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ELECTROANALISIS ( Elektrometri ). Potensiometri , Amperometri and Voltametri. Electroanalysis. Mengukur berbagai parameter listrik ( potensial , arus listrik , muatan listrik , konduktivitas ) dalam kaitannya dengan parameter kimia ( reaksi ataupun konsentrasi dari bahan kimia )

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Electroanalisis elektrometri

ELECTROANALISIS(Elektrometri)

Potensiometri, Amperometri and Voltametri


Electroanalysis
Electroanalysis

  • Mengukurberbagai parameter listrik (potensial, aruslistrik, muatanlistrik, konduktivitas) dalamkaitannyadengan parameter kimia (reaksiataupunkonsentrasidaribahankimia)

  • Konduktimetri, Potensiometri(pH, ISE), Koulometri, Voltametri, Amperometri


Potensiometri
Potensiometri

PengukuranpotensiallistrikdarisuatuSelElektrokimiauntukmendapatkaninformasimengenaibahankimia yang adapadaseltsb(conc., aktivitas, muatanlistrik)

Mengukurperbedaanpotensiallistrikantara 2 electroda:

ElektrodaPembanding(E constant)

ElektrodaKerja/Indikator(sinyalanalit)


Elektroda pembanding
ElektrodaPembanding

Ag/AgCl:

Ag(s) | AgCl (s) | Cl-(aq) || .....


Elektroda pembanding1
ElektrodaPembanding

SCE:

Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) ||.....


Elektroda pembanding2
ElektrodaPembanding

  • Reaksi/Potensialsetengahselnyadiketahui

  • Tidakbereaksi/dipengaruhiolehanalit yang diukur

    • Reversible danmengikutipersamaan Nernst

    • PotensialKonstan

    • Dapatkembalikepotensialawal

    • stabil

  • Elektroda Calomel

    • Hg in contact with Hg(I) chloride (Hg/Hg2Cl2)

    • Ag/AgCl


Electroda kerja
ElectrodaKerja

  • Inert:

    Pt, Au, Carbon. Tidakikutbereaksi.

    Contoh: SCE || Fe3+, Fe2+(aq) | Pt(s)

  • ElektrodaLogam yang mendeteksi ion logamnyasendiri (1st Electrode)

    (Hg, Cu, Zn, Cd, Ag)

    Contoh: SCE || Ag+(aq) | Ag(s)

    Ag+ + e-  Ag(s) E0+= 0.799V

    Hg2Cl2 + 2e  2Hg(l) + 2Cl- E-= 0.241V

    E = 0.799 + 0.05916 log [Ag+] - 0.241 V


Electroda kerja1
ElectrodaKerja

  • Ecell=Eindicator-Ereference

  • Metallic

    • 1st kind, 2nd kind, 3rd kind, redox

      1st kind

    • respond directly to changing activity of electrode ion

    • Direct equilibrium with solution


2 nd kind
2ndkind

  • Precipitate or stable complex of ion

    • Ag for halides

    • Ag wire in AgCl saturated surface

  • Complexes with organic ligands

    • EDTA

      3rd kind

    • Electrode responds to different cation

    • Competition with ligand complex


Metallic redox indictors
Metallic Redox Indictors

  • Inert metals

    • Pt, Au, Pd

      • Electron source or sink

      • Redox of metal ion evaluated

    • May not be reversible


Membrane indicator electrodes
Membrane Indicator electrodes

  • Non-crystalline membranes:

    • Glass - silicate glasses for H+, Na+

    • Liquid - liquid ion exchanger for Ca2+

    • Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3-

  • Crystalline membranes:

    • Single crystal - LaF3 for FPolycrystalline

    • or mixed crystal - AgS for S2- and Ag+

  • Properties

    • Low solubility - solids, semi-solids and polymers

    • Some electrical conductivity - often by doping

    • Selectivity - part of membrane binds/reacts with analyte



  • Ion selective electrodes ises
    Ion selective electrodes (ISEs)

    A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potensialdifference being created across that membrane




