Electroanalytical chemistry
Potentiometry, Voltammetry and Polarography
Electroanalysis
• measure the variation of an electrical parameter (potential, current, charge, conductivity) and relate this to a chemical parameter (the analyte concentration)
• Conductimetry, potentiometry (pH, ISE), coulometry, voltammetry
Potentiometry
the measure of the cell potential to yield chemical information (conc., activity, charge)
Measure difference in potential between two electrodes:
reference electrode (E constant)indicator electrode (signal α analyte)
Reference electrodes
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
Reference Electrodes
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 -
Indicator Electrodes• Inert:
Pt, Au, Carbon. Don’t participate in the reaction.
example: SCE || Fe3+, Fe2+(aq) | Pt(s)
• Certain metallic electrodes: detect their ions(Hg, Cu, Zn, Cd, Ag)example SCE || Ag+(aq) | Ag(s)Ag+ + e- Ag(s)
E0+= 0.799VHg2Cl2 + 2e 2Hg(l) + 2Cl-
E-= 0.241V
E = 0.799 + 0.05916 log [Ag+] - 0.241 V
Ion selective electrodes (ISEs)
A difference in the activity of an ion on either side of a selective membrane results in a thermodynamic potential 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 isopotential 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 isopotentialwithout 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
Voltammetry
The measurement of variations in current produced by variations of the potential applied to a working electrode
polarography:• Heyrovsky (1922): first voltammetry experiments
using a dropping mercury working electrode
In voltammetry, once the applied potential is sufficiently negative, electron transfer occurs between the electrode and the electroactive species: Cu2+ + 2e → Cu(Hg)
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
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 charged particle in a potential 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 currents 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 = potential gradient at distance x from electrode(x) = velocity at which species i moves (cm/s)
DiffusionFick’s 1st Law
Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions
I = nFAJ
Simplest ExperimentChronoamperometry
time
i
Simulation
Recall-Double layer
Double-Layer charging
• Charging/discharging a capacitor upon application of a potential step
RCtc e
R
EI /
Itotal = Ic + IF
Working electrode choice
• Depends upon potential window desired– Overpotential– 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
• Potential window expanded for reduction (high overpotential 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 current at the end of the drop life. The average current is
obtained by integrating the current 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 potential is similar to formal potential. Also potential 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 currents
Normal Pulse Polarography
•current 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.
NPP advantage
• IL = nFAD1/2c/(tm)1/2
• (tm = current 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.
Differential pulse voltammetry
DPP
• current measured twice during the lifetime of each drop difference in current 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 currents. Traditionally, metals in the ppm range can be determined with DPP.
• Derivative improves contrast (resolution) between overlapping waves
DPP vs DCP
Ep ~ E1/2 (Ep= E1/2E/2)
1
-1
(
cnFAD1/2
mp tI
where E=pulse amplitude
= exp[(nF/RT)(E/2)]
Resolution depends on EW1/2 = 3.52RT/nF whenE0
Improved response because charging current is subtracted and adsorptive effects are discriminated against.l.o.d. 10-8M
Resolution
Square wave voltammetry
SWV
SWV Response
SWV
• advantage of square wave voltammetry 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
Stripping Voltammetry• Preconcentration technique.
1. Preconcentration or accumulation step. Here the analyte species is collected onto/into the working electrode
2. Measurement step : here a potential waveform is applied to the electrode to remove (strip) the accumulated analyte.
Deposition potential
ASV
ASV or CSV
Adsorptive Stripping Voltammetry
• Use a chelating ligand that adsorbs to the WE.
• Can detect by redox process of metal or ligand.
Multi-Element
Standard Addition