The stability of lead chloride and oxide as well as the electrochemical behaviour of Pb2+ ions in the ZnCI,-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) + 2c1-u Pb(I1) + Cl,(g) The standard potential of the redox couple Pb(II)/Pt$O) has been determined by potentiometry (E'Pb(II)/ Pb(0) = -1.383 f 0.01 V (vs. Cl,(l atm)/Cl-).
It was shown that PbO is not stable and enters into an acid-base reaction with the melt according to the reaction :
The solubility products of lead oxide have been determined (pKs = -2.6 k 0.1 on a molality scale). All these results have enabled the construction of the E-pO*-equilibrium diagram. Using a tungsten electrode, it has been observed that the Pb(II)//Pb(O) exchange is quasi-reversible; log kf and a values for this reaction are (-4.3 k 0.1) and (0.48 k 0.06) respectively. Mass transport towards the electrode is a simple diffusion process, the average diffusion coefficient D,,,,,, is (7.5 k 0.7) x 10e6, and the thickness of the diffusion layer 6 by thermal convection is (7.5 k 0.2) x lo-3 cm.