Metody elektroanalityczne
Meto
y bazujace na elektrochemicznych wlasnosciach roztworów,
a szczególnie na reakcjach przeniesienia elektronu zacho
zacych
na granicy faz cialo stale / roztwór elektrolitu.
Roztwory elektrolitów
Rodzaje oddzialywan w roztworach elektrolitów:
1) jon  jon (przyciaganie i odpychanie elektrostatyczne)
Prawo Coulomba
z+ z-e2
F "
2
,
Err
gdzie F  sila przyciagania, z  ladunki jonów, Er  przenikal
nosc elektryczna rozpuszczalnika, r  odleglosc miedzy jonami
2) jon  dipol (oddzialywania dipoli i jonów)
3) dipol  dipol
4) inne : wiazania wodorowe, sily van der Waalsa
Solwatacja
Solwatacja  oddzialywanie miedzy jonami lub molekulami
analitu i molekulami rozpuszczalnika nazywane jest
solwatacja (w przypadku wody hydratacja)
Struktura roztworu elektrolitu
a) pierwszorzedowa sfera solwatacyjna
b) wtórna sfera solwatacyjna
c) wolny rozpuszczalnik
Konsekwencje solwatacji i oddzialywan miedzyjonowych:
Teoria roztworów Debye a  H�ckela
� Kazdy jon w roztworze jest otoczony wirtualna chmura
jonów o przeciwnym znaku
� Grubosc warstwy (chmury) jonowej zalezy od stezenia (c
) i wartosciowosci (zi) jonów, przenikalnosci elektrycznej
i
1/ 2
ErT
�ł �ł
rozpuszczalnika (Er) i temperatury (T)
d "
�ł �ł
I
�ł łł
1
gdzie jest sila jonowa roztw
I =
"c zi2
i
2
oru
� Wlasciwosci roztworów sa takie jakby jonów w roztwor
ze bylo mniej niz wynika to ze stezenia molowego roztworu 
wlasciwosci roztworu nie zaleza od stezenia jonów a od ich
aktywnosci (ai)
ai = fici
gdzie fi jest wspólczynnikiem aktywnosci jonu i, fi 1, gdy
ci 0
Azi2I1/ 2
- log fi =
1+ I1/ 2
i
gdzie stala A dla wody w temp. 298K wynosi 0.51; dla
rozcienczonych roztworów (I < 0,1) wyrazenie sie upraszcza:
log fi = -0,51zi2I1/ 2
podwójna warstwa elektryczna
Potencjal elektrody  podwójna warstwa elektryczna
Potencjal elektrody jest efektem wystepowania podwójnej
warstwy elektrycznej na granicy faz elektroda /elektrolit
w pólogniwie.
Evolution of the Electrode Double
Layer Models
"




a 1946
" Specyficznie a
sorbowane jony są 
e
solwatowane i czują większy potenc
Bulk
jał
_
+
solution
_
_ +
+
"
+
_
+

_ _
_ +
"
 IHP
"
OHP
electro
e
Zapis reakcji elektrochemicznej
Reakcje anodowe
Reakcje półogniw
Reakcje katodowe
Reakcja sumaryczna
e-
(+)
(-)
e-
e-
A B
A A+ + e-
Anode
Katoda B + e- B-
A+
B-
Anode Katoda
A + B A+ + B-
A B
A B
e- e-
e-
utlenia się redukuje się
donor e- akceptor e-
A+ B-
reduktor utleniacz
Cathode
Anode
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Ion
(10-4 S m2/mol)@infinite
dilution, 250C
L �
H+ 350
� = ; = �
RA c
OH- 200
Ba2+ 127
"
K+ 74
Cl- 77
HCO3- 45
"
TBA+ 24
Taken from Table 2.3.2 in Bard, A.; Faulkner, L.
Electrochemical Methods Wiley: New York,
1980.
"
Ion
 (10-4 S  (10-4 S
m2/mol) in m2/mol) in
water, 250C CH3CN, 250C
Na+ 50 100
K+ 74 84
Cl- 77 98
"
n-Bu4N+
PF6- 104
Taken from Table 4-7 in Sawyer, D.T.; Roberts,
Jr., J.L. Experimental Electrochemistry for
Chemists Wiley: New York, 1976.
"
Ł
�C A  
=c + a
c a C A
"
� �  
� �  
"
" �+/ �
" �-/ �
" �+ + �- = �
"
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"
"
"
Electrolyte
t+
"
@ 0.01 @ 0.1 M @ 0.2 M
M
HCl 0.8251 0.8314 0.8337
"
NaCl 0.3918 0.3854 0.3821
"
 
