Chapter 24 Coulometry 1 Principles of electrolysis Current-voltage relationship 1.1 Electrolysis at constant potential

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

Containing more than one metal ions Example: [Cu2+] = 1.0 M, [Ag+] = 0.1 M in 0.05 M H2SO4

Notes: 1) For {univalent} [M+] decreases from 0.1 M to 10-6 M, Er changes by =0.30V -Separation requires 0.30-V difference 2) For {divalent} [M2+] decreases from 0.1 M to 10-6 M, Er changes by 0.0592/2log(1/10-6)-0.0592/2log(1/0.1)=0.15V -Selectivity: separate any elements doesn’t deposit within this 0.15V potential range; 3) For {trivalent} [M3+] decreases from 0.1M to 10-6 M, Er changes by 0.10V

4) In electrolysis, even Eappl holds constant, both Er and El are changing, and I decreases because of concentration polarization.  result in crude separation. Controlled potential electrolysis: measure the potential of working electrode against a reference electrode whose potential is known and holds constant. Eappl is adjusted such the potential difference between working electrode and references hold constant. Working electrode: place where redox occurs Reference electrode: constant potential reference Counter electrode: inert material (Hg, Pt) plays no part in redox but completes circuit Supporting electrolyte: alkali metal salt does not react with electrodes but has conductivity

Eappl E – Eref = constant Fig. 24-3 (p.701) Apparatus for controlled-potential electrolysis.

2 Coulometric methods 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)

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 water 2I-  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

Fig. 24-8 (p.708) An automated coulometric titrator. Fig. 24-9 (p.709) A typical coulometric titration cell