POLAROGRAPHY AND VOLTAMMETRY : BASIC PRINCIPLES APPLICATIONS
GTF205 Voltammetry n Polarography 2014a
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Transcript of GTF205 Voltammetry n Polarography 2014a
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POLAROGRAPHY AND
VOLTAMMETRY
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POLAROGRAPHY AND
VOLTAMMETRY..general
Electrochemical method: measure
currentas a function of applied
potential in electrochemical cell. Analytical signalcurrent (Faraday
current) which flows through cell during
the reaction of the analyte at theworking electrode with a small surface.
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POLAROGRAPHY AND
VOLTAMMETRY..general Analyte = cation/anion/molecule
Plot i as a function of potential (A vs V):
voltammogram(provide quantitative and
qualitative information) Voltammetry:an electrochemical method in which
we measure current as a function of the applied
potential
Voltammogram: a plot of current as a function of
applied potential
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POLAROGRAPHYgeneral
Earliest voltammetry technique: Polarography1920
by J. Heyrovsky - a type of voltammetry that use
Dropping Mercury Electrode (DME) or Static
Mercury Drop Electrode (SMDE) as the workingelectrode
Polarographycurrent-potential curve polarogram.
Current-potential curve is recorded by using a liquid
working electrode whose surface can be renewedperiodically or continuously: Dropping Mercury
Electrode (DME) and static mercury drop electrode
(SMDE)
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VOLTAMMETRYgeneral
Voltammetry
All methods in which current-potential
measurement are made at stationary and
fixed working electrode: Hanging mercury
drop electrode (HMDE), thin mercury film
electrode (TMFE), glassy carbon electrode
(GCE), carbon paste electrode (CPE) or othersolid electrodes
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POLAROGRAPHYgeneral
Polarography : a form of voltammetry using DME or
SMDE as the WE
Measure the current that flow through DME during
a linear/direct voltage- direct current polarography(DCP)together with counter electrode /reference
electrode
2 components of current - faradaic current (if
) and
capacitive current (ic).
ifprovides measuring signal and ic is interference
signal
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POLAROGRAPHY
2 components of current - faradaic current (if) andcapacitive current (ic).
faradaic current (if) : any current in an
electrochemical cell due to an oxidation orreduction reaction of analyte
capacitive current (ic): any current from a flow ofelectron that charge the mercury droplet with
respect to the solution ( + or -) (residual current ornonfaradaic currentno reduction/oxidationprocess)
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POLAROGRAPHY
From normal polarography;
1) Limiting current or diffusion
current is obtained fromreduction / oxidation of
analyte in solution2) Limiting current is measured from
the maximum current or average
current.
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POLAROGRAPHY
If if/ ic = 1 useful signal cannot be separated
from interference signalcan effect detection limit
of DCP = signal-noise ratio (LOD: 3 S/N)
maximum value for ifwhich is obtained when allthe analyte particles transported to the surface of
the mercury drop by diffusion (reduced or oxidised)
is called diffusion current (iD)
Relationship between iD and the analyte
concentration is given by Ilkovic Equation
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POLAROGRAPHY
Relationship between iDand the analyte sconcentration is given by Ilkovic Equation
iD= 0.607.n.D1/2.m2/3.td
1/6.Ca
iD: Diffusion current (max)
n: No of electron exchanged in the charge-transferelectron
D: Diffusion coefficient of the analytetd: Dropping time of the mercury drop
Ca: Concentration of the analyte
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POLAROGRAPHY
if/ ic can be improved to get a higher sensitivity.
To increase ifby stripping voltammetry in which
analyte is accumulated electrolytically at a
stationary working electrode before voltammetricdetermination
To eliminate icsampled DC polarography and
pulse method.
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POLAROGRAPHY
sampled DC polarography :
current is measured at the end of a potential step
e.g at a constant potential and at an electrode
surface that remain constant and reduces thecontribution of ic to the measuring signal to a
minimum
gives smooth polarogram and more sensitive
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POLAROGRAPHY
Types of pulse polarography
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POLAROGRAPHY
1) Pulse polarography (normal, staircase and square
wave):
Substantial improvement in senstivity and
detection limit from normal polarography.Limiting and peak currents are directly
proportional to the concentration of analyte. Half
wave and peqak potential are used for qualitative
purposes.
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POLAROGRAPHY
Differential pulse polarography
Uses a series of potential pulsecharacterised by a cycle of time,, a pulse time of tp, a potentialpulse of Epand potential stepper cycle, Es.
Typical = = 1 s, tp= 50 ms, Ep= 50 mv, Es= 2 mV
Current is measured twice,approximately 17 msbeforeforward pulse and forapproximately 17 msbefore the
reverse pulse. Difference in thetwo currents gives rise to peak-shape voltammogram
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POLAROGRAPHY
Differential pulse polarography
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POLAROGRAPHY
Peak currents are directlyproportional to the concentration ofanalyte.
