Voltammetric methods and electrodes
Introduction
Electroanalytical
methods
Interfacial
methods
Static methods
I~0
Potentiometry
(E)
Const.
electrode
potential
coulometry
(Q = ∫01 idt
Bulk
methods
Dinamic methods
I#0
Conductometry
(G = 1/R)
Potentiometric
titrations (volume)
Controlled
potential
Voltammetry
[ I = f (E) ]
Conductometric
Titrations (volume)
Constant
current
Coulometric
Titrations
(Q = It)
Amperometric
titrations (volume)
Electrogravimetry
(wt)
Electrogravimetry
(wt)
Five Important interrelated concepts to
understand electrochemistry:
(1) the electrode’s potential determines the analyte’s form at
the electrode’s surface;
(2) the concentration of analyte at the electrode’s surface
may not be the same as its concentration in bulk solution;
(3) in addition to an oxidation–reduction reaction, the analyte
may participate in other reactions;
(4) current is a measure of the rate of the analyte’s oxidation
or reduction; and
(5) we cannot simultaneously control current and potential.
L. Faulkner, Understanding electrochemistry: some distinctive concepts,” J. Chem.
Educ., 60, 262 (1983) ; P. T. Kissinger, A. W. Bott, Electrochemistry for the NonElectrochemist,” Current Separations, 20, 51 (2002)
1) The Electrode’s Potential Determines the Analyte’s Form
Fig. Redox ladder diagram for Fe3+/Fe2+ and for Sn4+/ Sn2+ redox couples. The
areas in blue show the potential range where the oxidized forms are the
predominate species; the reduced forms are the predominate species in the
areas shown in pink. Note that a more positive potential favors the oxidized
forms. At a potential of +0.500 V (green arrow) Fe3+ reduces to Fe2+, but Sn4+
remains unchanged.
2) Interfacial Concentrations May Not Equal Bulk Concentrations
Fig. Concentration of Fe3+ as a function of distance from the
electrode’s surface at (a) E = +1.00 V and (b) E = +0.500 V. The
electrode is shown in gray and the solution in blue.
Nernst Equation
E = Eo + 0.0592/n + log [ox]/[red]
Fe3+ + e
Fe2+
E = Eo + 0.0592/1 + log [Fe3+]/[Fe2+]
3) The Analyte May Participate in Other Reactions
Fe3+ + OH-
FeOH2+
4) We Cannot Simultaneously Control Both Current and Potential
5) Controlling and Measuring Current and Potential
Controlled Potential Methods (Voltammetry)
Fig. Flow patterns and regions of interest near the work electrode
in hydrodynamic voltammetry
Controlled Potential Methods (Voltammetry)
O + ne
R
E = Eo` + 0.0592/n + log CsO / CsR
(1)
(2)
where
E = potential applied to electrode (mV)
Eo`= formal reduction potential of the couple vs Eref
n = number of electrons in reaction (1)
CsO = surface concentration of species O
CsR = surface concentration of species R
Table. Relationship of E to surface concentrations#
E, mV
CsO / CsR
236
10,000/1
177
1,000/1
118
100/1
59
10/1
0
1/1
-59
1/10
-118
1/100
-177
1/1,000
-236
1/10,000
For a reversible system, n = 1, Eo`= V
The current at an electrode is related to the flux (rate of mass transfer)
of material to the electrode
(3)
Considering x =  and C = CO – CsO
Where  = Nernst diffusion layer
(4) 4
(5)
Where ilc is limiting cathodic current and CsO is zero
Cyclic Voltammetry
FeIII(CN)63- + e
FeII(CN)64-
FeII(CN)64-
(1)
FeIII(CN)63- (2)
Nernst Equation for a reversible system
E = Eo` + 0.0592/1 log [FeIII(CN)63-] / [FeII(CN)64-]
(3)
Eo` = E1/2 = (Epa + Epa)/ 2
(4)
Ep = Epa – Epc = 0.059/ n
(5)
The peak current for a reversible system is described by RandlesSevcic Equation for the forward sweep for the first cicle:
ip = 2.69  105 n2/3 A D1/2 1/2 C
(6)
Where: ip = peak current (A); n = number of electrons; A = electrode
area (cm2); D = diffusion coefficient (cm2 / s);  = scan rate (V /s) and
C = concentration (mol / cm3)
Cronoamperometria
Figura (A) Representação esquemática da aplicação de potencial em
voltametria de pulso diferencial. A corrente é amostrada em S1 e S2 e a
diferença entre elas é registrada; (B) Voltamograma de pulso diferencial.
SCHOLZ, F., ed (2005). Electroanalytical methods. New York: Springer.
