Basics of Electrochemistry and Electrochemical Measurements

Basics of Electrochemistry and
Electrochemical Measurements
1
Mr. Halavath Ramesh
Department of Chemistry
Loyola College-Chennai
University of Madras

Introduction
• A potentiostat is an electronic instrument that controls the voltage difference between a Working Electrode and a
Reference Electrode. The potentiostat implements this control by injecting current into the cell through an
Auxiliary, or Counter, electrode.
• Involve the use of a potentiostat for applying a potential (relative to a reference electrode) and measuring the
current (flowing from the working electrode to the counter or auxiliary electrode)
• In almost all applications, the potentiostat measures the current flow between the Working and Counter electrodes.
The controlled variable in a potentiostat is the cell potential and the measured variable is the cell current.
2

Electrodes
• A potentiostat requires an electrochemical cell with three electrodes.
Working Electrode
• The Working Electrode is the electrode where the potential is controlled and where the current is measured.
• For many physical electrochemistry experiments, the Working Electrode is an “inert” material such as gold,
platinum, or glassy carbon. In these cases, the Working Electrode serves as a surface on which the
electrochemical reaction takes place.
• In corrosion testing, the Working Electrode is a sample of the corroding metal. Generally, the Working Electrode
is not the actual metal structure being studied. Instead, a small sample is used to represent the structure. This is
analogous to testing using weight-loss coupons. The Working Electrode can be bare metal or coated.
• For batteries, the potentiostat is connected directly to the anode or cathode of the battery.
4

Counter (Auxiliary) Electrode
• The Counter, or Auxiliary, Electrode is a conductor that completes the cell circuit.
• The Counter Electrode in lab cells is generally an inert conductor like platinum or graphite. In field probes, it is
generally another piece of the Working Electrode material. The current that flows into the solution via the Working
Electrode leaves the solution via the Counter Electrode.
• The electrodes are immersed in an electrolyte (an electrically conductive solution). The collection of the
electrodes, the solution, and the container holding the solution are referred to as an electrochemical cell.
Reference Electrode
• The Reference Electrode is used to measure the Working Electrode potential.
• A Reference Electrode should have a constant electrochemical potential as long as no current flows through it.
• The most common lab Reference Electrodes are the saturated calomel electrode (SCE) and the silver/silver
chloride (Ag/AgCl) electrodes. In field probes, a pseudo-reference (a piece of the Working Electrode material) is
often used.
5

6
Basic Diagram of a Potentiostat/Galvanostat
The connections available on the cell cables and
the color code used

7
• Total of five connectors: WE, CE, RE, S and ground. The potential is always measured between the
RE (blue) and the S (red) and the current is always measured between the WE (red) and CE (black).
The ground connector (green) can be used to connect external devices to the same ground.
• In potentiostatic mode, a potentiostat/galvanostat (PGSTAT) will accurately control the potential of the
Counter Electrode (CE) against the Working Electrode (WE) so that the potential difference between
the working electrode (WE) and the Reference Electrode (RE) is well defined, and correspond to the
value specified by the user. In galvanostatic mode, the current flow between the WE and the CE is
controlled. The potential difference between the RE and WE and the current flowing between the CE
and WE are continuously monitored.
• Some Autolab instruments are not fitted with the S connector (like the μAutolab type III). In these
instruments, the potential difference is measured between the RE and the WE.

8
Electrochemical Cell Setups
1.Two electrode setup
2.Three electrode setup
3.Four electrode setup
Overview of some commonly used
reference electrodes
Potential scale of commonly used
reference electrodes (at 25 °C, liquid
junction potentials are not considered)

ELECTROANALYTICAL METHODS
Bulk Method
Conductometry
• Electroanalytical methods are a class of
techniques in analytical chemistry, which study
an analyte by measuring the potential (volts)
and/or current (amperes) in an electrochemical
cell containing the analyte.
• The three main categories are
• Potentiometry (the 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).
9

Electrodes and Potentiometry
Introduction
1.) Potentiometry
 Use of Electrodes to Measure Voltages that Provide Chemical Information
- Various electrodes have been designed to respond selectively to specific analytes
 Use a Galvanic Cell
- Unknown solution becomes a ½-cell
- Add Electrode that transfers/accepts
electrons from unknown analyte
- Connect unknown solution by salt bridge to
second ½-cell at fixed composition and
potential
 Indicator Electrode: electrode that
responds to analyte and donates/accepts
electrons
 Reference Electrode: second ½ cell at a
constant potential
 Cell voltage is difference between the
indicator and reference electrode 10

