Approaches to estimate the ionic radius of hydrated Po

Due to the relativistic stabilisation of 6p1/2 electrons polonium – usually present in the 4+ oxidation state – can be reduced to the 2+ oxidation state. This cation has – outside it's filled [Xe]4f5d electron shells two electron pairs: 6s and 6p1/2. No information is available about the ionic radius of Po and about it's coordination number of water molecules in aqueous solutions. The measurement of the ionic radius could be one first step towards a better understanding of the contribution of Po electron orbitals to the chemical bonding. The goal of our studies was to estimate the ionic radius of hydrated Po from a comparison of the distribution coefficients (Kd) for Po with Kd values of divalent metal cations of the second group of the Periodic Table (Ca, Sr and Ba). This is based upon a well established linear correlation between reciprocal ionic radii (known for Ca, Sr and Ba) and Kd values. This correlation is valid for ions with similar coordination numbers. Kd values can be obtained from the maxima of elution peaks in liquid-chromatography experiments. Earlier a cation exchange study of Po in acid solutions (HClO4, H2SO4, H3PO4, CH3COOH and oxalic acid) has been reported [1]. We eluted polonium from the cation exchange resin Dowex 50W-X8, 200-400 mesh, with 3 M HClO4 and 3.3 M CF3SO3H (triflic acid), both in SO2 water solution. Perchloric acid and triflic acid were used because of their non-complexing properties for metal cations [2]. In order to reduce Po(IV) to Po(II) SO2 was applied. It is commonly known that sulfur dioxide and hydrazine reduce polonium to oxidation state +2 in acidic solutions [3]. As we were using low-level radioactive tracer solutions of Po the measurement of α-activity from these samples was mandatory. Therefore, we were only able to use SO2 as a reducing agent. Using hydrazine was impossible due to white salt residues which appeared during the evaporation step for α-sample preparation. The used radiotracers Ca (40 Bq/mg) and Sr (3.3 kBq/mg) were produced at the Mainz TRIGA reactor. While the Ca tracer had a very low specific activity, commercially available Ba was carrier free. Tracer solutions were prepared by dissolving the irradiated oxides in perchloric acid and aliquots were loaded onto the column (46 mm length and 3.2 mm inner diameter). Elutions were performed at a rate of about 3.3 mL/min. From the maxima of the elution position in 3M HClO4 we determined Kd values of 2, 7 and 24 for Ca, Sr, and Ba, respectively. The corresponding literature [4] values are 10.4, 13.4 and 25.1. Presumably, lower Kd values for Ca and Sr in comparison with values from [4] result from the use of relatively large amounts of carrier material in our experiment. It should also be noted here that our Ca and Sr elution curves were rather wide and exhibited considerable tailings. For the elution with 3.3 M CF3SO3H the Kd values were 11 for Sr and 37 for Ba. Then we studied the behaviour of polonium during the elution using 3 M HClO4 with and without SO2. The elution curves for both solutions were similar. Peaks were very broad with maxima appearing only after 65-75 mL giving incomprehensibly large Kd values (~ 450-550). As an additional surprise, we measured a (too) large elution maximum of about 25 mL, corresponding to a Kd of ~ 150, with 3.3 M CF3SO3H in SO2 water. One would not expect a large or any difference in the elution of Po with 3.3 M CF3SO3H and 3 M HClO4. We interpret the large Kd values for polonium in both systems such that we were not able to reduce polonium effectively to the +2 oxidation state by SO2. The observation of the different elution position in the two acids can possibly be explained by the oxidizing properties of HClO4. In perchloric acid solution we most likely could not reduce Po(IV) to the lower oxidation states at all. The next step in these experiments will be the application of hydrazine as a reductant when using γemitting Po, or liquid scintillation counting to measure α-active Po in solution.