1. Electroanalytical chemistry
Potentiometry, Voltammetry and Polarography

2. 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

3. 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)

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

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

6. 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+= 0.799V
Hg2Cl2 + 2e  2Hg(l) + 2Cl-
E-= 0.241V
E = log [Ag+] V

7. 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

8. ISEs

9. Combination glass pH Electrode

10. 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
Slope/temp control pivots
line around isopotential
without changing it mV mV
Calibrate knob raises
and lowers the line
without changing slope pH pH

11. Liquid Membrane Electrodes

12. Solid State Membrane Electrodes
Ag wire
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+
Filling
solution
with fixed
[Cl-] and
cation that
electrode
responds to
Ag/AgCl
Solid state membrane
(must be ionic conductor)

13. Solid state electrodes

14. 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)

16. Why Electrons Transfer
Reduction
Oxidation EF
Eredox
Eredox E E EF
Net flow of electrons from M to solute
Ef more negative than Eredox
more cathodic
more reducing
Net flow of electrons from solute to M
Ef more positive than Eredox
more anodic
more oxidizing

17. 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)

18. 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 electrode
Difficult to get perfect reproducibility with stirring, better to move the electrode
Convection is considerably more efficient than diffusion or migration = higher currents for a given concentration = greater analytical sensitivity

19. Nernst-Planck Equation
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)

20. Diffusion I = nFAJ Fick’s 1st Law
Solving Fick’s Laws for particular applications like electrochemistry involves establishing Initial Conditions and Boundary Conditions

21. Simplest Experiment Chronoamperometry

22. Simulation

23. Recall-Double layer

24. Double-Layer charging
Charging/discharging a capacitor upon application of a potential step
Itotal = Ic + IF

25. Working electrode choice
Depends upon potential window desired
Overpotential
Stability of material
Conductivity
contamination

26. electron transfer to the electroactive species.
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.

27. Dropping Mercury Electrode
Renewable surface
Potential window expanded for reduction (high overpotential for proton reduction at mercury)

28. 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 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

29. 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.

32. 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

33. 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.

34. NPP advantage IL = nFAD1/2c/(ptm)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.

35. Differential pulse voltammetry

36. 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

37. DPP vs DCP Ep ~ E1/2 (Ep= E1/2±DE/2) s = exp[(nF/RT)(DE/2)]
where DE=pulse amplitude
s = exp[(nF/RT)(DE/2)]
Resolution depends on DE
W1/2 = 3.52RT/nF when DE0
Improved response
because charging current
is subtracted and adsorptive
effects are discriminated against.
l.o.d M

38. Resolution

39. Square wave voltammetry

40. SWV

41. SWV Response

42. 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

43. 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.

44. Deposition potential

45. ASV

46. ASV or CSV

47. Adsorptive Stripping Voltammetry
Use a chelating ligand that adsorbs to the WE.
Can detect by redox process of metal or ligand.

50. Multi-Element

51. Standard Addition