    Proper ph calibration
    Proper pH Calibration

    • E = constant – constant.0.0591 pH

    • Meter measures E vs pH – must calibrate both slope & intercept on meter with buffers

    • Meter has two controls – calibrate & slope

    • 1st use pH 7.00 buffer to adjust calibrate knob

    • 2nd step is to use any other pH buffer

    • Adjust slope/temp control to correct pH value

    • This will pivot the calibration line around the isopotensialwhich is set to 7.00 in all meters

    Slope/temp control pivots

    line around isopotensial

    without changing it

    mV

    mV

    Calibrate knob raises

    and lowers the line

    without changing slope

    4 7

    4 7

    pH

    pH



    Solid State Membrane Electrodes

    Ag wire

    Filling

    solution

    with fixed

    [Cl-] and

    cation that

    electrode

    responds to

    Ag/AgCl

    Solid state membrane

    (must be ionic conductor)



    Voltametri
    VOLTAMETRI

    Pengukuranarussebagaifungsiperubahanpotensial

    POLAROGRAFI:

    • Heyrovsky (1922): melakukanpercobaanvoltametri yang pertamadenganelektrodamerkuritetes (DME)

      Cu2+ + 2e → Cu(Hg)


    EF

    Eredox

    Mengapaelektronberpindah

    Reduction

    Oxidation

    Eredox

    E

    E

    EF


    Steps in an electron transfer event
    Steps in an electron transfer event

    • O must be successfully transported from bulk solution (mass transport)

    • O must adsorb transiently onto electrode surface (non-faradaic)

    • CT must occur between electrode and O (faradaic)

    • R must desorb from electrode surface (non-faradaic)

    • R must be transported away from electrode surface back into bulk solution (mass transport)


    Mass transport or mass transfer
    Mass Transport or Mass Transfer

    • Migration – movement of a muatanlistriklistrikparticle in a potensialfield

    • Diffusion – movement due to a concentration gradient. If electrochemical reaction depletes (or produces) some species at the electrode surface, then a concentration gradient develops and the electroactive species will tend to diffuse from the bulk solution to the electrode (or from the electrode out into the bulk solution)

    • Convection – mass transfer due to stirring. Achieved by some form of mechanical movement of the solution or the electrode i.e., stir solution, rotate or vibrate electrode

      Difficult to get perfect reproducibility with stirring, better to move the electrode

      Convection is considerably more efficient than diffusion or migration = higher aruslistriksfor a given concentration = greater analytical sensitivity


    Nernst planck equation

    Diffusion

    Migration

    Convection

    Nernst-Planck Equation

    Ji(x) = flux of species i at distance x from electrode (mole/cm2 s)

    Di = diffusion coefficient (cm2/s)

    Ci(x)/x = concentration gradient at distance x from electrode

    (x)/x = potensialgradient at distance x from electrode

    (x) = velocity at which species i moves (cm/s)


    Diffusion
    Diffusion

    Fick’s 1st Law

    Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions

    I = nFAJ


    Simplest experiment chronoamperometri
    Simplest ExperimentChronoamperometri




    Double layer charging
    Double-Layer charging

    • Charging/discharging a capacitor upon application of a potensialstep

    Itotal = Ic + IF


    Working electrode choice
    Working electrode choice

    • Depends upon potensialwindow desired

      • Overpotensial

      • Stability of material

      • Conductivity

      • contamination


    The polarogram
    The polarogram

    points a to b

    I = E/R

    points b to c

    electron transfer to the electroactive species.

    I(reduction) depends on the no. of molecules reduced/s: this rises as a function of E

    points c to d

    when E is sufficiently negative, every molecule that reaches the electrode surface is reduced.