Figura (1) Forma de aplicação de potencial na voltametria de onda quadrada; (2)
Voltamogramas de onda quadrada esquemáticos para um sistema reversível (A) e para
um sistema totalmente irreversível (B).
SOUZA, D.; MACHADO, S. A. S.; AVACA, L. A. Química Nova, Vol. 26, 81-89, 2003.
LOVRIC, M.; KOMORSKY- LOVRIC, S.; MIRCESKI, V. Square Wave Voltammetry. ed.
(2007), Berlin: Springer.
Glassy carbon electrode
Eletrodo de diamante dopado com boro
8000 ppm; 0,72 cm2
Eletrodos de carbono vítreo da Tokai Carbon Co
Approximate potential ranges for platinum, mercury, carbon and
boro-doped diamond electrodes
0
3.0
-3.0
1M H2SO4
Pt
1M NaOH
1M H2SO4
1M KCl
Hg
1M NaOH
1M HClO4
C
0.1M KCl
0.5 M H2SO4
- 1.5
+2.5
BDD
Glassy carbon electrode application
Aplicação de EQM em sistema FIA
Eletrodo base:
Eletrodo de carbono vítreo
Preparação do Eletrodo:
Ciclagem de potencial entre -0,2 e 0,6 V
(vs. Ag/Cl) em solução de 1,0 mmol L-1
FeCl3.6H2O e 10 mmol L-1 de K3[Fe(CN)6]
Funcionamento
[Fe(CN)6]4-
[Fe(CN)6]3- + AA
[Fe(CN)6]3- + e-
[Fe(CN)6]4-
Comportamento voltamétrico do
sistema
Aplicação de EQM em sistema FIA
Anodic stripping voltammetric determination of
copper(II) using a functionalized carbon nanotubes
paste electrode modified with crosslinked chitosan
Janegitz, B.C., Marcolino-Junior, L.H., Campana-Filho, S.P.,Faria, R.C.,
Fatibello-Filho,O. Sensors and Actuators B, 142, 260 (2009)
Comparison: with and without carbon nanotube
functionalization
Anodic stripping voltammetry
•80 % CNTs (w/w) + 20 % nujol (w/w)
•-0.2V for 270 s
• 25 mV
Carbon Nanotubes
Functionalized
s-1
(B)
300
300
250
250
200
200
I/  A
I/A
(A)
150
100
150
100
50
50
0
0
-0.2
0.0
0.2
0.4
0.6
E/ V vs. Ag/AgCl
0.8
-50
-0.2
0.0
0.2
0.4
0.6
0.8
E / V vs. Ag/AgCl
Figure XX - Linear voltammograms obtained with electrodes containing functionalized
nanotubes not (A) and functionalized (B), in 0.1 mol L-1 NaNO3 solution in the
presence of Cu2 + 9.0 x 10-5 mol L-1.
49
Anodic stripping voltammetry
•-0.2V por 270 s
•25 mV s-1
EPNM-QTS-ECH
30
D
I/A
20
10
A
0
-10
-0.2
0.0
0.2
0.4
0.6
0.8
E/V vs. Ag/AgCl
Figura XX -. Stripping voltammetry for EPN (A), EPNM-QTS (B), EPNM-QTS-GA
(C) and EPNM-QTS-ECH (D) in 0.1 mol L-1 NaNO3 solution in the presence of Cu2 +
9.0 x 10-5 mol L-1.,  = 25mV s-1, a 25ºC.
50
Analytical Curve
300
6
12
250
8
200
1
4
I/  A
I/  A
16
0
150
100
50
-4
0
-0.3
-0.2
-0.1
0.0
0.1
0.2
E/ V vs. Ag/AgCl
Figura XX - Voltammograms obtained
for the construction EPNM-QTS-ECH
with 15% (w/w) QTS-ECH in
0.05 mol L-1 NaNO3 solution.
-50
-0.2
0.0
0.2
0.4
0.6
0.8
E / V vs. Ag/AgCl
Figura XX - Analytical curve:
• 7.93 x 10-8 a 1.6 x 10-5 mol L-1
L D =1.06 x10-8 mol L-1 ,
LQ= 7.93 x 10-8 mol L-1
RSD= 3.12%
Determination of Cu2+
Concentration de Cu (II) (mol L-1)
Sample
Urine samples
Industrial Waste
Method
Method
Erro relativo %
comparative*
proposed
0.50 ± 0.03
0.52 ± 0,09
4,.0
2.4 ± 0.2
2.3 ± 0.1
-4.1
3.5 ± 0.2
3.6 ± 0.1
1.0
10.7 ± 0.2
11.1 ± 0.1
3.6
Voltammetric determination of ciprofibrate using a glassy carbon
electrode modified with functionalized carbon nanotube within a poly
(allylamine hydrochloride) film
o The ciprofibrate is a fibrate and present Antilipemic effect (lipid lowering);
o Fibrates are indicated for patients who, after tests confirmed that the
increase in endogenous triglecirideos is due to poor nutrition;
o A possible interest in determining the ciprofibrate addition to quality
control of drugs;
CH3
O
Cl Cl
COOH
CH3
Figure XXX - Ciprofibrate molecular estructure.
