Measurement of the conductivity of iodides in liquid and solid iodine

The electrolytic conductivity of solutions of iodides in liquid and solid iodine is measured as a function of concentration, temperature and the nature of the cation. In the liquid the molar conductivity is comparable with that in aqueous solutions and increases with concentration in the range of 0.01 M to 1.0 M, due to the formation of aggregates which are stabilized by the charge on the cation. it is independent of temperature, lending support to a non-Stokesian mechanism of conductivity. In the solid phase, below the melting point of the eutectic mixture of iodine with an alkali metal iodide, the molar conductivity is of the order of 0.01 S cm² mole^-1, corresponding to an effective diffusion coefficient of 10^-9 cm² s^-1. This is too high for diffusion through the lattice, and may be due either to movement of the ions along grain boundaries or to a hopping mechanism of conductivity in the solid. Between the melting point of the solution and that of the eutectic, solid iodine and liquid solution of iodide in iodine are at equilibrium. Conductivity occurs in this region through the liquid phase which wets the solid and provides a tortuous path for the ions to move, although the volume of the liquid may be less than a few percent of the total volume. The conductivity is relatively high for the low dielectric constant of the medium (the specific conductivity of all salts varies from 5 × 10^-3 to 5 mScm^-1 for the concentration range of 10^-2 to 10 mol%). The two-phase region may be very useful in further studies in electrochemistry, since it allows convenient electrochemical measurements to be conducted at temperatures well below the melting point of the solvent.