Electrochemistry Tools - Basic
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ELECTROCHEMISTRY TOOLS:
Basic concepts
MSC. JAQUELINE RUIZ MALUTA
http://gmeme.iqsc.usp.br/
1UNIVERSITY OF SÃO PAULO / SÃO CARLOS CHEMISTRY INSTITUTE
GROUP OF ELECTROCHEMISTRY MATERIALS AND ELECTROANALYTICAL METHODS
WHAT IS ELECTROCHEMISTRY?
Interrelation of electrical and chemical effects.
Study of chemical changes caused by the passage of an
electric current
Types of research
The production of electrical energy by chemical reactions
Phenomena: Corrosion, electrophoresis
Devices: electrochromic displays, electroanalytical sensors,
batteries, and fuel cells
Technologies: the electroplating of metals and the large-scale
production of aluminum and chlorine
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WHICH TYPE OF INFORMATION
DO YOU HAVE?
Thermodynamic data about a reaction;
Analyze a solution for trace amounts of metal ions or organic species;
The design of a new power source;
Electro-synthesis of some product;
Electro-degradation of some molecule;
REQUIRES AN UNDERSTANDING OF THE FUNDAMENTAL PRINCIPLES OF ELECTRODE REACTIONS AND THE ELECTRICAL PROPERTIES OF
ELECTRODE-SOLUTION INTERFACES.
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MY FIELD OF RESEARCH
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ELECTROANALYSIS Electrochemistry in analytical context
Useful for quantitative purposes, based on measurements of the
peak current
The current is proportional to the concentration;
Environmental monitoring, industrial quality control, or biomedical
analysis.
Advantages
high sensitivity (~10-9 mol L-1), selectivity toward electroactive species, a
wide linear range, portable and low-cost instrumentation.
PORTABLE AND LOW-COST
INSTRUMENTATION Home testing of blood glucose
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Electrochemical test strips have
electrodes where a precise
voltage is applied and a current
proportional to the glucose in the
blood is measured as a result of
the electrochemical reaction on
the test strip.
http://www.docstoc.com/docs/69965634/Accu-Chek-Advantage-Electrochemistry-for-Diabetes-Management
WHAT DO YOU NEED?
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Voltammetric analyser
Potentiostatic circuitry and a voltage ramp generator
Cell:
Covered beaker of 5–50 mL volume;
Three electrodes:
working, reference, auxiliary/counter;
Electrolyte:
conductive sample solution
Analyte:
electroative species
Plotter
WORK ELECTRODE
The working electrode is the electrode at which the investigated
processes occur.
Electrode materials:
solid metals (e.g.,Pt, Au),
liquid metals(Hg, amalgams),
carbon
and semiconductors (indium-tin oxide).
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COUNTER ELECTRODE
The current flows between the working and the counter
electrode.
Material: usually platinum or titanium
The area of the counter electrode should be larger than that of
the working electrode.
15
REFERENCE ELECTRODE
The reference electrode is keep at a constant potential.
It is used to control the potential of a working electrode
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Saturade calomelane
electrode
Ag/AgCl electrode
ELECTROLYTE
Electrolyte phase charge is carried by the movement of ions;
Ionic species ( H+, Na+, Cl-) in either water or a nonaqueous solvent
Electrolyte is usually added to the test solution to ensure sufficient
conductivity.
Migration mass transport influence is eliminated
Unstirring There is no convection mass transport
Just diffusion mass transport
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POTENTIAL WINDOW
The available potential window is determined by the currents of
reduction/oxidation of the supporting electrolyte/solvent
We should avoid entering potentials where these processes visibly
occur
The products generated at the potential limits may interfere with the
system under investigation and may affect the electrode surface.
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Too positive:H2O(l) ½ O2 + 2 H+ + 2 e-
Too negative:2 H2O(l) + 2 e-
2 H2 + OH-
POTENTIAL WINDOW
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Surfaces are partially oxidized;
Thin layers of oxides are formed at gold and platinum;
Functional groups (–C=O; –OH) are attached to the carbon materials.
The voltammograms of solid phases are specific fingerprints, even when the assignment of signals is not obvious.
