Chemical and electrochemical behaviour of lead ions in the ZnCl 22NaCl mixture at 450 °C

Abstract

The stability of lead chloride and oxide as well as the electrochemical behaviour of Pb 2+ ions in the ZnCl 22NaCl mixture at 450 °C has been studied using the techniques of potentiometry, cyclic voltammetry, chronoamperometry and chronopotentiometry. The lead oxidation states 0 and II have been shown to exist in this melt, showing that Pb(IV) is a powerful oxidizing agent which oxidizes the chloride ion of the melt according to the reaction: Pb( IV) + 2 Cl − ↔ Pb( II) + Cl 2( g) The standard potential of the redox couple Pb(II) Pb(0) has been determined by potentiometry ( E 0 Pb(II) Pb(0) = −1.383 ± 0.01 rmV) (vs. Cl 2(1 atm) Cl − ). It was shown that PbO is not stable and enters acid-base reaction with the melt according to the reaction: Zn( II) + PbO( s) ↔ ZnO( s) + Pb( II) The solubility products of lead oxide have been determined ( pKs = − 2.6 ± 0.1 on a molality scale). All these results have enabled the construction of the E- pO 2− equilibrium diagram. Using a tungsten electrode, it has been observed that the Pb(II)//Pb(0) exchange is quasi-reversible; log k s 0 and α values for this reaction are (−4.3 ± 0.1) and (0.48 ± 0.06) respectively. Mass transport towards the electrode is a simple diffusion process, the average diffusion coefficient D pb( II) is (7.5 ± 0.7) × 10 −6, and the thickness of the diffusion layer δ by thermal convection is (7.5 ± 0.2) × 10 −3cm.