GTF205 Voltammetry n Polarography 2014a

8/10/2019 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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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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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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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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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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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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Application:

Bi, Cd, Cu, Ga, In, Pb, Sn, Zn

Can be determined simultanouesly


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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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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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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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Adsorptive stripping (CSV)

Application:

Bilirubin

Codiene

Cocaine

TestosteroneAflatoxins


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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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GTF205 Voltammetry n Polarography 2014a

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Transcript of GTF205 Voltammetry n Polarography 2014a