CHEM 312: Lecture 13 Neptunium Chemistry From: Chemistry of actinides http://radchem.nevada.edu/classes/rdch710 /lectures%20and%20chapters.html Nuclear properties and isotope production Aqueous phase chemistry Separation and Purification Metallic state Compounds Structure and coordination chemistry Analytical Chemistry 13-1
Neptunium nuclear properties 22 known Np isotopes 237 Np longest lived Neutron irradiation of U * Consecutive neutron capture on 235 U * 238 U(n,2n) 237 U 237 Np + β - * Alpha decay of 241 Am Used at target for 238 Pu production by neutron irradiation Reaction with 23 MeV and 30 MeV electrons to produce 236 Pu Critical mass is 73 kg 2500 kg in environment from fallout 238,239 Np Short half-life, useful radiotracers * From neutron irradiation of 237 Np and 238 U 235,236 Np Cyclotron irradiation of 235 U * 235 U(d,n) 236 Np * 235 U(p,n) 235 Np Np isotopes formed in Earth s crust Dynamic equilibrium established 13-2
Np solution chemistry and oxidation states Np exists from 3+ to 7+ Stable oxidation state favored by acidity, ligands, Np concentration 5+ and 6+ forms dioxocations Redox potentials Basic solutions Difficulty in understanding data Chemical forms of species Determine ratios of each redox species from XANES Use Nernst equation to determine potentials http://www.webelements.com/webelements/elements/text/np/redn.html 13-3
Np solution chemistry Disproportionation NpO 2 + forms Np 4+ and NpO 2 2+ Favored in high acidity and Np concentration 2NpO 2 + +4 H + Np 4+ + NpO 2 2+ + 2H 2 O K for reaction increased by addition of complexing reagents K=4E-7 in 1 M HClO 4 and 2.4E-2 in H 2 SO 4 * Suggested reaction rate -d[npo 2+ ]/dt=k[npo 2+ ][H + ] 2 Control of redox species Important consideration for experiments 13-4
Np solution chemistry Oxidation state control Redox reagents Adjustment from one redox state to another Best for reversible couples * No change in oxo group * If oxo group change occurs need to know kinetics Effort in PUREX process for controlled separation of Np focused on organics * HAN and derivates for Np(VI) reduction * Rate 1 st order for Np in excess reductant 1,1 dimethylhydrazine and tertbutylhydrazine selective of Np(VI) reduction over Pu(IV) 13-5
Np solution chemistry Applied to Np(III) to Np(VII) and coordination complexes Np(V) spin-orbit coupling for 5f 2 Absorption in 2 M HClO 4 Np(III): 786 nm, ε=45 Np(IV): 960 nm, ε=160 Np(V): 980 nm, ε=395 Np(VI): 1223 nm, ε=45 Np(VII) only in basic media NpO 6 5-2 long (2.2 Å) and 4 short (1.85 Å) Absorbance at 412 nm and 620 nm * O pi 5f * Number of vibrational states Between 681 cm -1 and 2338 cm -1 Range of complexation constants available Oxidation state trends same as hydrolysis Stability trends for inorganic F - >H 2 PO 4- >SCN - >NO 3- >Cl - >ClO - 4 CO 2-3 >HPO 2-4 >SO 2-4 NpO + 2 forms cation-cation complexes 13-6 Fe>In>Sc>Ga>Al
Np solution chemistry Np hydrolysis Np(IV)>Np(VI)>Np(III)>Np(V) For actinides trends with ionic radius Np(III) below ph 4 Stable in acidic solution, oxidizes in air Potentiometric analysis for determining K No K sp data Np(IV) hydrolyzes above ph 1 Tetrahydroxide main solution species in equilibrium with solid based on ph independence of solution species concentration Np(V) not hydrolyzed below ph 7 Np(VI) below ph 3-4 Np(VII) No data available Most separation methods exploit redox chemistry of Np 13-7
