Aula 3- Breve histórico dos eletrodos, conceitos fundamentais e

Aula 3- Breve histórico dos eletrodos, conceitos fundamentais e

Aula 3- Breve histórico dos eletrodos, conceitos fundamentais e
potenciometria
Introduction
Historic overview, Classification of (bio)chemical electrodes, definitions of sensors and
biosensors and basic measuring techniques
Fundamental Concepts
Electrical conduction, electrodes, electrolytic cells, faraday’s law of electrolysis, Voltaic or
Galvanic cells, The Nernst equation, reference and indicator electrodes, standard electrode
potentials, liquid-junction potentials
Potentiometric Methods and Electrodes
Principles of potentiometric electrodes, experimental set-up and instrumentation (Galvanic
cell),
ll) Indicator
I di t electrodes:
l t d 1) metallic
t lli electrodes:
l t d first
fi t (cation
( ti or redox
d electrode),
l t d ) secondd andd
third class or specie; 2) membrane electrodes: glass electrodes, polymer (liquid membranes)
electrodes, crystalline and pressed powder solid electrodes, gas electrodes, enzymatic
electrodes (biosensors),
(biosensors) practical aspects and applications
References
1)R. W. Cattrall, Chemical Sensors, Oxford, Oxford Press, 1997
2) A. Evans, Potentiometry and Ion Selective Electrodes, Chichester, Wiley, 1987
3) F. Scholz, Electroanalytical methods-guide to experiments and applications, Berlin,SpringerVerlag, 2009
4) V. A. Gault, N. H. McClenaghan, Understanding Bioanalytical Chemistry- Principles and
Applications, Oxford, Wiley-Blackwell, 2009
5) A. P. F. Turner, I. Karube, G. S. Wilson, Biosensors- Fundamentals and
applications,Oxyford, Oxyford University Press, 1987
6) G. G. Guilbault, A. A. Suleiman, O. Fatibello-Filho, M. A. Nabirahni, Immobilized
Bioelectrochemical Sensors, In: D. L. Wise, Bioinstrumentation and Biosensors, New York,
Dekker,, 659-692
7) O. Fatibello-Filho et al, Chapter 17 - Electrochemical biosensors based on vegetable
y
In: S.
tissues and crude extracts for environmental, food and ppharmaceutical analysis,
Alegret & A. Merkoçi, Comprehensive Analytical Chemistry, Vol. 49-Electrochemical Sensor
Analysis”, Amsterdam, Elsevier , 355-375, 2007
Breve histórico sobre os sensores
Tabela- Breve histórico sobre o desenvolvimento dos sensores
químicos
Ano
Tipo de sensor
Investigador
1888
Eletrodos metálicos/íons
W. Nernst
1906
Eletrodo de vidro
M. Cremer
1922
Eletrodo gotejante de Hg
J. Heyrovsky
1930
Eletrodo de vidro/Corning 015
D. Innes & M. Dole
1934
El t d de
Eletrodo
d vidro
id para Na(I)
N (I)
B Lengyel
B.
L
l & E.
E Blum
Bl
1936
Eletrodo c/ CaF2 para Ca(II)
H.J.C. Tenderloo
1956
Eletrodo para Oxigênio
L C Clark
L.C.
1958
Eletrodo para CO2
W. Severinghaus & A. Bradley
1958
Eletrodo de pasta de carbono
R.N. Adams
1959
Sensor piezelétrico
G.Z. Sauerbrey
3
Ano
Tipo de Sensor
Investigador
1961
Sensor AgI(s) parafina p/ I-
E. Pungor
1962
para glicose
g
Biossensor p
L.C. Clark & W. Lyons
y
1964
Sensor piezelétrico
W.H. King
1966
Sensor de LaF3/EuF2 para F-
M.S. Frant & J.W. Ross
1967
Sensor de membrana liq. p/ Ca2+
J.W. Ross
1970
Sensor de membrana de PVC p/ Ca2+
G. Moody et al
1970
ChemFET
P. Bergveld
1974
Calorimétrico (enzimático)
K. Mosbach & B. Danielson
1975
ENFET
J. Janata
1979
Biossensor PC c/ ADH e LDH
T. Yao & S. Musha
1979
Biossensor de tecido
Rechnit et al
Rechnitz
1980
Sensor de fibra-óptica
J.I. Peterson et al
1997
Eletroantenograma
J Pickett et al
J.
