ELECTROANALYTICAL METHODS

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ELECTROANALYTICAL METHODS
Dr. ABRAHAM GEORGE READER IN CHEMISTRY, MAR
IVANIOS COLLEGE, TRIVANDRUM.
abraham.mankavilatgmail.com
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APPLICATION OF ELECTROCHEMISTRY TO SOLVE
ANALYTICAL PROBLEMS
USED FOR THE ESTIMATION OF ELEMENTS OR SPECIES
CAN BE USED FOR IDENTIFICATION AND ESTIMATION CAN
BE USED TO STUDY THE KINETICS OF SOME REACTIONS
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• ADVANTAGES
• SELECTIVITY AND SPECIFICITY (From the choice of
  electrode and potential)
• HIGH SENSITIVITY AND LOW DETECTION LIMIT
• READY TO USE COMPACT INSTRUMENTS
• SPEED AND ACCURACY
• NO TECHNICAL SKILL REQUIRED

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MOST WIDELY USED TECHNIQUE
ECG, EEG,ERG GLUCOMETER UREA ANALYSIS BLOOD
ANALYSIS SOIL TESTING CLARKS OXYGEN SENSOR
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ALL THE FREE ENERGY FOR LIFE PROCESS COMES FROM
AN ELECTRON TRANSFER PROCESS O2 4e 4H ?
2H2O A RESTING HUMAN WILL USE 0.2 L O2 /MIN
9mmol OF O2 36 mmol OF ELECTRONS/MIN
0.6 mmol OF ELECTRONS PER SECOND
60 AMPERES OF CURRENT. THE MAIN CURRENT PANEL IN
MANY HOUSES HAVE ONLY A CAPACITY AROUND THIS.
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GLUCOSE SENSOR THE MEMBRANE CONSISTS OF THREE
LAYERS. OUTER LAYER IS A POLYCARBONATE FILM
PERMEABLE TO GLUCOSE BUT NOT OTHER CONSTITUENTS
OF BLOOD. THE MIDDLE LAYER IS IMMOBILISED ENZYME
(GLUCOSE OXIDASE). THE INNER LAYER IS CELLULOSE
ACETATE PERMEABLE TO H2O2 GLUCOSE O2 GLUCOSE
OXIDASE GLUCONIC ACID H2O2 H2O2 2OH-
O2 H2O 2e THE
RESULTING CURRENT IS DIRECTLY PROPORTIONAL TO
GLUCOSE CONCENTRATION
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GLASS ELECTRODE AND pH Liquid membrane Ca2
electrode A GAS SENSING PROBE IS A GALVANIC CELL
WHOSE POTENTIAL IS RELATED TO THE CONCENTRATION
OF A GAS IN A SOLUTION. PORTABLE CLINICAL
ANALYSER TO DETECT AND ESTIMATE VARIOUS BLOOD
COMPONENTS LIKE Na, K, CO2, BICARBONATE,O2, pH
etc.
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BIOELECTROANALYSIS
APPLICATION OF ELECTROANALYSIS TO BIOLOGICAL
COMPOUNDS MANY IMPORTANT ANALYTES ARE ORGANIC OR
BIOLOGICAL ONE MAIN GOAL IS TO PERFORM in vivo
ELECTROCHEMISTRY
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TYPES OF ELECTROANALYSIS

• CONDUCTOMETRIC NOT SPECIES SPECIFIC- GIVES THE
  CONC. OF TOTAL CHARGES
• POTENTIOMETRIC THE EQUILIBRIUM POTENTIAL OF AN
  INDICATOR ELECTRODE IS MEASURED AGAINST A
  REFERENCE ELECTRODE
• AMPEROMETRIC AND VOLTAMMETRIC
• IN AMPEROMETRY, A FIXED POTENTIAL IS APPLIED AND
  THE CURRENT IS MEASURED.
  IN VOLTAMMETRY CURRENT IS
  MEASURED AS A FUNCTION OF APPLIED POTENTIAL.

