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)

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

Potensiometri

Pengukuran potensial listrik dari suatu Sel Elektrokimia untuk mendapatkan informasi mengenai bahan kimia yang ada pada sel tsb (conc., aktivitas, muatan listrik)

Mengukur perbedaan potensial listrik antara 2 electroda:

Elektroda Pembanding (E constant)Elektroda Kerja/Indikator(sinyal analit)

Elektroda Pembanding

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

- +

Ag/AgClSalt bridge

KCl

Pt

Fe2+, Fe3+

- +

Ag

Soln. aq. satdin KCl + AgCl

Pt

Fe2+, Fe3+AgCl + KCl

AgCl

Porous glass

AgCl(s) + e - <=> Ag(s) + Cl -

E0=0.222V

Fe3+ + e - <=> Fe2+

E0=0.771VE(KCl sat.)=0.197V

Elektroda Pembanding

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

Hg(l)

Soln. sat. in KCl

Pt

KCl

Hg, Hg2Cl2 et KCl

Porous glass

E0=0.268V

E(KCl sat.)=0.241VGlass wool

Hg2Cl2 + 2e - <=> 2Hg(l) + 2Cl -

Elektroda Pembanding

• Reaksi/Potensial setengah selnya diketahui• Tidak bereaksi/dipengaruhi oleh analit yang diukur

– Reversible dan mengikuti persamaan Nernst– Potensial Konstan– Dapat kembali ke potensial awal– stabil

• Elektroda Calomel– Hg in contact with Hg(I) chloride (Hg/Hg2Cl2)– Ag/AgCl

Electroda Kerja• Inert:

Pt, Au, Carbon. Tidak ikut bereaksi.

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

• Elektroda Logam yang mendeteksi ion logamnya sendiri (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 Kerja

• 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

2nd kind• 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

Inert metals – Pt, Au, Pd

• Electron source or sink• Redox of metal ion evaluated

– May not be reversible

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+

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

Glass Membrane Electrode

Ion selective electrodes (ISEs)

A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic

potensial difference being created across that membrane

C a 2 + C a 2 + 0 . 0 1 M C a 2 +

0 . 0 2 M C l -

0 . 1 M C a 2 +

0 . 2 M C l -

( 0 . 1 + ) M C a 2 + ( 0 . 1 - ) M C a 2 +

0 . 0 2 M C l - 0 . 2 M C l -

+

+

+

+

-

-

-

-

Calcium selective molecular recognition ligand

ISEs

25C) (@

log0592.0

ln

ln

2

1

2

1

2

1

A

A

nA

A

nF

RTE

nFEA

ARTG

Combination glass pH Electrode

Ag

Soln. aq. satdin KCl + AgCl

AgCl(s) + KCl(s)

AgCl porousglass

+ -

0.1M HCl inAgCl sat.

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 isopotensial which is set to

7.00 in all meters

mV

pH 4 7

Calibrate knob raisesand lowers the linewithout changing slope

mV

pH 4 7

Slope/temp control pivots line around isopotensialwithout changing it

Liquid Membrane Electrodes

Solid State Membrane Electrodes

Ag wire

Filling solutionwith fixed[Cl-] andcation thatelectroderesponds to

Ag/AgCl

Solid state membrane(must be ionic conductor)

Solid State Membrane Chemistry

Membrane Ion Determined

LaF3 F-, La3+

AgCl Ag+, Cl-

AgBr Ag+, Br-

AgI Ag+, I-

Ag2S Ag+, S2-

Ag2S + CuS Cu2+

Ag2S + CdS Cd2+

Ag2S + PbS Pb2+

Solid state electrodes

VOLTAMETRI Pengukuran arus sebagai fungsi perubahan potensial

POLAROGRAFI:• Heyrovsky (1922): melakukan percobaan voltametri

yang pertama dengan elektroda merkuri tetes (DME)

Cu2+ + 2e → Cu(Hg)

Mengapa elektron berpindah

EF

Eredox E

F

Eredox

Reduction Oxidation

E E

Steps in an electron transfer eventO 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• Migration – movement of a muatan listrik listrik particle in a

potensial field• 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 electrodeDifficult to get perfect reproducibility with stirring, better to move the electrodeConvection is considerably more efficient than diffusion or migration = higher arus listriks for a given concentration = greater analytical sensitivity

Nernst-Planck Equation

xx

x

RT

F

x

xx CCDzCDJ iii

iiii

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 = potensial gradient at distance x from electrode(x) = velocity at which species i moves (cm/s)

Diffusion

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

Fick’s 1st Law

I = nFAJ

Simplest ExperimentChronoamperometri

time

i

Simulation

Recall-Double layer

Double-Layer charging

• Charging/discharging a capacitor upon application of a potensial step

RCtc e

R

EI /

Itotal = Ic + IF

Working electrode choice

• Depends upon potensial window desired– Overpotensial– Stability of material– Conductivity– contamination

The polarogrampoints a to b

I = E/Rpoints 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 Epoints c to d

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

surface is reduced.

Dropping Mercury Electrode

• Renewable surface• potensial window expanded for reduction

(high overpotensial for proton reduction at mercury)

PolarographyA = 4(3mt/4d)2/3 = 0.85(mt)2/3

Mass flow rate of dropDensity of drop

We can substitute this into Cottrell Equation

i(t) = nFACD1/2/ 1/2t1/2

Giving the Ilkovich Equation:

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 arus listrik at the end of the drop life. The average arus listrik is

obtained by integrating the arus listrik over this time period

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

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

Polarograms

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

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

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 arus listriks

Normal Pulse Polarography

NPP advantage

Differential pulse voltametri

DPP vs DCP

Ep ~ E1/2 (Ep= E1/2±DE/2)

1

-1

(

cnFAD1/2

mp tI

where DE=pulse amplitude

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

Resolution depends on DEW1/2 = 3.52RT/nF when DE0

Improved response because charging arus listrik is subtracted and adsorptive effects are discriminated against.l.o.d. 10-8M

Resolution

Stripping voltametri• Preconcentration technique.

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

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

Deposition potensial

ASV

ASV or CSV

Multi-Element

Standard Addition

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 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 redox potensials, detection of chemical reactions that precede or follow the electrochemical reaction and evaluation of electron transfer kinetics.

Cyclic voltametri

Cyclic voltametri

For a reversible process

Epc – Epa = 0.059V/n

The Randles-Sevcik equation Reversible systems

The Randles-Sevcik equation Reversible systems

214463.0 RTnFvDnFACip

• 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

ACDvnip212123510687.2

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 voltametri

Cyclic voltametri

Cyclic voltametri

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 potensial Ep/2 = E1/2 0.028/n

• Sign is + for a reduction