Neptunium(IV) oxalate is a chemical compound made up of neptunium(IV) (Np4+) and oxalate (C2O2−4) ions with the formula Np(C2O4)2. It has been known at least since 1947. It is known to form several hydrates with formulas Np(C2O4)2·xH2O (x = 1, 2, 6). The hexahydrate (x = 6) is a highly insoluble solid prepared by adding oxalic acid to aqueous solutions containing neptunium(IV) and nitric acid, and forms green crystals. The structure of the compound was determined through X-ray diffraction. At high temperatures, the hexahydrate decomposes, forming other hydrates and eventually neptunium(IV) oxide, NpO2. Due to this, the high-temperature calcination of neptunium(IV) oxalate is used to produce NpO2, which can be used for nuclear power applications or preparation of other neptunium compounds. Neptunium(IV) oxalate is also used to separate neptunium from other metals. The presence of neptunium in this compound causes it to be radioactive.
Synthesis Neptunium(IV) oxalate production usually starts from neptunium-nitric acid (HNO3) solutions, though solutions with hydrochloric acid (HCl) can be used instead. The presence of higher neptunium oxidation states like +5 reduces the amount of oxalate filtered out, so it must be ensured that neptunium is in the +4 oxidation state in the initial nitric acid solution. Hydrazine (N2H4) or hydrazinium nitrate (N2H5NO3) is added to the initial solution to stabilize the +4 oxidation state and prevent oxidation. Iron(II) sulfamate (Fe(NH2SO3)2) or ascorbic acid (C6H8O6) is then added to reduce any neptunium(V) to neptunium(IV). With ascorbic acid, the reduction happens as follows:
2 NpO2+2 + H2A → 2 NpO+2 + A + 2 H+ 2 NpO+2 + 6 H+ + H2A → 2 Np4+ + A + 4 H2O Ascorbic acid is a slow reducing agent, but the process is sped up rapidly 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 + 6 H2O → Np(C2O4)2·6H2O + 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.
Physical properties Neptunium(IV) oxalate forms several hydrates, with formulas Np(C2O4)2·xH2O (x=1, 2, 6). When neptunium(IV) oxalate is precipitated from aqueous solutions, green crystals of the hexahydrate (Np(C2O4)2·6H2O) are formed. 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. 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:
Np(C2O4)2·6H2O (s) ⇌ Np(C2O4)2 (aq) + 6 H2O Np(C2O4)2 + 2 H+ ⇌ Np(C2O4)2+ + H2C2O4 Np(C2O4)2 + H2C2O4 ⇌ Np(C2O4)2−3 + 2 H+
Structure The structure of the hexahydrate (Np(C2O4)2·6H2O) was determined via X-ray diffraction. While initial studies from Grigor'ev et al. suggested that each neptunium atom was bonded to four oxalate groups with a coordination geometry of cubic, later studies from Sockwell et al. 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)2]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)2]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.
Decomposition Neptunium(IV) oxalate decomposes when heated. 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 converted 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 quality between 500–550 °C.
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
Applications
Neptunium separation and purification Neptunium(IV) oxalate is an intermediate in neptunium separation and purification. Adding oxalic acid to a solution of other metals, such as alkali, alkaline earth, or transition metals, will precipitate neptunium as neptunium(IV) oxalate, allowing it to be separated from most other metals. It is also used in the separation of neptunium from other actinides, where a modification of the PUREX process with adjusted acid and tributyl phosphate concentrations is used. Neptunium(IV) oxalate is the form of neptunium which is isolated in this process, and it can then be calcined to produce neptunium(IV) oxide, NpO2.
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