Neptunium hexafluoride
Orthorhombic
A = 990.9 pm b = 899.7 pm c = 520.2 pm
Neptuniumhexafluorid
Orange crystalline solid
Fixed
54.4 ° C
55.18 ° C
-1970.0 ± 20.0 kJ · mol -1
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Neptunium (VI ) fluoride ( NpF6 ), usually called Neptuniumhexafluorid is a compound of the elements neptunium, and fluorine. It is an orange crystalline solid; which is volatile, radioactive and corrosive. Neptuniumhexafluorid is stable in dry air but reacts violently with water. At atmospheric pressure, it melts at 54.4 ° C and boils at 55.18 ° C.
In most cases, it is obtained from the neptunium (IV ) fluoride ( NpF4 ) by reaction with elemental fluorine (F2). The volatility of Neptuniumhexafluorid, which pointed out a separation of neptunium from spent fuel, quickly led to an interest in its presentation as well as the detailed investigation of its properties.
- 2.1 Physical Properties
- 2.2 Crystal and Molecular Structure
- 2.3 Spectroscopic Properties
- 2.4 Chemical Properties
Representation
Neptuniumhexafluorid was first represented by Alan E. Florin in 1943. It heated a sample of neptunium (III ) fluoride in a nickel crucible in a fluorine gas stream and condensed volatile Neptuniumhexafluorid in a thin-walled glass capillary. The representation of Neptuniumhexafluorid of neptunium (III ) fluoride and neptunium (IV ) fluoride was submitted by Glenn T. Seaborg and Harrison S. Brown on October 17, 1947, patented on April 25, 1961.
Standard method
The common method for the preparation of Neptuniumhexafluorid is the reaction of neptunium (IV ) fluoride ( NpF4 ) with elemental fluorine (F2 ) at 500 ° C.
For comparison produced uranium hexafluoride ( UF6) relatively quickly even at 300 ° C from uranium tetrafluoride ( UF4 ) and fluorine ( F2), plutonium hexafluoride ( PuF6 ) only at 750 ° C from plutonium tetrafluoride ( PuF4 ) and F2.
In other starting substances
Neptuniumhexafluorid was prepared by fluorination of neptunium (III ) fluoride or neptunium ( IV) oxide.
In other fluorine compounds
The presentation also succeeds with strong fluorinating reagents such as bromine trifluoride ( BrF 3 ) or bromine pentafluoride ( BrF5 ). The conversion rates of the reaction of NpF4 at 350 ° C have the sequence: F2 (100 mol%) > BrF 3 (6-13 mol%) > BrF5 ( 33-35 mol%).
Neptuniumdioxid and tetrafluoride are virtually completely by Disauerstoffdifluorid ( O2F2 ) converted to volatile Neptuniumhexafluorid. This is achieved both in gas - solid reactions at moderate temperatures and in liquid anhydrous hydrogen fluoride at -78 ° C:
This reaction temperatures are in marked contrast to the otherwise high temperatures of over 200 ° C, which are previously required for the representation of NpF6 using elemental fluorine or a halogen fluoride. In this reaction neptunyl (VI ) fluoride ( NpO2F2 ) was identified by Raman spectroscopy as a key intermediate in the reaction with Neptuniumdioxid. Direct reaction between NpF4 with liquid O2F2 other hand, leads to a violent decomposition of O2F2 of low to no reaction to give NpF6.
Properties
Physical Properties
Neptuniumhexafluorid forms orange, orthorhombic crystals; under normal pressure ( 1013.25 hPa) it melts at 54.4 ° C and boils at 55.18 ° C.
The triple point where the three phases, liquid and gas are determined in equilibrium, is at a temperature of 55.10 ° C at a pressure of 1010 hPa ( 758 Torr), this means that below this pressure - the only slightly below the Normal pressure is - Neptuniumhexafluorid solid when heated by sublimation goes directly into the gaseous state.
The volatility of NpF6 is similar to that of uranium hexafluoride ( UF6) and plutonium hexafluoride ( PuF6 ); they belong together to the three known hexafluorides the actinoid. The of formation ( S0m ) is for NpF6 229.1 ± 0.5 J · K-1 · mol -1. Solid NpF6 is paramagnetic; the molar magnetic susceptibility is χmol 165 · 10-6 cm3 · mol -1.
