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The Temperature Dependence of the Isotope Effect for Electromigration of Rubidium Ions in Molten Rubidium Nitrate A R N O L D LUNDÉN, A L L A N FLOBERG, a n d R O N N Y M A T T S S O N
Department of Physics, Chalmers Institute of Technology, Göteborg, Sweden (Z. Naturforsch. 27 a, 1135—1138 [1972] ; received 1 May 1972)
The relative difference (Ablb) between the internal electromigration mobilities of 85 Rb and 87 Rb in molten RbNOj has been measured over the range 355 to 500 °C. The mass effect fi= (Ab/b)/(Am/m) has a complicated temperature dependence. Thus, the largest mass effect, - ^ = 0 . 0 6 1 , was obtained at 445 °C, while it is about 0.033 at 350 °C and 0.041 at 500 °C. A similar temperature dependence was found by SAITO et al. for / u i n pure N a N 0 3 , and for both ^Ußb and (x Na maxima have been found also in nitrate mixtures (in K N 0 3 — R b N 0 3 and NaN0 3 —KNO s ).
In molten salts (and in solid phases with a high cation mobility) isotope effects of electromigration should be correlated with the different modes of interaction between the ions of the melt, and a study of temperature dependence of isotope effects might give information on how the relative importance of different modes of interaction changes. A number of studies of temperature effects have been made. Thus, this has been done by KLEMM and different coworkers 1 for both cations and anions in molten halides, and both solid and molten sulfates have been studied by us 2 . Regarding nitrates, we have previously considered temperature effects in pure LiN0 3 , 1. c. 3 , and K N 0 3 , 1. c. 4 , while other groups have worked on pure N a N 0 3 , 1. c . 5 and on the equimolar NaN0 3 — KN0 3 mixture 6> 7 . In the present investigation we have studied pure RbN0 3 over a range above the temperature (350 °C) at which we previously had measured the isotope effect 8 . Reprint requests to Dr. A. LUNDÉN, Department of Physics, Chalmers Institute of Technology, Göteborg, Sweden.
Experimental The experimental procedure was essentially the same as previously 3' 4 , i. e. a Pyrex cell was used where a separation column (length 20 cm, inner diam. about 4 mm) separates a small compartment at the platinum anode from a large compartment on the cathode side. An excess of N0 2 gas was bubbled through the aluminium cathode. In our previous work on LiN0 3 the isotope abundance ratio for the initial salt and for the sample with the highest enrichment differed by 7 to 26%, and in the KN0 3 case this difference was 4 to 15%, while in the present work it is only some 2 to 3%, except for one experiment where it was 5%, cf the separation factors quoted in Table 1. For this reason the accuracy of the calculated mass effect (relative difference in isotope mobility, divided by the relative difference in mass) depends very much on how accurately the initial composition is determined. In order to compensate for the inaccuracy due to the low degree of isotope separation, we made three independent series of mass analyses for each experiment. (We have discussed these evaluation problems previously2.) The results are summarized in Table 1, where we also have included our early experiments 8 . Concerning these, it should be remembered that the temperature was at that time only determined by means of a single thermo-
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Table 1. Data of experiments, and results of the present experiments (No. 1 — 7) as well as of previous ones by us 8 (A and B) and by OKADA
No.
Exp.
Temp. °C
Duration hours
Transport charge Ah
Separation factor a