Taken from Table 2.3.1 in Bard, A.; Faulkner, L.
Electrochemical Methods Wiley: New York,
1980.
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Transfer elektronów na elektrodzie
Równanie Nernsta
RT [ox]
0
E = E + ln
nF [red ]
Or
2.303RT [ox]
E = E0 + log
nF [red]
Electrochemical Cells:
Galvanic  cell is employed to produce electricity.
Electrolytic  cell consumes electricity from an external source.
Electrochemical Cells
Consists of two conductors (called electrodes) each immersed in
a suitable electrolyte solution.
For electricity to flow:
" The electrodes must be connected externally by means of a
(metal) conductor.
" The two electrolyte solutions are in contact to permit
movement of ions from one to the other.
Electrochemical Cells
Cathode is electrode at which reduction occurs.
Anode is electrode at which oxidation occurs.
Junction potential is small potential at the interface between
two electrolytic solutions that differ in composition.
Ohm s Law:
I = E/R
I = current in amperes
E = potential difference in volts responsible for movement of ions
R = the resistance in ohms of the electrolyte to the current
Electrode Potentials:
Standard potential for cell, E0ccll, follows Nernst Equation:
E0ccll = (RT/nF) lnK
R is gas law constant, T is temperature, K is equilibrium
constant, F is the Faraday, and n is number of
equivalents of electricity
Electrode potential is sum of two half-reactions.
Cannot determine potential of a single electrode, measure
differences in potential.
Potentiometric Methods:
To perform potentiometry, the following is needed:
" Reference Electrode
" Indicator Electrode
" Potential Measuring Device
Characteristics of Ideal Reference Electrode:
" Reversible and follow Nernst equation
" Potential should be constant with time
" Should return to original potential after being subjected to
small currents
4) Little hysteresis with temperature cycling
5) Should behave as ideal nonpolarized electrode
Membrane Electrodes:
Two types:
" Responsive to ionic species
" Applied to determination of molecular analytes
" gas-sensing probes
" enzymatic electrodes
Properties of Ion-Selective Electrodes:
Minimal solubility of ion-selective medium in analyte solution
Electrical conductivity
Selective reactivity with analyte, usually ion-exchange,
crystallization, or complexation.
Typical pH Electrode
Molecular-Selective Electrodes:
Gas-Sensing Probes: Examples, hydrophobic membranes for
CO2 and NH3
Enzyme Substrate Electrodes: Example, urease membrane for
blood urea
Example of Activity vs. Concentration Discrepancy:
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 Electro
es in Daniell cell




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Electrode Acronym Potential vs.
SHE
Hg(l)/Hg2Cl2(s)/KCl (0.1 M) 0.3337
Hg/Hg2Cl2(s)/KCl (1 M) �CE 0.2801
Hg(l)/Hg2Cl2(s)/KCl (sat'd) SCE 0.2412
Hg(l)/Hg2Cl2(s)/ �aCl (sat'd) SSCE 0.2360
Note: concentrations typically high �! " concentrations small �! electrode doesn t
become polarized �! potential constant
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From BAS www-
site:
http://www.bioanal
ytical.com/
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From BAS www-
site:
http://www.bioanal
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ytical.com/
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From BAS www-
site:
" http://www.bioanal

ytical.com/
" Pt or Ag wire (inert)
" Idea:
in medium of high resistance, low conductivity,
wire will assume reasonably steady, highly repr
oducible potential (+ 20 mV)
" Advantage: no solution contamination
" Limitation: must use internal potential standard
(ferrocene)
Can Aqueous References Be Used i
n �onaqueous Media?
" Yes with caution!
 May be significant junction potentials
" Requires use of internal stan
ar

 May be greater noise
" Electrolyte may precipitate/clog electro
e frit
 Don t forget about your chemistry
" Chemistry may be water sensitive