Download - ELECTROANALYSIS 2010
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ELECTROANALISIS(Elektrometri)
Potensiometri, Amperometri and Voltametri
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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
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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)
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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
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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 -
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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
• Elektroda Calomel– Hg in contact with Hg(I) chloride– Ag/AgCl
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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
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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
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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
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Metallic Redox Indictors
Inert metals – Pt, Au, Pd
• Electron source or sink• Redox of metal ion evaluated
– May not be reversible
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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
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Glass Membrane Electrode
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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
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ISEs
25C) (@
log0592.0
ln
ln
2
1
2
1
2
1
A
A
nA
A
nF
RTE
nFEA
ARTG
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Combination glass pH Electrode
Ag
Soln. aq. satdin KCl + AgCl
AgCl(s) + KCl(s)
AgCl porousglass
+ -
0.1M HCl inAgCl sat.
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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
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Liquid Membrane Electrodes
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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+
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Solid state electrodes
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voltametri The measurement of variations in arus listrik produced by variations of the potensial applied to a working electrode
polarography:• Heyrovsky (1922): first voltametri experiments using
a dropping mercury working electrode
In voltametri, once the applied potensial is sufficiently negative, electron transfer occurs between the electrode and the electroactive species:
Cu2+ + 2e → Cu(Hg)
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Why Electrons Transfer
EF
Eredox E
F
Eredox
• Net flow of electrons from M to solute
• Ef more negative than Eredox
• more cathodic • more reducing
Reduction Oxidation
• Net flow of electrons from solute to M
• Ef more positive than Eredox
• more anodic • more oxidizing
E E
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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)
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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
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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)
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Diffusion
Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions
Fick’s 1st Law
I = nFAJ
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Simplest ExperimentChronoamperometri
time
i
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Simulation
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Recall-Double layer
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Double-Layer charging
• Charging/discharging a capacitor upon application of a potensial step
RCtc e
R
EI /
Itotal = Ic + IF
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Working electrode choice
• Depends upon potensial window desired– Overpotensial– Stability of material– Conductivity– contamination
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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.
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Dropping Mercury Electrode
• Renewable surface• potensial window expanded for reduction
(high overpotensial for proton reduction at mercury)
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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
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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.
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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
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Normal Pulse Polarography
• arus listrik measured at a single instant in the lifetime of each drop.
• higher signal because there is more electroactive species around each drop of mercury.
• somewhat more sensitive than DC polarography.
• data obtained have the same shape as a regular DCP.
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NPP advantage
• IL = nFAD1/2c/(ptm)1/2
• (tm = arus listrik sampling t)
• IL,N.P./IL,D.C. = (3t/7tm)1/2
• Predicts that N.P.P. 5-10 X sensitive than D.C.P.
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Differential pulse voltametri
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DPP
• arus listrik measured twice during the lifetime of each drop difference in arus listrik is plotted.
• Results in a peak-shaped feature, where the top of the peak corresponds to E1/2, and the height gives concentration
• This shape is the derivative of the regular DC data. • DPP has the advantage of sensitive detection limits and
discrimination against background arus listriks. Traditionally, metals in the ppm range can be determined with DPP.
• Derivative improves contrast (resolution) between overlapping waves
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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
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Resolution
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Square wave voltametri
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SWV
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SWV Response
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SWV
• advantage of square wave voltametri is that the entire scan can be performed on a single mercury drop in about 10 seconds, as opposed to about 5 minutes for the techniques described previously. SWV saves time, reduces the amount of mercury used per scan by a factor of 100. If used with a pre-reduction step, detection limits of 1-10 ppb can be achieved, which rivals graphite furnace AA in sensitivity.
• data for SWV similar to DPP
• height and width of the wave depends on the exact combination of experimental parameters (i.e. scan rate and pulse height
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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.
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Deposition potensial
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ASV
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ASV or CSV
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Adsorptive Stripping voltametri
• Use a chelating ligand that adsorbs to the WE.
• Can detect by redox process of metal or ligand.
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Multi-Element
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Standard Addition
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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
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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.
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Cyclic voltametri
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Cyclic voltametri
For a reversible process
Epc – Epa = 0.059V/n
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The Randles-Sevcik equation Reversible systems
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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
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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.
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Cyclic voltametri
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Cyclic voltametri
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Cyclic voltametri
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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
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Cyclic voltametri – Stationary Electrode
• The shape of the voltammogram depends on the transfer coefficient
• When deviates from 0.5 the voltammograms become asymmetric -cathodic peak sharper as expected from Butler Volmer eqn.