?
Pâncreas artificial implantável
J Jaremko & O. Rorstad
Electrical energy
energ
Electrolytic cells
Non-electrolytic
cells (gavanic)
Non-Spontaneous
p
process
Spontaneous
process
ΔG < 0
ΔE > 0
Potenciometry
I~0
ΔG > 0
ΔE < 0
Chemical energy
Electrolysis Coulometry
P l
Polarography/Voltammetry
h /V l
5
Electrogravimetry
Electroanalytical
methods
Interfacial
methods
Static methods
I 0
I=0
Potentiometry
(E)
Const.
electrode
potential
coulometry
(Q = ∫01 idt
Bulk
methods
Dinamic methods
I>0
Conductometry
(G = 1/R)
Potentiometric
titrations
Controlled
potential
Voltammetry
[ I = f (E) ]
Conductometric
Ti i
Titrations
Constant
current
Coulometric
Titrations
((Q = It))
Amperometric
titrations
i i
Electrogravimetry
(wt)
Electrogravimetry
6
Esquema de (Bio)sensor químico 7
Detectores eletroquímicos
Potenciométricos
Medidas realizadas sem a
passagem de corrente
elétrica
Baseada na mudança de
potencial
t
i l da
d superfície
fí i do
d
eletrodo de trabalho
Amperométricos
Medidas de corrente elétrica
realizadas sob a aplicação
de um potencial elétrico
constante
Baseada nas reações
oxidação e redução que
ocorrem na superfície do
eletrodo
8
2 Fundamental Concepts
2.
11
Electrical conduction
Materiais
Isolantes
Condutores
Eletrô nicos
Metais, Óxidos Inorgâ nicos,
Polí meros Condutores
Iô nicos
Soluç õ es de Eletró litos
Cristais Dopados
I = dQ/dt
e-
Electronic conductor
Ionic conductor
12
Condutores Eletrônicos e Iônicos
Eletrônicos: Obedecem a lei de Ohm (E = IR)
E = Diferença
f
ç de Potencial ((volts)) devido ao movimento
de elétrons
passagem
g
de Corrente
R = Resistência (ohms) do condutor à p
I = Corrente (amperes)
Iônicos: Obedecem a lei de Ohm para pequenos
valores
l
de
d corrente
E = Diferença
Dif
de
d Potencial
P t
i l ((volts)
lts) devido
d id ao movimento
i
t
de íons
R = Resistência (ohms) do eletrólito à passagem de corrente
13
I = Corrente(amperes)
Reações de Oxi-Redução
Transferência de elétrons de um reagente para outro
2 Ag+ + Cu(s)
2Ag(s) + Cu2+
Esta reação pode ser realizado por dois caminhos
fi i
fisicamente
t dif
diferentes
t
Caminho 1: Colocar os reagentes em contato direto
Cuo
Cuo
Ag+
Ag+
Ago
A
Cu2+
14
Célula Eletroquímica
Caminho 2: Separar os reagentes em um arranjo apropriado
e-
e-
Ponte
Salina
(KCl sat.)
Eletrodo de Cobre
Eletrodo de Prata
[Cu2+] = 1.00 mol/L
Cu(s)
[Ag+] = 1.00 mol/L
Cu2+ + 2e-
Ânodo (oxidação)
Ag+ + e-
Ag(s)
Cátodo (redução)
C p
Componentes
de uma C
Célula Eletroquímica
q
• 2 condutores imersos em uma solução contendo eletrólitos (eletrodos)
• 1 condutor eletrônico externo para permitir o fluxo de elétrons
• 1 condutor iônico para evitar o contato direto dos reagentes e permitir
15
o fluxo de íons
Célula Eletroquímica – Movimento de cargas
eee-
e-
e-
Oxidação
Redução
e- Cu2+
NO3
2-
Ag+
SO4
eee-
Cu2+
NO3
2-
SO4
Cu2+
Cl-
CuSO4
K+
Interface Eletrodo/solução
K+
AgNO
g
3
ee-
Ag+
NO3
Cl-
e--
e-
Interface Eletrodo/solução
Voltaic or Galvanic Cell
16
17
Ecellll = Ecatt – Ean + Ej
18
Schematic diagram showing the
standard
t d d hydrogen
h d
electrode
l t d
19
Voltaic or Galvanic Cell
20
21
2
Galvanic Cell: Zno + Cu2+
2
Cuo + Zn2+
22
Schematic
diagram
showing the saturated
calomel electrode
Schematic diagram showing a
Ag/AgCl electrode.
