UME’s:
Fast scan rates 30 V/s
planar diffusion
Slow scan rates 5 mV/s
radial diffusion
Fe3+
0.1 m
0.1 m
UME’s Radial vs. Planar Diffusion
Radial Diffusion • Redox wave:
– Reducing salt/film and forming soluble anion
• Record voltammogram
HDME CSV
• Can study halides, sulfides, selenides, cyanides, molybdates, vanadates
• EX: FDA 1982-1986 used to confirm CN- (-0.1 V) in Tylenol Crisis
t
to
time
– i ____
– slope I
time • Deficiency
to
What Experiment Is This?
Excitation
• Name of experiment
E
• Type of excitation
t
to
time • Response
I
• Concentrate analyte
• Stop stirring = equilibration/rest
Ip
period
• Scan E in anodic sense and record voltammogram
Pb Pb2+ + 2e-
– oxidize analyte (so redissolution occurs)
Electroanalytical Chemistry
Lecture #6 An Introduction to Electrochemical
Methods (cont’d)
Q: What Experiment is This?
Excitation • Name of experiment
E
• type of excitation
Criteria for Nernstian Process
• Ep independent of scan rate • ip 1/2 (diffusion controlled) • Ipc/Ipa = 1 (chemically reversible)
Quasi-reversible or Irreversible
Eapp • If ideal Nernstian, Epa = Epc and DEp/2 = 90.6 mV/n at 250C
EXAMPLE 2: Oxidation of Cysteine at BDD
Nicolae Spãtaru, Bulusu V. Sarada, Elena Popa, Donald A. Tryk, and Akira Fujishima* ; Voltammetric Determination of L-Cysteine at Conductive Diamond Electrodes, Analytical Chemistry; 2001; 73(3); 514-519.
Stripping Analysis or Stripping Voltammetry
• 2 Flavors:
– Anodic (ASV)
• Good for metal cations
– Cathodic (CSV)
• Good for anions and oxyanions
Stripping Voltammetry - Steps
Stripping Voltammetry - Quantitation
• Ip Co*
• Concentrations obtained using either
– Standard addition – Calibration curve
HDME ASV
• Usually study M with Eo more negative than Hg
– EX: Cd2+, Cu2+, Zn2+, Pb2+
• Study M with Eo more positive than Hg at GC
– EX: Ag+, Au+, Hg
• Can analyze mixture with DEo 100 mV
CSV
• Anodic deposition
Response Q
– Q ____ – intercept
Qdl
– slope
to time
Eapp, V
Excitation E2
Eo X
E1 Time, s
Response
Ep
I, A
E1
X Eo
E2
Eapp, V
Q: What Is This Experiment?
• Name of experiment
EXAMPLE: Electrocatalytic Oxidation of Guanine in DNA
• Top: non-faradaic contribution
• Bottom: shape and magnitude of redox waves
P.M.Armistead; H.H.Thorp Anal. Chem. 2000, 72, 3764-70.
– Specific short-range interactions important – Chemical identity of species important
CV and Adsorption
• If electroactive adsorbed species:
90 mV I
– Ep = Eo - (RT/nF) ln (bo/bR) – ip = (n2F2/4RT) A o*
• Excitation
• Response
– i ____ – Ep ____ of – E’ = _____________
Excitation E2
Cyclic Voltammetry (CV)
Eapp, V
E1 Time, s Response Epa
I, A
E1
E2
Epc Eapp, V R - ne- = O
Annette R. Howells, Pedro J. Zambrano, and Maryanne M. Collinson* ; Diffusion of Redox Probes in Hydrated Sol-GelDerived Glasses, Analytical Chemistry; 2000; 72(21); 5265-5271.
• Many organic functional groups are reducible: C=O C=C C=N N=N S-S
• see Handbook of Organic Compounds
Applications of CV
• Many functional group are not reducible so we can derivatize these groups
• Learn Do from CA
• Obtain Co*from slow scan rate CV (Iss)
Annette R. Howells, Pedro J. Zambrano, and Maryanne M. Collinson* ; Diffusion of Redox Probes in Hydrated Sol-GelDerived Glasses, Analytical Chemistry; 2000; 72(21); 5265-5271.
• [S2Mo18O62]4- + e- = [S2Mo18O62]5- + e- = [S2Mo18O62]6-
• BAS 100-A
• 3-electrode cell:
– GC macrodisk/Pt wire/ Pt wire
– ACN with no electrolyte
Applications of CV
– convert them into electroactive groups by chemical modification
• EXAMPLES:
– alcohols + chromic acid = aldehyde group – phenyl + nitration = nitro group
Adsorption Phenomena
• Non-specifically adsorbed
– No close-range interaction with electrode – Chemical identity of species not important
• Specifically adsorbed
• Ipc/Ipa = 1
For Nernstian Process
• Potential excitation controls [R]/[O] as in Nernst equation: Eapp = E0- 0.059/n log [R]/[O]