Cyclic voltametri

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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)
• 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) || .....
Elektroda Pembanding
SCE:
Pt(s) | Hg(l) | Hg2Cl2 (l) | KCl(aq., sat.) ||.....
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)
Hg2Cl2 + 2e  2Hg(l) + 2ClE = 0.799 + 0.05916 log [Ag+] - 0.241 V
E0+= 0.799V
E-= 0.241V
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
rd
3
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+
 Properties
o Low solubility - solids, semi-solids and polymers
o Some electrical conductivity - often by doping
o 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
0 .0 1 M Ca2 +
0 .0 2 M Cl -
Ca2 +
0 .1 M Ca2 +
0 .2 M Cl -
+
2
( 0 . 1 + ) M Ca +
+
+
0 .0 2 M Cl +
Calcium selective
molecular
recognition ligand
Ca2 +
( 0 . 1 - ) M Ca2 +
0 .2 M Cl -
ISEs
A1
G   RT ln
 nFE
A2
RT A1 0.0592
A1
E
ln

log
nF A2
n
A2
(@ 25C)
Combination glass pH Electrode
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
Slope/temp control pivots
line around isopotensial
without changing it
mV
Calibrate knob raises
and lowers the line
without changing slope
4
7
pH
Liquid Membrane Electrodes
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)
Solid State Membrane Chemistry
Membrane Ion Determined
LaF3
F-, La3+
AgCl
Ag+, ClAgBr
Ag+, BrAgI
Ag+, IAg2S
Ag+, S2Ag2S + 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
Reduction
Oxidation
EF
Eredox
E
E
Eredox
E
F
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
•
•
•
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 electrode
Difficult to get perfect reproducibility with stirring, better to
move the electrode
Convection is considerably more efficient than diffusion or
migration = higher arus listriks for a given concentration =
greater analytical sensitivity
Nernst-Planck Equation
J x    D
i
i
 C i x 
x
Diffusion
 x 
 C i x 
D
i Ci
RT
x
F

z

i
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
Fick’s 1st Law
I = nFAJ
Solving Fick’s Laws for
particular applications
like electrochemistry
involves establishing
Initial Conditions and
Boundary Conditions
Simplest Experiment
Chronoamperometri
i
time
Simulation
Recall-Double layer
Double-Layer charging
• Charging/discharging a capacitor upon application
of a potensial step
E t / RC 
Ic 
e
R
Itotal = Ic + IF
Working electrode choice
• Depends upon potensial window desired
– Overpotensial
– Stability of material
– Conductivity
– contamination
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
• Renewable surface
• potensial window expanded for reduction
(high overpotensial for proton reduction at
mercury)
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 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
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±E/2)
where E=pulse amplitude
nFAD1/2 c 1 - 
Ip 
(t m  1  
 = exp[(nF/RT)(E/2)]
Resolution depends on E
W1/2 = 3.52RT/nF when E0
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
i p  0.4463nFACnFvD RT 
12


i p  2.687105 n3 2v1 2 D1 2 AC
•
•
•
•
•
•
•
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
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
• Sign is + for a reduction
0.028/n

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