Alpha spectroscopy was used to study the mechanism of electrostatic collection of thoron daughters from the air. The results indicate that ThA atoms are primarily collected on negatively charged surfaces. Atoms of ThB produced by decay of ThA already attached to the collecting surface are injected by recoil into the surface layers of solids and are suitable for use in radiotracer experiments.

The physicochemical state of trace cerium (III) in aqueous chloride solutions was studied using centrifugation, free-liquid electrophoresis and continuous capillary diffusion methods and 141 Ce. It has been found that cerium is present predominantly as Ce 3 + cations in 0.1 -0.001 Μ solutions of hydrochloric acid. Colloidal forms of carrier-free 141 Ce exist in the solutions of lower acidity (pH > 3). They are formed by adsorption of cerium ions'on colloidal impurities in the solutions or by coprecipitation of cerium with colloidal ferric hydroxide, present in the solutions as an impurity. Tervalent cerium does not undergo a hydrolysis at pH < 5.Knowledge of the physicochemical state of trace elements in aqueous solutions is of considerable importance for prediction and interpretation of behaviour of trace elements in many physicochemical and biochemical processes [1,2]. The physicochemical state of trace cerium in aqueous solutions is not sufficiently known. Particularly the hydrolysis and the colloidal behaviour of tervalent cerium are subject to controversies among various authors. Thus for instance STEPANOV and SHVEDOV [3], SHALINEC [4] and PUSTOVALOV et al. [5] claim that hydrolysis of trace cerium is significant already at pH > 2-3, whereas it should be negligible (less than 10%) at pH < 8 according to the hydrolytic constants published by other authors [6,7], Formation of colloidal forms of trace cerium(III) in aqueous solutions has been reported by several authors [8 -16]. However, the authors largely disagree as to the nature and the region of existence of the colloids. This is mainly due to different conditions and methods used for the studies and sometimes also due to inadequate interpretation of the data obtained. It seemed therefore useful to reinvestigate these problems using well defined conditions and combining several methods of study. Electrophoresis, centrifugation and capillary diffusion were used to elucidate the physicochemical state of trace cerium in aqueous solutions labelled with carrier-free 141 Ce. Experimental Reagents and solution preparationTriply distilled water and p. a. reagents were used throughout. All stock solutions were prepared by dissolving or diluting exact amounts of chemicals and were stored in clean polyethylene bottles. Stock solutions of sodium hydroxide were prepared from carbonate-free, 50% solution of NaOH. Preparations of 141 Ce (carrier-free, CeCl 3 in 1 Μ HCl), supplied by Zentralinstitut für Kernforschung, Rossendorf near Dresden, were used as radiotracers. They were suitably diluted with 0.01 Μ HCl. For some experiments, a standard inactive solution of CeCl 3 was added.The experimental solutions were prepared by gradually mixing the stock solutions of radiocerium, 0.01 Μ HCl and 0.01 Μ or 0.1 Μ NaOH so as to obtain the desired pH. All the mixing was carried out at room temperature in a glove box to avoid an uncontrolled contamination of the solutions with dust. Unless stated otherwise, the ionic strength of the solutions was 0.01 (HCl + NaCl + NaOH). The solu...

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