Polonium IV

Hataye et al. (1981) studied the hydrolysis of polonium(IV) in l.Omoll (HjNalClO at ambient temperature using solvent extraction. For the reaction, 

The cited data can be combined to determine the solubility constant, log K q, and the first four monomeric hydrolysis constants, log to log 

These are the values determined by Brown (2001) where the uncertainties have been assigned in the present review. 

Brown (2001) also provided thermodynamic data for Po(s) and PoO +. From these values, thermodynamic data for the hydrolysis species and Po02(s) can be derived, as was done by Brown (2001). These data are listed in Table 15.6. 

Ampelogova, N.I. (1973) Ion-exchange study of the complexing of polonium. Radiokhimiya, 15, 813-820. 

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P0O2 is obtained by direct combination of the elements at 250° or by thermal decomposition of polonium(IV) hydroxide, nitrate, sulfate or selenate. The yellow (low-temperature) fee form has a fluorite lattice it becomes brown when heated and can be sublimed in a stream of O2 at 885°. However, under reduced pressure it decomposes into the elements at almost 500°. There is also a high-temperature, red, tetragonal form. P0O2 is amphoteric, though appreciably more basic than Te02 e.g. it forms the disulfate Po(S04)2 for which no Te analogue is known. 

The treatment of polonium(lV) with nitric acid/potassium permanganate under reflux yields a sludge of manganese dioxide which contains all the polonium originally present the valency state is uncertain. Polonium (IV) in weighable amounts is not oxidized by persulfate, ceric salts or chlorine in alkaline solution (12), although trace scale work indicates that both ceric salts and dichromate do oxidize polonium to polonium(VI) (94). 

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. 

A white solid, possibly polonium tetrafluoride, is obtained by treating polonium hydroxide or tetrachloride with dilute aqueous hydrofluoric acid treatment of this solid, in suspension in dilute hydrofluoric acid, with sulfur dioxide yields a bluish grey product (possibly PoF2) which rapidly reverts to the original white solid on standing, presumably owing to radio-lytic oxidation 12). The solubility of polonium(IV) in aqueous hydrofluoric acid increases rapidly with acid concentration, indicating complex ion formation (/ft), p. 48). 

The compound is produced by evaporating hydrochloric acid solutions of polonium (IV) 6, 26, 74), by heating the dioxide in carbon tetrachloride vapor 74), in hydrogen chloride, thionyl chloride or with phosphorus pentachloride 6) and by heating the metal in dry chlorine at 200°C (6, 25, 74). It is hygroscopic and hydrolyzes in moist air to a white solid, possibly a basic chloride (7)). The tetrachloride is soluble in thionyl chloride and in water with hydrolysis, and is moderately soluble in ethanol, acetone, and... 

Bipositive polonium in hydrochloric acid solution (pink) is oxidized to polonium(lV) by hydrogen peroxide, by hypochlorous acid or by the radiolysis products of the alpha bombardment of the solvent. Solutions of polonium(II) in acid are obtained by the reduction of polonium(lV) with sulfur dioxide or hydrazine in the cold, or with arsenious oxide on warming. Polonium (IV) is not reduced in hydrochloric acid by either hydroxylamine or oxalic acid, even on boiling 6). 

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-... 

Solutions of polonium(IV) in hydrobromic acid deposit a blackish brown solid on cooling to — 30°C this is unstable at room temperature and appears to be the hydrated acid, II2PoBr6. The ammonium bromopolonite is obtained in small yield by heating polonium tetrabromide in ammonia gas at 100°C on heating more strongly in a sealed tube, this salt blackens and detonates, possibly owing to the formation of an explosive nitride (7). 

Metallic polonium is not affected by 15% bromic acid, even at 70°C, and polonium dioxide or polonium(IV) hydroxide are only slightly soluble (2.5 mg Po02/liter) in 10% bromic acid. Polonium tetrachloride is converted to a white solid of unknown composition by 15% bromic acid this could be a basic chloride (12). 