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ELECTROCHEMICAL PRINCIPLE
ELECTROCHEMISTRY IS THE STUDY OF CHARGE TRANSFER
PROCESSES AT THE ELECTRODE/SOLUTION INTERFACE
Ox n e Red
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Michael Faraday, FRS (September 22, 1791 August
25, 1867) was a British scientist (a physicist
and chemist) who contributed significantly to the
fields of electromagnetism and electrochemistry.
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VOLTA, AMPERE AND OTHER SCIENTISTS CONTRIBUTED A
LOT TO THE DEVELOPMENT OF ELECTROCHEMISTRY
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CYCLIC VOLTAMMETRY
Cyclic voltammetry is a highly versatile electro
analytical technique and in recent years, it has
become the most popular technique for studying
electrochemical reactions. The versatility of the
technique coupled with the ease of measurement
makes it an important analytical technique in
electrochemistry, organic chemistry, inorganic
chemistry, biochemistry etc.
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Organic chemists have applied the technique to
the study of biosynthetic reaction pathways and
to the study of electro-chemically generated free
radicals. Inorganic chemists have used cyclic
voltammetry to evaluate the effects of ligands on
the oxidation/ reduction potential of the central
metal ion in complexes and multinuclear clusters.
This type of information gained from cyclic
voltammetry is highly helpful in solar energy
research and the studies of enzymatic catalysis.
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Cyclic voltammetry consists of cycling the
potential of a working electrode immersed in an
unstirred solution and measuring the resulting
current. The linear scan of the potential can
start from an initial value Ei to a predetermined
limit E1, where the direction of the scan is
reversed to another predetermined limit E2, where
again the scan is reversed to reach Ei.
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Thus the potential is cycled between two
predetermined limits E1 and E2 called the
switching potentials. The potential of the
working electrode is controlled with reference to
a reference electrode such as saturated calomel
electrode or Ag/AgCl electrode. The current
response is measured as a function of the applied
voltage. The applied potential can be considered
as an excitation signal and the current can be
considered as the response signal.
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The voltammogram is a graphical representation of
the current along the vertical axis and the
potential along the horizontal axis. As the
potential scan varies linearly with time, the
horizontal axis can be considered as a time axis
also. A typical voltammogram can be considered
to be analogous to a conventional spectrum in
which information is given as a function of an
energy scan.
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Modern instruments have the flexibility of a wide
choice of switching potentials at different scan
rates. The choice of the scan rate depends on
the lifetime of the electroactive species.
Though successive scans may not be different, any
difference in the CV profile can give fresh
insights into the electrode reactions.
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Similarly, a variation in the scan rate can also
lead to additional information at times.
Generally, in CV, the fate of an electroactive
species produced in a forward scan can be
followed in the backward scan. This scrutiny of
the fate shows, whether the equilibrium is
reversible, quasi reversible or irreversible.
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The important parameters of a cyclic voltammogram
are the magnitudes of the anodic peak current
(ipa) and cathodic peak current (ipc) and the
anodic peak potential (Epa) and cathodic peak
potential (Epc). A redox couple, in which both
the species rapidly exchange electrons with the
working electrode is termed an electrochemically
reversible couple.
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IN THE GRAPH, FROM b TO d THE ELECTRODE IS
SUFFICIENTLY REDUCTANT AND REDUCES FeIII(CN)63-
TO FeII(CN)64- . ON THE REVERSE SCAN THE
ELECTRODE BECOMES SUFFICIENTLY OXIDANT AND
OXIDATION TAKES PLACE.
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The formal reduction potential (E0) for a
reversible couple is centered between Epa and
Epc. The number of electrons transferred in
the electrode reaction (n) for a reversible
couple can be determined from the separation
between the peak potentials.
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For a simple reversible fast couple,
Electrochemical irreversibility is caused by slow
electron exchange of the redox species with the
working electrode.
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COULOMETRY
Coulometry is an analytical method
for measuring an unknown concentration of an
analyte in solution by completely converting the
analyte from one oxidation state to another.
