H. W. KIRBY, G. D. NELSON', and J. H. PAYNE, J R . ~ Mound Laboratory, Monsanto Chemical Co., Miamisburg, Ohio
Reduction of Bismuth Chloride I N T E R E S T in polonium-210 has paralleled the interest in radioisotopes generally. Because of its relatively high specific activity TI,^ = 138 days) and its nearly monoenergetic alpha emission, it has found use as a calorimetric standard (4, as fuel in a thermal battery (6, 7), and in the preparation of alpha and neutron sources (7 7). The recovery of polonium from uranium ores is tedious and difficult because of its low occurrence and the abundance of interfering elements. A ton of uranium contains, a t most, approximately 0.1 mg. of polonium. However, when bismuth is irradiated with thermal neutrons, polonium is produced by the reaction ( 3 ) :
If 1 kg. of bismuth is irradiated in a thermal reactor having an average flux of 1013 neutrons/second/square cm., it will contain approximately 2 mg. of polonium after 138 days. Under these conditions, the concentration of polonium in bismuth is considerably greater than in uranium, but still extremely small. Even after longer irradiation, the magnitude of the problem is not appreciably changed. Fortunately, polonium is readily reduced to the metal by stannous chloride even in moderately strong hydrochloric acid (5). If conditions could be found in which bismuth would not also be reduced, a separation procedure based on the Present address, Monsanto Chemical
Co., Inorganic Division, St. Louis 4, Mo. 2
Present address, Monsanto Chemical
Co., Inorganic Research Department, Day-
ton, Ohio.
Figure 1. Equilibrium values at various temperatures and chloride concentrations
by Stannous Chloride
stannous chloride reduction of polonium would be feasible. The investigation described was carried out to determine to what extent and under what conditions bismuth chloride is reduced by stannous chloride in hydrochloric acid solution. Reagents and Equipment
Bismuth chloride (2.OM) stock solution was prepared by dissolving reagent grade bismuth trioxide in dilute hydrochloric acid. The acid concentration was maintained a t a level just high enough to prevent the precipitation of bismuth oxychloride. The solution was standardized by the precipitation and weighing of bismuth oxychloride (9). Bismuth powder was prepared from spectrographically pure bismuth metal by mechanical grinding or by sprayng from a metal-spraying gun. The powder used passed through a 250-mesh screen and collected on a 325-mesh screen. The oxide film was removed by washing the powder in hydrochloric acid and water until the wash water showed no further precipitation of bismuth oxychloride. The powder was dried in acetone and ether and stored in nitrogen. Stannous chloride (0.5M) stock solution was prepared by dissolving reagent grade stannous chloride dihydrate in 1 : 10 hydrochloric acid. The solution was boiled with pure granular tin metal, cooled, diluted to volume, and filtered through glass wool into a storage bottleburet assembly. Stannic chloride (0.5M) stock solution was prepared by dissolving reagent grade stannic chloride pentahydrate in 1 : 10 hydrochloric acid. Total tin in the stock solutions was determined by a modification of Scott's method (70). Standard potassium iodate was substituted for iodine, and the reduction was carried out in the presence of 0.5 gram of powdered antimony and 1 gram of granular lead. An inert atmosphere was maintained by a continuous stream of carbon dioxide or nitrogen. Stannous tin was determined by the same procedure, with the reduction step omitted. Potassium iodate (0.1N) standard solution was prepared from reagent grade potassium iodate and standardized against reagent grade tin metal (99.95'% pure), which was weighed, dissolved in hydrochloric acid, and analyzed for total tin as above. The standard solution showed no change after 3 months. All experiments were made in sealed glass ampoules fastened to the rotating drum of a constant temperature tumbler.