Long treatment of polonium(IV) hydroxide with water saturated with carbon dioxide yields an unstable white solid, apparently a carbonate (12). 

Aqueous 1 M chromium trioxide (12) does not react with metallic polonium, but with polonium(IV) hydroxide or tetrachloride yields an orange-yellow solid, thought to be Po(Cr04)2. This is insoluble in an excess of the reagent and is easily hydrolyzed by water or wet acetone to a dark brown basic chromate with a composition close to 2Po02 Cr()3 (cf., the basic sulfate and selenate). On long standing in an excess of aqueous chromium trioxide, oxidation to polonium(VI) may occur (Section VI,A). 

The white basic sulfate, 2Po02-S03, results when polonium(IV) hydroxide or chloride is treated with 0.02 N-0.25 N sulfuric acid. Like the selenate, it is yellow above 250°C and decomposes to the dioxide at 550°C. Solubility studies indicate that it is metastable in contact with 0.1 N-0.5 N sulfuric acid (10). 

This salt is a white crystalline solid made by treating polonium (IV) hydroxide or chloride with dilute acetic acid. Its solubility in the latter increased from 0.2 mg (of Po210)/liter in 0.1 N acid to 82.5 mg/liter in 2 N acid, indicating complex ion formation. The acetato complex is colorless in solution and appears to be more stable than the hexachloro complex (11). 

This is a white crystalline solid obtained by treating polonium(IV) hydroxide or chloride with aqueous oxalic acid solubility studies indicate complex ion formation (11). 

The basic salts of quadrivalent polonium, such as the sulfate and sele-nate, show a marked resemblance to those of tellurium and further resemblances appear in the quadrivalent halides, particularly in their complexing with halide ions in solution, while complexing of polonium(IV) with weak acids, such as acetic, oxalic and tartaric, seems to be more marked than is the case with tellurium. 

Although there are similarities between the chemistry of the chalcogenide elements, the properties of selenium and tellurium clearly lie between those of non-metallic sulfur and metallic polonium. The enhancement in metallic character as the group is descended is illustrated in the emergence of cationic properties by polonium, and marginally by tellurium, which are reflected in the ionic lattices of polonium(IV) oxide and tellurium(IV) oxide and the formation of salts with strong acids. 

Tellurium(IV) sulfato complexes of composition 2(2Te02 S03),MHS04-2H20 have been reported,46 from which the anhydrous compounds were obtained by calcination. Carboxylic acids have also been found to form anionic complexes with tellurium(IV) and polonium(IV). For example, the silver salts of the citrato- and tartrato-tellurates(IV) have been described47 as insoluble in water but soluble in nitric acid. 

Studies of the solubility of polonium(IV) in formic, acetic, oxalic and tartaric acids have provided evidence of complex formation,48 with the acetato complex emerging as more stable than the hexachloro anion. Other studies of the solubility of polonium(IV) hydroxide in carbonate49 and nitrate50 solution, together with investigations51 of the ion exchange behaviour of polonium(IV) at high nitrate ion concentration, have been discussed in terms of the formation of anionic complex species. 

Although tellurium tetrachloride has been reported55 to form a 1 2 complex with acetamide, there appears to be little other information available on the complex. Similarly, the reported tributyl phosphate complex of polonium tetrachloride56 has received little attention. The formation of a polonium(IV) perchlorate complex with tributyl phosphate has been suggested57 in the solvent extraction of polonium from perchloric acid. 

Polonium has more isotopes than any other element. The most common isotope is °Po, which has a half-life of 138.4 days. Other isotopes have half-lives ranging from less than 1 second to 102 years ( °To). Polonium burns in air to form polonium (IV) oxide (P0O2) and reacts with halogens to form tetrahalides (e.g., P0CI4, PoBr4, P0I4). It also dissolves readily in dilute acids. 

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