Functionalization of MWCNTs in acid solution
(H2SO4/HNO3 3:1)
Carbon nanotubes dispertion in PAH solution
[dispertion]=1mg mL-1
Film formation on the elcetrode
by casting technique (20 μL)
(a)
(b)
(c)
(d)
Figure XX - PAH SEM images (a) and (b); MWCNTs/PAH SEM
image (c) and (d)
Analytical curve
6
6
Linear equation:
i= -0.700 + 4.75 x 104 x C
i/A
4
2
4
0
0,84
0,91
0,98
1,05
1,12
Concentration range:
13.3 to132  mol L-1
i/A
E/V vs. Ag/AgCl
2
Detection Limit: 8.34 mol L-1
0
0
40
80
[ciprofibrate]/mol L
120
-1
Figure XX - Analytical curve obteined for
ciprofibrate determination in phosphate buffer
solution 0,01 mol L-1 by VPD. = 12 mV s-1,
A= 60 mV, t= 100 ms
Table XX - Ciprofibrate determination in pharmaceuticals formulations using GCEMWCNTs/PAH and standard method
Sample
Label value (mg)
DPV
HPLC
method
method
REc1
A
100
100 ± 3
98±5
2
B
100
99 ± 4
100±7
-1
C
100
99 ± 6
100±4
-1
D
100
100 ± 6
104±2
-4
REc1 = 100 x (VPD value – Reference method value) / Reference method value
Frutas e vegetais empregados como fontes da PFO (POLIFENOL OXIDASE) e
PER (PEROXIDADE) em bioreatores e biossensores.
Abacate
(Persea americana)
Abobrinha
(Cucurbita pepo)
Alcachofra
(Cynara scolymus L.)
Berinjela
Cara
Batata inglesa
(Solanum tuberosum) (Solanum melongena) (Dioscorea bulbifera)
Jaca
(Artocarpus integrifolia L.)
Mandioca
(Manihot utilissima)
Nabo
(Brassica
campestre ssp.)
Banana
(Musa paradisiaca)
Coco
(Cocus nucifera L.)
Pêssego
(Prunus persica)
Batata doce
(Ipomoea batatas L.
Lam.)
Inhame
(Alocasia macrorhiza)
Rabanete
(Raphanus sativus)
Enzimas
São proteínas que agem como catalisadores biológicos:
enzima
Composto A
Composto B
Centro ativo ou sítio
catalítico
Não há consumo ou modificação
permanente da enzima
Emil Fisher, década 50
Modelo
chave-fechadura
E e S se deformam quando em
contato (alteração
conformacional), para otimizar
o encaixe
Daniel Kosland, 1970
Modelo
Encaixe induzido
Biossensor para glicose - Radiometer®
Fig. Esquema de um biossensor
OH
OH
OH
+
+ O2+ 3 H PFO
+
Fenol
Catecol
OH
O
OH
+ 1/2 O2
Catecol
PFO
O
+
o-quinona
H2O
H2O
Screen-printed electrodes
Low cost
Portability
Practicality
64
Screen-printed electrodes
“screen-printed” or “silk-screen” Technology
 the possibility of mass production
 Extremely low cost
 Simplicity
 Complete electrochemical system
Referen Count
er
ce
Work
Figura 5: Struture of screen printed electrodes
65
Screen-printed electrodes
Substrates
Work electrode
 Plastic materials
(Polyester)
 Ceramics
 Metals
Metalic films
Nanoparticles
•Addition
Carbon nanotubes
•Deposition
Enzymes
Polymers
Complexation agents
66
New Materials
Carbon nanotubes
Copper
Boron-doped diamond
(BDD)
Gold
Carbon glassy (CG)
Iridium
Metallic films
Antimony
etc
Bismuth
Etc.
67
Bismuth film
2002
Vytras et al.
Pauliukaite et al.
2003
Wang et al.
Carbon paste modified
with Bi2O3
Bismuth film electrode
(BiFE) electrodeposited in
CG
68
Bismuth film
• Good cathodic potential window
• Interference of dissolved oxygen is minimal
• Low toxicity
• Electrochemical behavior is similar to that of mercury
69
Bismuth film electrode
MEV-FEG
A)
B)
Figura 10: Micrographs of the BiFE A) 10000x B) 50000x
70
Bismuth film
determination
electrode
for
anodic
stripping
SWV
lead
B
A
C
Bi
deposit
=
Copper
plate
3-electrodes
scheme
Insulating
film
Definition
of the
superficial
area
Ag
deposit
=
Bi Film
mini-sensor
(A): PalmSens and (B): DropSens potentiostats and (C) BiSPE preparation
Confecção do minissensor
120 °C
durante
200 s
FeCl3 0,50 mol L-1 em meio
de HCl 0,10 mol L-1 durante
15-20 minutos.
