Polonium preparation

The discovery that jS-rays are deflected by a magnetic field was made simultaneously by GieseR with a polonium preparation, BecquereR with radium, and S. Meyer and E. R. von Schweidler with polonium and radium. (Pure polonium preparations emit only a-rays.) Attempts by BecquereR and P. Curie to deflect a-rays in a magnetic field showed that the effect was inappreciable. M. and Mme. Curie found that j8-rays carry a negative charge by collecting them in a metal Faraday cylinder. BecquereR by deflection in... 

For almost half a century scientists had to be satisfied to work only with polonium compounds (usually in rather small amounts). The pure metal was prepared only in 1946. High-density layers of metallic polonium prepared by vacuum sublimation have a silvery colour. Polonium is a pliable low-melting metal (melting point 254°C, boiling point 962°C), its density is about 9.3 g/cm . When polonium is heated in the air it readily forms a stable oxide its basic and acidic properties are weakly manifested. Polonium hydride is unstable. Polonium forms organometallic compounds and alloys with many metals (Pb, Hg, Ca, Zn, Na, Pt, Ag, Ni, Be). When we compare Mendeleev s predictions with these properties we see how close they are to the truth. 

In 1934, scientists discovered that when they bombarded natural bismuth (209Bi) with neutrons, 210Bi, the parent of polonium, was obtained. Milligram amounts of polonium may now be prepared this way, by using the high neutron fluxes of nuclear reactors. 

Twenty five isotopes of polonium are known, with atomic masses ranging from 194 to 218. Polonium-210 is the most readily available. Isotopes of mass 209 (half-life 103 years) and mass 208 (half-life 2.9 years) can be prepared by alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron, but these are expensive to produce. 

Metallic polonium has been prepared from polonium hydroxide and some other polonium compounds in the presence of concentrated aqueous or anhydrous liquid ammonia. Two allotropic modifications are known to exist. 

Gr. aktis, aktinos, beam or ray). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees G. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons. 

The chemistry of sulfur is a broad area that includes such chemicals as sulfuric acid (the compound prepared in the largest quantity) as well as unusual compounds containing nitrogen, phosphorus, and halogens. Although there is an extensive chemistry of selenium and tellurium, much of it follows logically from the chemistry of sulfur if allowance is made for the more metallic character of the heavier elements. All isotopes of polonium are radioactive, and compounds of the element are not items of commerce or great use. Therefore, the chemistry of sulfur will be presented in more detail. 

The rarity of polonium is evident from a calculation (1) which shows that the outermost mile of the earth s crust contains only 4000 tons of the element, whereas radium, usually classed as rare, is present to the extent of 1.8 X 107 tons. The abundance of polonium in uranium ores is only about 100 Mg per ton and hence separation of the element from such mineral sources cannot seriously be considered. However, radium, at equilibrium with its daughters, contains 0.02 wt % of polonium and, until recently, most of the element was obtained either from radium itself or, more usually, from expended radon ampoules which, after the radon decay is complete, contain radium-D and its daughters. Fortunately, however, the parent of polonium in these sources, bismuth-210, can be synthesized by neutron bombardment of natural bismuth [Bi209 (n,y) Bi210] and with the advent of the nuclear reactor it has become practicable to prepare milligram amounts of polonium. Almost all of the chemistry of the element recorded in the recent literature has been the result of studies carried out with polonium-210 prepared in this way. 

Reactions used for the preparation of polonium compounds are straightforward, but the experimental techniques are strictly determined by the small amount of the commonly used polonium-210 which is available and by the exceptionally high specific activity of the isotope (4.5 curics/mg, i.e., 1013 disintegrations/min/mg). Apart from the major effects of the alpha bombardment to be described, the separation of polonium-210 from its lead daughter, which grows in at a rate of 0.5%/day, constitutes a major chemical problem. It calls for rapid and efficient methods of purifying the polonium stock before each experiment the best of these is the sulfide process described in Section III. 

The black monoxide appears to be formed by the spontaneous decomposition of polonium sulfotrioxide and selenotrioxide (10). The corresponding hydroxide (or hydrated oxide) is obtained as a dark brown precipitate when alkali is added to a freshly prepared solution of bipositive polonium (6). Both are rapidly oxidized to polonium(IV) in air or in contact with water. 

It is readily decomposed by aqueous bromine, aqua regia or by hypochlorite and is comparatively soluble in concentrated hydrochloric acid it decomposes to the elements at 275°C under vacuum, a property utilized for the preparation of pure polonium metal (14). 

There have been some unsuccessful attempts to prepare a volatile hexafluoride from fluorine and polonium-210 26, 104), but recently such a fluoride has been prepared in this way from polonium-208 plated on platinum 132). The product appears to be stable while in the vapor phase, but on cooling a nonvolatile compound is formed, probably polonium tetrafluoride resulting from radiation decomposition of the hexafluoride. Analytical data are not recorded for any polonium fluoride, largely owing to the difficulty of determining fluoride ion accurately at the microgram level. 