Half wave and peak potential areused for qualitative purpose.
Pulse polarography is more populardue to its sensitivity and detectionlimit are improved
Is used for analysis of metal ions,inorganic ions (IO
3
-and NO3
-) andorganic compounds contains easilyreducible or oxidisable functional(carbonyl, carboxylic acid and C=C)
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POLAROGRAPHY: application 1
Example:
The DPP analysis of mixtures of indium and cadmium in0.1 M HCl is complicated by the overlap of theirrespective polarograms. The peak potential for indium
is at -0.557 V and that for cadmium occurs at a potentialof -0.597 V. When a 0.800 ppm indium standard isanalysed, the peak current is found to be 200.5 at -0.557 V and 87.5 at -0.597 V. A standard solution of0.793 ppm cadmium gives peak current of 58.5 at
0.557 V and 128.5 at -0.597 V. What is the concentrationof indium and cadmium in a sample if the peak currentis 167.0 at potential of -0.557 V and 99.5 at a potentialof -0.597 V ?
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POLAROGRAPHY: application 2
Example:
The concentration of As(III) in water can be
determined by DPP in 1.0 M HCl. The initial
potential set to -0.1 V versus the SCE, and isscanned toward more negativepotential at a rate of
5 mV/s. Reductionof As(III) to As(0) occurs at a
potential of approximately -0.44 V versus a SCE. The
peak currents corrected for residual current, for aset of standard solution are shown in the following
table:
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POLAROGRAPHY
As(III) / M Ip/ A
1.00 x 10-6 0.298
3.00 x 10-6 0.947
6.00 x 10-6 1.83
9.00 x 10-6 2.72
What is the concentration of As(III) in a sample ofwater if the peak current (Ip) under the same
conditions is 1.37 A (answer: 4.49 x 10-6M)
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VOLTAMMETRY
- Time dependent potential is applied to an
electrochemical cell and the current flowing
through the cell is measured as function of that
potential. Voltammogram is recorded: qualitative and
quantitative information about the species involved
in oxidation or reduction reaction
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VOLTAMMETRY
Use to study:
a) oxidation and reduction processes in various
media
b) adsorption processes on surfaces
c) electron transfer mechanisms
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VOLTAMMETRY
Voltammetric instruments:
Use 3-electrode immersed in a solution containing
the analyte and an excess of a non-reactive
electrolyte called a supporting electrolytea)WE : Hg, (HMDE or Hg film), DME / SMDE
b) AUX: Pt
c) RE: SCE, Ag/AgCl
Current is measured at WE versus std /ref electrode.
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VOLTAMMETRY
Voltammetric instruments:
a)WE : kept small to enhance its tendency tobecome polarised. Its potential versus a RE is
varied linearly with time
b) AUX: passes current between WE and AUX
electrode
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VOLTAMMETRYE/chemical cell
Typical electrochemical
cell for voltammetry
3 electrodes (AUX, WE,
RE) N2 purge line
Stir bar
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VOLTAMMETRY: electrodes
WE: Hg
3 types:
a) Hanging mercury drop electrode (HMDE),b) Dropping mercury drop electrode (DME)
c) Static mercury drop electrode (SMDE)
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VOLTAMMETRY: Hg as WE
Advantageous of Hg as WE:
a) High overpotential for reduction of H3O+to
H2(e.g -1.0 V)
b) Ability of metals to dissolve in the mercury
forming an amalgam
c) Ability to easily renew the surface of
electrode by extruding a new drop.
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VOLTAMMETRY: Hg and others
Use other electrodes: solid electrode (Pt, gold,
silver, C).
C
Hg
Pt
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VOLTAMMETRY: e/chemical cell
Other arrangement in voltammetric cell:
a) N2purge line for removing dissolved O2.
(Why ?)
b) Optional stir bar (Why?)
c) Electrochemical cell in various sizes(50 l to 100 ml)
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VOLTAMMETRY: e/chemical cell
N2purge line forremovingdissolved O2.
To avoid any O2peaks appeardue to reductionof dissolved
oxygen
(2 peaks)
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VOLTAMMETRY
Voltammetric wave:
A S shape wave obtained in currentvoltage
plot in voltammetry: linear sweep
voltammetry.
Limiting current:
Constant current which is limited by the rate at
which the reactant can be brought to the
electrode by mass transport processes
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VOLTAMMETRY
Half wave potential:
Occur when the current is equal to one half of
the limiting current
Hydrodinamic voltammetry:
Type of voltammetry in which the analyte
solution is kept in continuous motion.
