文档介绍：§5 Voltammetry
§5-1 Basic principle of polarograpy
Voltammetry
A group of analytical methods based on determining current flow – voltage curve during electrolysis
Several types of methods based on
Electrode type
How the potential is applied
How the current is measured.
When applied voltage reaches the position voltage of metal ion:
Reduction reaction on cathode:
Cd2+ + 2e- Cd
Oxidation reaction on anode:
2OH- -2e H2O + 1/2 O2
U外∝ i
Where U外 denotes applied potential, Ud is position potential, R is total resistance of electrolysis circuit, i is current through the circuit.
U外- Ud= iR
(Cd2+)
When following conditions are obeyed, U外– i graph is showed in following Figure
Lower current density during electrolysis
Adequately stirring to remove concentration gradient
No quantitative relationship between current (or potential) and concentration of interested ions
To find the analytical relationship between the current and the concentration of sought-for ions , following measurements are adopted:
●MICROPLATINUM ELECTRODE
Or DROPPING MERCURY ELECTRODE
( in mon use)
to insure high current density
● NO STIRRING
to insure enough high concentration difference between the electrode surface and the solution bulk
Then
E = Eo + ln CM
RT
nF
When the current flowed for only a short period through the electrode, ion concentration on the surface of the electrode reduce suddenly. The difference in ion concentrations between the surface and the bulk solution is equivalent to an electrochemical cell, which is called concentration polarization(浓差极化). thus, there is a voltage that is equivalent to this concentration change called concentration polarization potential(浓差电势)
When a microplatinum electrode or dropping mercury electrode is used and no stirring is carried out, concentration polarization takes place soon, following polarogram is obtained
Limiting current
A
B
C
D
When applied voltage does not reach reduction voltage of metal ion, only a little current