Polonium tetrabromide is a bright red solid which melts, in bromine vapor, at about 330°C (7, 75), and boils at 360°C/200 mm 75). It is prepared by heating polonium metal in bromine vapor at 200 mm pressure for 1 hour at 250°C (7, 75) or, more rapidly, in a stream of nitrogen saturated with bromine vapor at 200°-250°C, and by heating polonium dioxide in hydrogen bromide or by evaporating a solution of polonium(IV) in hydro-... 

Both polonium derivatives are chemically very stable, requiring hot fuming nitric acid for their decomposition. However, they char rapidly under the intense alpha bombardment and attempted analyses with acetyl-acetone labeled with carbon-14 in the 1 and 3 positions were unsuccessful. It is interesting that the corresponding yellow oxide, prepared by treating (VI) with aqueous hydrogen peroxide, reverts to (VI) on treatment with aqueous alkali (12). 

These compounds have only been studied on the trace scale. An ether soluble polonium dibenzyl is reported (80, 118) to be formed by the action of dimethylphenylbenzy 1-ammonium chloride on sodium polonide/telluride mixtures in water saturated with hydrogen. The dimethyl was prepared in the same way, but with dimethyl sulfate (118) and may also be formed in... 

In the past 15 years a large number of polonium compounds have been prepared in visible quantities for the first time and as a result of these investigations it has been shown that polonium behaves chemically very much as would be expected from its position in the Periodic Table, with the inert-pair effect, likely to be more marked in polonium than in tellurium, still little in evidence. 

The group VIB cyanides, thiocyanates and selenocyanates and their complexes with species such as thiourea have been described.1,45 For example, the tellurium dithiocyanate complex has been prepared45 by treatment of tellurium dichloride or tellurium dibromide with ammonium thiocyanate. It seems that little information exists on the preparation of tellurocyanates and there is a sparsity of data on polonium derivatives. Indeed, the only known cyanide of polonium is probably a salt of the quadrivalent element.1... 

The selenides and tellurides are similar to the sulfides, being preparable from ammonia solutions of the alkali metals. They are water-soluble yet partially hydrolyzed like the sulfides, but are more susceptible to oxidation back to the element. Not every member of the class M cSe3,/Te3, has been fully investigated, but the many that have promise few surprises see Selenium Inorganic Chemistry and Tellurium Inorganic Chemistry). The polonides are similar, and also have their own article see Polonium Inorganic Chemistry). 

Solutions containing the metal, most commonly in nitric acid, will deposit the metal on a platinnm electrode by electrodeposition. Polonium metal is deposited spontaneously from such solutions on to metals such as silver or nickel. The metal can be sublimed off such support metals at low pressures. Thermal decomposition of polonium sulfide also yields the metal. In much the same way as tellurium, the metal can be obtained from its solutions by the action of reducing agents such as hydrazine, tin(II) ion, titanium(III) ion, and dithionite. Such metal precipitates appear as gray-black powders. Thin foils, silvery in color, have been prepared by vacuum sublimation of the metal. 

The deposition of polonium on metal wires gives rise to a useful a-source. Tips of metal wires having a length 10 mm and a diameter of 0.2 mm were utilized. They were made of Al, Ni, Pd, Pt or An. Each was immersed in 100 pi of a solution containing °Po (300 Bqml ) for 15 h at 27°. Alpha particle emission was measured using a liquid scintillation system. There was an observed diminution in the a-pulse spectra for all of the wires except Al. This was attributed to the mutual diffusion between the wire metal and °Po. The °Po deposited on the Al wire had a tendency to be eluted with the liquid scintillator. This was attributed to physical absorption on the porous metal oxide layer on the Al wire and °Po. The °Po deposited by the Al wire had a tendency to be eluted with the liquid scintillator. It was possible to prepare a °Po -Al wire as a useful a-source by heating at 120° for 30 minutes. 

Trioxides of sulfur, selenium, and tellurium are well recognized, but polonium trioxide has yet to be prepared in weighable quantities. 

Evidence for the existence of PoS is much stronger. Aqueous solutions in HCl containing Po and Po yield a precipitate of PoS. During the course of this reaction, Po is reduced to Po with the concurrent oxidation of sulfide to free sulfur. The same sulfide can be prepared by the reaction between polonium hydroxide and ammonium sulfide. The compound has not been successfully prepared by the direct reaction between the elements. 

The tetrachloride of polonium is prepared by dissolving the metal in hydrochloric acid and evaporating to dryness. P0CI4, is a hydroscopic yellow solid which melts at 300° and is soluble in ethanol and acetone. The tetrachloride can be converted to dichloride, P0CI2, by reduction with sulfur... 

The tetrabromide of polonium has been prepared in several ways, which parallel those described for the tetrachloride. These include the direct reaction between the elements at 200 °C, dissolution of the metal in aqueous hydrobromic acid solution followed by evaporation, and by the reaction between gaseous HBr and P0O2. Its existence has been coirfirmed by chemical analysis. 

The preparation of polonium tetrabromide has been achieved in several ways. These include the following ... 

Polonium dibromide is purple-brown in color and sublimes at 110° at reduced pressure with some decomposition. It is prepared by the reduction of PoBr4 with H2S or by heating PoBr4 to 200° in vacuo. The dibromide is soluble in ketones as well as in dilute, aqueous HBr in which it forms a purple solution. In the latter solution, the dibromide is oxidized rapidly to Po(IV). 

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