23
24
25
26
27
Relationship between the potential of an Fe3+/Fe2+ half-cell
half cell relative
to the reference electrodes. The potential relative to a standard
hydrogen
y g electrode is shown in blue,, the ppotential relative to a
saturated silver/silver chloride electrode is shown in red, and the
potential relative to a saturated calomel electrode is shown in
green.
28
Junction Potentials
Fig. Origin of the junction potential between a solution of 00.1
Fig
1
29
M HCl and a solution of 0.01 M HCl.
Potential of Junction
Potencial de Junção Líquida: Origem e Cálculos
Devido a diferença de mobilidade dos íons que transportam pela
junção (ponte salina)
Max Planck (Ann. Phys., 39, 161 (1890); 40, 561 (1890)
sendo Zi = número atômico
ti = número de transporte
µi = mobilidade
bilid d iônica
iô i (cm
( 2 s-11V-11)
31
Caso 1
P. Henderson (Z.
P
(Z Physik.
Physik Chem.,
Chem 59,
59 118 (1907); 63,
63
325 (1908)): Cálculo da junção líquida entre 2
soluções com concentrações diferentes (C1 e C2),
) mas
de mesma valência
Ag │ AgCl │ MCl(C1)│ │ MCl(C2) │AgCl │Ag
Ej = 0,0591 (2 t+ -1) log C1/C2
32
Caso 2
Lewis & Sargent (J. Am. Chem. Soc., 31, 363 (1909)):
Cálculo do potencial de junção líquida entre 2
eletrólitos univalentes diferentes de mesma
concentração com um íon comum
Ag │ AgCl │ M1Cl(C)│ │ M2Cl(C) │AgCl │Ag
Onde Λ é a condutância iônica equivalente a diluição
infinitaa de cada eeletrólito
e ó o
34
3. Potentiometric Methods and Electrodes
35
Potentiometric Methods and electrodes: Principles
¾ In ppotentiometry
y the ppotential
of an electrochemical cell is
measured
under
static
conditions. Because no current
or only a negligible current
flows
through
the
electrochemical cell.
36
Potenciometria
E = E* + 0,059 / n log [ox] / [red]
Equação de Nernst
Metallic Indicator Electrodes
1a)Electrodes of the First Kind or Class responds to the
activity of Mn+
Examples: Ag, Hg, Cu, Pb in contact with Ag+, Hg2+, Cu2+,
Pb2+
Cu2+(aq) + 2 e
Cuo
Eo = 0.34 V
ECu2+/Cu = EoCu2+/Cu + 0.0592/2 + log [Cu2+]
Ecell = Eind – Eref + Ej
39
1b) Redox Electrodes
Pt, Au and Pd electrodes
40
Potentiometric Methods: Redox Electrodes
¾ Treatment of platinum electrode
The platinum electrode generally form a monolayer of PtO on its surface that causes
changes in its potential and also delay its response,
Th electrode
The
l
d can b
be attacked
k db
by:
a) Oxidizing solutions containing Cl- with formation of PtCl4-;
b) Natural waters or other oxidant solutions with formation of PtO;
c) Cr(II) in acid medium reduces H+ by formation of adsorbed H2 on Pt
¾ Pre-treatment of Pt electrode
a) Mechanical with Al2O3 or CeO2 or diamond powder;
b) Chemical (alcoholic KOH , sulphochromic, aqua regia or Ce(SO4)2 solution) ;
c) Electrolytic: a) Cathodic - remove the oxide layer; b) Anodic - remove adsorbed H2
41
Potentiometric Titration
42
Potentiometric Methods and electrodes: Electron activity and pE
43
Potentiometric Methods and electrodes: Electron activity and pE
Continuing...
