The Temperature Dependence of the Isotope Effect for Electromigration of Lithium Ions in Molten Lithium Nitrate

Abstract

The relative difference (Δb/b) between the internal electromigration mobilities of 6Li and 7Li in molten LiNO3 has been measured over the range 313° to 488 0C. The mass effect,μ =(Δb/b) /(Δm/m), is found to be μ = 0.0845+ (0.00002 ±0.00002) (t-300) where t is the temperature in °C and the quoted error is the standard deviation. There is thus in this case no significant influence of the temperature upon the mass effect. Some previous investiga­tions of molten LiNO3 gave smaller values for the mass effect. The reasons for these apparent dis­crepancies are discussed. A slight, but significant, anomalous enrichment of the light isotope is ob­tained in the middle of the cell, as has been previously observed in experiments with KNO3 . The mass effects have now been measured for all five alkali ions in the pure molten nitrates. For a corresponding temperature chosen 5% above the melting point none of the experimental values differ more than 14% from the relation μ+ =-0.0848 (1+m +/46) -1.The obtained relations are interpreted in terms of cooperative motion of the ions in a molten salt.