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 (non-faradico)

– 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 non-metallo in contatto con suoi anioni

• ESEMPI:– Cu2+ /Cu(s)– Zn2+/Zn(s)– SHE– Ag+/Ag (Elettrodo di riferimento non acquoso)

– Cl-/Cl2(g)/Pt

Electtrodi nella pila Daniell

• Risposta dell’elettrodo data dalla equazione di Nernst (Nernstiano):

• N.B.: elettrodi di Fe, Al, e W NON sono elettrodi di I specie– spessa ricopertura superficiale da parte degli ossidi

zMa

nF

RTEE ln0

Elettrodi di I specie

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:

Cl

PSAg

ClAgPS

a

Ka

aaClAgK

ClAgAgCl

]][[

La reazione redox corrispondente è: ClAgeClAg

Essendo insolubile è presente come corpo di fondo o deposito sul metallo

aAg+=1

Il potenziale redox è relativo alla coppia Ag/Ag+:

ClAgAgAgAg

PSAgAgAgAg

ClPSAgAgCl

PSAgAgAgAg

AgAgAgAgAg

aF

RTEE

KF

RTEE

aF

RTK

F

RTE

a

K

F

RTEE

aF

RTEE

ln

ln

lnlnln

ln

'0

//

0

/

'0

/

0

/

0

//

0

//

Electrode of the Second Kind

• Electrode response given by:

• NOTES: – anion activity determines potential

– make great reference electrodes because of low solubility of salt (potential very stable)

ClAgAgAgAg

aF

RTEE ln'0

//

The Calomel Reference Electrode

Electrode Acronym Potential vs.SHE

Hg(l)/Hg2Cl2(s)/KCl (0.1 M) 0.3337Hg/Hg2Cl2(s)/KCl (1 M) NCE 0.2801

Hg(l)/Hg2Cl2(s)/KCl (sat'd) SCE 0.2412Hg(l)/Hg2Cl2(s)/ NaCl (sat'd) SSCE 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• International standard

E0 0 V• One of most

reproducible potentials + 1 mV

Disadvantages• Convenience

– Pt black easily poisoned by organics, sulfide, cyanide, etc.

– Hydrogen explosive

– 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

Disadvantages

• Hg toxic

• solubility of KCl temperature dependent dE/dT = -0.67 mV/K (must quote temperature)

Advantages

• Most polarographic data ref’d to SCE

From BAS www-site: http://www.bioanalytical.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

Disadvantages

• solubility of KCl/NaCl temperature dependent dE/dT = -0.73 mV/K (must quote temperature)

Advantages

• chemical processing industry has standardized on this electrode

• convenient

• rugged/durable

From BAS www-site: http://www.bioanalytical.com/

Ag+/Ag• Ag+ + e-= Ag(s)• requires use of internal potential

standard

Disadvantages

• Potential depends on– solvent

– electrolyte (LiCl, TBAClO4, TBAPF6, TBABF4

• Care must be taken to minimize junction potentials

Advantages

• Most widely used

• Easily prepared

• Works well in all aprotic solvents:– THF, CAN, DMSO,

DMF

From BAS www-site: http://www.bioanalytical.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

• Disk– area: r2

• wire (cylinder)– area: l(2 r) r2

• Mesh– optically transparent

• Sheet

Note: Geometric area < 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.