44
Potentiometric Methods and electrodes:
Principles Electron activity and pE
Table. Standard EMF`s and p
pEO for some common redox couples
p
45
Potentiometric Methods: Electrodes of the Second kind
The electrode of the second kind responds to the activity of another specie in
equilibrium with Mn+ (precipitate or stable complexes).
complexes) Generally this species are
anions.
Ex: the
h potential
i l off a Ag
A electrode
l
d in
i a solution
l i off Ag
A + is:
i
This first kind electrode in presence of AgI has the following half-cell equation:
Where:
46
Potentiometric Methods: Electrodes of the Second kind
Another examples: Ag/AgCl(s), Ag/AgBr(s), (Hg/HgY2-).
Where:
47
Potentiometric Methods: Electrodes of the Second kind
Electrode of the second kind: metal-metal oxide electrodes
Ex: Ti, Pb, Nb, Sb, W when passivated chemically responses to H3O+ (pH).
Ex:
Ex:
Potentiometric Methods: Electrodes of the third kind
Electrode of the third kind responds to the activity of another Cation in specific
conditions Some times when an electrode of the second kind is subjected to a redox
conditions.
reaction and another reaction, such as a solubility reaction (involving more than two
reactions) an electrode of the third kind is found.
found
Ex:
Where :
Electrode Membranes
50
Origin of a membrane potential
If the smaller ions are able to diffuse through the membrane but
the larger ions cannot, a potential difference will develop between
the two solutions. This membrane potential can be observed by
introducing a pair of platinum electrodes.
51
52
Potentiometric Methods: Membrane potential
A typical potentiometric electrochemical cell equipped with an ion-selective
electrode.
The electrochemical cell includes two reference
electrodes: one immersed in the ion-selective
electrode’s internal solution and one in the
sample. The cell potential, therefore, is:
Variable
K- Constant
K
The analyte
analyte’ss interaction with the membrane generates a membrane potential if
there is a difference in its activity on the membrane’s two sides.
53
Potentiometric Methods: Glass Selective Electrodes
9Fritz Haber (1901): discovered that there is a change in potential across a
glass membrane when its two sides are in solutions of different acidity.
g
y
9 Harber & Klemensiewicz (1909): First glass electrode + Nernst equation;
9 Mc-Innes & Dole (1930): Corning 015 (commercial);
9 Lengyel & Blum (1934): Na+ electrode;
9 Nikolsky
&
Eisenman
(1960-1975):
Alkaline
and
alkaline-earth
electrodes;
54
Potentiometric Methods: pH Glass Electrode
Reference
electrolyte
Inner
solution
Sample
solution
E1 = Outer potential of the glass membrane (a1);
E2 = Asymmetry potential:
9 glass thickness;
9 asymmetry;
9wearing;
E3 = Inner potential of the membrane (a1‘);
E4 = Inner reference electrode potential (aCl-);
E5 = Outer reference electrode potential;
E6 = Junction potential;
Glass membrane (0.2-0.5 mm)
Gel layer (10-4 mm)
55
Potentiometric Methods: Glass Selective Electrodes
Schematic representation of the
reactions in a glass membrane
9 aq = aqueous solution;
g = surface g
gel ;
9 sg
9 g = gel layer;
9 v = dry glass layer
layer.
56
Potentiometric Methods: Glass Selective Electrodes
Schematic representation of the atomic structure of (a) soda silica glass; (b) soda aluminosilica glass
57
Potentiometric Methods: Combined pH Glass Electrode
Screw Cap
Filling port
Connecting plug
Reference element
Lead-off electrode
Reference electrolyte
Diaphragm
Internal buffer
Membrane
58
An early Beckman pHmeter
Arnold Beckman
(1934 -1939)
59
Potentiometric Methods: Glass Selective Electrode
Asymmetry potential and pHmeter calibration
61
Potentiometric Methods: Glass Selective Electrodes
pHmeter calibration
Dependence of the factor pre-Nernstian with T
K is found using buffer solutions
62
63
64
65
Potentiometric Methods: Glass Ion-Selective Electrodes
¾ Corning 015 (first commercial) = 22% Na2O, 6% CaO and 72% SiO2. When
immersed in an aqueous solution for 7 h, the outer approximately 10 nm of the
membrane’s surface becomes hydrated, resulting in negatively charged sites, —SiO–.