Reference Electrodes
1.) Overview
 Potential change only dependent on one ½ cell concentrations
 Reference electrode is fixed or saturated  doesn’t change!
Reference electrode,
[Cl-] is constant
Potential of the cell
only depends on [Fe2+]
& [Fe3+]
Pt wire is indicator
electrode whose
potential responds
to [Fe2+]/[Fe3+]
 
]
Cl
log[
.
.
]
Fe
[
]
Fe
[
log
.
.
Ecell
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
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

 05916
0
222
0
1
05916
0
771
0
3
2
Unknown solution of
[Fe2+] & [Fe3+]
11

 The hydrogen electrode is used as a reference for electrode potential
measurements . Theoretically , it is the most important electrode for use in
aqueous solutions.
• The reversible hydrogen electrode in a solution of hydrogen ions at unit
activity exhibits a potential, which is assumed to be zero at all
temperatures .The electrode consists of a platinum wire immersed in a
solution containing hydrogen ions and saturated with hydrogen gas .
Platinum is immersed completely in aqueous arsenic free hydrochloric
acid, and hydrogen gas free from oxygen and carbon monoxide is bubbled
to platinum surface.
• Slowly the air is displaced by hydrogen and the reversible potential is
achieved. Unfortunate, this electrode has some drawbacks . First the
reversibility of hydrogen electrode cannot be maintained in oxidizing
media. Second if a current is withdrawn from the electrode the electrode
acts an anode ,because of the ionization of gas molecules.
Hydrogen Electrode
Reference Electrode: Hydrogen Electrode
13

Reference Electrode: Silver – Silver chloride Electrode
This electrode is composed of a silver chloride and immersed in a solution of
chloride ions .The chloride equilibrium is given by :
Two other reactions involve a dynamic equilibrium between deposition and
dissolution of silver together with solubility equilibrium between silver
chloride and its ions .The metallic silver reaches equilibrium with silver ions
according to the following reaction :
The overall electrode reaction is therefore given by :
Silver – Silver chloride Electrode
14

2.) Silver-Silver Chloride Reference Electrode
 Convenient
- Common problem is porous plug becomes clogged
Eo = +0.222 V
Activity of Cl- not 1E(sat,KCl) = +0.197 V
15

Reference Electrode: Calomel Electrode
• It is the most commonly used reference electrode. It has a constant and reproducible potential. The electrode
basically consists of a platinum wire dipped into pure mercury which rests in a paste of mercurous chloride and
mercury. The paste is in contact with a solution of potassium chloride.
Calomel Electrode
16

3.) Saturated Calomel Reference Electrode (S.C.E)
 Saturated KCl maintains constant [Cl-] even with
some evaporation
 Standard hydrogen electrodes are cumbersome
- Requires H2 gas and freshly prepared Pt surface
Eo = +0.268 V
Activity of Cl- not 1E(sat,KCl) = +0.241 V
17

4.) Observed Voltage is Reference Electrode Dependent
 The observed potential depends on the choice of reference electrode
- Silver-silver chloride and calomel have different potentials
 Use Reference Scale to convert between Reference Electrodes
Observed potential relative to SCE
Observed potential relative to Ag|AgCl
Observed potential relative to SHE
18

Junction Potential
1.) Occurs Whenever Dissimilar Electrolyte Solutions are in Contact
 Develops at solution interface (salt bridge)
 Small potential (few millivolts)
 Junction potential puts a fundamental limitation on the accuracy of direct
potentiometric measurements
- Don’t know contribution to the measured voltage
Again, an electric potential is generated by a separation of charge
Different ion mobility results in
separation in charge
19

Indicator Electrodes
1.) Two Broad Classes of Indicator Electrodes
 Metal Electrodes
- Develop an electric potential in response to a redox reaction at the metal surface
 Ion-selective Electrodes
- Selectively bind one type of ion to a membrane to generate an electric potential
Remember an electric potential is generated by a separation of charge
20

2.) Metal Electrodes
 Platinum
- Most common metal indicator electrode
- Inert: does not participate in many chemical reactions
- Simply used to transmit electrons
 Other electrodes include Gold and Carbon
 Metals (Ag, Cu, Zn, Cd, Hg) can be used to monitor their aqueous ions
- Most metals are not useable
- Equilibrium not readily established at the metal surface
E+
o = +799 V
E(sat,KCl) = +0.241 V
½ Reaction at Ag indicator electrode:
½ Reaction at Calomel reference electrode:
 
