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/HF	reference without Sm2+	`reduced' fraction	reduced
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 %

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