Abstract

The high-dilution diffusion of Tl + in molten NaN03 , KN03 and RbN03 has been measured with wave-front-shearing interferometry. For very low concentrations of T1N03 the results are Z) = 6.21X10-® exp [-(18400± 800)/R T] in NaN03 (316-387 °C), D = 5.39X10" 8 exp [-(18800 ±1400) /R T] in KN03 (341-408°C), D = 1.87X10-7 exp [ — (25800±2200) JR T] in RbN03 (327-419 °C), where D is the diffusion coefficient in m 2 s —1 , R the gas constant in J K —1 mol -1 and T the tem-perature in K. Comparisons are made with high-dilution diffusion of Na + , K + , Rb + , Cs + and Ag + ions in alkali nitrates, and it is found that both Ag + and Tl + diffuse faster than expected if the mass and the ionic radius were the sole parameters of importance, i. e. other parameters such as the polarizability also have an influence on the diffusion rate of an ion. For the system NaN03 — T1N03 , the inter-diffusion coefficient has also been measured at 88.8, 73.8, 51.5 and 31.6 mol % NaN03 .