Voltammetric methods and electrodes. Electroanalytical methods Interfacial methods Bulk methods...
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Voltammetric methods and electrodes
Transcript of Voltammetric methods and electrodes. Electroanalytical methods Interfacial methods Bulk methods...
Voltammetric methods and electrodes
Electroanalytical methods
Interfacial methods
Bulk methods
Static methodsI ~ 0
Dinamic methodsI # 0
Conductometry(G = 1/R)
Conductometric Titrations (volume)
Potentiometry(E)
Potentiometric titrations (volume)
Constant current
Coulometric Titrations(Q = It)
Electrogravimetry(wt)
Controlled potential
Electrogravimetry (wt)
Amperometric titrations (volume)
Voltammetry[ I = f (E) ]
Const. electrode potential
coulometry (Q = ∫0
1 idt
Introduction
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 Non-Electrochemist,” 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 (1)
E = Eo` + 0.0592/n + log CsO / Cs
R (2)
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
where
Table. Relationship of E to surface concentrations#
E, mV CsO / Cs
R
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
Considering x = and C = CO – CsO
Where = Nernst diffusion layer
(3)
4(4)
(5)
Where ilc is limiting cathodic current and CsO is zero
Cyclic Voltammetry
FeIII(CN)63- + e FeII(CN)6
4- (1)
FeII(CN)64- FeIII(CN)6
3- (2)
E = Eo` + 0.0592/1 log [FeIII(CN)63-] / [FeII(CN)6
4-] (3)
Eo` = E1/2 = (Epa + Epa)/ 2 (4)
Nernst Equation for a reversible system
Ep = Epa – Epc = 0.059/ n (5)
The peak current for a reversible system is described by Randles-
Sevcic 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.
Eletrodo de diamante dopado com boro8000 ppm; 0,72 cm2
Glassy carbon electrode
Eletrodos de carbono vítreo da Tokai Carbon Co
-3.0 3.0
0
1M H2SO4
1M NaOH
1M H2SO4
1M KCl
1M NaOH
Pt
Hg
1M HClO4
0.1M KCl
C
0.5 M H2SO4BDD
- 1.5 +2.5
Approximate potential ranges for platinum, mercury, carbon and boro-doped diamond electrodes
Glassy carbon electrode application
Aplicação de EQM em sistema FIAAplicação 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- + e-
[Fe(CN)6]3- + AA [Fe(CN)6]4-
Comportamento voltamétrico do sistema
Aplicação de EQM em sistema FIAAplicação 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)
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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 s-1
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.
-0.2 0.0 0.2 0.4 0.6 0.8-50
0
50
100
150
200
250
300
I/
A
E / V vs. Ag/AgCl
-0.2 0.0 0.2 0.4 0.6 0.8
0
50
100
150
200
250
300
I/A
E/ V vs. Ag/AgCl
Carbon Nanotubes Functionalized
(A) (B)
50
Anodic stripping voltammetry
•-0.2V por 270 s•25 mV s-1
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.
-0.2 0.0 0.2 0.4 0.6 0.8-10
0
10
20
30
I/A
E/V vs. Ag/AgCl
A
D
EPNM-QTS-ECH
Analytical CurveAnalytical Curve
Figura XX - Voltammograms obtained for the construction EPNM-QTS-ECH with 15% (w/w) QTS-ECH in 0.05 mol L-1 NaNO3 solution.
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%
-0.3 -0.2 -0.1 0.0 0.1 0.2
-4
0
4
8
12
16
I/
A
E/ V vs. Ag/AgCl
1
6
-0.2 0.0 0.2 0.4 0.6 0.8-50
0
50
100
150
200
250
300
I/
A
E / V vs. Ag/AgCl
Concentration de Cu (II) (mol L-1)
Sample Method
comparative*
Method
proposed
Erro relativo %
Urine samples0.50 ± 0.03 0.52 ± 0,09 4,.0
2.4 ± 0.2 2.3 ± 0.1 -4.1
Industrial Waste3.5 ± 0.2 3.6 ± 0.1 1.0
10.7 ± 0.2 11.1 ± 0.1 3.6
Determination of Cu2+
Voltammetric determination of ciprofibrate using a glassy carbon electrode modified with functionalized carbon nanotube within a poly (allylamine hydrochloride) film
ClCl
O
CH3
CH3
COOH
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;
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)
Linear equation: i= -0.700 + 4.75 x 104 x C
Concentration range: 13.3 to132 mol L-1
Detection Limit: 8.34 mol L-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
Analytical curve
Sample Label value (mg) DPV
method
HPLC
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
Table XX - Ciprofibrate determination in pharmaceuticals formulations using GCE-MWCNTs/PAH and standard method
REc1 = 100 x (VPD value – Reference method value) / Reference method value
Inhame (Alocasia macrorhiza)
Abacate (Persea americana)
Batata inglesa (Solanum tuberosum)
Frutas e vegetais empregados como fontes da PFO (POLIFENOL OXIDASE) e PER (PEROXIDADE) em bioreatores e biossensores.
