Polonium metal

The usual methods of obtaining the metal are the electrodeposition from acid solution (usually nitric, sometimes hydrofluoric) on to platinum 

The precipitated metal is a grey-black powder, but when sublimed, polonium is silvery in thick layers [ 0.2 mg (1 curie)/cm2] and a transparent smoked film in thin ones. When the thicker metal mirrors, obtained by vacuum sublimation onto a glass surface, are treated with a little 8 N nitric acid, a part of the metal can be detached from the glass as a thin foil. 

The physical properties of the metal (Table II) resemble those of thallium, lead and bismuth, its neighbors in the Periodic Table, rather than those of tellurium, its lower homologue. The low melting and boiling points are particularly noteworthy an attempted study of the Hall effect in polonium metal has also been reported (90). In chemical properties the metal is very similar to tellurium, the most striking resemblance being in its reactions with concentrated sulfuric acid (or sulfur trioxide) and with concentrated selenic acid. The products are the bright red solids, PoSOs and 

PoSe03, both of which decompose rapidly at room temperature to the unstable black monoxide (10). 

Earlier observations of some of the L lines of polonium (69, 124), obtained with trace amounts of the element, have been confirmed by more recent work on its K and L spectra, in which a copper target was coated with 2-3 mg of the element (112). 

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Metals typically form basic oxides and nonmetals typically form acidic oxides, but what about the elements that lie on the diagonal frontier between the metals and nonmetals Along this frontier from beryllium to polonium, metallic character blends into nonmetallic character, and the oxides of these elements have both acidic and basic character (Fig. 10.7). Substances that react with both acids and bases are classified as amphoteric, from the Greek word for both. For example, aluminum oxide, A1203, is amphoteric. It reacts with acids ... 

Another method to separate polonium from bismuth involves heating at 650°C to convert the metals into their oxides. This is followed by further heating to about 800° C at reduced pressure in which polonium metal is removed by volatilization. 

Self-heating, due to the stoppage of the alpha particles within the solid, is a well known phenomenon and calculation shows that the energy release from one gram of polonium metal would be about 140 watts. This high energy output affords a useful and absolute method for the rapid determination of polonium in large sources by calorimetry. 

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). 

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-... 

Its solutions in dilute hydrobromic acid are a carmine-red (0.025 M PoBr4) and in more dilute solution (10 3 M), orange red. The tetrabromide is soluble in ethanol, acetone and some other ketones, and is sparingly soluble in liquid bromine. It is hygroscopic and is easily hydrolyzed to a white, basic bromide of variable composition. It forms a yellow ammine in ammonia gas and this yields polonium dibromide and polonium metal on standing, presumably because of radiation decomposition of the ammonia and subsequent hydrogen reduction of the tetrabromide (7). 

Polonium tetraiodide (8) is a black solid which sublimes in nitrogen at 200°C with partial decomposition to the metal. It is formed from the elements at 40°C/1 mm, by treating polonium dioxide or hydroxide with 0.1 N hydriodic acid, and is precipitated from solutions of polonium(lV) in dilute hydrochloric acid on the addition of 0.1 N hydriodic acid. It is also obtained as a black sublimate by heating polonium dioxide in hydrogen iodide at 200°C a black addition compound (PoCVxHI) is formed in the cold. Polonium metal does not react with iodine dissolved in carbon tetrachloride, but with iodine dissolved in benzene it does react to some extent. 

Polonium tetranitrate, with at least one molecule of dinitrogen tetroxide of crystallization, is formed as a white crystalline solid by the action of liquid dinitrogen tetroxide on polonium dioxide or tetrachloride polonium metal does not react with this reagent or with its solution in ethyl acetate. The dinitrogen tetroxide is rapidly lost on standing and the resulting tetranitrate decomposes to the basic salt (1) in l i 2 hr. under vacuum (16). 

PROBLEM 10.12 Polonium metal crystallizes in a simple cubic arrangement, with the edge of a unit cell having a length d = 334 pm. What is the radius (in pico-meters) of a polonium atom ... 

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. 

Polonium metal has a melting point of 489°F (254°C), a boiling point of 1,764°F (962°C), and a density of 9.4 grams per cubic centimeter, nearly 10 times that of water. 

Polonium metal was formed by the reduction of P0CI4, P0OCI2 as well as from the nitrate or the acetate. The solvents involved were hydrochloric acid and polar organics such as acetone, ethanol, and methyl ethyl ether. Mercury served as the reducing agent and a polonium-mercury amalgam is formed. 

A crystal ofone form of polonium metal has Po atoms at the corners of a simple cubic unit cell that is 3.35 A on edge (Example 13-8). 

Polonium is known to form a number of oxidation states including Po ". Brown (2001) showed that the stability region ( j,-pH) of polonium(II) was quite narrow, occurring in a region that does not exceed a pH of 6 and is bounded by polonium metal at lower and polonium(IV) at higher E. The hydrolytic properties of polonium(II) are expected to be similar to those of lead(II). As a consequence, polonium(II) is not expected to hydrolyse until a pH higher than 6, in a region where it is not expected to be stable. It is not expected, therefore, that polonium(II) hydrolysis species would exist and none have been reported. 

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