1. Analytical Chemistry II Spring 2017
Dr Violeta Jevtovic

2. Introduction to Electrochemical Methods of Analysis

3. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
Oxidation-Reduction (Redox) reactions involve the transfer of electrons (e-)
Results in electric current (amps) and potential (volts) generated in an electrochemical cell
OIL RIG – Oxidation Is Loss e-, Reduction Is Gain e-
Oxidation occurs at the anode (-ve terminal)
Electrons flow from anode to cathode (+ve terminal)

4. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
e.g. lead-acid battery used in cars:
Pb(s) + SO42-(aq) ® PbSO4(s) + 2e- [OX]
PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- ® PbSO4(s) + 2H2O(l) [RED]
Net: Pb(s) + PbO2(s) + 2H2SO4 ⇌ 2PbSO4(aq) + 2H2O(l)
Chemical energy is converted into electrical energy, each cell produces 2 V, six cells will give familiar 12 V battery

5. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
What are the oxidation numbers of lead in the net reaction?
Pb(s) + SO42-(aq) ® PbSO4(s) + 2e- [OX]
PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- ® PbSO4(s) + 2H2O(l) [RED]
Pb(s) + PbO2(s) + 2H2SO4 ⇌ 2PbSO4(aq) + 2H2O(l)

6. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
e.g the Daniell cell
Zn(s) ⇌ Zn2+(aq) + 2e- (anode)
Cu2+(aq) + 2e- ⇌ Cu(s) (cathode)
Cu2+(aq) + Zn(s) ⇌ Cu(s) + Zn2+(aq)
2 half cells: Zinc electrode in ZnSO4 solution
Copper electrode in CuSO4 solution
Generates potential of 1.1 V

7. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
Circuit must be completed with external metal wire and a salt bridge
e- flow through the metal wire from one electrode to the other (anode to cathode)
Salt bridge allows charge (ions) to transfer through the solutions (completes circuit), prevents the complete mixing of the solutions, maintains electro-neutrality
Sodium sulfate, sodium chloride, potassium nitrate or similar

8. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
As the oxidation-reduction reaction occurs, the anode becomes more positive as Zn2+ is produced whilst the cathode becomes more negative as Cu2+ is removed from solution
Salt bridge allows for the migration of ions in both directions to maintain neutrality

9. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells → →
Spontaneous (used in potentiometric analysis)
Non-spontaneous Used in voltammetric analysis

10. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
Potential energy between two electrodes is called an electrode potential or electromotive force (emf) measured in volts (V)
emf depends on concentration of REDOX species, described by the Nerst equation:
E = Eo – RT x ln [OX] substituting for R, T, F and
nF [RED] converting to log10 (x 2.303)
E = Eo – log [OX]
n [RED]
Where E = electrode potential (V), E0 = standard potential for the reaction under standard conditions, R = gas constant ( J/K.mol), T = temperature (K), n = no. moles of charge transferred, F = Faraday constant (96,485 C/mol)

11. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
For the Daniell cell:
E = Eo – log ([Zn2+]/ [Cu2+])
E0 can be calculated from tables
e.g the Daniell cell
Zn(s) ⇌ Zn2+(aq) + 2e- E0 = V
Cu2+(aq) + 2e- ⇌ Cu(s) E0 = V
Cu2+(aq) + Zn(s) ⇌ Cu(s) + Zn2+(aq) E0overall = E0OX + E0RED = V
+ve E value indicates a spontaneous reaction

12. Tro, Chemistry: A Molecular Approach

13. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
Potential energy between two electrodes is called an electrode potential or electromotive force (emf) measured in volts (V)
E = Eo – log [OX]
n [RED]
What is the emf of a Daniel cell when [OX] = [RED]?
log (1) = 0
E = Eo – x log (1) = E0 - 0 2
E = E0 = 1.10 V

14. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
Notation: shorthand description of Voltaic cell
electrode | electrolyte || electrolyte | electrode
oxidation half-cell on left, reduction half-cell on the right
single | = phase barrier
if multiple electrolytes in same phase, a comma is used rather than |
double line || = salt bridge

15. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
What would be the notation for the following cell?
Zn | Zn2+ || Cu | Cu2+

16. Electrochemical Methods Introduction to Electrochemical Theories
Review of Redox Chemistry and Electrochemical Cells
Daniell Cell Demo

17. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Potentiometry
Coulometry
Voltammetry
Conductometry

18. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Potentiometry
Two types:
Indirect potentiometry - titration
Direct potentiometry
Indirect:
In a potentiometric titration an abrupt change in potential (E) is used for the end-point
Potentiometric titrations can be automated and used where color-changing indicators are difficult to see
Direct:
Measurement of analyte concentration according to Nernst equation using potential (E)
Potential of indicator electrode w.r.t. a reference electrode
e.g. pH meter

19. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Coulometry
Measures quantity of electricity or the charge generated by a reaction at an electrode
Voltage is applied to drive the nonspontaneous redox reaction (electrolytic cell)
Charge (Q) is related to current (I) according to:
Q = It (constant current)
Q = ∫ I dt (controlled potential)
Where I = current (A) and t = time (s).

20. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Coulometry
Constant current techniques:
‘Colulometric titrations’
Reagent is generated an one electrode, reacts stoichiometrically with analyte (titrant)
Controlled potential techniques:
Current resulting from reaction of analyte is monitored w.r.t. time
Equation relating Q to quantity of analyte (m) is Faraday’s law of electrolysis:
m = Q M
F n
Where F = Faraday constant (96,485 C/mol e-), M = molec. mass, and n = no. moles e- per mole analyte

21. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Coulometry
Constant current techniques:
Controlled potential techniques:
e.g. 0.5 A was used to deposit Cu2+ in 10 mins in the Daniell Cell, the amount of copper in solution
m = Q M
F n
m = Q M = (0.5 A x 10 mins x 60s/min) x 63.5 g/mol
F n ,485 C/mol e- x 2 mol e-/mol Cu
m = = 9.87 x 10-2 g C

22. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Voltammetry
Current (I) is measured as a function of changing potential (E)
Electrolytic cell consisting of 3 electrodes:
Micro indicator electrode
Reference electrode
Auxillary counter electrode

23. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Voltammetry
Changing potential (E) is applied on the indicator electrode (working electrode) to drive a nonspontaneous redox reaction
Counter electrode serves to conduct electricity between the two electrodes
Reference electrode has a constant potential throughout

24. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Conductometry
Measures conductivity – ability of a solution to carry electric current
Conductivity cell has two electrode plates placed in a solution
Ohm’s law:
R = V/I
Where R = resistance (ohms), V = voltage (volts), I = current (amps)
e.g. Chloride determination using conductivity meter
Mohr method uses potassium chromate as indicator with a difficult to see end-point, conductometric method is faster and more accurate

25. Electrochemical Methods Introduction to Electrochemical Theories
General Principles of Electroanalytical Methods
Conductometry
Conductance (G), ohms-1:
G = 1/R (units ohms-1 or mho or Siemens)
Conductivity (k):
k = GK = GL/A = L/AR (units ohms-1 cm-1, or S cm-1)
Where K (cm-1) = cell constant (L/A)

26. Electrochemical Methods Introduction to Electrochemical Theories
Types of Electrodes and Notations for Electrochemical Cells
Electrodes are important for electrochemical analysis!
Reference electrodes
Indicator electrodes

27. Electrochemical Methods Introduction to Electrochemical Theories
Types of Electrodes and Notations for Electrochemical Cells
Reference electrodes
Provide constant potential – not affected by sample composition
Follows Nernst equation
Two common reference electrodes:
1. Saturated calomel electrode (SCE)
Mercury in contact with a solution saturated with mercury chloride || KCl (saturated), Hg2Cl2(s)|Hg(s) (E0 = vs. SHE at 25 ºC)
2. Silver wire coated with silver chloride in a saturated KCl solution
|| KCl (saturated), AgCl(s)|Ag(s) (E0 = vs. SHE at 25 ºC)
E0 above is calculated based on reference to the standard hydrogen electrode (SHE)

28. Electrochemical Methods Introduction to Electrochemical Theories
Types of Electrodes and Notations for Electrochemical Cells
Indicator electrodes:
Respond to changing analyte concentrations
Two types of indicator electrodes:
1. Metallic electrodes – metal/metal ion, metal/metal salt/anion no longer used
2. Membrane electrodes (AKA ion-selective electrodes) – glass electrodes, polymer membrane electrodes (liquid-phase), crystalline membrane electrodes (solid-state)
ISE has both reference and indicator electrodes