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ELECTROANALISIS ( Elektrometri )

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ELECTROANALISIS(Elektrometri)

Potensiometri, Amperometri and Voltametri

- Mengukurberbagai parameter listrik (potensial, aruslistrik, muatanlistrik, konduktivitas) dalamkaitannyadengan parameter kimia (reaksiataupunkonsentrasidaribahankimia)
- Konduktimetri, Potensiometri(pH, ISE), Koulometri, Voltametri, Amperometri

PengukuranpotensiallistrikdarisuatuSelElektrokimiauntukmendapatkaninformasimengenaibahankimia yang adapadaseltsb(conc., aktivitas, muatanlistrik)

Mengukurperbedaanpotensiallistrikantara 2 electroda:

ElektrodaPembanding(E constant)

ElektrodaKerja/Indikator(sinyalanalit)

Ag/AgCl:

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

SCE:

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

- 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

- 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

- 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

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

- 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

- EDTA

- Inert metals
- Pt, Au, Pd
- Electron source or sink
- Redox of metal ion evaluated

- May not be reversible

- Pt, Au, Pd

- 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+

- Low solubility - solids, semi-solids and polymers
- Some electrical conductivity - often by doping
- Selectivity - part of membrane binds/reacts with analyte

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

- 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)

Pengukuranarussebagaifungsiperubahanpotensial

POLAROGRAFI:

- Heyrovsky (1922): melakukanpercobaanvoltametri yang pertamadenganelektrodamerkuritetes (DME)
Cu2+ + 2e → Cu(Hg)

EF

Eredox

Mengapaelektronberpindah

Reduction

Oxidation

Eredox

E

E

EF

- 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)

- 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

Diffusion

Migration

Convection

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)

Fick’s 1st Law

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

I = nFAJ

- Charging/discharging a capacitor upon application of a potensialstep

Itotal = Ic + IF

- Depends upon potensialwindow desired
- Overpotensial
- Stability of material
- Conductivity
- contamination

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.

- Renewable surface
- potensialwindow expanded for reduction (high overpotensialfor proton reduction at mercury)

A = 4(3mt/4d)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

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.

- 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 DE0

Improved response

because charging aruslistrik

is subtracted and adsorptive

effects are discriminated against.

l.o.d. 10-8M

- 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.

- 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 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.

For a reversible process

Epc – Epa = 0.059V/n

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

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.

- 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