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    Potentiometry

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    api-19959578
    Potentiometry is an electroanalytical technique that measures potential (voltage) in electrochemical cells containing indicator and reference electrodes. It can be used to determine analyte concentrations without a current flowing. The key components of a potentiometric cell are the reference electrode, which has a known and stable potential, the indicator or working electrode, which generates a potential that depends on the analyte concentration, and a salt bridge. Common applications of potentiometry include pH and ion-selective electrode measurements using glass, metallic, liquid membrane, or crystalline membrane electrodes. Potentiometric techniques can perform quantitative analyses via direct measurement, titrations, or standard addition methods.

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    Potentiometry

    Uploaded by

    api-19959578
    743 views48 pages
    Potentiometry is an electroanalytical technique that measures potential (voltage) in electrochemical cells containing indicator and reference electrodes. It can be used to determine analyte concentrations without a current flowing. The key components of a potentiometric cell are the reference electrode, which has a known and stable potential, the indicator or working electrode, which generates a potential that depends on the analyte concentration, and a salt bridge. Common applications of potentiometry include pH and ion-selective electrode measurements using glass, metallic, liquid membrane, or crystalline membrane electrodes. Potentiometric techniques can perform quantitative analyses via direct measurement, titrations, or standard addition methods.

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    Potentiometry is an electroanalytical technique that measures potential (voltage) in electrochemical cells containing indicator and reference electrodes. It can be used to determine analyte concentrations without a current flowing. The key components of a potentiometric cell are the reference electrode, which has a known and stable potential, the indicator or working electrode, which generates a potential that depends on the analyte concentration, and a salt bridge. Common applications of potentiometry include pH and ion-selective electrode measurements using glass, metallic, liquid membrane, or crystalline membrane electrodes. Potentiometric techniques can perform quantitative analyses via direct measurement, titrations, or standard addition methods.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    743 views48 pages

    Potentiometry

    Uploaded by

    api-19959578
    Potentiometry is an electroanalytical technique that measures potential (voltage) in electrochemical cells containing indicator and reference electrodes. It can be used to determine analyte concentrations without a current flowing. The key components of a potentiometric cell are the reference electrode, which has a known and stable potential, the indicator or working electrode, which generates a potential that depends on the analyte concentration, and a salt bridge. Common applications of potentiometry include pH and ion-selective electrode measurements using glass, metallic, liquid membrane, or crystalline membrane electrodes. Potentiometric techniques can perform quantitative analyses via direct measurement, titrations, or standard addition methods.

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    POTENTIOMETRY

    Potentiometric Analysis
    • Based on potential
    measurement of
    electrochemical cells without
    any appreciable current
    • The use of electrodes to
    measure voltages from
    chemical reactions
    Applications of Potentiometric Analysis
    Components of a
    Potentiometric Cell
    1. Reference electrode
    2. Salt bridge
    3. Analyte
    4. Indicator electrode
    RE SB A IE
    – Eref + Ej + Eind
    Reference electrode
    • Half-cell with known potential (Eref )
    • Left hand electrode (by convention)
    • Easily assembled
    • Rugged
    • Insensitive to analyte concentration
    ▫ Reversible and obeys Nernst equation
    ▫ Constant potential
    ▫ Returns to original potential
    Indicator electrode
    • Generates a potential (Eind ) that
    depends on analyte
    concentration
    • Selective
    • Rapid and reproducible response
    Salt bridge
    • Prevents mixing up of analyte
    components

    • Generates potential (Ej) =


    negligible
    Reference Electrodes
    1. Standard Hydrogen Electrode
    2. Calomel Reference Electrode
    3. Silver/Silver Chloride
    Reference Electrode
    SHE
    Hydrogen Gas Electrode
    •

    Pt (H2 (1 atm), H+ (1M)


    •
    Saturated Calomel Electrode
    Hg2Cl2(s)+2e-⇔2Hg(l)+2Cl-(aq)

    • Aka SCE
    • Easy to prepare
    • Easy to maintain
    • 0.2444 V at 25°C
    • Dependent on temp
    • Toxic
    SCE
    Ag/AgCl Ref. Electrode

    AgAgCl (satd),KCl (satd)

    AgCl(s) + e-⇔Ag(s)+Cl-(aq)
    E° = 0.199 V
    Liquid Junction Potential
    • Liquid junction - interface between
    two solutions containing different
    electrolytes or different
    concentrations of the same
    electrolyte
    • A junction potential occurs at every
    liquid junction.
    ▫ Caused by unequal mobilities of the +
    and - ions.
    Indicator Electrodes
    I. Metallic IE
    A. Electrodes of the First Kind
    B. Electrodes of the Second Kind
    C. Inert Metallic Electrodes (for Redox Systems)

    II. Membrane IE
    A. Glass pH IE
    B. Glass IE for other cations
    C. Liquid Membrane IE
    D. Crystalline-Membrane IE

    III. Gas Sensing Probes


    METALLIC
    INDICATOR
    ELECTRODES
    Electrodes of the First Kind
    • Pure metal electrode in direct equilibrium with its
    cation
    • Metal is in contact with a solution containing its
    cation.

