try polarography and related methods

8/2/2019 try Polarography and Related Methods

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The previous chapter (potentiometry) dealt

with potential of electrodes in which no

current was allowed to follow.

In voltammetry investigate phenomena areaccompanied by the passage of appreciable

current(i.e. we supply a potential to the cell)


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We are forcing a certain electrochemicalreaction(oxidation or reduction).

There are electron exchange between thesolution and the electrode.

As a result of electron transfer a current isproduced.

In voltammetry we concerned with plottingcurves showing the current produced as afunction of the voltage on the electrode of

interest(working)


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A three electrode cell is used in generalvoltammetry ,

The working electrode is the electrode on which

the reaction of interest occurs, and on which

measurements are taken.

The Auxiliary electrode, is an electrode used inan electrochemical cell with a workingelectrode.

The auxiliary electrode changes in polarityopposite to that of the working electrode, but itscurrent and polarity is not measured.


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It exists to ensure that current does not run

through thereference electrode.

The auxiliary electrode passes all the current

needed to balance the current observed at the

working electrode.

Inert material(Hg, Pt).
http://en.wikipedia.org/wiki/Reference_electrodehttp://en.wikipedia.org/wiki/Auxiliary_electrodehttp://en.wikipedia.org/wiki/Auxiliary_electrodehttp://en.wikipedia.org/wiki/Reference_electrode


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The reference electrode is a half cell with a

known reduction potential. Its only role is to

act as reference in measuring and controlling

the working electrodes potential and at no

point does it pass any current.
http://en.wikipedia.org/wiki/Reference_electrodehttp://en.wikipedia.org/wiki/Reference_electrode


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The Nernst equation is fundamental to voltammetryand can be used for a reversible reaction. In thisequation R represent the reduced species and O the

oxidized.

R = Molar gas constantT = temperature in K

n = number of electrons transferredF = Faraday constantE = applied potentialE0 = standard reduction potential
http://en.wikipedia.org/wiki/Nernst_equationhttp://en.wikipedia.org/wiki/Nernst_equation


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When a potential is applied to the cell,

whats happen?

If an electroactive species present, a

current will be recorded when the applied

potential become sufficientlyve or +ve

for it to be electrolysed ( on both sides of

the standard potential (E) for the redox

reaction of species)


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The applied potential control the concentration ofthe electroactive species according to Nernsts eq.

On the +ve side of E the oxidized form of the redoxcouple is stable, whereas the reduced form tend toundergo oxidation if it reaches the electrode.

The current results from a change in oxidation stateof the electroactive species(electron transfer).


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Voltammetric cell configuration


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Polarography is a special case ofvoltammetry when the working electrode is

dropping mercury (has high over voltage of hydrogen as faras 1.8V in acid medium& -2.3V in basic medium).

In polarographic analysis, the cell containing

a Hg-microelectrode at which the analytereacts.(the working electrode in voltammetry is inert responding towhatever electroactive species may be present in solution)


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Dropping mercury electrode


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Both qualitative and quantitative

information is obtained from

plots of the current generated in

the cell as a function of applied

voltage, polarogram.


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Current against applied voltage

Half wave potential (E1/2) is

close to E0 for reduction reaction

Limiting current proportional to

analyte activity


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Polarographic spectrum- curve of polarographicreduction of seven different cations

Current,A

Applied potential, V


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Polarographic curves showing constancy of half wave potential of Cd

Ion reduction at different concentrations of CdCl2

CurrentA

Applied potential, V


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Polarographic of blood sera of different patients with following diagnoses

1- status febrilis 2-tumer hepatitis 3- ca.ventriculi susp 4- normal serum

5- cirrhosis hepatic 6- atherosclerosis.


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The height of the wave ,limiting current,

represent the maximum rat at which the

particular ions can be discharged underthe given experimental conditions

(M++ M.

)

Residual current: a small current which

exist even in the absence of theanalyte,(blank experiment).


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id, is the current at which the rapid increase of

potential is observed.

Id = limiting current residual current

It measure the maximum rate at which the

discharged ions can be brought up to the electrode

by diffusion alone.


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The potential at the center of the rising part of thewave is the half wave potential.

It is the point at which the concentration of the ionsbeing discharged is half the value in the bulk of thesolution.

(E1/2), it is characteristic of the species beingdischarged.

E1/2 is independent of the concentration of thesolution.


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E1/2 is used to identify the particular species.

E1/2 depend upon the solution composition andmay change with variation in the pH or thesolvent system or with addition of complexingagents.

E1/2 is a characteristic property of the givenoxidation-reduction system and can used for

1- identification

2- determination of pH

3-Estimation of standard potential(E1/2 = E)


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A polarographic wave suitable for analysisis obtained in the presence of a large excess

of a supporting electrolyte e.g. KCl,(supporting electrolyte).

The linear relationship between the

diffusion current (id) and the concentrationof electroactive species is shown by theIlkovic equation


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id: is the max. current in A

n: No.of electrons required per molecule ofelectroactive substance

D: is the diffusion coefficient in cm2 /sec.C: is theconc. In m moles/L

m: is the rate of mercury flow from DME inmg/sec.

t: is the drop time, in sec.


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All the terms can be kept constant therefore:

id= KC

i.e. id is proportional to the conc. Of solution


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1- Temperature controlFor accurate polarographic analysis, solution should be

thermostated.

2- Oxygen removalDissolved oxygen undergoes a two steps irreversible

reduction at the dropping electrode

1- 2H2O + O2 + 2e H2O2 + 2OH-

2- H2O2 + 2e H2O


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Two waves of equal size result, the first with ahalf wave potential at about 0.14 V and thesecond at about 0.9V(vs SCE)

The two half reactions are some what slow.

While these polarographic waves are convenientfor the determination of oxygen in solution, thepresence of this element often interferes with the

accurate determination of other species.

Thus oxygen removal is ordinarily the first step inpolarographic analysis.


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A simplistic look at the effects ofdeareation of the Cd2+ wave


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Deareation of the solution for several minuteswith an inert gas accomplishes this end.

A stream of the same gas usually, nitrogen , ispassed over the surface during the analysis toprevent reabsorbtion.

3- supporting electrolyte

A supporting electrolyte as KCl must be used in

high concentration ( if we work in aqueous

medium)


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Polarographic waves are frequently distorted by

the so called current maxima. Maxima are troublesome because they interfere

with the accurate evaluation of diffusion currentsand half wave potentials.

The cause of maxima are not fully understood .

The addition of traces of high molecular weightsubstances as gelatin or commercialsurfactant(trikon x-100) are useful for removal ofcurrent maxima, they are called maximumsuppressors.


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Advantages of DME clean surface and constant

mixing

constant current duringdrop growth

No H2

formation (highhydrogen overvoltage,-o.14 _ -1.8 V).

Mercury amalgamates

Readily with most metals. Disadvantages of DME:

Hg easily oxidized

cumbersome to use

try polarography and related methods

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