PUREX separations Np(V) not extracted in PUREX Np(V) slowly disproportionates in high acid Formation of extractable Np(IV,VI) Variation of Np behavior based on redox * Need to understand redox kinetics * Reduction of Np(VI) by a range of compounds Back extraction of Np(V) can be used to separate from Pu and U * Controlled Np(VI) reduction in presence of Pu(III) Hydrazine derivatives N-butyraldehyde Hydroxamic acids Acetohydroxamic acid shows preferential complexation with tetravalent Np and Pu O H 3 C C N OH 13-8 H
Np solvent extraction Tributylphosphate NpO 2 (NO 3 ) 2 (TBP) 2 and Np(NO 3 ) 4 (TBP) 2 are extracted species Extraction increases with increase concentration of TBP and nitric acid * 1-10 M HNO 3 Separation from other actinides achieved by controlling Np oxidation state CMPO (Diphenyl-N,N-dibutylcarbamoyl phosphine oxide) Usually used with TBP Nitric acid solutions Separation achieved with oxidation state adjustment Reduction of Pu and Np by Fe(II) sulfamate Np(IV) extracted into organic, then removed with carbonate, oxalate, or EDTA 13-9
Np solvent extraction HDEHP (Bis(2-ethyl-hexyl)phosphoric acid ) In 1 M HNO 3 with addition of NaNO 2 U, Pu, Np, Am in most stable oxidation states Np(V) is not extracted Oxidized to Np(VI) then extracted Reduced to Np(V) and back extracted into 0.1 M HNO 3 Tri-n-octylamine Used for separation of Np from environmental samples Extracted from 10 M HCl Back extracted with 1 M HCl+0.1 M HF HDEHP 13-10
Metallic Np First synthesis from NpF 3 with Ba at 1473 K Current methods NpF 4 with excess Ca NpO 2 in a molten salt process Can also use Cs 2 NpO 2 Cl 4 and Cs 3 NpO 2 Cl 4 LiCl/KCl as electrolyte at 723 K NpC reduction with Ta followed by volatilization of Np Electrodepostion from aqueous solution Amalgamation with Hg from 1 M CH 3 COOH and 0.3 M CH 3 COONa at ph 3.5 Distillation to remove Hg 13-11
Metallic Np data Melting point 912 K Boiling point estimated at 4447 K Density 19.38 g/ml Three metallic forms Enthalpies and entropies of transitions α β * Transition T 553 K * ΔS=10.1 JK -1 mol -1 * ΔH=5.607 kjmol -1 β γ * Transition T 856 K * ΔS=6.23 JK -1 mol -1 * ΔH=5.272 kjmol -1 13-12
Two known anhydrous oxides Np 2 O 5 and NpO 2 NpO 2 From thermal decomposition of a range of Np compounds Isostructural with other actinides Fluorite lattice parameter Stable over a range of temperatures Phase change from fcc to orthorhombic at 33 GPa Stable to 2.84 MPa and 673 K Np 2 O 5 From thermal decomposition of NpO 3. H 2 O or NpO 2 OH (am) Np 2 O 5 decomposes to NpO 2 from 693 K to 970 K Neptunium oxides 13-13
Np halides Fluorides NpF 3, NpF 4, NpF 5, and NpF 6 Prepared from reactions with HF at 773 K NpO 2 +1/2H 2 +3HF NpF 3 + 2H 2 O NpF 3 +1/4O 2 +HF NpF 4 + 1/2H 2 O NpO 2 +4HF NpF 4 + 2H 2 O 10NpF 6 +I 2 10NpF 5 +2IF 5 * Other route where Np(VI) is reduced NpF 6 is volatile Melting point at 327.8 K * Higher vapor pressure that U and Pu compound Can form Np(V) species upon reaction with NaF * NpF 6 +3NaF Na 3 NpF 8 + 1/2F 2 U will stay as hexavalent compound Range of monovalent species with Np fluorides Synthesis similar to U compound NpO 2 F 2 intermediate species KrF 2 used as fluorinating agent for some synthetic routes 13-14
Np halides NpCl 4 From the reaction of NpO 2 with CCl 4 Addition of H 2 yields NpCl 3 Similar to U reactions Several melting point reported Heating for NpOCl 2 NpBr 4 NpO 2 with AlBr 3 Reaction of elements Same for AlI 3 for NpI 3 Synthesis reactions similar to U species Measured data on Np compounds limited 13-15
