Electroanalytical Chemistry

Un processo all’interfaccia
• Per: O + ne- = R
• 5 eventi separati devono verificarsi:
– O devono essere trasportati con successo dal bulk
della soluzione (transporto di massa)
– O deve adsorbirsi in modo transiente sulla
superficie dell’elettrodo (non-faradico)
– dveve avvenire un trasferimento di carica tra
elettrodo ed O (faradico)
– R deve desorbirsi dalla superfiie elettrodica (nonfaradico)
– R deve essere trasportato lontano dall’elettrodo nel
bulk della soluzione (transporto di massa)
Classificazione degli Elettrodi
• si basa sulla natura e sul numero delle fasi
tra cui avviene il trasferimento elettronico
• 3 Classi:
– Elettrodi di I specie
– Elettrodi di II specie
– Elettrodi di III specie
Elettrodi di I specie
• Metallo in contatto con suoi cationi o nonmetallo in contatto con suoi anioni
• ESEMPI:
–
–
–
–
–
Cu2+ /Cu(s)
Zn2+/Zn(s)
Electtrodi nella pila Daniell
SHE
Ag+/Ag (Elettrodo di riferimento non acquoso)
Cl-/Cl2(g)/Pt
Elettrodi di I specie
• Risposta dell’elettrodo data dalla equazione di
Nernst (Nernstiano):
RT
EE 
ln aM z
nF
0
• N.B.: elettrodi di Fe, Al, e W NON sono elettrodi
di I specie
– spessa ricopertura superficiale da parte degli ossidi
Electrode of the Second Kind
• Metal in contact with sparingly soluble salt
of the metal
• Common name: anion electrodes
• EXAMPLES:
– Ag/AgCl(s)
– Hg/Hg2Cl2(s)/Cl- (saturated calomel electrode;
SCE)
Ag/AgCl, KCl
Il sale insolubile è AgCl sottoposto all’equilibrio di
solubilità:
AgCl  Ag   Cl  


K PS  [ Ag ][Cl ]  a Ag  aCl  
a Ag 
K PS

aCl 
La reazione redox corrispondente è:



Ag  Cl  e  Ag  Cl
Essendo insolubile è presente
come corpo di fondo o deposito
sul metallo
aAg+=1

Il potenziale redox è relativo alla coppia
Ag/Ag+:
E Ag / Ag   E
0
Ag / Ag 
0
E Ag / Ag   E Ag
/ Ag 
E
0'
Ag / Ag 
E
0
Ag / Ag 
0'
E Ag / Ag   E Ag
/ Ag 
RT
ln a Ag  

F
RT
RT
RT K PS
0
ln aCl 
ln K PS 
 E Ag / Ag  
ln

F
F
aCl 
F
RT
ln K PS

F
RT
ln aCl 

F
Electrode of the Second Kind
• Electrode response given by:
E Ag / Ag   E
0'
Ag / Ag 
RT

