Voltammetry and Polarography

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LECTURE of a UP MANILA Professor on advanced analytical chemistry

Transcript of Voltammetry and Polarography

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    Voltammetry and Polarography

    Voltammetric methods of Analysis

    What is Voltammetry?

    An electrochemical techniqye where time-dependent

    potential is applied to an electrochemical cell, and

    the current flowing through the cell is measured as

    a function of that potential.

    A plot of current as a function of applied potential is called a voltammogram (the electrochemical equivalent of a spectrum in spectroscopy) which

    provides both quantitative and qualitative

    information about the species involved in the

    oxidation or reduction reaction.

    Voltammetric Measurements

    Three electrode system potentiostat mentioned earlier is used as a device that

    measures the current as a function of

    potential

    Working electrodes used: Hg, Pt, Au, Ag, C or others

    Reference electrode: SCE or Ag/ AgCl;

    Auxiliary electrode: Pt wire

    Polarography

    In polarography, the current flowing through the cell is measured as a function of the potential of the working electrode.

    Usually this current is proportional to the concentration of the analyte.

    Apparatus for carrying out polarography is shown below.

    The working electrode is a dropping mercury electrode or a mercury droplet suspended from a bottom of a glass capillary tube.

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    Analyte is either reduced (most of the cases) or oxidized at the surface of the mercury drop.

    The current carrier auxiliary electrode is a platinum wire.

    SCE or Ag/AgCl reference electrode is used.

    The potential of the mercury drop is measured with respect to the reference electrode.

    Typical electrochemical

    cell used in polarography

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    Why Dropping Mercury Electrode?

    Hg yields reproducible currentpotential data. This reproducibility can be attributed to the continuous

    exposure of fresh surface on the growing mercury drop.

    With any other electrode (such as Pt in various forms), the potential depends on its surface condition and therefore on its previous treatment.

    The vast majority of reactions studied with the mercury electrode are reductions.

    At a Pt surface, reduction of solvent is expected to compete with reduction of many analyte species, especially in acidic solutions.

    The high overpotential for H+ reduction at the mercury surface. Therefore, H+ reduction does not interfere with many reductions.

    Problems with mercury electrode

    A mercury electrode is not very useful for performing oxidations, because Hg is too easily oxidized.

    In a non-complexing medium, Hg is oxidized near + 0.25 V (versus S.C.E.).

    For most oxidations, some other working electrode must be employed.

    Pt electrode vs SCE works for a range of +1.2 to 0.2 in acidic solution +0.7 V to 1 V in basic solution. Carbon paste electrode is also used in voltammetry

    Problems with mercury electrode

    Mercury is toxic and slightly volatile, and spills are almost inevitable.

    To remove residual mercury, sprinkle elemental zinc powder on the surface and dampen the powder with 5% aqueous H2S04

    Mercury dissolves in the zinc. After working the paste into contaminated areas with a sponge or brush, allow the paste to dry and then sweep it up. Discard the powder appropriately as contaminated mercury waste

    Current in Voltammetry

    When an analyte is oxidized at the working electrode, a current passes electrons through the external electric circuitry to the auxiliary electrode.

    This current flows from the auxiliary to the working electrode, where reduction of the solvent or other components of the solution matrix occurs .

    The current resulting from redox reactions at the working and auxiliary electrodes is called a faradaic current.

    Sign Conventions: A current due to the analyte's reduction is called a cathodic current and by convention, is considered positive. Anodic currents are due to oxidation reactions and carry a negative value.

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    Influence of applied potential on the

    faradaic current

    When the potential applied to the working electrode exceeds the reduction potential of the

    electroactive species, a reduction will take place

    at the electrode surface

    Thus, electroactive species diffuses from the bulk solution to the electrode surface and the

    reduction products diffuse from the electrode

    surface towards the bulk solution. This creates

    what is called the faradaic current.

    The magnitude of the faradaic current is determined by the rate of the resulting oxidation or reduction reaction at the electrode surface.

    Two factors contribute to the rate of the electrochemical reaction:

    the rate at which the reactants and products are transported to and from the surface of the electrode (mass transport)

    and the rate at which electrons pass between the electrode and the reactants and products in solution. (kinetics of electron transfer at the electrode surface)

    Influence of Mass Transport on the Faradaic Current

    There are three modes of mass transport to and from the electrode

    surface: diffusion, migration, and convection.

    Diffusion from a region of high concentration to a region of low concentration occurs whenever the concentration of an ion or

    molecule at the surface of the electrode is different from that in bulk solution.

    Convection occurs when a mechanical means is used to carry reactants toward the electrode and to remove products from the electrode.

    The most common means of convection is to stir the solution using a stir bar. Other methods include rotating the electrode

    and incorporating the electrode into a flow cell.

    Migration occurs when charged particles in solution are attracted or repelled from an electrode that has a positive or negative

    surface charge.

    Unlike diffusion and convection, migration only affects the mass transport of charged particles

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    The flux of material to and from the electrode surface is a complex function of all three modes of mass transport.

    In the limit in which diffusion is the only significant means for the mass transport of the reactants and

    products, the current in a voltammetric cell is given

    by

    where n is the number of electrons transferred in the redox reaction, F

    is Faraday's constant, A is the area of the electrode, D is the diffusion

    coefficient for the reactant or product, CbuIk and Cx=o are the

    concentration of the analyte in bulk solution and at the electrode

    surface, and is the thickness of the diffusion layer.

    For the above equation to be valid, migration and convection must not interfere with formation of diffusion layer around the electrode surface.

    Migration is eliminated by adding a high concentration of an inert supporting electrolyte to the analytical solution.

    The large excess of inert ions, ensures that few reactant and product ions will move as a result of migration.

    Although convection may be easily eliminated by not physically agitating the solution, in some situations it is desirable either to stir the solution or to push the solution through an electrochemical flow cell. Fortunately, the dynamics of a fluid moving past an electrode results in a small diffusion layer, typically of 0.001 - 0.01-cm thickness, in which the rate of mass transport by convection drops to zero.

    Influence of the Kinetics of Electron Transfer on the

    Faradaic Current When electron transfer kinetics at the electrode surface are fast,

    the redox reaction is at equilibrium, and;

    the concentrations of reactants and products at the electrode are those specified by the Nernst equation.

    Such systems are considered electrochemically reversible.

    When electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible.

    Non faradaic Currents Currents other than faradaic may also exist in an electrochemical cell that are unrelated to any redox reaction.

    The most important example of a nonfaradaic current occurs whenever the electrode's potential is changed.

    When mass transport takes place by migration, negatively charged particles in solution migrate toward a positively charged electrode, and positively charged particles move away from the same electrode.

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    Non faradaic Currents

    When an inert electrolyte is responsible for migration, the result is a structured electrode-surface interface called the electrical double layer, or EDL,

    The movement of charged particles in solution, gives rise to a short-lived, nonfaradaic charging current.

    Changing the potential of an electrode causes a change in the structure of the EDL, producing a small charging current.

    Residual Current

    Even in the absence of analyte, a small current flows through an electrochemical cell.

    This current, called the residual current, has of two components:

    a faradaic current due to the oxidation or reduction of trace impurities; and

    a charging current, the current needed to charge or discharge the capacitor formed by the electrode surface-solution interface.

    Also called the condenser current or charging current.

    It is present in all voltammetric and polarographic experiments, regardless of the pu