Np coordination compounds Interests driven from different Np oxidation states and systematic studies of actinides Np 3+ Very little data Instable in aqueous solutions under air Trivalent state stabilized by sodium formaldehyde sulfoxylate (NaHSO 2. CH 2 O. 2H 2 O) Formation of oxalate and salicylate species * 2 Np, 3 ligands * No O 2 in synthesis Np 4+ Et 4 NNp(NCS) 8 Isostructural with U complex Range of nitrate compounds Np(V) Exhibit cation-cation interaction Na 4 (NpO 4 ) 2 C 12 O 12 Dissolve neptunium hydroxide in solution with mellitic acid Adjust to ph 6.5 with base Slowly evaporate 13-16
Np coordination compounds Np(VI) Some simple synthesis Oxalic acid to Np(VI) solutions * Reduction of Np over time Ammonium carbonate species * Excess (NH 4 ) 2 CO 3 to nitrate solutions of Np(VI) Np(VII) Some disagreement on exact species Mixed species with Co, Li, NH 3 and OH 13-17
Np Organometallic compounds Mainly cyclopentadienyl and cyclooctatetraenyl compounds Np cyclopentadienyl Reduction of Np 4+ complex with Na Np(C 5 H 5 ) 3 Cl + Na Np(C 5 H 5 ) 3. 3THF + NaCl CP Difficult to remove THF * Heating and vacuum Np 4+ NpCl 4 +4KC 5 H 5 Np(C 5 H 5 ) 4 +4KCl Dissolves in benzene and THF * Less sensitive to H 2 O and O 2 than tetravalent Pu and Am compound Halide salt of Np compound reported * NpX 4 + 3 KC 5 H 5 Np(C 5 H 5 ) 3 X +3KX * Can use as starting material and replace X with ligands Inorganic (other halides); NC 4 H 4-, N 2 C 3 H 3-, CH - 13-18
Analytical methods Environmental levels General levels 1E-15 g/l Elevated levels up to 1E-11 g/l Radiometric methods Alpha 2.6E7 Bq/g Isolation from seawater * Hydroxide co-precipitation, ion-exchange, LaF 3, solvent extraction with HTTA Liquid scintillation Activation analysis Formation of 238 Np * 170 barns, 2.117 day half life for 238 Np * 500 more sensitive than alpha spectroscopy 13-19
Analytical methods Spectrophotometric methods Direct absorbance Detection limit in M (1 cm cell, 0.02 absorbance) * Np(III) 5E-4, Np(IV) 1E-4, Np(V) 5E-5, Np(VI) 5E-4 Laser induced photoacoustic spectroscopy (LIPAS) Increase factor by over an order of magnitude Indicator dyes Fluorescence New work in tetrachlorides and solids Luminescence at 651 nm and 663 nm from Np in CaF 2 at 77 K X-ray fluorescence Mass spectroscopy 13-20
Analytical methods: 237 Np Moessbauer spectroscopy 68 ns excited state lifetime Isomer shift suitable for analysis of chemical bonds Can record radiation spectrum from absorber 60 kev from 241 Am Shift correlated with oxidation state and number of 5f electrons present 13-21
Review Oxidation states of Np in solution Role of different oxidation states in separations Np separations Distribution with ligands in solvent extraction Synthesis of Np metal Np oxides and fluorides Coordination and organometallic compounds Analytical methods 13-22
Questions What is the primary solution phase oxidation state of Np? What is the NpO + 2 disproportionation reaction? How does NpO + 2 disproportionation impact its behavior in the PUREX process? Details on PUREX in lecture 17 What are the Np binary oxides? 2NpO 2 + +4 H + Np 4+ + NpO 2 2+ + 2H 2 O High acidity in process: NpO 2 + does not extract Np 4+ and NpO 2 2+ extracts Np 2 O 5 and NpO 2 13-23
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