edited by MARY VIRGINIA ORNA, 0.S.U College of New RmheMe New Rochelle, NY 10801
thumbnail ~ketchef Polonium W. Conard Fernelius
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Kent State University Kent, OH 44240 Although polonium was discovered in 1898, little is said about i t in the textbooks except that the Curies found it some davs hefore thev identified radium. The reason for this is that poionium was ihentified by its radioactivity and not by the isolation of a weighable and visible amount of some compound. In any ore of uranium, polonium is present in "radioactive equilibrium" (really steady state) to the extent of one part in 1.19 X 101° parts of uranium, whereas radium is present to the extent of one part in 3 X lo6. This difference arises from the enormous differences in half lives: 238U,4.51 X 109years; 22Wa,1590 years; 210Po, 138.4 days. Any attempt to isolate polonium results in the concentration of certain minor constituents in the source to a greater extent than that of polonium. Thus, it is not surprising that the fmt sample of pure polonium was prepared in March 1944, at the forerunner of the Mound Laboratory in Dayton, Ohio. However, the extreme energy of the a-rays emitted by polonium (5.298 Mev, 3.84 cm range in air, 27.4 cal/hr/curie (or 0.222 g)) makes possible the detection of very minute amounts of the element. The .'natural" sources of polonium herome evident from a consideration of the disinteeration producLs of urwium: 2?RU by loss of three a-particies a n d two @-particles yields
(Among the members of the series not shown above are two isotopes 21"Po and 214P0, hut their half lives are very short sec. resnectivelv.) In the workiue-UD. 3.05 min and 1.6 X of uranium ores, the hulk ofthe ioloniu&(90%) remains with a final residue of hydrated silica from which it can he removed only with great difficulty. Hence, it is better to allow polonium to "grow" into a sample of an earlier memher of the disintegration series after that member has been separated out in reasonably pure form. Thus a solution containing 210Ph (T112 = 21 years) may he "milked" of its polonium content periodicallv hv suitable treatment. (Of course. all of the inactive lead originaily present in the uranium mineral would also he present.) At an earlier time, radium salt solutions were "milked" of their,content of radon and small amounts of the gas sealed in small glass tubes. When implanted in body tissue these were much safer than using radium itself. After the radon had decayed (TI/%= 3.823 days) these "seeds"contained only 210Phtogether with its disintegration products and were an excellent source for very small amounts of polonium. Today the mast convenient and only practical way to obtain 210Po is to expose hismuth to neutrons in an atomic pile: Bi (n, y)Z g B ifollowed by disintegration (TI/z = 5 days) 8). With a neutron flux of 5 X t o polonium ('if Bi 2i:Po 10" neutrons/cm2/sec and a 30-dav irradiation. the hismuth will contain 0.02 mglkg. Fluxes of 10'4and longer irradiatiun ha\,e yielded 4 mg~kg(4 ppml. The polon~umis concentrated hy dissolving in aqua regia, removing the last traces of nitric and nitrousacids,and agitation with hismuthpowder ( ' 5 0 ~ f the original weight of bismuth). A repetition of these steps brings about another IOO-fold ronrentration. The ~ulonium is then separated frum the bismuth and the impuriiies which have concentrated with the polonium. The first step is either
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converting the hismuth to the oxide and volatilizing the polonium oxide by heating in a vacuum or forming a hlack nrecinitate hv treatine a denitrated solution of HBiCL with 10% tint111 chloride. ~ h e p r i n r i ~ a l i m p u r iisilver) ty is removed hv dissolvina the oxide or metal in nitrir acid and . precipitating . hidrated polonium oxide. Pure polonium is ohtained h i electrodeposition from 1.5N nitric acid at a controlled cathode potential. Polonium has a few specific uses. I t is a "pure" alpha emitter (only 2 y-quanta in 105alpha emissions). As such, it is useful for studying the effects of alpha particles and for calibration Purposes. Small neutron sources are prepared hv mixing &l&ium with beryllium or other low a& t& numhe; element (2 Be ( a , n ) C). Polonium is also a small hut concentrated sburce of power (heat). The disintegration of polonium yields 206Pb,a pure isotope not readily ohtained otherwise. ~ x ~ e r i m e n t a t i owith n pol~,n'~um is difficult because of its intense radiation. Elaborate precaution.i muat be taken to protect workers from ingestion and skin contact and to prevent contamination of the work area. Even the exposure of a crystal of polonium (or one of its compounds) to-a beam of X-rays can he a difficult and dangerous operation. The disruotive effect of the aloha-bombardment on the elass canillarv " causes it to "craze" after a few days and become very fragile. Further, hoth helium and lead are growing into the sample and after a week the pressure of the former in a small capillary could be sufficient to cause the capillary to explode. Even worse, however, from the experimental side is the disruption of the lattice hy the intense bombardment of alpha-particles and by the recoil of product heavy nuclei. On the average, probably every polonium atom would change its lattice site once in 24 hours. In solution, the intense radiation causes decom~ositionof the solvent. Thus. an aoueous solution of polonikm will slowly liberate small bubbles of gas. The hydroeen ~eroxideformed in this decom~asitionwill oxidize the polonium from a lower to a higher oxidation state. Nevertheless. much is known about oolonium and its compounds. The element is silvery gray in iolor similar to lead and forms a bright mirror if pure. I t is soft enough t o he scratched by a dissecting needle and is a good conductor of electricity. I t is readily volatile in a vacuum and can he moved about hy heating and cooling. I t forms two allotropic modifications: low temperature cubic (d = 9.496) and high temperature hexagonal (d = 9.398) with a transition temperature near 36'. The molecule in the gaseous state is Po? and the heat of vaporization 24,597 f 3 i callmole. ~olon&nnforms a number of solid polonides: Na*Po, AgzPo, BePo, Capo, ZnPo, PbPo, NiPo, and PtPoz. The only oxide known is pale yellow, feebly amphoteric Po02 which does not suhlime hut decomposes to elemental polonium on heating in a vacuum. Yellow Pock, ruby red PoC12, bright red PoBr4, purple brown PoBr2, and hlack Po14 are known and have the properties characteristic of halides of nonmetals (volatility, organic solvent solubility, etc.), including the formation of halosalts such as (NH&PoBr6. Other interesting compounds are 2P002.S03, 2PoO&e03. P o (SO& (all white). and PoS (black). The octahedal co&ent rad& of PoIVis ahout 1.52 A. he standard electrode potential for polonium in nitric acid (EF)oPo'V) is +0.76 V and in hydrochloric is 0.55 V. From the latter value K for the reaction Po4+ 6C1- e PoCli- is -1014. The potential for the couple PoCG-, PoCli- is -0.72 V from which it is estimated that E; Po/F'oCIi- is -0.38 V. Equilibrium
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