    Dropping mercury electrode
    Dropping Mercury Electrode

    • Renewable surface

    • potensialwindow expanded for reduction (high overpotensialfor proton reduction at mercury)


    Polarography
    Polarography

    A = 4(3mt/4d)2/3 = 0.85(mt)2/3

    Density of drop

    Mass flow rate of drop

    We can substitute this into Cottrell Equation

    i(t) = nFACD1/2/ 1/2t1/2

    We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding drop

    Giving theIlkovichEquation:

    id = 708nD1/2m2/3t1/6C

    I has units of Amps when D is in cm2s-1,m is in g/s and t is in seconds. C is in mol/cm3

    This expression gives the aruslistrikat the end of the drop life. The average aruslistrikis obtained by integrating the aruslistrikover this time period

    iav = 607nD1/2m2/3t1/6C


    Polarograms
    Polarograms

    E1/2 = E0 + RT/nF log (DR/Do)1/2 (reversible couple)

    Usually D’s are similar so half wave potensialis similar to formal potensial. Also potensialis independent of concentration and can therefore be used as a diagnostic of identity of analytes.


    Other types of polarography
    Other types of Polarography

    • Examples refer to polarography but are applicable to other votammetric methods as well

    • all attempt to improve signal to noise

    • usually by removing capacitive aruslistriks





    DPP vs DCP

    Ep ~ E1/2 (Ep= E1/2±DE/2)

    where DE=pulse amplitude

    s = exp[(nF/RT)(DE/2)]

    Resolution depends on DE

    W1/2 = 3.52RT/nF when DE0

    Improved response

    because charging aruslistrik

    is subtracted and adsorptive

    effects are discriminated against.

    l.o.d. 10-8M



    Stripping voltametri
    Stripping voltametri

    • Preconcentrationtechnique.

      1. Preconcentrationor accumulation step. Here the analyte species is collected onto/into the working electrode

      2. Measurement step : here a potensialwaveform is applied to the electrode to remove (strip) the accumulated analyte.


    Deposition potensial
    Deposition potensial






    Cyclic voltametri
    Cyclic voltametri

    • Cyclic voltametri is carried out at a stationary electrode.

    • This normally involves the use of an inert disc electrode made from platinum, gold or glassy carbon. Nickel has also been used.

    • The potensial is continuously changed as a linear function of time. The rate of change of potensial with time is referred to as the scan rate (v). Compared to a RDE the scan rates in cyclic voltametri are usually much higher, typically 50 mV s-1


    Cyclic voltametri1
    Cyclic voltametri

    • Cyclic voltametri, in which the direction of the potensial is reversed at the end of the first scan. Thus, the waveform is usually of the form of an isosceles triangle.

    • The advantage using a stationary electrode is that the product of the electron transfer reaction that occurred in the forward scan can be probed again in the reverse scan.

    • CV is a powerful tool for the determination of formal redoxpotensials, detection of chemical reactions that precede or follow the electrochemical reaction and evaluation of electron transfer kinetics.


    Cyclic voltametri2
    Cyclic voltametri


    Cyclic voltametri3
    Cyclic voltametri

    For a reversible process

    Epc – Epa = 0.059V/n



    The randles sevcik equation reversible systems1

    n = the number of electrons in the redox reaction

    v = the scan rate in V s-1

    F = the Faraday’s constant 96,485 coulombs mole-1

    A = the electrode area cm2

    R = the gas constant 8.314 J mole-1 K-1

    T = the temperature K

    D = the analyte diffusion coefficient cm2 s-1

    The Randles-Sevcik equation Reversible systems


    The randles sevcik equation reversible systems2
    The Randles-Sevcik equation Reversible systems

    As expected a plot of peak height vs the square root of the scan rate produces a linear plot, in which the diffusion coefficient can be obtained from the slope of the plot.


    Cyclic voltametri4
    Cyclic voltametri


    Cyclic voltametri5
    Cyclic voltametri


    Cyclic voltametri6
    Cyclic voltametri


    Cyclic voltametri stationary electrode
    Cyclic voltametri – Stationary Electrode

    • Peak positions are related to formal potensial of redox process

    • E0 = (Epa+ Epc) /2

    • Separation of peaks for a reversible couple is 0.059/n volts

    • A one electron fast electron transfer reaction thus gives 59mV separation

    • Peak potensials are then independent of scan rate

    • Half-peak potensialEp/2 = E1/2  0.028/n

    • Sign is + for a reduction


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