Crystal and Molecular Structure
Neptuniumhexafluorid is a covalent compound and no salt. It crystallizes in the orthorhombic crystal system, space group Pnma ( D162h ) with the lattice parameters a = 990.9 pm, b = 899.7 pm and c = 520.2 pm and four formula units per unit cell. In the gaseous state it is composed of regular octahedral molecules (OH) with a uniform Np -F bond length of 198.1 pm.
Bond lengths and angles in the gaseous Neptuniumhexafluorid
Spectroscopic properties
Neptuniumhexafluorid has six fundamental vibrations. ν1, ν2 and ν3 are stretching vibrations and ν4, ν5 and ν6 are bending vibrations. Of these, ν1, ν2 and ν5 Raman - active ν3 and ν4 IR active, ν6 is IR and Raman inactive.
Chemical Properties
Neptuniumhexafluorid is resistant to dry air. It reacts violently with water, however (eg, humidity), wherein the water- soluble neptunyl (VI ) fluoride ( NpO2F2 ) and hydrogen fluoride (HF ) are produced.
It can be stored indefinitely at room temperature in quartz or Pyrex ampoules, if it is ensured that no traces of moisture are present, the glass itself from all gas inclusions and is free of any existing hydrogen fluoride ( HF) has been completely removed.
NpF6 and PuF6 are light sensitive and decompose to form the tetra fluoride and fluorine.
NpF6 forms with alkali metal fluorides fluoro. With cesium fluoride ( CsF ), it is at 25 ° C, the CsNpF6, sodium fluoride ( NaF) reacts reversibly to Na3NpF8. In both cases a reduction of the hexavalent neptunium to pentavalent occurs.
In the presence of chlorine trifluoride ( ClF 3 ) as a solvent and at lower temperatures, there are signs of a possible formation of an unstable Np (VI ) complex.
The hydrolysis of Neptuniumhexafluorid in nearly anhydrous hydrogen fluoride solution leads to the oxide - fluoride NpOF4. It has the same structure with the trigonal form of the UOF4. Oxidation of NpOF4 but by Kryptondifluorid ( KrF2 ) in anhydrous HF to the maximum possible oxidation state Np (VII) does not occur. The exchange reaction between NpF6 and boron trichloride ( BCl 3 ) leads to the formation of NpF4.
Use
Neptuniumhexafluorid plays a role in the separation of neptunium from both uranium and plutonium in particular.
For the reprocessing of spent fuel (95% of total mass ) has been proposed for the separation of uranium to crush the material fine and with elemental fluorine to treat ( " direct fluorination "). The resulting volatile fluorides (mainly UF6 and small amounts NpF6 ) can easily be of the non-volatile fluorides, such as plutonium (IV ) fluoride ( PuF4 ), americium (III ) fluoride ( AMF3 ), curium (III ) fluoride ( CMF3 ) and the fluorides of most fission products separate.
From a mixture of uranium and Neptuniumhexafluorid is selectively reduced to Neptuniumhexafluorid Neptuniumtetrafluorid pelleted with cobalt (II ) fluoride, at temperatures in the range of 93 to 204 ° C. The existing as a mixture of uranium hexafluoride does not address this reaction.
Safety
Neptuniumhexafluorid acts as follows on the human body:
- On contact with body fluids of the gas is hydrofluoric acid, which causes the skin and the mucous membranes of the respiratory tract is burns. The exposure of humans to the gas acts first on the eyes and respiratory tract and causes irritation, loss of vision, cough and excessive formation of saliva and sputum. After prolonged exposure, this leads to pneumonitis and pulmonary edema and may lead to death.
- Like all Neptuniumverbindungen it is radioactive. The activity is dependent on the isotopic composition of the neptunium. The most eligible 237Np has a half -life of 2,144,000 years and is an α - emitter. 235Np and 236Np have half-lives of 396.1 days or 154,000 years and are therefore more active.
Classifications according to the Hazardous Substances Ordinance are not available because they only include the chemical danger and play a very minor role compared to the risks based on the radioactivity. Even the latter applies only if there is a relevant material for this amount.