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ANALYTICAL
CHEMISTRY 1polarography AND METAL ION
ANALYSIS
Introduction to polarography
Polarography is a type of electroanalytical method used in
studying analyte by measuring its current as potential is varied in
an electrochemical cell containing the analyte. It is different from
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the other voltammetric method in the sense that its electrode is
formed by a rhythmically dropping mercury from a tip of fine
capillary, while the others have solid electrodes. Polarography is
particularly important in quantitative analysis of electroactivespecies . Trace metal ions(at g/L level or less)in a solution could
be analysed.
Criteria of polarography
Small sample size(dependent on the type of technique )
Presence of electroactive species (oxidizable or reducible)
which are soluble in appropriate liquid solvent which ischemically inert.
Dropping Mercury Electrode(DME) which consists of a glass
capillary through which mercury flows under gravity to form a
succession of mercury drops.It is used to increase the sensitivity of
analysis .The mercury surface is altered by the reduced species
formed on It(due to discharging). Therefore, the mercury surface is
constantly renewed by knocking off the drop of mercury in aninterval of time. This causes the data oscillation between maximum
and minimum value during data(measured current) recording.(referfigure 1). This has causing the interpretation of polarogram
difficult.(refer figure 2)
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Figure 1: Changes of current with the area, showing the current oscillates
between a near-zero minimum value as drop falls from the capillary to a
maximum value as the surface area of a new drop increases.
Figure 2:polarogram.The average response is used for subsequent analysis.
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Steps in quantitative polarography
(1.) Sample treatment:The analyte must be in solution form. Therefore if it is in solid
form, it is dissolved in a suitable solvent. Sparingly soluble solid
or liquid are to be extracted or digested. In the end of treatment,the solution should be free of suspended solid, colloids and
surfactant.
(2.) Addition of supporting electrolyte:
Examples of supporting electrolyte are salt,acid,base,buffer or
chelating reagent. The supporting electrolyte should of high
concentration relative to analyte usually at least 10 times the
concentration of the analyte.
(3.) Bubbling of the solution with N2
This step has to be carried out to remove oxygen. Oxygen always
present in solution and it is reducible at the electrode and couldinterfere with the experimental data. After bubbling , pause a
while, the solution is left to quiescent.
(4.) Electrode cleaning.
(5.) Scanning/registration of sample polarograms.
Instead of keeping the electrode potential(voltage) constant, we do
scanning (vary it at time by increase or decrease it).
(6.) Addition of the standard analyte.
(7.) Scanning/registration of sample polarograms after addition.This step is usually repeated 2 to 8times.
(8.) Measurement of all peak/wave heights.
The cell potential is measured during the redox reaction at various
potentials.
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(9.) Drawing of the graphs (polarograms).
A current versus potential curve is drawn.
(10.) Calculation of the unknown concentration.
This is done using formulae.
Principle of polarography
There are many different polarographic techniques used but the
underlying fundamental principle is the same. The electrochemical
cell, where the voltammetric experiment is carried out, consists of
a working (indicator) Electrode(DME), a reference electrode, andusually a counter (auxiliary) electrode.
At the working electrode surface, there is an interface wherebytransfer of electron takes place when potential is imposed on it.
This interface is called diffusion layer, the poorer concentration of
electroactive species in the layer due to its discharging onelectrode, creating concentration gradient, results in the mass
transport(diffusion) of other electroactive species from bulk
solution to the electrode surface and this generates current. This is
the current measured by the electrons movement in the externalcircuit connecting the two electrodes.
When an applied potential of about 200 mV is applied to a
solution containing only analyte then a very small amount of
current will flow to convert a small amount of analyte to reducedform so that Nernst equation is obeyed at the electrode surface.
The reduced form(usually metal) would form amalgam with
surface mercury of mercury drop.