Abobrinha (Cucurbita pepo)
Alcachofra (Cynara scolymus L.)
Banana (Musa paradisiaca)
Batata doce (Ipomoea batatas L.
Lam.)
Berinjela (Solanum melongena)
Cara (Dioscorea bulbifera)
Coco (Cocus nucifera L.)
Jaca (Artocarpus integrifolia L.)
Mandioca (Manihot utilissima)
Nabo (Brassica
campestre ssp.)Pêssego
(Prunus persica)Rabanete
(Raphanus sativus)
Enzimas São proteínas que agem como catalisadores biológicos:
enzimaComposto AComposto A
Composto BComposto B
Centro ativo ou sítio catalítico Não há consumo ou modificação
permanente da enzima
Emil Fisher, década 50
Daniel Kosland, 1970
Modelo chave-fechadura
Modelo Encaixe induzido
E e S se deformam quando em contato (alteração
conformacional), para otimizar o encaixe
Biossensor para glicose - Radiometer®
OH
+O2+ H+3 PFOOH
OH
+ H2O
OHOH
+ 1/2 O2
OO
H2O+
o-quinona
Catecol
Catecol
Fenol
Fig. Esquema de um biossensor
PFO
Low cost PortabilityPracticality
64
Screen-printed electrodes
“screen-printed” or “silk-screen” Technology the possibility of mass production Extremely low cost Simplicity Complete electrochemical system
WorkCounter
Reference
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Screen-printed electrodes
Figura 5: Struture of screen printed electrodes
Substrates
Plastic materials (Polyester)
Ceramics
Metals
Work electrode
•Addition
•Deposition
Metalic films
Nanoparticles
Carbon nanotubes
Enzymes
Polymers
Complexation agents
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Screen-printed electrodes
Carbon nanotubes
Boron-doped diamond (BDD)
Carbon glassy (CG)
Metallic films
etc
Copper
Gold
Iridium
Antimony
Bismuth
Etc.
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New Materials
2002Vytras et al.Pauliukaite et al.
Carbon paste modified with Bi2O3
2003Wang et al.
Bismuth film electrode (BiFE) electrodeposited in CG
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Bismuth film
• Good cathodic potential window
• Interference of dissolved oxygen is minimal
• Low toxicity
• Electrochemical behavior is similar to that of mercury
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Bismuth film
70
MEV-FEG
Figura 10: Micrographs of the BiFE A) 10000x B) 50000x
A) B)
Bismuth film electrode
= =
Copperplate
3-electrodesscheme
Insulatingfilm
Definitionof the
superficial area
Agdeposit
Bideposit
Bi Filmmini-sensor
= =
Copperplate
3-electrodesscheme
Insulatingfilm
Definitionof the
superficial area
Agdeposit
Bideposit
Bi Filmmini-sensor
Bismuth film electrode for anodic stripping SWV lead determination
A B
C
(A): PalmSens and (B): DropSens potentiostats and (C) BiSPE preparation
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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.
electrode
Bismuth film electrode
tt-type connector for printers
Bismuth redox process
-0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2-0,03
-0,02
-0,01
0,00
0,01
0,02
I
II
I /
E/ V vs. Ag/AgCl ( KCl 3,0 mol L-1)
Figura 7: Cyclic voltammogram for 0.02 mol L-1 Bi(NO3)3 in 0.10 mol L-
1 acetate buffer (pH 4,5) solution as electrolyte support; the work electrode is a platinum foil and scan rate of 10 mV s-1.
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I Bi3+ + 3e- Bi0 -0.30 V II Bi3+ Bi0 + 3e- 0.08 V
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.
-0,8 -0,7 -0,6 -0,5 -0,40
4
8
12
16
20
24
28
pa
/ A
E / V vs. Ag/AgCl
0 1 2 3 4 50
4
8
12
16
pa
/ A
[Pb2+] / mol L-1
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.
Determination of lead
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 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
*The SD (±) was calculated from three replicates.
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
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Confecção do minissensor
Figura- Etapas da confecção do minissensor
• Atrazina (ATZ) (2-cloro-4-etilenodiamino-6-isopropilamino-s-triazina)
• Pertence a classe das triazinas
• Composto polar, fracamente básico de coloração branca
N
N
N
NHHN CHC2H5
Cl
CH2 3
Figura. Fórmula estrutural da Atrazina80
Atrazina
-1,2 -1,0 -0,8 -0,6 -0,4
-9
-6
-3
0
I /
E / V vs. Ag/AgCl ( 3,0 mol L-1)
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
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Comportamento eletroquímico da Atrazina (ATZ)
N
N
N
NHCHCH3CH2NH
Cl
CH3
CH3
H+N
N+
N
NHCHCH3CH2NH
Cl
CH3
CH3H
e-
N
H
Cl
NHCHCH3CH2NHCH3
CH3
*e- N
N
N
NHCHCH3CH2NH
CH3
CH3
+ Cl-