72
Bismuth film electrode
electrode
tt-type connector for printers
Bismuth redox process
II
0,02
I / 
0,01
I Bi3+ + 3eBi0
II Bi3+
Bi0 + 3e-
0,00
-0.30 V
0.08 V
-0,01
-0,02
-0,03
-0,6
I
-0,5
-0,4
-0,3
-0,2
-0,1
0,0
0,1
0,2
-1
E/ V vs. Ag/AgCl ( KCl 3,0 mol L )
Figura 7: Cyclic voltammogram for 0.02 mol L-1 Bi(NO3)3 in 0.10 mol L1 acetate buffer (pH 4,5) solution as electrolyte support; the work
electrode is a platinum foil and scan rate of 10 mV s-1.
Filme de bismuto
-0.18 V vs. Ag/AgCl (3.0 mol L-1 KCl) during 200 s
0.02 mol L-1 Bi(NO3)3, 1.0 mol L-1 HCl in 0.15 mol L-1 Sodium citrate.
74
Determination of lead
16
28
12
20
pa / A
pa / A
24
16
12
8
8
4
4
0
-0,8
-0,7
-0,6
-0,5
E / V vs. Ag/AgCl
-0,4
0
0
1
2
2+
3
4
5
-1
[Pb ] / mol L
Anodic stripping voltammograms of 9.9 x 10-8 – 8.3 x 10-6 lead (LD of 5.8 x 10-8
M) in 0.1 M acetate buffer (pH 4.5), using square-wave mode. Deposition at 1.1 V for 2 min; pulse amplitude of 28 mV; increment of potential of 3 mV and
frequency of 15 Hz.
Bismuth film electrode (BiFE) for paraquat determination
In 0.1 mol L-1 HAC pH
4,5, using differential
pulse voltammetry.
Figueiredo-Filho, L. C. et al, Electroanalysis, 22, 1260 (2010)
DPV para determinação de Paraquat
Besides of paraquat can
be
determined
simultaneously Cd2+ e
Pb2+.
Determination of PQ in six natural water samples by BIFE and
HMDE (reference).
Samples*
/µ mol L-1
*The
HMDE
BIFE
ER (%)
A1
59.03 ± 0.06 58.73 ± 0.03
-0.51
A2
58.74 ± 0.01 59.23 ± 0.00
0.84
A3
58.35 ± 0.03 57.41 ± 0.02
-1.61
A4
29.36 ± 0.08 29.56 ± 0.03
0.68
A5
29.23 ± 0.05 27.97 ± 0.02
-4.31
A6
27.95 ± 0.05 29.51 ± 0.02
5.58
SD (±) was calculated from three replicates.
Confecção do minissensor
Figura- Etapas da confecção do minissensor
Filme de bismuto
-0,18 V vs. Ag/AgCl (KCl 3,0 mol L-1) durante 200 s
Bi(NO3)3 0,02 mol L-1, HCl 1,00 mol L-1 e citrato de sódio 0,15 mol L-1
Cola de prata
Bright Silver Epoxy (BSE) + Gray Silver Hardener (GSH). Após a aplicação
na superfície de cobre esperou-se 24 horas para a cura da cola
79
Atrazina
• Atrazina (ATZ) (2-cloro-4-etilenodiamino-6isopropilamino-s-triazina)
• Pertence a classe das triazinas
• Composto polar, fracamente básico de coloração
branca
Cl
N
C2H5 HN
N
N
NH CH CH2
3
Figura. Fórmula estrutural da Atrazina
80
Comportamento eletroquímico da Atrazina (ATZ)
0
I / 
-3
-6
-9
-1,2
-1,0
-0,8
-0,6
-0,4
-1
E / V vs. Ag/AgCl ( 3,0 mol L )
Figura 11- Voltamograma obtido para uma solução de Atrazina 4,00 x 10-5 mol L-1,
utilizando tampão acetato 0,10 mol L-1, pH *4,5 em 15 % v/v de etanol como
eletrólito suporte. * pH condicional
Cl
N
CH3CH2NH
Cl
N
N
CH3
NHCH
CH3
N
H+
CH3CH2NH
Cl
N
N+
H
CH3
NHCH
CH3
*
e-
CH3
CH3CH2NH
N
H
NHCH
CH3
N
eCH3CH2NH
N
N
CH3 + ClNHCH
CH3
81