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Figure 3. Polarogram with standard reduction potential of analyte.Given E = potential of the working electrode,
E = the standard reduction potential of analyte
Referring Figure 3:
The amount of current rises when the E approached theE, when
finally (at B) E = E, [reduced form] = [analyte] at the electrode
surface. In other words, at E= E, half of the total analyte
reaching the electrode is converted to reduced form, meaning thecurrent is half way to its limiting current.
If E is further lowered, all analyte would be reduced as soon as it
reaches at the electrode. At C the [analyte] at electrode surface is
negligibly small compared to the one of bulk solution. At C the
current does not increases linearly with applied potential but
approaches a steady limiting value at D which corresponds to the
diffusion rate of analyte ions from the bulk solution to theelectrode surface.The number of analyte ions diffusing from the
bulk solution to the electrode surface is equal to the number that isreduced at diffusion layer, meaning The diffusion rate = reduction
rate.
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Figure 3: polarogram and general terms used in polarography
Half wave potential (E) is the potential at the point when the
current is exactly half of the total current and this value is used to
identify the unknown species (qualitative analysis) and limitingcurrent is the difference in the value of the current from the start to
the maximum value (that is the sum of migration current, residualcurrent and diffusion current).
However, according to Ilkovic equation, keeping all other factors
of the equation constant( which corresponds to the elimination of
the first two current) the diffusion current (id) which is the
wave/peak height is directly proportional to the concentration of
the analyte(CA) and this makes quantitative analysis of metal ionspossible.
id =KcA K=constant
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As drop forms:
(a) At the electrode surface, the concentration polarizationoccurs and is described by Cottrell equation:
(b) Area increases with the size of drop.
The effects of (a) and (b) cancel each other out. Combining bothequations giving the Ilkovic equation:
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Quantitative analysis
Quantitative analysis is usually done with qualitative analysis.
Elimination of the background current
Polarographic analysis can be used when concentrations ofelectroactive species is between 10
3to 10
5M. If solutions have
more than one electroactive species with their E in supporting
electrolyte differ by about 0.4 V for single charged ions and 0.2 V
for doubly charged ions they can be determined. However, if the
E of two ions are close they could be removed by interaction with
certain reagents, the common two methods are as followed :
a) precipitationeg. In determination of zinc in Lead and Zinc mixture lead can
be removed as lead sulphate precipitate.
b) Complexation: Metal ions have complexing ability. By adding
complexing agent( e.g. EDTA),E1/2 of one of the ions may be
shifted to more negative potential than the other electroactivespecies.
The concentration of analyte can be examined only if certaininterference is removed.
If an analyst prepare a calibration curve using standard solution in
distilled water, problems could be encountered if the sample to beanalysed has a complex matrix (a matrix rich of general chemical
compounds) because these compounds can (i)vary significantly
some physical properties of the solution (e.g. viscosity, refractionindex etc), or they can (ii)vary the non-specific background
instrumental signal or the analyte signal, or, (iii) they could
increase or decrease the analytical signal, reacting with the analyte
or reagent added to the solution giving rise to the instrumental
signal.
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One way is to prepare standard solutions in a matrix similar to the
samples. Standard addition method is good to avoid matrixinterference although cannot compensate the non-specific
interference which raise the background signal. Non-specific
interference could be solved by measuring the signal as differencebetween the analytical signal and a background of a blank solution,
or, by zeroing the signal with an appropriate blank solution.
Methods in Quantitative analysis
The two methods usually employed are (i)standard addition
method and (ii)internal standard method. The former is the best
method for lowering the matrix interference. When the sample
matrix is very simple or reproducible the other method calledcalibration curve method is used.
(i)Internal standard methodThis is a method to improve the precision of quantitative analysis.
