Un processo all’interfaccia

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1 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)

2 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

3 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

4 Elettrodi di I specie 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

5 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)

6 aAg+=1 La reazione redox corrispondente è: Ag/AgCl, KCl
Il sale insolubile è AgCl sottoposto all’equilibrio di solubilità: La reazione redox corrispondente è: Essendo insolubile è presente come corpo di fondo o deposito sul metallo aAg+=1

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

8 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)

9 The Calomel Reference Electrode
Note: concentrations typically high   concentrations small  electrode doesn’t become polarized  potential constant

10 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 = E (V) log ([Fe2+]/[Fe3+])

11 Electrode of the Fourth Kind
Electrodes that cannot be classified as 1-3 EXAMPLES: Chemically modified electrodes (CME’s)

12 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

13 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

14 Practical Reference Electrodes
Aqueous SCE Ag/AgCl Nonaqueous Ag+/Ag pseudoreferences Pt, Ag wires Ferrocene

15 SCE Cl-(aq)/Hg2Cl2/Hg(l) Hg22+ + 2e- = 2Hg(l)
E0 = 0.24 V vs. 250C Advantages Most polarographic data ref’d to SCE Disadvantages Hg toxic solubility of KCl temperature dependent dE/dT = mV/K (must quote temperature) From BAS www-site:

16 Ag/AgCl Ag wire coated with AgCl(s), immersed in NaCl or KCl solution
Ag+ + e- = Ag(s) E0 = 0.22 V vs. 250C Advantages chemical processing industry has standardized on this electrode convenient rugged/durable Disadvantages solubility of KCl/NaCl temperature dependent dE/dT = mV/K (must quote temperature) From BAS www-site:

17 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 www-site:

18 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)

19 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

20 Electrodes Metal Semiconductors solid liquid Si, GaAs
Pt, Au, Ag, C liquid dropping mercury electrode (DME) Semiconductors Si, GaAs In-SnO2/glass (optically transparent)

21 Carbon Paste Glassy carbon (GC)
With nujol (mineral oil) Glassy carbon (GC) Amorphous Pyrolytic graphite - more ordered than GC Basal Plane Edge Plane (more conductive)

22 Electrode Materials Different Potential Windows
Can affect electron transfer kinetics

23 Electrodes Size Analytical macro Micro 1.6 - 3 mm diameter
From BAS www-site:

24 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

25 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)

26 Electrochemical Cleaning
Taken from Table 4-7 in Sawyer, D.T.; Roberts, Jr., J.L. Experimental Electrochemistry for Chemists Wiley: New York, 1976.