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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