Coulometry is an absolute measurement similar to
gravimetry or titration and requires no chemical
standards or calibration.
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It is therefore valuable for making
absolute concentration determinations of
standards. Coumetry uses a constant
current source to deliver a measured amount of
charge. One mole of electrons is equal to 96,485
coulombs of charge, and is called a faraday.
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Schematic of a coulometric cell
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Coulometric Titration Due to concentration
polarization it is very difficult to completely
oxidize or reduce a chemical species at an
electrode. Coulometry is therefore usually done
with an intermediate reagent that quantitatively
reacts with the analyte. The intermediate reagent
is electrochemically generated from an excess of
a precursor so that concentration polarization
does not occur. An example is the electrochemical
oxidation of I- (the precursor) to I2 (the
intermediate reagent). I2 can then be used to
chemically oxidize organic species such as
ascorbic acid.
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The point at which all of the analyte has been
converted to the new oxidation state is called
the endpoint and is determined by some type of
indicator that is also present in the solution.
For the coulometric titration of ascorbic acid,
starch is used as the indicator. At the endpoint,
I2 remains in solution and binds with the starch
to form a dark purple complex. The analyte
concentration is calculated from the reaction
stoichiometry and the amount of charge that was
required to produce enough reagent to react with
all of the analyte.
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CONSTANT CURRENT COULOMETRY IS ALSO CALLED
COULOMETRIC TITRIMETRY
COULOMETRIC TITRATIONS ARE SIMILAR TO OTHER
TITRIMETRIC METHODS IN THAT ANALYSES ARE BASED ON
MEASURING THE COMBINING CAPACITY OF THE ANALYTE
WITH A STANDARD REAGENT. IN COULOMETRY, THE
REAGENT IS ELECTRON AND THE STD. SOLN. IS A
CONSTANT CURRENT OF KNOWN MAGNITUDE. THE
MAGNITUDE OF CURRENT IN AMPERES IS LIKE MOLARITY
OF A SOLN.
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A FUNDAMENTAL REQUIREMENT IS 100 CURRENT
EFFICIENCY. ONE FARADAY SHOULD BRING ONE
EQUIVALENT OF CHEMICAL CHANGE. THIS CAN BE
ACHIEVED EVEN WITHOUT THE DIRECT PARTICIPATION OF
THE ANALYTE. Cl- CAN BE ESTIMATED BY GENERATING
Ag AT A SILVER ELECTRODE.
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KARL FISCHER COULOMETRIC TITRATOR
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VOLTAMMETRY
VOLTAMMETRIC METHODS ARE BASED ON MEASUREMENT OF
CURRENT AS A FUNCTION OF THE POTENTIAL APPLIED TO
A SMALL ELECTRODE. POLAROGRAPHY IS VOLTAMMETRY AT
THE MERCURY ELECTRODE. THE IMPORTANCE OF
POLAROGRAPHY INCREASED AFTER THE DME WAS
DESIGNED BY JAROSLAV HEYROVSKY.
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JAROSLAV HEYROVSKY BORN IN PRAGUE IN 1890 NOBEL
PRIZE 1959 DIED IN 1967
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Voltammetry refers to the measurement of current
that results from the application of a potential.
Unlike potentiometric measurements, which employ
only two electrodes, voltammetric measurements
utilize a three electrode electrochemical cell.
The use of the three electrodes (working,
auxillary, and reference) along with the
potentiostat instrument allow accurate
application of potential functions and the
measurement of the resultant current. The
different voltammetric techniques that are used
are distinguished from each other primarily by
the potential function that is applied to the
working electrode to drive the reaction, and by
the material used as the working electrode.