An internal standard is actuallya substance (with known
concentration) which is present in every analyzed sample .It ispresent in a constant amount in samples, blank and calibration
standards. Internal standards are usually used with the calibration
curve. Calibration curve is prepared by plotting the ratio of the
analyte signal to the internal standard signal versus analyte
concentration of the standards. This is done to correct for the loss
of analyte during preparation of sample . Internal standard behaves
similarly to the analyte but still it provides a signal that can be
differentiated from that of the analyte. In other words, factorsaffecting the analyte signal would affect the internal standard
signal to a more or less same degree. Therefore, the ratio of the itssignals will has less variability than the analyte signal. Also,standard must have a wave before or after the unknown and the
wave must be displaced enough for the two limiting currents to be
determined.
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Polarogram (currentvoltage curve) of the unknown solution in a
supporting electrolyte is recorded. A known amount of standardion is added into the same solution and polarogram is recorded.
(As mentioned before, the only condition is that E 1/2 of standard
ion and unknown should be differed by 0.4 V.) In other words, todetermine K, a known mixture of standard and analyte (Ck& Cu )
is prepared to measure the relative signals ((id)k& (id)u )of the two
species.
(k=known; u=unknown)
The known amount of internal standard is added to every sample to
be analysed and polarogram is analysed.
(ii) standard addition method
With any kind of the analytical method is used, the interference
affects has to be removed, avoiding analytical errors. The best
method to avoid some matrix interference is the standard addition
methodalthough cannot compensate the non-specific interference
which raise the background signal. The only valid strategy to
compensate this kind of interference is (i) to measure the signal as
difference between the analytical signal and a background signal of
a blank solution, or(ii) to zero the signal with an appropriate blanksolution.
Conditions for the use of the standard addition in Voltammetry is
(i)to make the scanning related to each addition using the same
parameters to be considered so as to keep the proportionalitybetween concentration and peak height among different scannings
constant. (ii) to work in the linear range of the relation between
concentration and peak height. In some cases, this range is very
narrow. In this case the standard is the same compound as the
unknown.
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Firstly, the polarogram of the unknown is recorded.
Measure (id)u at some potential { (id)u = K.Cu }
Then, a known amount of standard solution of the
same ion (i.e.standard Cs & Vs) is added and a second
polarogram is taken.(id)mix is measured under the same
conditions.
Cu=concentration of unknown
Vu=Volume of the unknown started with
Cs=concentration of the standard solution
Vs=Volume of standard added
Two equations with two unknowns is obtained,
therefore solve forKand Cu
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Experimental Set-up of polarography :
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The polarographic circuit:
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References:
1) Protti P. (2001). Amel electrochemistry: Introduction toModern Voltammetric and Polarographic Analisys Techniques.
Retrieved 21 November, 2011, fromhttp://www.amelchem.com/download/items/voltammetry/manuals/e
ng/manual_eng.pdf
2) UNIT 7: VOLTAMMETRY. Retrieved 21 November, 2011,
from vedyadhara.ignou.ac.in/wiki/images/c/ca/Unit_07-
Voltammetry.pdf
3) UNIT 8: POLAROGRAPHY AND AMPEROMETRICTITRATIONS. Retrieved 21 November, 2011, fromhttp://vedyadhara.ignou.ac.in/wiki/images/5/58/UNIT_8_POLAR
OGRAPHY_AND_AMPEROMETRIC_TITRATIONS.pdf
4) Practical Exercises in Physical Chemistry Advanced Level.Retrieved 21 November, 2011, from pcprakt.userpage.fu-
berlin.de/SKRIPT/rastertunnel.pdf
5) CHAPTER 1 POLAROGRAPHY. Retrieved 21
November, 2011, from
www.newagepublishers.com/samplechapter/001166.pdf
6) Electroanalytical Chemistry, Retrieved 21 November,
2011, fromfaculty.uml.edu/David_Ryan/84.../ElectrochemLecture10-
2005.pdf
7) Instrumental Methods, Retrieved 21 November, 2011, from
http://chemistry.olivet.edu/classes/chem301/pdf/Instrumental%20Section.PDF