chapter 24 coulometry 1principles of electrolysis

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

Fig. 24-1c (p.698) Change in (a) current and (b) potential during deposition of Cu2+.

Fig. 24-3 (p.701) Apparatus for controlled-potential electrolysis.

E – Eref = constant

Eappl

nFQMm

nFQn

AA

A

Faraday’s Law (relating the number of moles of the analyte nA to the charge

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

Fig. 24-6 (p.705) Schematic of a system for controlled-potential coulometry

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

Fig. 24-8 (p.708) An automated coulometric titrator.

Fig. 24-9 (p.709) A typical coulometric titration cell