¾ Na+, serve as counter ions. H+ displace the Na+, giving rise to the membrane’s
selectivity for H+.
¾ Corning 015 obeys the following equation:
9 Ideal application: a pH range of approximately 0.5 to 9;
9 At more basic pH levels the glass membrane is more responsive to other cations,
cations
such as Na+ and K+ (alkaline error).
66
Potentiometric Methods: Selectivity of Membranes
Most membranes are not selective toward a single analyte. Instead, the membrane
potential is proportional to the concentration of each ion that interacts with the
membrane’s active sites.
9 zA and zI = charges of the analyte and the interferent;
9 KA,I
A I = selectivity coefficient.
9 (aA)e and (aI)e = activities of analyte and interferent;
9 If KA,I is 1.0, the membrane responds equally to the analyte and the interferent;
9If a membrane shows good selectivity, KA,I << 1.0.
67
68
Nikolsky’ss equation:
Nikolsky
NAS 11-18 from Na+
S l ti id d
Seletividade:
Na2O
11% (mol)
pH > 7 KNa,K = 10-3
Al2O3
18%
pH
H = 7 KNa,K = 3.3.10
3 3 10-33
SiO2
71%
pH < 7 KNa,H > 1
NAS 27-4 from K+
N 2O
Na
27% ((mol)
l)
Al2O3
4%
SiO2
69%
Seletividade:
H+ > Ag+ > K+ = NH4+ > Na+ >Li+ ....>>
>> Ca2+
70
Potentiometric Titration
Solid-based graphite-epoxy electrodes for potentiometric
measurements of pH and acid-base titration
Graphite-epoxy
composite
pH range
Slope /
mV pH-1
Lifetime /
mon (det)
Ref.
40% m/m PbO2
1.0 – 11
-58.7 + 0.3
> 8 (> 1200)
1
30% m/m silica gel
2.0 – 13
-40.5 + 0.4
> 12 (> 6000)
2
30% m/m λ-MnO2
2.0 – 13
-53.6 + 0.5
> 4 (1500)
3
30% m/m Fe2O3
1.7 – 12.5
-39.7 + 0.6
> 6 (2000)
4-6
20% magnesium
silicate
1.0 – 12.0
-39.2 + 0.3
> 8 (1500)
7
Cu/Cu2S film
acid-base
titrations
-59.0 + 0.5
> 3 (400)
8
74
Potentiometric Methods: Liquid-Based Selective Electrodes
¾ This class of ISEs uses a hydrophobic membrane containing a liquid organic
complexing agent that reacts selectively with the analyte. Three types of organic
complexing agents have been used:
9 Cation exchangers;
9 Anion exchangers;
9 Neutral ionophores.
¾ One example of a liquid-based ion-selective electrode is that for Ca2+, which
uses a porous plastic
l ti membrane
b
saturated
t t d with
ith the
th cation
ti exchanger
h
di ( d l)
di-(n-decyl)
phosphate.
75
Potentiometric Methods: Kind of ionic exchangers
Cationic exchangers:
9 Calcium di-(n-decyl) phosphate: (Ca[PO2 (CH3(CH2)9O)2];
9 Sodium tetraphenylborate: (NaBØ4);
Anionic exchangers:
g
9 Tricaprylylmethylammonium chloride (Aliquat 336) : CH3N[(CH2)7CH3]3Cl ;
9 Protonated tertiary
y amine ((tri-n-octylamine);
y
);
9 Tri-n-benzylamine;
9 Tetraphenylarsonium chloride ((AsPh4)Cl)
Neutral exchangers:
9 Valinomycin;
9Crown ether.
76
Preparation of the ionic pair and polimeric membrane
P
Preparation
ti off the
th active
ti material
t i l
a)Cationic
)
Species:
p
Ionic Pair
a)Anionic Species:
Ionic Pair
¾ Preparation of the polimeric membrane with the active material
9 2 - 10 % (m/m)
( / ) off ionic
i i pair;
i
960 – 68 % (m/m) of plasticizer DBP (dibutylphtalate), DOP (dioctylphtalate), or
2-nitrofeniloctileter (o-NPOE);
9 30 % (m/m) PVC;
9 dissolve the mixture in 10 mL THF (tetrahydrofuran)
Potentiometric Methods: Liquid-Based Selective Electrodes
First L-ISE selective to Ca2+ developed
p by
y Ross ((Science,, 156,, 1378 ((1967).