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VOLTAMMETRY: Faradaic current
Current in voltammetry:
Faradaic current:
any current in an electrochemical cell due to an
oxidation or reduction reaction.
Due to reductioncathodic current
Due to oxidationanodic current
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VOLTAMMETRY: Factors effect FC
Magnitude of FC is influenced by 2 factors:
a) Rate of the electrochemical reaction: the rate at
which the reactant and products are transportedto and from the electrode surface
(mass transport)
b) The rate at which electrons pass between electrodeand the reactants and products in solution.
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VOLTAMMETRY: mass transport
Mass transport(reactant andproducts are transported toand from electrode surface):3 modesdiffusion,
migration and convection.
Diffusion
Diffusion creates diffusionlayer(DL).Width of DL ()increase with time as theconcentration of reactantnear the electrode decrease.
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VOLTAMMETRY: mass transport
Migration:charged particles in solution are
attracted or repelled from electrode that has
a positive and negative charge.
Convection:when a mechanical means is
used to carry reactants towards electrode
and remove products from electrode. (stir
the solution): hydrodynamic voltammetry.
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VOLTAMMETRY: mass transport
Concentration gradient for the analyte
showing the effects of difussion and
convection as methods of mass transport
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VOLTAMMETRY: other currents
Mass transport:the movement of materialtoward or away from the electrode.
Diffusion: the movement of material inresponse to a concentration gradient
Diffusion layer: the layer of the solutionadjacent to the electrode in which diffusion isthe only means of transport
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VOLTAMMETRY: other currents
Nonfaradaic current:result from unrelated toany redox reaction.
Residual current: a small inevitably flowseven in the absence of analyte (2components/sources: i) faradaic current dueto oxidation or reduction of trace impuritiesand ii) charging current). = backgroundcurrent
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VOLTAMMETRY: other currents
Electrical double layer:interface between a
positively or negatively charged electrode
and the negatively or positively charged layer
of the solution in contact with the electrode.
Charging current: a current in an
electrochemical cell due to the electricitydouble layers formation.
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VOLTAMMETRY
Quantitative and qualitative aspects
Quantitative: relating current to [analyte] in
bulk solution.
Qualitative: extracting std-state potential for
redox reaction
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VOLTAMMETRY
Cyclic voltammetry; introductory step involtammetry analysis. To obtain information onelectrochemical behaviour of species in certainsolution.
Stripping voltammetry: a form of voltammetry inwhich the analyte is first deposited on the electrodeand then removed or stripped electrochemicallywhile monitoring the current as a function of
applied potential
(anodic, cathodic and adsorptive strippingvoltammetry)
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CYCLIC VOLTAMMETRY
Introductory step in voltammetry analysis.Function;
To obtain information on electrochemical behavior ofspecies in certain solution.
To study electron transfer and to probe subsequent chemicalreaction
Study the oxidation or reduction reaction, the detection ofreaction of intermediate and the observation of follow upreaction of products formed at electrode.
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CYCLIC VOLTAMMETRY
Electrode is imposedwith a cyclic linearpotential sweep andobtaining current-potential curve (cyclicvoltammogram)
Apply triangularwaveform producesforward (1) and thenreverse (2) scan
Schematic diagram of E
versus time for CV technique
1 2
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CYCLIC VOLTAMMETRY
Potential is 1stvaried linearly from +0.8 V to-0.15 V vs SCE at which point the scandirection is reversed and the potential isreturned to its original value of +0.8 V.
Scan rate = 50 mV/s.
Cycle = single/repeated Potentials at which reversal take place are
called switching potentials. (-0.15 V =switching potential)
A scan in direction of more negativepotential = forward reaction and more
positive potential = reverse reaction(depend on direction of first scan)
Cycle time: 1 ms or less to 100 s or more
Example: 40 s
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CYCLIC VOLTAMMETRY
Sweep /scanInitial (Ei), switching (Es) and
Final (Ef) potentials.
E = Ei+ t (forward sweep)
E = Est (reverse sweep)
( = sweep rate /scan rate in V/s).
Multiple cycles also used rather than singlecycle
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CYCLIC VOLTAMMETRY
Possible cyclesCVwaveforms:
A) reversible
B) irreversible C) quasi-reversible
at Hg electrode
O =oxidised formR = reduced form
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CYCLIC VOLTAMMETRY
CV- Important parameters: A) Cathodic peak potential, Epc B) Anodic peak potential, Epa C) Cathodic peak current, Ipc
D) Anodic peak current, Ipa
Reversible electrode reaction:
Anodic and cathodic peak currents are approximately equal
Ep = | EpaEpc| = 0.059/n (in V) where n = no ofelectron involved in redox reactionIrreversible:
Epexceeding expected value
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CYCLIC VOLTAMMETRY
CV responsecurrent vs potential
Forward: oxidised form is reduced while on reversesweep, the reduced form near electrode is re-oxidised.
