Lecture 5: Protactinium Chemistry From: Chemistry of actinides Nuclear properties Pa purification Atomic properties Metallic state Compounds Solution chemistry Analytical Chemistry 5-1
Pa Nuclear Properties 29 known isotopes 2 naturally occurring 231,234 Pa Reactor produced 233 Pa From irradiation of 232 Th 231 Pa Longest lived Pa isotopes Large thermal capture σ=211 b Small fission branch (t 1/2 =1.1E16 a) Complex alpha and gamma spectra Photopeak at 27.35 kev 234 Pa Metastable state 5-2
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Preparation and purification Pa is primarily pentavalent Pa has been separated in weighable amounts during U purification Diethylether separation of U Precipitation as carbonate Use of Ta as carrier * Ta is pentavalent, does not form yl species Sulfate precipitation of Ra at ph 2 Inclusion of H 2 O 2 removes U and 80 % of Pa Isolated and redissolved in nitric acid Pa remains in siliceous sludge Ability to separate Pa from Th and lanthanides by fluoride precipitation Pa forms anionic species that remain in solution Addition of Al 3+ forms precipitate that carries Pa 5-4
Pa purification Difficult to separate macro amounts of Pa from Zr, Ta, and Nb Precipitation Addition of KF K 2 PaF 7 * Separates Pa from Zr, Nb, Ti, and Ta NH + 4 double salt * Pa crystallizes before Zr but after Ti and Ta * Selective precipitation Reduction in presence of fluorides Zn amalgam in 2 M HF PaF 4 precipitates * Redissolve with H 2 O 2 or air H 2 O 2 precipitation No Nb, Ta, and Ti precipitates * Selective formation of peroxide actinide species Silicates Pa for K, Na silicates with alumina 5-5
Pa purification: Ion exchange Anion exchange with HCl Pa sorbs to column in 9-10 M HCl Fe(III), Ta, Nb, Zr, U(IV/VI) also sorbs Elute with mixture of HCl/HF HF Sorbs to column Elute with the addition of acid Suppresses dissociation of HF Lowers K d Addition of NH 4 SCN Numerous species formed, including mixed oxide and fluoride thiocyanates 5-6
Pa purification: Solvent extraction At trace levels (<1E-4 M) extraction effective from aqueous phase into a range of organics Di-isobutylketone Pa extracted into organic from 4.5 M H 2 SO 4 and 6 M HCl Removal from organic by 9 M H 2 SO 4 and H 2 O 2 Di-isopropylketone Used to examine Pa, Nb, Db * Concentrated HBr * Pa>Nb>Db TTA 10 M HCl PaOCl 6 3- With TBP, Tri-n-octylphosphine oxide (TOPO), or triphenylphosphine oxide (TPPO) Triisooctylamine Mixture of HCl and HF 0.5 M HCl and 0.01 M HF * Used to examine the column extraction Sorbed with 12 M HCl and 0.02 M HF Elute with 10 M HCl and 0.025 M HF, 4 M HCl and 0.02 M HF, and 0.5 M HCl and 0.01 M HF Extraction sequence Ta>Nb>Db>Pa 5-7
Pa purification Aliquat 336 Methyltrioctylammonium chloride Extraction from HF, HCl, and HBr 5-8
Application of Pa Scintillator for x-ray detection Oxides of Gd, Pa, Cs, and lanthanides Cathode ray Green fluorescence Dating 231 Pa/ 235 U Use of gamma spectroscopy Range of 100K a Geology 231 Pa/ 235 U ratios related to formation conditions 5-9
Atomic properties Pa ground state [Rn] 5f 2 6d 1 7s 2 Relativistic calculations favor [Rn] 5f 1 6d 2 7s 2 by 0.9 ev Pa + [Rn] 5f 2 7s 2 Confirmed by experiment and calculations Calculation for other ions * Pa 2+ [Rn] 5f 2 6d 1 * Pa 3+ [Rn] 5f 2 * Pa 4+ [Rn]5f 1 Emission spectra of Pa 231 Pa Numerous lines, hyperfine splitting * 3/2 nuclear spin Moessbauer effect Beta decay of 231 Th produces 84.2 kev excited state in 231 Pa Use of Pa 2 O 5 and PaO 2 5-10
Pa atomic properties X-ray energy in ev 5-11