Experimental Procedure
Each sample was prepared in a 100-ml. volumetric flask; 50 ml. of the 2.OM bismuth chloride stock solution and various amounts of hydrochloric acid were added. The mixture was diluted with distilled water to approximately 95 ml. The solution was mixed periodically during the dilution to prevent precipitation of bismuth oxychloride. Nitrogen was passed through the mixture, and a measured amount of 0 . 5 M tin solution was added. The tin solution was, in most case, pure stannous or stannic chloride, but in one series, where approach to equilibrium was extremely slow, mixtures of stannous and stannic chloride of known composition were added. Except in preliminary experiments at room temperature, samples were prepared in complementary pairs, the two samples in each pair being similar except for the oxidation state of the added tin. The nitrogen bubbling tube was removed from the flask briefly while the solution was diluted to the mark. The tube was reinserted and the solution was kept under a nitrogen atmosphere while the tumbling flask was being prepared. Either 2 or 20 grams of bismuth metal powder was weighed into a dry tumbling flask and the flask atmosphere was swept out with nitrogen. The mixture in the volumetric flask was transferred as rapidly as possible to the tumbling flask. The nitrogen tube was removed, and the neck of the flask was sealed in a flame. The flask was attached to the drum of the tumbler, and the mixture was agitated continuously for a time depending upon chosen conditions. (The approach to equilibrium was more INITIAL TIN MNCENTRlTlON 0 0 0 2 4 s M P" U I I D
5n i l V i 01226 M 3" ( I l l 01226 M 5" 1,")
00245M
.
Figure 2. Effect of varying bismuth and tin concentrations at 40.1 "C. VOL. 48, NO. 10
OCTOBER 1956
1949
Table 1.
Experiments at 40.1 44.7 Days’ Agitation
C.-
(Initial bismuth concentration = 1 .OOM)
5.73 6.32 6.88 7.50 8.73 5.58 6.18 6.81 7.36 8.60
45.0 21.9 6.0 2.1 0.1 39.1 17.8 5.0 0.6 0.0
100 100 100 100 100 0.5 0.5 0.5 0.5 0.5
concentration in moles per liter. Total tin concentration = 0.0245 mole per liter. a
C1-
rapid a t higher temperatures, and agitation time was adjusted accordingly.) The mixtures were sampled periodicalIy. The flask was removed from the bath and immersed in cold water. It was opened by application of a hot glass rod to a scratch on the neck, and a sample was taken rapidly by a volumetric pipet. The sample was transferred immediately to the titrating flask, which had been prepared with 100 ml. of 6iV hydrochloric acid and starch solution swept free of oxygen by a stream of nitrogen introduced through a side arm. Titration for stannous chloride was then made as described under “Reagents.” While the titration was carried out, nitrogen was passed through the solution in the tumbling flask, so that another sample could be taken when necessary. Total chloride was determined by a modified Volhard titration (2), but 100 ml. of concentrated nitric acid was added to prevent precipitation of bismuth oxychloride during the titration. If analysis showed that additional agitation time was needed, the tumbling flask was resealed, and returned to the tumbler. Samples were taken and analyzed periodically, until it became evident that the approach to equilibrium had become so slow that little advantage could be gained by continuing agitation. Preliminary experiments carried out at room temperature showed that the reaction between bismuth metal and stannic chloride proceeded so slowly that the time required to reach equilibrium wouId be unreasonably long. For example, in 7.5N chloride, extrapolation of the data obtained during the first 27 days of agitation indicated that at least 90 days would be required before equilibrium. Similar results were obtained in the reaction between bismuth chloride and stannous chloride. Accordingly it was decided to approach equilibrium from both directions-to reduce bismuth chloride with stannous chloride on one hand and to oxidize bis-
1950