Major use: provide qualitative information aboutelectrochemical processes under various conditions.
E.g: CV of parathion in 0.5 M pH 5 sodium acetate
buffer in 50% ethanol. HMDE as WE at scan rate =200 mV/s.
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CYCLIC VOLTAMMETRY
CV of parathion in 0.5 M pH 5 sodium acetate buffer in 50% ethanol.
HMDE as W.E, s.rate = 200 mV/s.
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CYCLIC VOLTAMMETRY
Switching potentials:-1.2 V and +0.3 V
Initial forward scan from 0.0 V.
Produces 3 peaks: A, B and C.
A= cathodic peak from reductionof parathion to give hydroxylaminederivative
B = anodic peak from oxidation ofhydroxylamine to a nitrosoderivative during reverse scan
C= cathodic peak from reduction ofthe nitroso compound to thehydroxylamine
1
2
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CYCLIC VOLTAMMETRY
Azo dye (reactive dye group)
Chemical name
[2,7-naphthalenedisulfonic acid, 4-amino-5-hydroxy-3,6-bis ((4 - ( (2 (sulfoxy) ethyl)sulfonyl) phenyl) azo)-tetrasodium salt],
Gives 3 cathodic peaks - 2ndpeak is the welland characteristic peak.
No oxidation peak.
[ref: M.H.Yaacob and Z. Nursyamimi (2012)J of Health & Environmental Sc ]
1
2
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CYCLIC VOLTAMMETRY
Azo dye (reactive dye group)
The first two obtained reduction peaks are
suggested due to subsequence reduction
process of the two azo groups to amines.
Both hydroxyl and amino groups are electrondonating substituent in the RB5 dye
compound. The first reduction peak is
suggested from reduction process of the azo
with the hydroxyl group and followed with
the reduction process of the other azo withamino group, which gave second reduction
peak on the voltammograms.
1
2
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STRIPPING VOLTAMMETRY
3 related techniques: anodic, cathodic and adsorptive strippingvoltammetry
Anodic stripping (ASV)
2 steps:
Step A)controlled potential electrolysisusing HMDE or Hg filmelectrode - is held at a cathodic potential sufficient to deposit metalion on the electrode.E.g Cu2+- deposition reaction : Cu2++ 2e Cu (Hg)
Copper is amalgamated with Hg.
Function : pre-concentrating the analyte from larger volume of thesolution to a smaller volume of electrode.
Solution is stirred during electroylsis (why?)Near the end of deposition time, stirrer is stopped (why?)
Deposition time = 1-30 min.
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STRIPPING VOLTAMMETRY
Anodic stripping (ASV) .. cont 2 steps:
Step B) potential is scanned anodically towardmore positive potentials. With sufficientpositive potential, the analyte is stripped from
electrode, returning to solution as its oxidisedform:
E.g Cu2+- deposition reaction :
Cu(Hg) Cu2+(aq) +2e
Current (i) during stripping step is monitoredas a function of potential giving rise to peak-
shaped voltammogram
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STRIPPING VOLTAMMETRY
Application:
Bi, Cd, Cu, Ga, In, Pb, Sn, Zn
Can be determined simultanouesly
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STRIPPING VOLTAMMETRY
Cathodic stripping (CSV)
2 steps:
Step A)deposition step involves the
oxidation of Hg electrode to 2Hg +1 which
then reacts with analyte to form insoluble
film at the surface of electrode.
E.g Cl-- deposition reaction :
2Hg (l) + 2Cl- Hg2Cl2(s) + 2e
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STRIPPING VOLTAMMETRY
Step B)stripping is accomplished by
scanning cathodically toward more
negative potential, reducing Hg+ back to
Hg and returning the analyte to solution;Hg2Cl2(s) + 2e 2Hg (l) + 2Cl
-(aq)
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STRIPPING VOLTAMMETRY
Adsorptive stripping (CSV)
deposition step occures without electrolysis.Analyte adsorbs to the electrode surface. Duringdeposition the electrode maintained at a potential
that enhances adsorption.
E,g: adsorption of a neutral molecule on a Hg dropis enhanced if the electrode is held at -0.4 V (SCE).When deposition is complete, the potential isscanned in an anodic or cathodic directiondepending on whether we wish to oxidise orreduce the analyte
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STRIPPING VOLTAMMETRY
Adsorptive stripping (CSV)
Application:
Bilirubin
Codiene
Cocaine
TestosteroneAflatoxins
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STRIPPING VOLTAMMETRY
DPP voltammogram for 5 x 10-10M
riboflavin
Preconcentration for
(A) 5 min
(B) 30 min
at -0.2 V.
Ep almost constant but Ipincresead.
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