Electromigration in Molten and Solid Binary Sulfate Mixtures: Relative Cation Mobilities and Transport Numbers

Abstract

The electrolytic displacement of the two cations relative to each other has been studied in solid (Li, Ag)2SO4 (93 eq. % Li) at 700 °C, solid (Li, K)2SO4 (91% Li) at 575 °C and molten (Li, K)2SO4 (41 — 90% Li). For melts containing more than about 52% Li the Li ions have a higher mobility (at about 740 °C) than the K ions, while the latter have the highest mobility in mixtures where K is the more abundant cation. The relative difference (Δb/b) between the mobilities of the two cations exceeds 15% already at concentrations some 8% off from the equimobility concentration. For the eutectic composition, 80% Li, Δb/b was about 60% independent of the temperature (590 — 835 °C). For mixtures containing 90% Li it was established that Δb/b changes its sign at the melting point. Thus, the mobility of the Li ion was of the order of half the mobility of the other ion in the solid systems, while it was about twice as large in the melt. The cation transport numbers (relative to the anion) were calculated for all experiments ,and for the melts also the difference between the cation mobilities. An estimation is made of the obtainable accuracy of electromigration experiments. The experiments with melts were analysed for isotope effects. The light isotopes were always enriched towards the cathode, independent of whether Li or K had the higher mobility. The magnitude of the isotope effects shows a concentration dependence, i. e. the mass effect (relative difference in mobility divided by relative difference in mass) tends to increase when an ion is dilute, as previously found for other systems. Thus for the melts with 80% Li, the mass effect is higher for K than for Li.