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Current-voltage relationship
1.1 Electrolysis at constant potential
electrode (left)right at on polarizati kinetic from overltages :)(electrode (left)rght at on polarizatiion concentrat from overltages :)(
source exteranl from voltageapplied :
)()()(
rk
rc
lk
lc
appl
lklcrkrclrappl
E
IREEE
V 94.023.134.0E
V 23.1E 22)(21 :
34.0E )(2Cu :
2)(21)(
cell
022
02
222
OHeHgOanode
VsCuecathode
HgOsCuOHCu
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Fig. 24-1c (p.698) Change in (a) current and (b) potential during deposition of Cu2+.
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Fig. 24-3 (p.701) Apparatus for controlled-potential electrolysis.
E – Eref = constant
Eappl
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nFQMm
nFQn
AA
A
Faraday’s Law (relating the number of moles of the analyte nA to the charge
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2.1 Controlled-potential coulometry
Can also be used for determination of organic compounds (if they can be reduced at mercury cathode whose potential is suitably controlled)
tidtQ
0current variableafor
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Fig. 24-6 (p.705) Schematic of a system for controlled-potential coulometry
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2.2 Coulometric titration – controlled current coulometry
Notes:1. current efficiency = 100%2. need a end-point detection (color changes, potentiometric, photometric measurement)
Karl Fisher determination of water2I- I2 + 2e-
I2 + SO2 + 2H2O 2 HI + H2SO4
2HI + H2O + SO2 + 3C5H5N 2(C5H5N+H)I- + C5H5N.SO3
C5H5N.SO3 + CH3OH (C5H5N+H)O.SO2.OCH3
currentconstant afor tIQ
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Fig. 24-8 (p.708) An automated coulometric titrator.
Fig. 24-9 (p.709) A typical coulometric titration cell