Pressure dependence of electrical conductivity for fused mercuric halides

Abstract

The electrical conductivities of molten HgCl₂, HgBr₂ and HgI₂ have been measured over the pressure range 1–1000 bar at temperatures between 260°C and 370°C. In each case the conductivity increased with pressure; at 300°C, the specific conductance “activation volume”ΔV_κ=–RT(∂ ln x/∂P)_T, was –50 cm³ mol^–1 for HgCl₂, –65 cm³ mol^–1 for HgBr₂ and –102 cm³ mol^–1 for HgI₂. These increases in conductivity are attributed to the displacement of ionization equilibria of the type 2HgX₂⇌HgX⁺+HgX^–₃(X = halogen). The Born equation was used to estimate the volume changes associated with ionization, with results in reasonable accord with the experimental values. The pressure required to cause complete ionization is estimated to be in the range 5–10 kbar. For molten HgI₂, d.c. electrolysis experiments showed that the increase in conductivity with pressure was not caused by the onset of electronic conductivity. Although the conductivity at constant pressure decreases with temperature for HgI₂, and passes through a maximum with rising temperature for HgBr₂ and HgCl₂, the conductivity at constant density increases with temperature for all three compounds.