Transcript chem341_17_chp24_slides

Current-voltage relationship
E appl  ( E r  E l )  ( rc   rk )  ( lc   lk )  IR
E appl : applied
voltage
from exteranl source
 rc ( lc ) : overltages
from concentrat ion polarizati
 rk ( lk ) : overltages
from
kinetic
polarizati
on at rght (left) electrode
on at right (left) electrode
1.1 Electrolysis at constant potential
Cu
2
 H 2 O  Cu ( s ) 
cathode : Cu
anode :
1
2
2
1
2
O2 ( g )  2 H

 2 e  Cu ( s )
O2 ( g )  2 H



E  0 . 34 V
0
 2 e  H 2 O E  1 . 23 V
E cell  0 . 34  1 . 23   0 . 94 V
0
Fig. 24-1c (p.698) Change in (a) current and (b) potential
during deposition of Cu2+.
Eappl
E – Eref =
constant
Fig. 24-3 (p.701) Apparatus for controlled-potential electrolysis.
Faraday’s Law (relating the number of moles of the analyte nA to the charge
nA 
mA 
Q
nF
M AQ
nF
2.1 Controlled-potential coulometry
Q 

t
idt
for a variable
current
0
Can also be used for determination of organic compounds (if they can be
reduced at mercury cathode whose potential is suitably controlled)
Fig. 24-6 (p.705) Schematic of a system for controlled-potential
coulometry
2.2 Coulometric titration – controlled current coulometry
Q  I  t for a constant
current
Notes:
1. current efficiency = 100%
2. need a end-point detection (color changes, potentiometric, photometric
measurement)
Karl Fisher determination of water
2I-  I2 + 2eI2 + SO2 + 2H2O 2 HI + H2SO4
2HI + H2O + SO2 + 3C5H5N  2(C5H5N+H)I- + C5H5N.SO3
C5H5N.SO3 + CH3OH  (C5H5N+H)O.SO2.OCH3
Fig. 24-8 (p.708) An
automated coulometric titrator.
Fig. 24-9 (p.709) A typical coulometric
titration cell