Neptunium(IV) oxalate
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_oxalate.png) | |
| Identifiers | |
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3D model (JSmol) |
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CompTox Dashboard (EPA) |
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| Properties | |
| Np(C2O4)2 | |
| Molar mass | 413 g·mol−1 |
| Appearance | Green crystals |
| slightly soluble | |
| Related compounds | |
Other cations |
Thorium oxalate Uranium(IV) oxalate Plutonium(IV) oxalate |
Related neptunium oxalates |
Neptunyl(V) oxalate Neptunyl(VI) oxalate |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Neptunium(IV) oxalate is an inorganic compound and a salt of neptunium and oxalic acid, with the formula Np(C2O4)2. It is known to form several hydrates with formulas Np(C2O4)2·xH2O (x = 1, 2, 6). The hexahydrate (x = 6) is prepared by adding oxalic acid to aqueous solutions containing neptunium(IV) and nitric acid, and forms green crystals. It is frequently burned as part of the production of neptunium(IV) oxide, which is used in nuclear waste, the preparation of other neptunium compounds, and the production of plutonium-238.
Neptunium(IV) oxalate has been known at least since 1947.[1] A method of producing neptunium(IV) oxide (NpO2) from neptunium(IV) oxalate first appeared in 1961.[2]
Synthesis
Neptunium(IV) oxalate production starts from neptunium-nitric acid solutions. The presence of higher neptunium oxidation states like neptunium(V) reduces the amount of oxalate filtered out, so it must be ensured that neptunium is in the +4 state in the initial nitric acid solution. Hydrazine is added to the initial solution to stabilize the +4 state and prevent oxidation. Ascorbic acid is then added to reduce any neptunium(V) or neptunium(VI) to neptunium(IV):
- 2 NpO2+2 + H2A → 2 NpO+2 + A + 2 H+
- 2 NpO+2 + 6 H+ + H2A → 2 Np4+ + A + 4 H2O
Here, H2A represents ascorbic acid, and A represents dehydroascorbic acid. Ascorbic acid is a slow reducing agent, and the process is sped up with increased nitric acid concentration and/or temperature. After neptunium(IV) solution is prepared, neptunium(IV) oxalate is produced via the addition of oxalic acid:
- Np4+ + 2 H2C2O4 → Np(C2O4)2 + 4 H+
Oxalic acid is often added in a two-step precipitation process. After addition, the solid neptunium(IV) oxalate can easily be filtered out.[3][4][2][5][6][7]
Physical properties
Neptunium(IV) oxalate forms several hydrates, with formula Np(C2O4)2·xH2O (x=0, 1, 2, 6). When neptunium(IV) oxalate is precipitated from aqueous solution, green crystals of the hexahydrate (Np(C2O4)2·6H2O) are formed.[4] Other hydrates, such as the monohydrate (Np(C2O4)2·H2O) and dihydrate (Np(C2O4)2·2H2O), are formed by heating this compound. The dihydrate can also be produced by the dehydration of the hexahydrate with sulfuric acid. An anhydrous (lacking water) form, Np(C2O4)2, is formed on further heating.[8][9]
Neptunium(IV) oxalate hexahydrate is highly insoluble in water. A study on its solubility in nitric acid-oxalic acid solutions found that its value ranges between 4.0 and 37.4 mg/L at 22 °C, but it increases with increasing temperature (19.0–373.5 mg/L at 45 °C, 32.4–420.1 mg/L at 60 °C). When the ratio of neptunium(IV) ions to oxalate ions is close to 1:2, solubility decreases as neptunium(IV) oxalate is precipitated out, but when it is less than or more than 1:2, solubility increases due to the formation of Np(C2O4)2+ and Np(C2O4)2−3 ions, respectively. Equilibrium equations:[10]
Reactions
Reaction with ozone
When reacted with ozone while suspended in water, it is oxidized to neptunyl(V) oxalate, and then to neptunyl(VI) hydroxide.[11]
Decomposition
Neptunium(IV) oxalate decomposes on heating. Starting with the hexahydrate, Np(C2O4)2·6H2O, four water molecules per formula unit are lost between 80 and 90 °C. The fifth one is split off at 100–120 °C, and the last is split off at 190–200 °C to form anhydrous neptunium(IV) oxalate. Between 270 °C and 300 °C, this compound begins to decompose. At 270–330 °C, 70% of the neptunium is concerted to the +5 oxidation state, mainly due to the formation of neptunyl(V) oxalate, (NpO2)2C2O4. This compound further decomposes to neptunium(IV) oxide (NpO2), which is formed at satisfactory purity between 500–550 °C.[8][9][4]
- Np(C2O4)2·6H2O → Np(C2O4)2·2H2O + 4 H2O (between 80 and 90 °C)
- Np(C2O4)2·2H2O → Np(C2O4)2·H2O + H2O (between 100 and 120 °C)
- Np(C2O4)2·H2O → Np(C2O4)2 + H2O (between 190 and 200 °C)
- 2 Np(C2O4)2 + 2 O2 → (NpO2)2C2O4 + 6 CO2
- (NpO2)2C2O4 → 2 NpO2 + 2 CO2
Structure
In the hexahydrate (Np(C2O4)2·6H2O), initial studies suggested that each neptunium atom was bonded to four oxalate groups in a cubic fashion, but later studies[12] suggest that each neptunium atom is bonded to two water molecules as well. Neptunium, oxalate, and water molecules join together to form layers of composition [Np(C2O4)2(H2O)]n. Three of the oxalate groups per neptunium atom lie perpendicular to the layers, while the other lies parallel, providing room to fit the two water molecules. Between these [Np(C2O4)2(H2O)]n layers lie the rest of the water molecules, four per formula unit. Each oxalate group donates two oxygen atoms to neptunium, and each water molecule donates one atom, so neptunium atoms are bonded to ten oxygen atoms total.[12]
2_6H2O_layer_new.png)
2_6H2O.png)