文档介绍：Electroanalytical Chemistry
Lecture 3: Electrolyte - the Unassuming and Unappreciated Electrochemical Workhorse
Junction Potentials
Whenever we change electrolyte or solvent there is a cost in that we create a double layer (electrode) which has a small but often measureable potential
We call this potential a junction potential
General - Junction Potentials
To solve this we must know 3 things:
How the concentration of each ion changes
How the ion activity varies with concentration
How the transport number varies with the concentration of the ions
The Henderson Equation
We cannot solve analytically so we make 2 simplifying assumptions:
concentration of each ion varies linearly from region 1 to region 2 (1)
i = ci, ., i 1 for each ion (2)
Then we obtain the Henderson equation:
Classification of Junction Potentials
3 Categories of Junction Potentials:
Type 1: 2 solutions of same electrolyte with different concentrations in contact
Type 2: 2 solutions of same concentrations of electrolytes in contact which share mon univalent (z=1) ion
Type 3: 2 different solutions containing different electrolytes and/or different concentrations in contact
Type I Liquid Junctions
For Type 1, potential, in V, given by:
Note: RT/F = V at 298 K
Checks:
When c2=c1, EJ = 0
When t+=t-, EJ = 0 (good electrolyte!)
Observations about Type I Liquid Junction Potentials
Consider:
If t+ > t- EJ _______
If c1 > c2 EJ _______
So, this tells us if we want to minimize EJ we must:
Keep c’s ___________________
Use electrolytes with t+______t-
EXAMPLE (B&F )
Ag/AgCl(s)/K+,Cl-(1 M)/ K+, Cl-( M)/AgCl(s)/Ag
Left (1), right (2) so c2 = and c1 =
t+ = /(+) =
EJ = - ((2 * )-1)log() = - (- ) (-1) = - V
Note: this represents one approach to measurement of junction potentials
M
KCl
M
KCl
K+
Cl-
Same junction -HCl not KCl: Ag/AgCl(s)/H+,Cl-(1 M)/ H+, Cl-( M)/AgCl(s)/Ag
Left (1), right (2) s