    M+n(aq) + ne- ⇔ M(s)


    Disadvantages of First Kind Electrodes
    • Not very selective
    ▫ Ag+ interferes with Cu+2

    • May be pH dependent
    ▫ Zn and Cd dissolve in acidic solutions

    • Easily oxidized (deaeration required)


    • Non-reproducible response
    Electrodes of the Second Kind
    • Respond to anions by forming precipitates or stable complex

    • Examples:
    1. Ag electrode for Cl- determination

    2. Hg electrode for EDTA determination


    Inert Metallic (Redox) Electrodes
    • Inert conductors that respond to redox
    systems
    • Electron source or sink
    • An inert metal in contact with a solution
    containing the soluble oxidized and
    reduced forms of the redox half-reaction.
    • May not be reversible

    • Examples:
    ▫ Pt, Au, Pd, C
    MEMBRANE
    ELECTRODES
    • Aka p-ion electrodes
    • Consist of a thin membrane separating 2 solutions of
    different ion concentrations
    • Most common: pH Glass electrode
    Glass pH Electrode
    Properties of Glass pH electrode
    • Potential not affected by the presence
    of oxidizing or reducing agents
    • Operates over a wide pH range
    • Fast response
    • Functions well in physiological
    systems
    • Very selective
    • Long lifespan
    Theory of the glass membrane potential
    • For the electrode to become operative, it must be soaked in water.
    • During this process, the outer surface of the membrane becomes
    hydrated.
    • When it is so, the sodium ions are exchanged for protons in the
    solution:
    • The protons are free to move and exchange with other ions.

    Charge is slowly carried


    by migration of Na+
    across glass membrane

    Potential is determined
    by external [H+]
    Alkaline error
    • Exhibited at pH > 9
    • Electrodes respond to
    H+ and alkali cations
    • C,D,E and F: measured
    value is < true value
    ▫ Electrode also responds
    to other cations
    • Higher pH at lower [Na+]
    Acid error
    • Exhibited at pH <
    0.5

    • pH readings are
    higher (curves A
    and B)
    ▫ Saturation effect
    with respect to H+
    Selectivity Coefficient
    • No electrode responds exclusively to one kind of ion.
    ▫ The glass pH electrode is among the most selective, but it also
    responds to high concentration of Na+.
    • When an electrode used to measure ion A, also responds
    to ion X, the selectivity coefficient gives the relative
    response of the electrode to the two different species.

    response to X
    k A, X =
    resp
    ▫ The smaller the selectivity onsethe lessto
    coefficient, A by X.
    interference
    Selectivity Coefficient
    • Measure of the response of an ISE to other ions

    Eb = L’ + 0.0592 log (a1 + kHB b1)

    • kHB = 0 means no interference

    • kHB ≥ 1 means there is interference

    • kHB < 1 means negligible interference


    LIQUID MEMBRANE
    ELECTRODES
    Liquid Membrane Electrodes
    • Potential develops across the interface between the
    analyte solution and a liquid ion exchanger (that bonds
    with analyte)
    • Similar to a pH electrode except that the membrane is
    an organic polymer saturated with a liquid ion
    exchanger
    • Used for polyvalent ions as well as some anions

    • Example:
    • Calcium dialkyl phosphate insoluble in water, but binds
    Ca2+ strongly
    0.1 M CaCl2

    Responsive to Ca2+
    Characteristics of Ca ISE +2

    • Relatively high sensitivity


    • Low LOD
    • Working pH range: 5.5 – 11
    • Relevant in studying physiological processes
    A K -selective electrode
    +

    • Sensitive membrane
    consists of
    valinomycin, an
    antibiotic
    CRYSTALLINE-
    MEMBRANE
    ELECTRODES
    Crystalline-Membrane Electrodes
    • Solid state electrodes
    • Usually ionic compound
    • Crushed powder, melted and formed
    • Sometimes doped to increase conductivity
    • Operation similar to glass membrane
    Crystalline-Membrane Electrodes
    • AgX membrane: Determination of X-
    • Ag2S membrane: Determination of S-2
    • LaF3 membrane: Determination of F-
    F Selective Electrode
    -

    • A LaF3 is doped with EuF2.


    • Eu2+ has less charge than the La3+, so an
    anion vacancy occurs for every Eu2+.
    • A neighboring F- can jump into the vacancy,
    thereby moving the vacancy to another site.
    • Repetition of this process moves F- through
    the lattice.
    Fluoride Electrode
    GAS SENSING
    PROBES
    Gas Sensing Probes
    • A galvanic cell whose potential is related to the
    concentration of a gas in solution
    • Consist of RE, ISE and electrolyte solution
    • A thin gas-permeable membrane (PTFE)
    serves as a barrier between internal and
    analyte solutions
    • Allows small gas molecules to pass and
    dissolve into internal solution
    • O2, NH3/NH4+, and CO2/HCO3-/CO32-
    Gas
    Sensing
    Probe
    DIRECT POTENTIOMETRY
    • A rapid and convenient method of
    determining the activity of cations/anions
    Potentiometric Measurement
    • Ionic composition of standards must be
    the same as that of analyte to avoid
    discrepancies

    • Swamp sample and standard with inert


    electrolyte to keep ionic strength
    constant

    • TISAB (Total Ionic Strength Adjustment


    Buffer) = controls ionic strength and pH
    of samples and standards in ISE
    measurements
    Potentiometric Measurement
    1. Calibration Method

    2. Standard Addition
    Method
    Special Applications:
    Potentiometric pH Measurement
    using Glass electrode
    • One drop of solution
    • Tooth cavity
    • Sweat on skin
    • pH inside a living cell
    • Flowing liquid stream
    • Acidity of stomach
    Potentiometric Titration
    • Involves measurement of the potential
    of a suitable indicator electrode as a
    function of titrant volume
    • Provides MORE RELIABLE data than
    the usual titration method
    • Useful with colored/turbid solutions
    • May be automated
    • More time consuming
    Potentiometric Titration Curves

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