CHEMICAL AND ELECTROCHEMICAL-BEHAVIOR OF LEAD IONS IN THE ZNCL2-2NACLMIXTURE AT 450-DEGREES-C

The stability of lead chloride and oxide as well as the electrochemica l behaviour of Pb2+ ions in the ZnCl2-2NaCl mixture at 450 degrees C h as been studied using the techniques of potentiometry, cyclic voltamme try, chronoamperometry and chronopotentiometry. The lead oxidation sta tes 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) + 2Cl(-) <-> Pb(II) + Cl-2(g) The standard potential of the redox couple Pb(II)/Pb(0) has been deter mined by potentiometry (E(0)Pb(II)/Pb(0) = -1.383 +/- 0.01 V (vs. Cl-2 (1 atm)/Cl-). It was shown that PbO is not stable and enters into an a cid-base reaction with the melt according to the reaction: Zn(II) + Pb O(s) <-> ZnO(s)+ Pb(II) The solubility products of lead oxide have bee n determined (pKs = - 2.6 +/- 0.1 on a molality scale). All these resu lts have enabled the construction of the E-pO(2-) equilibrium diagram. Using a tungsten electrode, it has been observed that the Pb(lI)//Pb( 0) exchange is quasi-reversible; log k(s)(0) and alpha values for this reaction are (-4.3 +/- 0.1) and (0.48 +/- 0.06) respectively. Mass tr ansport towards the electrode is a simple diffusion process, the avera ge diffusion coefficient D-Pb(II) is (7.5 +/- 0.7) x 10(-6), and the t hickness of the diffusion layer delta by thermal convection is (7.5 +/ - 0.2) x 10(-3) cm.