Martínez-Huitle, J. Brazilian Chem Society, 2008, 19: 150-156. Maluta, J.P., J Solid State Electrochem. 2014, 18:2497–2504
EXEMPLE: pH INFLUENCE
NO detection in
phosphate buffer solution
pH influence, all the
others parameters are
the same.
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0,60 0,65 0,70 0,75 0,80 0,85 0,90 0,95 1,00
0,00
4,50x10-4
9,00x10-4
1,35x10-3
1,80x10-3
I / m
A c
m-2
E / V (vs Ag/AgCl)
pH 2
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
WHAT IS HAPPENING?
Oxidation reaction
R O + ne-
The e- formed will be received by an electrode
Reduction reaction
O + ne- R
The electrode is the source of the electrons
Redox changes current passage
The amout of material electrochemical oxidized or reduced isproportional to the current passage
REDOX
REACTIONS
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TECHNIQUES
POTENTIOMETRY: difference in electrode potentials is measured
COULOMETRY: the cell's current is measured over time
VOLTAMMETRY: the cell's current is measured while actively altering
the cell's potential.
CYCLIC VOLTAMMETRY
SQUARE WAVE VOLTAMMETRY
DIFFERENCIAL PULSE VOLTAMMETRY
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THE ANALYTICAL SENSITIVITYCAN BE IMPROVED
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(A) Cyclic voltammograms obtained at the OMC electrode with, 0.5, 1, 1.5 mM CySH (from top to bottom) in the PBS pH 7.16
(B) Current–time response curves of the OMC electrode in PBS pH 7.16 with successive addition of 0.1 mM CySH
CYCLIC-VOLTAMMETRY
Cyclic voltammetry is often the first experiment performed in an
electroanalytical study:
Qualitative information about electrochemical reactions
Thermodynamics of redox processes
Kinetics of heterogeneous electron transfer reactions
Location of redox potentials
Effect of media on the redox process
The occurrence of chemical reactions, that precede/succeed the
redox process
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ELECTROCHEMICALLY
REVERSIBILITY
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a) Reversible
Ep = independent of scan rate
IPA / IPC = 1
IP vs √scan rate = linear
Current is controlled by mass transport
Described by Nerst Equation
b) Quasi-reversible:
Depend of both electron transfer and mass transport
c) Irreversible
The electron transfer is slow
31EFFECTS DUE TO CAPACITANCE
AND RESISTANCE
a) Reversible cyclic voltammogram
b) With the effect of capacity
c) With additional resistance
COMPLICATIONS
The electrode reaction is rarely simple;
The product is either insoluble, or partly adsorbed at the electrode
surface.
Coupled chemical reactions can happen
The results should be carefully analyzed
In situ UV-vis
Scan rate test
Change the window potential
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SCAN RATE TEST
Run experiments in which you selectively vary the potential scan rate;
Plot the peak current versus the square root of the scan rate;
For a reversible system, the peak height will increase linearly with the square root of the scan rate. Also, de current is controlled by diffusion transport
The slope of the resulting line will be proportional to the diffusion coefficient (Randles-Sevcik equation)
Ip = 269 n2/3 AD1/2 v1/2 C
• ip= peak height (amp)
• n = number of electrons
• A = area (cm2)
• D = diffusion coefficient (cm2/sec)
• v = scan rate (V/sec)
• C = concentration of solution (M)
• F = Faraday constant
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SCAN RATE TEST
34
D. Zheng et al. / Journal of Electroanalytical Chemistry 625 (2009) 82–87
The current linearly increases with
scan rate (not √scan rate)
suggesting a adsorption-controlled
process
NOT controlled by diffusion transport
COUPLED REACTIONS
The product R is chemically
‘removed’ from the surface
Smaller reverse peak
In the extreme case, the chemical
reaction may be so fast that all of R
will be converted, and no reverse
peak will be observed.
In faster scan rates, we can see the
reverse peak
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37
-0,6 0,0 0,6
-0,2
0,0
i / m
A
E / V (vs Ag/AgCl)
1
2
3
4
5
N+
OO-
+ 4 e- +
N
OHH
4 H+
nitrobenzene N-phenylhydroxylamine
N
OHH
N
O
+ 2 e- + 2 H+
N-phenylhydroxylamine nitrosobenzene
N
O
+ 2 e- + 2 H+
N
OHH
nitrosobenzene N-phenylhydroxylamine
PROGRESSIVE ADSORPTIVE
ACCUMULATION
Repetitive CV for riboflavin in a
sodium hydroxide solution.
