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Plutonium(III) oxalate
Chemical compound From Wikipedia, the free encyclopedia
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Plutonium(III) oxalate is a compound consisting of plutonium and oxalate with the formula Pu2(C2O4)3. It is produced by adding oxalic acid to plutonium solution, and is often used as a starting material for other plutonium compounds, like plutonium(IV) oxide, plutonium(III) fluoride, plutonium(III) chloride, or plutonium(III) bromide.
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Synthesis
Upon carefully adding oxalic acid or adding sodium oxalate to an acidic solution containing plutonium(III), often nitric acid, plutonium(III) oxalate hydrates are precipitated.[1][2]: 836
Properties
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It is a turquoise-blue solid which is quite insoluble, like the related compounds americium(III) oxalate and curium(III) oxalate, and it is easily filterable.[3][2]: 836 It forms two main hydrates: the decahydrate, Pu2(C2O4)3·10H2O, and the hexahydrate, Pu2(C2O4)3·6H2O.[2]: 1174 Other hydrates are known, such as Pu2(C2O4)3·9H2O, Pu2(C2O4)3·2H2O, and Pu2(C2O4)3·H2O, as well as anhydrous Pu2(C2O4)3 (contains no water). Sources vary as to whether it is the decahydrate, Pu2(C2O4)3·10H2O, or the nonahydrate, Pu2(C2O4)3·9H2O, that is found upon precipitation from aqueous solution.[4][5]
Decomposition
Data on the thermal decomposition of plutonium(III) oxalate is reported for the nonahydrate. Upon heating, it loses water between 30–225 °C, going through various different hydrates of compositions Pu2(C2O4)3·8H2O, Pu2(C2O4)3·2H2O, and Pu2(C2O4)3·H2O before reaching anhydrous plutonium(III) oxalate (Pu2(C2O4)3):[6][7]
- Pu2(C2O4)3·9H2O → Pu2(C2O4)3·8H2O + H2O (between 30–80 °C)
- Pu2(C2O4)3·8H2O → Pu2(C2O4)3·2H2O + 6 H2O (between 80–140 °C)
- Pu2(C2O4)3·2H2O → Pu2(C2O4)3·H2O + H2O (between 140–180 °C)
- Pu2(C2O4)3·H2O → Pu2(C2O4)3 + H2O (between 180–225 °C)
Starting at 225 °C, Pu2(C2O4)3 begins to decompose to plutonium dioxide, PuO2, though complete conversion only occurs at 400 °C.[6][7] The intermediates in this process have been studied as part of the thermal decomposition of plutonium(IV) oxalate, which produces anhydrous plutonium(III) oxalate. The transition from Pu2(C2O4)3 to PuO2 goes through several carbonate oxalate phases, with compositions Pu2(C2O4)2CO3 and Pu2C2O4(CO3)2, releasing carbon monoxide, before forming plutonium(IV) oxycarbonate PuOCO3. The compound then finally decomposes to from plutonium dioxide:[8][9]
- Pu2(C2O4)3 → Pu2(C2O4)2CO3 + CO
- Pu2(C2O4)2CO3 → Pu2C2O4(CO3)2 + CO
- Pu2C2O4(CO3)2 → 2 PuOCO3 + 2 CO
- PuOCO3 → PuO2 + CO2
At room temperature in air, plutonium(III) oxalate also decomposes, albeit much more slowly. It eventually degrades to a product proposed to be either a colloidal PuO2 polymer or PuOCO3.[10]
Structure
While the structure of anhydrous plutonium(III) oxalate is unknown, two structures have been predicted by density functional theory (DFT) calculations. One structure consists of plutonium-oxalate layers in a honeycomb-like arrangement, and the other consists of 2D sheets which are connected into a 3D network via bridging oxalate groups. Both of these structures have been calculated to be stable.[8][11]
On the other hand, the structures of plutonium(III) oxalate nonahydrate (Pu2(C2O4)3·9H2O) and decahydrate (Pu2(C2O4)3·10H2O) are known. Plutonium(III) oxalate decahydrate has been determined to adopt the same structure as lanthanum(III) oxalate decahydrate, La2(C2O4)3·10H2O.[12] Both it and the nonahydrate, Pu2(C2O3)3·9H2O, are made up of repeating honeycomb-like layers of composition Pu2(C2O3)3(H2O)6. Within these layers, each plutonium atom is bonded to nine oxygen atoms, and has a coordination geometry of distorted tricapped trigonal prismatic. Three oxygen atoms come from three water molecules, and six oxygen atoms come from three oxalate groups. Per formula unit, there are either three or four water molecules disjoint from each layer (three for the nonahydrate, four for the decahydrate).[5][13]
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Uses
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Plutonium(III) oxalate is frequently used as a starting material for the preparation of other plutonium compounds.[2]: 836–837, 1031–1032, 1077, 1093, 1095 For example, plutonium(IV) oxide is mainly produced by the high-temperature heating of either plutonium(III) oxalate or the related compound plutonium(IV) oxalate. During this process, it is slowly heated up to 700 °C to avoid rapid decomposition and evolution of gases, and afterwards it is heated to 1000 °C to remove any carbon left behind.[2]: 1031–1032 For information about the reactions which occur, see above.
Plutonium(III) oxalate can also be used to produce plutonium halides. The compound plutonium(III) fluoride has been prepared at the 150–300 gram scale by heating plutonium(III) oxalate in a stream of hydrogen between 150–600 °C, and then in a stream of hydrogen fluoride between 200–300 °C:[2]: 1077–1078
- Pu2(C2O4)3 + 6 HF → 2 PuF3 + 3 CO + 3 CO2 + 3 H2O
Plutonium(III) oxalate, in the form of the decahydrate (Pu2(C2O4)3·10H2O), can also be used to prepare plutonium(III) chloride (PuCl3) by reaction with chlorinating agents. Reaction with hydrogen chloride is considered the best method for PuCl3 for medium-scale reactions (between 1 to 10 grams):[2]: 1093
- Pu2(C2O4)3·10H2O + 6 HCl → 2 PuCl3 + 3 CO2 + 3 CO + 13 H2O
Alternatively, the liquid hexachloropropene can be used as a chlorinating agent to avoid working with hazardous gases:[14]
- Pu2(C2O4)3·10H2O + 3 C3Cl6 → 2 PuCl3 + 3 C3Cl4O + 3 CO2 + 3 CO + 10 H2O
To prepare plutonium(III) bromide, it can be reacted with hydrogen bromide between 400 °C and 600 °C.[2]: 1095 Plutonium(III) oxalate can also be used to synthesize various other compounds; e.g. it reacts with rhenium(VII) oxide to produce plutonium(III) perrhenate, Pu(ReO4)3.[2]: 1065
References
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