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THE POTENTIAL OF THE WORKING ELECTRODE IS
INCREASED OR DECREASED AT A TYPICAL RATE OF 2 TO
5 Mv/s. THE CURRENT WHICH IS USUALLY IN THE
MICROAMPERE RANGE, IS THEN RECORDED TO GIVE A
VOLTAMMOGRAM. THE WORKING ELECTRODE IS THE
ELECTRODE AT WHICH THE ANALYTE IS OXIDISED OR
REDUCED. THE POTENTIAL BETWEEN THE WORKING
ELECTRODE AND REFERENCE ELECTRODE IS CONTROLLED.
ELECTROLYSIS CURRENT PASSES BETWEEN THE WORKING
ELECTRODE AND A COUNTER ELECTRODE.
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IN ADDITION TO THE ANALYTE THERE IS AN EXCESS OF
A NONREACTIVE ELECTROLYTE CALLED A SUPPORTING
ELECTROLYTE WHICH CARRIES THE CURRENT. MOST
COMMONLY IT IS AN ALKALI METAL SALT THAT DOES NOT
REACT AT THE WORKING ELECTRODE AT THE POTENTIALS
BEING USED. THE SALT REDUCES THE EFFECT OF
MIGRATION AND LOWERS THE RESISTANCE OF THE
SOLUTION.
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VOLTAMMETRIC ELECTRODES
DME CAN INDUCE POLARISATION AND REDUCE SURFACE
CONTAMINATION. LARGE NEGATIVE POTENTIALS CAN BE
USED WITH MERCURY ELECTRODES AS HYDROGEN HAS A
VERY HIGH OVERVOLTAGE ON MERCURY. METALS THAT
ARE SOLUBLE IN MERCURY FORMS LIQUID ALLOYS KNOWN
AS AMALGAMS. THE SURFACE AREA OF THE DROP CAN BE
CALCULATED FROM ITS WEIGHT. BUT THE
ELECTROCHEMISTRY IS COMPLICATED BY THE CHANGING
SIZE OF THE DROP.
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A COMPUTER CONTROLLED POLAROGRAPH
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BIG ELECTRODES DO NOT POLARISE AND HAVE SURFACE
CONTAMINATION. THE VOLTAMMOGRAM IS A GRAPHICAL
REPRESENTATION OF THE POLARISATION OF THE DME AND
HENCE THA APPARATUS IS CALLED A
POLAROGRAPGH. VOLTAMMETRIC CURRENTS DEPEND ON THE
CONCENTRATION GRADIENT THAT IS ESTABLISHED VERY
NEAR THE ELECTRODE DURING ELECTROLYSIS.
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A VOLTAMMETRIC WAVE IS A SIGMOIDAL (? - SHAPED)
CURVE OBTAINED IN CURRENT VOLTAGE PLOTS. THE
LIMITING CURRENT il IS PROPORTIONAL TO THE
ANALYTE CONCENTRATION AND IS USED FOR
QUANTITATIVE ANALYSIS. THE HALF WAVE POTENTIAL E½
IS RELATED TO THE STANDARD POTENTIAL FOR THE HALF
REACTION AND IS USED FOR QUALITATIVE
IDENTIFICATION OF SPECIES. THE HALF WAVE
POTENTIAL IS THE APPLIED POTENTIAL AT WHICH THE
CURRENT i il /2
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SPARGING IS A PROCESS IN WHICH DISSOLVED GASES
ARE SWEPT OUT OF A SOLVENT BY BUBBLING AN INERT
GAS LIKE NITROGEN, ARGON OR HELIUM THROUGH THE
ANALYTE. THEN THE SOLUTION IS KEPT FOR SOMETIME
TO AVOID ANY MOVEMENT IN THE SOLUTION.
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THEORETICAL PRINCIPLES
il id ic im MIGRATION CURRENT (im) CURRENT
DUE TO MIGRATION OF IONS. ELIMINATED USING THE
INDIFFERENT ELECTROLYTE CONVECTION CURRENT (ic)
ELIMINATED USING UNSTIRRED SOLUTION DIFFUSION
CURRENT (id) DIFFUSION IS GOVERNED BY FICKS
LAW dc/dt D d2c/dx2
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Diffusion current is proportional to the
concentration as given by the Ilkovic eqn. id
607 n D1/2Cm2/3t1/6 n no. of faradays required
per mole of analyte D a constant known as
diffusion coef. C analyte conc.