muth metal with stannic chloride on the other. Thus, determination of the final concentrations of the reactants would establish upper and lower limits for their concentrations at equilibrium. It was found in the preliminary series that the use of 20 instead of 2 grams of bismuth metal powder accelerated the reduction of stannic tin by a factor of 3 to 4. In all later experiments, the greater amount of bismuth was used. Typical of the data obtained from experiments carried out at different temperatures are those shown in Table I for the series at 40.1 ’ C. The equilibrium values estimated from these data are shown graphically in Figure 1. Figure 1 shows that, at a given chloride concentration, bismuth chloride is more readily reduced a t lower temperatures (higher per cent stannic tin indicates more reduction of bismuth); at a given temperature, the reduction of bismuth is favored at lower chloride concentrations. The effects of varying the bismuth concentration and of quintupling the tin concentration are shown in Figure 2. While a twofold change in the bismuth concentration has a significant effect on the equilibrium, the effect of variation in the tin concentration is small under the conditions of these experiments. Higher bismuth concentrations favor the reduction of bismuth, as expected from the mass action law. An increase in the bismuth concentration would decrease the concentration of “free” chloride because of the formation of a bismuth chloride complex, such as BiC14- (8). However, this mechanism cannot be used to explain the shift; the difference in chloride concentration between points on the 0.58M bismuth curve and those on the 1.00M bismuth curve is about 4.8 moles of chloride per mole of bismuth, whereas the shift from 1.00M bismuth to 1.92M bismuth corresponds to a change of 3.6 moles of chloride per mole of bismuth. An effort was made by the authors and others (7) to determine the activity coefficients of the concentrated solutions used in these experiments, but the results were inconsistent and could not be interpreted. Attempts to calculate an equilibrium constant without activity coefficients for the reaction 2BiCla-
+ 3Sn++
-t
2Bi0
+
3Sn+4f 8C1as well as for several other hypothetical reactions were also unsuccessful. This failure is not surprising, as bismuth forms not one but a number of complexes with chloride ions, the ionic species being dependent upon the hydrochloric acid concentration (8). That the reduction of bismuth by stannous chloride is extremely sensitive to the chloride concentration is probably
INDUSTRIAL AND ENGINEERING CHEMISTRY
due, at least in part, to changes in the nature of the bismuth complex. Probably, in the chloride concentrations used, more than one bismuth complex was present in any given solution. The small change in the equilibrium when the total tin concentration was increased by a factor of five (Figure 2) was probably the result of a corresponding change in the amount of free chloride. Presumably, increasing the tin concentration still further would shift the equilibrium more significantly, as the oxidation-reduction changes in the bismuth concentration would then become more apparent. However, the concentrations of stannous chloride used were more than adequate to reduce the anticipated amounts of polonium. Conclusions
1 The reduction of bismuth by stannous chloride is slow at all temperatures between 25’ and 80’ C. 2. The reduction of bismuth by stannous chloride is favored by low chloride concentration, by high bismuth concentration, and by low temperature. 3. The equilibrium is not significantly affected by the total tin concentration, if the tin concentration is not greater than one tenth that of the bismuth. 4. The wide variety of conditions under which bismuth will not be reduced by stannous chloride indicates that the separation of small quantities of polonium from large quantities of bismuth is feasible. a
Literature Cited
(1) Blanke, B. C., Hendricks, J., unpublished work. (2) Caldwell, J. R., Moyer, H. V., IND.ENG. CHEM.,ANAL. ED. 7, 38 (1935). ( 3 ) Colmer, F. C. W., Littler, D. J., Proc. Phys. Sac. London A63, 1175 (1951 \. (4) Eichelberger, J. F., Jordan, K. C., Orr, S. R., Parks, J. R., Phys. Rev. 96,719 (1954). ( 5 ) Guillot, M., Camp. rend. 190, 1553 (1930). ( 6 ) Hevd. J. 147.. Jordan. K. C.. U. S. Atomic ’Energy ’ Commission, Mound Laboratory Report MLM1060, Dec. 1, 1954. ( 7 ) Jordan, K. C., Birden, J. H., Ibid., MLM-984, June 2, 1954. (8) Noyes, A. A., Hall, F. W., Beattie, J. A., J . Am. Chem. Sur. 39, 2526 (1917). ( 9 ) Scott, W. W., “Standard Methods of Chemical Analvsis.” 5th ed.. vol. 1, p. 153, Van ’Nostrand, ’New York, 1939. (10) Zbid., p. 969. (11) U. S. Atomic Energy Cornmission Report TID-5087, July 1952.
.,
RECEIVED for review November 15, 1955 ACCEPTEDJune 13, 1956
Mound Laboratory is operated by Monsanto Chemical Co. for the United States Atomic Energy Commission under Contract Number AT-33-1-GEN-53.