241
0
1
1
05916
0
799
0 .
]
Ag
[
log
.
.
E
E
Ecell 
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
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Potential of Ag
indicator electrode
Cell voltage changes as a function of [Ag+]
Example:
Cell Potential from Nernst Equation:
21

]
[
constant 

 C
log
n
.
E
05916
0
4.) Ion-Selective Electrodes
 Responds Selectively to one ion
- Contains a thin membrane
capable of only binding the
desired ion
 Does not involve a redox process
Electrode Potential is defined as:
where [C+] is actually the activity of the analyte and n is
the charge of the analyte
22

pH Electrodes
1.) pH Measurement with a Glass Electrode
 Glass electrode is most common ion-selective electrode
 Combination electrode incorporates both glass and
reference electrode in one body
Ag(s)|AgCl(s)|Cl-(aq)||H+(aq,outside) H+(aq,inside),Cl-(aq)|AgCl(s)|Ag(s)
Outer reference
electrode
[H+] outside
(analyte solution)
[H+] inside Inner reference
electrode
Glass membrane
Selectively binds H+
Electric potential is generated by [H+] difference across glass membrane 23

pH Electrodes
3.) Calibration
 A pH electrode should be calibrated with two or more standard buffers
before use.
 pH of the unknown should lie within the range of the standard buffers
Measured voltage is correlated
with a pH, which is then used to
measure an unknown.
24

pH Electrodes
4.) Errors in pH Measurements
 Standards
- pH measurements cannot be more accurate than standards (±0.01)
 Junction potential
- If ionic strengths differ between analyte and standard buffer, junction potential will
differ resulting in an error of ±0.01
 Junction Potential Drift
- Caused by slow changes in [KCl] and [AgCl] re-calibrate!
 Sodium Error
- At very low [H+], electrode responds to Na+ and the apparent pH is lower than the
true pH
 Acid Error
- At high [H+], the measured pH is higher than actual pH, glass is saturated
 Equilibration Time
- Takes ~30s to minutes for electrode to equilibrate with solution
 Hydration of glass
- A dry electrode will not respond to H+ correctly
 Temperature
- Calibration needs to be done at same temperature of measurement
 Cleaning
- Contaminates on probe will cause reading to drift until properly cleaned or
equilibrated with analyte solution 25

Reference Electrodes Potential
Hg/HgO/NaOH (0.1 M) 0.930 V
Hg/Hg2SO4/K2SO4 (sat) 0.650 V
Fe/Fe+TBAP(0.1M) ACN 0.624 V
Ag/Ag+TBAP(0.1M) ACN 0.542 V
Hg/Hg2Cl2 KCl (sat) 0.241 V
Hg/Hg2Cl2 NaCl (sat) 0.236 V
Ag/AgCl/KCl (3.5 M) 0.205 V
Ag/AgCl/KCl (sat) 0.197 V
Ag/AgCl/NaCl (sat) 0.194 V
NHE (Normal Hydrogen
Electrode)
0.000 V
Potentials of Reference Electrodes
E/V Vs NHE
At 250C
Relationship between the potential of an half-cell relative to
the reference electrodes
26

 Firstly, when it is not in use, the recommended way to maintain the reference electrode capability and lifetime, is to
keep it in a sealed preservative vial with a solution. This storage solution should be identical to the filling solution of
the reference electrode.
• Secondly, it is also important to replace the porous glass frit when needed. A Yellowish discoloration can indicate
contamination, caused by the adsorption of organic compounds into their pores.
• Thirdly, to prevent the contamination of the electrode itself, it is recommended to use a new sample holder when
necessary.
Maintenance of Small Reference Electrodes
• Yellowish discoloration indicates contamination.
This is caused by the adsorption of organic compounds into their pores
Replace the porous glass frit when needed
• To prevent contamination of the reference electrode, a sample holder can be used.
Prevent contamination
Ref: http://www.bio-
logic.net/en/accessories/electrodes/re
ference-electrodes-porous-glass-frits/ 27

A galvanostat, (also known as amperostat) is a control and measuring device capable of keeping
the current through an electrolytic cell in coulometric titrations constant, disregarding changes in
the load itself
28

Basics of Electrochemistry and Electrochemical Measurements

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