The electrolysis of an acidic NaCl solution with a graphite anode : III. mechanism of chlorine evolution

The mechanism of Cl evolution on a graphite anode in acid chloride soln. has been investigated. Chloride ions are discharged according to the Volmer reaction Cl- -> Clad + e. Mol. Cl is formed by discharge of chloride ions on Cl atoms according to the Heyrovsky reaction Cl- + Clad -> Cl2 + e. For an aged electrode the Heyrovsky reaction is the rate-detg. step, whereas for a new electrode both the Volmer and the Heyrovsky reactions det. the relation between potential and c.d. For Cl evolution on an aged electrode in a soln. of 4M NaCl and 1M HCl, the activation energy of the Heyrovsky reaction and also of the Cl evolution at the reversible potential at 1 atm Cl is 8.4 +- 0.3 Kcal/mole mol. Cl; the transfer coeff. of the Heyrovsky reaction is 0.5; the exchange c.d. of the Heyrovsky reaction at 25 Deg is 0.009 +- 0.002 mA/cm2 real surface area; the exchange c.d. of the Volmer reaction at 25 Deg is 0.18 +- 0.04 mA/cm2 real surface area; and the degree of coverage with at. Cl is 0.05. For the Cl evolution on a new electrode, the ratio between the exchange c.d. of the Volmer reaction and that of the Heyrovsky reaction lies between 1 and 10; and the total exchange c.d. at 25 Deg is 0.12 mA/cm2 real surface area. [on SciFinder (R)]