Potentiometry and Coulometry

1
Chapter 23 24

• Potentiometry and Coulometry
• CHM 411
• Spring 2013

2
I. Reference Electrode

• It is the ½ cell potential that is known as a
  constant and completely insensitive to the
  composition of the solution under study

3
Saturated calomel electrode
2 Hg2Cl2(s) 2e- ? 2Hg (l) 2Cl-
E (satd KCl) 0.241 V
A. Calomel Electrodes
4
Silver-Silver Chloride electrode
AgCl (s) e- ? Ag(s) Cl-
E (satd KCl) 0.179 V

• B. Ag/AgCl Electrodes

5
II. Membrane indicator electrodes

• ion selective electrodes

6
II. Membrane indicator electrodes

• pH electrodes

7
(No Transcript)
8
B. Liquid Membrane Electrode
Ca2 ion selective electrode
Ion exchanger in Ca2 electrode is calcium
didecylphosphate dissolved in diocylpphenylphosph
onate
(RO)2PO22Ca ? 2(RO)2PO2- Ca2 where R C10H21
9
C. Crystalline ion-selective electrode
Migration of F- through LaF3 doped with EuF2. A
neighboring F- can jump into the vacancies and
move the F- through the lattice and establishes
the potential difference.
10
D. Gas sensing probes
11
III. Coulometry

• Three electrochemical methods require no
  calibration against standards

12
II. Types of Coulometric Methods

• A. Potentiostatic

13
1. Operation of a cell at a fixed potential

• Simplest to perform, but has drawbacks

14
2. Current changes during electrolysis at
constant applied potential

• If potential is held at -2.5V the current is
  expected to decrease because of the depletion of
  the copper ions.

15
3. Potential changes during electrolysis
16
B. Coulometric titrations

• Employs a titrant that is electrolytically
  generated at a constant current

17
C. Constant Current Electrolysis

• The electrodeposition is carried out by
  maintaining a constant current