A gradual increase of the
cathodic and anodic peak
currents
indicating progressive
adsorptive accumulation at
the surface.
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INCREASE OF ANALYTE
CONCENTRATION
Run experiments in which you selectively
vary the analyte concentration;
Plot concentration versus peak current
Check the concentration where the graphic
is linear
The equation can be used to determine an
unknown solution concentration
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Maluta, J.P., J Solid State Electrochem (2014) 18:2497–2504
MODIFIED ELECTRODES
Methods based on attaching a certain compound, or a specific
chemical group, to the surface of electrode
by adsorption, by chemical reaction or by formation of a polymer film…
Electrocatalytic modified electrodes contain attached electron-
transfer mediators
They accelerate electrode reactions
The catalyst is readily regenerated by the fast and reversible electrode
reaction
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WHAT COMPOUNDS AND
MATERIALS CAN BE STUDIED?
Highly insoluble in the electrolyte solution used;
Possess electroactivity;
the ability to be either oxidized or reduced in the accessible potential
window of the experiment.
Three different kinds of compounds:
not electroactive,
irreversibly destroyed in the electrochemical reactions,
can be reversibly reduced and oxidized.
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SAMPLE PREPARATION
Physical casting in a base electrode:
Prepare a solution in a volatile solvent (Usually alcohols or
DMF);
Put a drop and let it dry;
Nafion® or glutamin can be use together as binder;
The compound should be stable under ambient
conditions.
Impurities, which may be electrochemically active,
should not be introduced by this procedure.
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SAMPLE PREPARATION
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Chen, J. Adv. Mater. 2012, 24, 4569–4573Bojorge, N., & Alhadeff, E. Graphite-Composites Alternatives for
Electrochemical Biosensor.
CARBON ELECTRODES
Carbon electrodes: graphite, glassy carbon, graphite powder with liquid or solid binders, carbon fibers, carbon nanotubes, boron-doped diamond,
titanium carbide, ordered mesoporous carbon, graphene
The glassy carbon:
Synthesis: Very slow carbonization in inert atmosphere at 300◦C to 1200◦C.
Inside a pre-modeled polymeric resin body
With the base polished to a mirror
Carbon nanotubes:
Synthesis: arc discharge, laser ablation, or chemical vapor deposition
They appear either as multi-walled, or as single-walled.
Boron-doped Diamond
Synthesis: Low-pressure chemical vapor deposition or high pressure and high temperature
Wide window potential
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ELECTRODE PRETREATMENT
A solid electrode requires very careful pretreatment.
The best cleaning method has to be chosen from applying either
inorganic or organic solvents.
If necessary, the electrode surface should be clean and polished on
a very wet pad to mirror gloss.
Using abrasive powders (such as diamond and alumina)
Often, the solid electrode needs an electrochemical
activation/regeneration.
Cycling the potential in an appropriate range and in an appropriate
solution.
Modified solid cannot be treated by polishing.
The only way is the electrochemical treatment
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ELECTRODE PRETREATMENT
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http://www.basinc.com/mans/LC_epsilon/Maintenance/Working/working.html
REMEMBER...
Before carrying out voltammetric experiments with analytes:
Check the available potential window
Check if there are no peaks of unwanted impurities in that range
After use, the working electrode should be thoroughly rinsed and
dried
Pay much attention to the reproducibility of results!
Each set of experiments should start and end with a blank
voltammogram in order to verify the cleanliness of the electrode itself
Pay attention in all variables which may affecting the electrode reaction
The starting potential should be carefully set to a value where no
reaction is expected.
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REFERENCES
Scholz, F. (2009). Electroanalytical methods: guide to experiments and
applications. Springer.
Wang, J. (2006). Analytical electrochemistry. John Wiley & Sons.
Bard, A. J., & Faulkner, L. R. (1980). Electrochemical methods: fundamentals
and applications. New York: Wiley.
Monk, Paul M.S., Fundamentals of Electroanalytical Chemistry (2002). John
Wiley & Sons.
http://chemwiki.ucdavis.edu/Analytical_Chemistry/Analytical_Chemistry_2.0
/11_Electrochemical_Methods/11D_Voltammetric_Methods
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