Reduction of W(VI) with the Automated Rapid Chemistry Apparatus ARCA II

E. Strub 1, W. Brüchle 2, H.W. Gäggeler 3,4, M. Gärtner 4, E. Jäger 2, D. Jost 3, J.V. Kratz 1, M. Schädel 2, B. Schausten 2, E. Schimpf 2, A. Türler 3

1 Institut für Kernchemie, Universität Mainz, 2 Gesellschaft für Schwerionenforschung, Darmstadt, 3 PSI, Villigen, 4 Universität Bern

W as a homologue of seaborgium (Sg) is used to develop a reduction experiment for Sg. Previous studies [1] with carrier free amounts of W had shown that it is possible to reduce W(VI) to W(III) with a solution of Sm2+. While W(III) sticks to a cation exchange column in diluted HCl/HF as cation, W(VI) forms anionic oxyfluoro complexes and runs through the column. In a second step, W(III) is stripped from the column. In previous works we used 4 M HCl/0.01 M HF solutions for this purpose. Off-line experiments with reducing agents weaker than Sm2+ showed quite low reaction rates. Hence, although the reduction of dissolved W(VI) with Sm2+ seemed to be quantitative even with short reaction times, it was to be suspected that the reaction rate could be too low for reaction times of about 1 s. On the other hand, the reduction might be assisted by the fact that on-line produced W would be dissolved directly into the reducing agent, while off-line experiments always involved the mixing of solutions. So, the main goal of the W experiments was to check whether under on-line conditions with ARCA the Sm2+ system would be as efficient in reducing W quantitatively as under off-line conditions. W (mainly 170 W) produced on-line at the Philips Cyclotron of the Paul Scherrer Institut was transported to ARCA II with a He/KCl gas jet system. After a collection time of 120 s, the KCl spot was dissolved in 250 l of a mixture of HCl, dilute HF and 0.005 M SmCl 2 solution. The solution was fed onto a CIX column (Aminex A4) at a flow rate of 1ml/min. As described above, W(VI) runs through this column and is collected as the first fraction. W(III) sticks to the column and is stripped from the column using a solution of 0.1 M HCl and 15% H2O2 as a second fraction. By this solution, W(III) is reoxidized to W(VI) and runs through the column, while Sm sticks to the column in both of its possible oxidation states. The more simple stripping with 4 M HCl/0.01 M HF would also elute the Sm from the column. In a Sg experiment, this would disturb the measurement by producing a thick sample. The -activities of the two fractions were determined. For the feeding solution, different concentrations of HCl and HF were used to optimize the separation of the different oxidation states of W. Further, for each concentration there was taken a reference value without reducing agent (see Table).

solvent HCl/HFreference without Sm2+ `reduced' fractionreduced
1M/0.001M 6.2 % 18.3 % 12.1 %
0.5M/0.0005M 13.1 % 23.3 % 10.2 %
0.05M/0.0005M 15.6 % 26.8 % 11.2 %
0.5M/0.001M 16.5 % 19.0 % 2.5 %
0.05M/0.001M 6.8 % 16.5 % 9.7 %

The percentage of reduction was not much more than 10 % at most HCl concentrations. Thus, the reaction rate seems to be too slow. Recent off-line experiments lead to a new concept using metallic Al as reducing agent. This system has the following advantages: The amount of Al 3+ dissolved is negligible and does not disturb the measurement. The Al column can be heated much more easily than a solution; higher temperatures generally lead to higher reaction rates. Off-line experiments with reaction times of less than 0.5 s (contact time with Al) yielded about 80% of reduced species. For this experiment, an anion exchange column is used with 0.1M HF/0.1M HCl (stripping with 4M HCl/0.01M HF) where the reduced species run through the column while the W(VI) sticks to the column. This should allow for a fast access to the reduced species in a future Sg experiment.

Fig.1: Temperature dependence of the reduction of W(VI) with Al. Reduced and unreduced species are separated by an anion exchange column (Dowex AG1x8)

Reference

[1] E. Strub, Diplomarbeit, Universität Mainz, 1997


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Februar 1998 by Erik Strub.