ln aCl 
F
• NOTES:
– anion activity determines potential
– make great reference electrodes because of low
solubility of salt (potential very stable)
The Calomel Reference Electrode
Electrode
Acronym
Hg(l)/Hg2Cl2(s)/KCl (0.1 M)
Hg/Hg2Cl2(s)/KCl (1 M)
Hg(l)/Hg2Cl2(s)/KCl (sat'd)
Hg(l)/Hg2Cl2(s)/ NaCl (sat'd)
NCE
SCE
SSCE
Potential vs.
SHE
0.3337
0.2801
0.2412
0.2360
Note: concentrations typically high   concentrations small  electrode
doesn’t become polarized  potential constant
Electrode of the Third Kind
• Electrodes that merely serve as sources or
sinks for electrons
• Common names: redox, inert, unattackable
• EXAMPLES:
– metals: Pt, Au, GC, graphite, HOPG, Hg
– semiconductors: Si, GaAs, In-SnO2/glass
• Response:
– for Pt in contact with Fe2+, Fe3+ in solution:
– E = E0- 0.059 (V) log ([Fe2+]/[Fe3+])
Electrode of the Fourth Kind
• Electrodes that cannot be classified as 1-3
• EXAMPLES:
– Chemically modified electrodes (CME’s)
Reference Electrodes
• Purpose: provide stable potential against
which other potentials can be reliably
measured
• Criteria:
– stable (time, temperature)
– reproducible (you, me)
– potential shouldn’t be altered by passage of
small current = not polarizable
– easily constructed
– convenient for use
SHE
Advantages
Disadvantages
• International standard • Convenience
E0  0 V
– Pt black easily
poisoned by organics,
• One of most
sulfide, cyanide, etc.
reproducible potentials
– Hydrogen explosive
+ 1 mV
– Sulfuric and
hydrochloric strong
acids
Practical Reference Electrodes
Aqueous
• SCE
• Ag/AgCl
Nonaqueous
• Ag+/Ag
• pseudoreferences
– Pt, Ag wires
• Ferrocene
SCE
• Cl-(aq)/Hg2Cl2/Hg(l)
• Hg22+ + 2e- = 2Hg(l)
• E0 = 0.24 V vs. SHE @ 250C
Advantages
• Most
polarographic
data ref’d to
SCE
Disadvantages
• Hg toxic
• solubility of KCl
temperature
dependent dE/dT = 0.67 mV/K (must
quote temperature)
From BAS wwwsite:
http://www.bioana
lytical.com/
Ag/AgCl
• Ag wire coated with
AgCl(s), immersed in NaCl
or KCl solution
• Ag+ + e- = Ag(s)
• E0 = 0.22 V vs. SHE @ 250C
Advantages
• chemical processing
industry has
standardized on this
electrode
• convenient
• rugged/durable
Disadvantages
• solubility of
KCl/NaCl
temperature
dependent
dE/dT = -0.73 mV/K
(must quote
temperature)
From BAS wwwsite:
http://www.bioana
lytical.com/
Ag+/Ag
• Ag+ + e-= Ag(s)
• requires use of internal potential
standard
Advantages
• Most widely used
• Easily prepared
• Works well in all
aprotic solvents:
– THF, CAN, DMSO,
DMF
Disadvantages
• Potential depends on
– solvent
– electrolyte (LiCl,
TBAClO4, TBAPF6,
TBABF4
• Care must be taken to
minimize junction
potentials
From BAS wwwsite:
http://www.bioana
lytical.com/
Pseudo-References
• Pt or Ag wire (inert)
• Idea:
in medium of high resistance, low
conductivity, wire will assume reasonably
steady, highly reproducible potential (+ 20
mV)
• Advantage: no solution contamination
• Limitation: must use internal potential
standard (ferrocene)
Can Aqueous References Be
Used in Nonaqueous Media?
• Yes with caution!
– May be significant junction potentials
• Requires use of internal standard
– May be greater noise
• Electrolyte may precipitate/clog electrode frit
– Don’t forget about your chemistry
• Chemistry may be water sensitive
Electrodes
• Metal
– solid
• Pt, Au, Ag, C
– liquid
• dropping mercury electrode (DME)
• Semiconductors
– Si, GaAs
– In-SnO2/glass (optically transparent)
Carbon
• Paste
– With nujol (mineral oil)
• Glassy carbon (GC)
– Amorphous
• Pyrolytic graphite - more ordered than GC
– Basal Plane
– Edge Plane (more conductive)
Electrode Materials
• Different Potential Windows
• Can affect electron transfer kinetics
Electrodes
• Size
– Analytical macro
• 1.6 - 3 mm diameter
– Micro
• 10-100 m diameter
From BAS www-site:
http://www.bioanalytical.com/
Electrode Geometry
Geometry is critical and affects how the data
are analyzed and interpreted
Note: Geometric area <
• Disk
– area: r2
• wire (cylinder)
– area: l(2 r) r2
• Mesh
– optically transparent
• Sheet
effective surface area
Cleanliness IS Next to Godliness
in Electrochemistry
• Working electrode must be carefully
cleaned before each experiment
– Mechanical
• Abrasion with alumina or “diamond” polish
– Chemical
• Sonicate in Alconox
• Soak in HNO3
– Electrochemical
• Cycle in 0.5 M H2SO4 (Pt)
Electrochemical Cleaning
Taken from Table 4-7 in Sawyer, D.T.; Roberts, Jr., J.L. Experimental
Electrochemistry for Chemists Wiley: New York, 1976.