Post on 06-Apr-2018
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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_electrode8/2/2019 try Polarography and Related Methods
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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_electrode8/2/2019 try Polarography and Related Methods
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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_equation8/2/2019 try Polarography and Related Methods
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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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http://images.google.com/imgres?imgurl=http://www.scielo.br/img/revistas/jbchs/v15n3/20195f2a.gif&imgrefurl=http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-50532004000300011&lng=pt&nrm=iso&tlng=pt&h=203&w=352&sz=4&hl=en&start=5&um=1&tbnid=BIeYiW_jQP2yEM:&tbnh=69&tbnw=120&prev=/images?q=polarography+and+current+maxima&um=1&hl=en&rls=com.microsoft:*:IE-SearchBox&rlz=1I7RNWN&sa=N8/2/2019 try Polarography and Related Methods
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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