)
¾ The membrane is placed at the end of a non-conducting cylindrical tube, and is in
contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in
di-n-octylphenylphosphonate, which soaks into the porous membrane. The inner
reservoir contains a standard aqueous solution of Ca2+ and a Ag/AgCl reference
78
electrode.
79
Potentiometric Methods: PVC membrane electrode
¾ PVC membrane
b
electrode
l t d
developed by Moody et al
(Analyst, 95, 910 (1970).
Electric conection
Silicone rubber
Glass junction
Ag/AgCl electrode
0.1 mol L-1 CaCl solution
PVC tube
PVC membrane
80
Potentiometric Methods: Polymeric membrane
Experimental arrangement for casting PVC membrane
81
82
83
84
Potentiometric Methods: Polymeric membrane with ionic exchanger
¾ First
Fi t Coated-wire
C t d i PVC ion-selective
i
l ti electrode
l t d
Coaxial cable
Outer conductor
Paraffin
Inner insulation
Inner cond
conductor
ctor
Polymeric membrane
with
ith ionic
i i exchanger
h
¾ Cattrall, R. W. & Freiser, H.; Anal. Chem.; 43, 1905 (1971).
85
Coated graphite ion-selective electrode
Coated graphite ion-selective electrode
88
Potentiometric Methods: Tubular electrode
¾ Tubular electrode made of graphite-epoxy with a PVC film
¾ Preparation
P
ti off the
th tubular
t b l electrode.
l t d A = acrylic
li mold
ld with
ith 8 and
d 10 mm off inner
i
and outer diameters, respectively, 7 mm of length, a = 1mm diameter hole. B =
adaptation of the central conductor of coaxial cable with welded copper plate. C =
mold filled with graphite-epoxy paste. D = view of the electrode showing the
89
channel where it was deposited the PVC membrane.
Potentiometric Methods: Tubular electrode
acrylic mold
copper plate
l
graphite-epoxy paste
coaxial cable
Shielded Connector
¾ Details of the tubular electrode based
on graphite-epoxy PVC film
A = View of the support; B = View of the tubular electrode holder on the
support. 1 = Acrylic holders. 2 = Screws, 3 = rings of silicone rubber, 4 =
90
coaxial cable, 5 = polyethylene tubes (input and output of fluid).
Potentiometric determination of saccharin using
coated-carbon rod ISEs and a graphite-epoxy ISE
H3C
O
N
N
H2N
+
S
N (CH3)2
-
SO2
Toluidine blue O-saccharinate
O saccharinate
Ion-pair: 5:30:65 % m/m ionpair:DBPh:PVC
1
2
3
6 cm
m
2
3
4
2 cm
1
5
0 5 cm
0.5
Coated-carbon rod ISEs (1 and 2) and graphite-epoxy ISE (3)
91
FI-Potentiometric determination of saccharin using a tubular ISE
pH
R
3
2
1
C
RE
L
S
W
6 mm
1 mm
TISE
W
2-3 cm
coating
Schematic diagram of the flow system and tubular ISE of carbon rod: 1)
PVC membrane with ionic pair; 2) epoxy resin coating;
3) electric connection.
Manual
FIA
8.1 x 10-5 – 1.4 x 10-2 mol L-1
1.0 x 10-4 – 1.0 x 10-2 mol L-1,
DL = 6.3 x 10-5 mol L-1
DL = 8.0 x 10-5 mol L-1
Slope = 58.9 + 0.9 mV dec-1
Slope = 53.1 + 0.4 mV dec-1
Lifetime = 9 months (over 1000
determinations)
Sampling frequency = 40 h-1
92
Transient potentiometric signals for saccharin determination
11.00 x 10-22 – 1.0
1 0 x 10-44 mol L-11,
DL= 8.0 x 10-5 mol L-1
Slope= 53.1 + 0.4 mV dec-1
Sampling frequency = 40 h-1
Fatibello-Filho, O.; Aniceto, C. Lab. Rob. and Autom., 11, 234 (1999).
Eletrodo para gases (Severinghaus e Bradley)
94