(mmol/L) m
mass of Hg dropping from the electrode per second

t the drop time (s)
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Anodic Stripping Voltammetry
Anodic stripping voltammetry is an electrolytic
method in which a mercury electrode is held at a
negative potential to reduce metal ions in
solution and form an amalgam with the electrode.
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The solution is stirred to carry as much of the
analyte metal(s) to the electrode as possible for
concentration into the amalgam. After reducing
and accumulating the analyte for some period of
time, the potential on the electrode is increased
to reoxidize the analyte and generate a current
signal.
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The concentration of the analyte in the Hg
electrode, CHg, is given by CHg (il td )/(n F
VHg ) where il is the limiting current during
reduction of the metal, td is the duration of
accumulation, n is the number of moles of
electrons transferred in the half reaction, F is
the Faraday constant and VHg is the volume of the
electrode.
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The expression for current produced by anodic
stripping depends on the particular type of Hg
electrode, but is directly proportional to the
concentration of analyte concentrated into the
electrode. The main advantage of stripping
analysis is the preconcentration of the analyte
into the electrode before making the actual
current measurement. Anodic stripping can achieve
detection of concentrations as low as 10-10 M.
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Ion-Selective Electrodes (ISE)
An Ion-Selective Electrode (ISE) produces a
potential that is related to the concentration of
an analyte. Making measurements with an ISE is
therefore a form of potentiometry.
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The most common ISE is the pH electrode, which
contains a thin glass membrane that responds to
the H concentration in a solution. ISEs for
other ions must have an appropriate membrane that
is sensitive to the ion of interest but not
sensitive to interfering ions.
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For example, a LaF3 crystal can function as an
electrode membrane for fluoride ions. Many of the
commercial ISEs use a polymer membrane to embed
ion-sensitive species that are sensitive to Ca2,
NO3-, NH4, or other common ions.
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GLASS ELECTRODE AND pH Liquid membrane Ca2
electrode A GAS SENSING PROBE IS A GALVANIC CELL
WHOSE POTENTIAL IS RELATED TO THE CONCENTRATION
OF A GAS IN A SOLUTION. PORTABLE CLINICAL
ANALYSER TO DETECT AND ESTIMATE VARIOUS BLOOD
COMPONENTS LIKE Na, K, CO2, BICARBONATE,O2, pH
etc.
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The potential difference across an ion-sensitive
membrane is E K (2.303RT/nF)log(a) where K
is a constant to account for all other
potentials, R is the gas constant, T is
temperature, n is the charge of the ion
(including the sign), F is Faraday's constant,
and a is the activity of the analyte ion. A plot
of measured potential versus log(a) will
therefore give a straight line.
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DIGITAL pH METER AND GLASS ELECTRODE
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ISEs are susceptible to several interferences.
Samples and standards are therefore diluted 11
with total ionic strength adjuster and buffer
(TISAB). The TISAB consists of 1 M NaCl to adjust
the ionic strength, acetic acid/acetate buffer to
control pH, and a metal complexing agent.
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ISEs consist of the ion-selective membrane, an
internal reference electrode, an external
reference electrode, and a voltmeter.
Schematic of an ISE measurement
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Picture of a commercial fluoride ISE
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The pH meter measures the pH of a solution using
an ion-selective electrode (ISE) that responds to
the H concentration of the solution. The pH
electrode produces a voltage that is proportional
to the concentration of the H concentration, and
making measurements with a pH meter is therefore
a form of potentiometry. The pH electrode is
attached to control electronics which convert the
voltage to a pH reading and displays it on a
meter.
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A pH meter consists of a H-selective membrane,
an internal reference electrode, an external
reference electrode, and a meter with control
electronics and display. Commercial pH electrodes
usually combine all electrodes into one unit that
are then attached to the pH meter.
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