Metallic Pa: Preparation Bombarding Pa 2 O 5 for several hours with 35 kv electrons at 5-10 ma Pentahalide heated on W filament at 10-6 torr PaF 4 treated with Ba, Ca, or Li vapors In crucible of single fluoride crystal supported by Ta foil i.e., Ba with BaF 2 of LiF About 15 mg of metal Larger amounts (500 mg) PaC from Pa 2 O 5 with C Heating PaC with I 2 form volatile PaI 5 PaI 5 decomposed on W filament 5-12
Preparation Pa precipitated with dilute H 2 SO 4, HF solution on metal plate (Zn, Al, Mn) Electrolytic reduction from HN 4 F solution with triethylamine at ph 5.8 Calculated phase transition at 1 Mbar Alpha to beta phase Valence electron transition spd to 5f * Similar to U Body-centered tetragonal High pressure fcc or bcc * As pressure increases f electron band broadens Metallic Pa 5-13
Metallic Pa reactions Metal attacked by 8 M HCl, 12 M HF, 2.5 M H 2 SO 4 Reaction starts quickly, slows due to formation of protective hydrolysis layer on Pa(IV) or Pa(V) Does not react with 8 M HNO 3 :0.01 M HF Very slow oxidation of metal Formation of Pa 2 O 5 from reaction with O 2, H 2 O, or CO 2 from 300-500 ºC Metal with NH 3 forms PaN 2 Metal with H 2 yields PaH 3 Formation of PaI 5 from metal with I 2 above 400 ºC Alloys with noble metal from reduction with Pa 2 O 5 5-14
Pa hydrides (PaH 3 ) H 2 with Pa at 250 ºC at 600 torr Black flaky, isostructural with β-uh 3 Cubic compound Two different phases found Prepared at 250 and 400 ºC Pa carbide (PaC) Reduction of Pa 2 O 5 with C, reduced temperature at 1200 ºC fcc NaCl type structure At 2200 ºC new lines from XRD attributed to PaC 2 5f electrons calculated to be important in bonding Pa 2 O 5 common oxide form Heat of formation 106 kj/mol PaO 2 from the reduction of Pa 2 O 5 with H 2 at 1550 ºC Did not dissolve in H 2 SO 4, HNO 3, or HCl Reacts with HF Pa 2 O 9 from Pa(V) in 0.25 M H 2 SO 4 with H 2 O 2 Ternary oxides PaO 2 or Pa 2 O 5 with oxides of other elements Pa compounds Rhombohedral (trigonal) orthorhombic hexagonal 5-15
Synthesis based on aqueous acidic solution of pentavalent Pa Volatile at relatively low temperatures Used in separation of Pa from Th Pa fluorides PaF 5 Fluorination of PaC (570 K) or PaCl 5 (295 K) * PaC used for formation of other halides PaI 5 with I 2 (400 ºC) PaI 4 from PaI 5 and PaC (600 ºC) Isostructural with β-uf 5 PaF 5. 2H 2 O Evaporation of Pa in 30% HF solution PaCl 5 Pa 2 O 5 with Cl 2 and CCl 4 (300 ºC), reduction at 400 ºC Pa halides 5-16
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Pa halides Number of alkali fluoro complexes formed K 2 PaF 7 MPaF 6 M= group 1, Ag, NH 4 * HF solutions equimolar Pa and M-fluorides M 2 PaF 7 M=K, HN 4, Rb, Cs Precipitated from 17 M HF with Pa(V) by addition of acetone and excess fluoride M 3 PaF 8 from M 2 PaF 7 and MF 450 ºC 5-18
Pa halides: Properties Paramagnetic resonance of PaCl 4 Confirm 5f 1 electronic structure 231 Pa nuclear spin of 3/2 PaCl 4 insoluble in SOCl 2 Electronic structures and optical properties calculated for PaX 6 2-5f 1 6d 1 transition Fluorescence and absorption spectra of ground and excited states evaluated Metal ligand covalent bonding with 5f and 6d Pa orbitals 6d atomic orbital characteristic increases with mass of fluoride Stabilization of 5f with np orbitals f-f transitions separated from charge transfer bands based on relativistic calculations 5-19
Pa Pnictides and other compounds PaP 2 Elemental P with PaH 3 Thermal dissociation forms Pa 3 P 4 PaAs 2 Tetragonal structure PaH 3 with elemental As at 400 ºC Heating to 800 ºC yields Pa 3 As 4 Body centered Electronic properties PaN and PaAs have about 1 f electron * Paramagnetic PaO(NO 3 ) 2 Dissolved Pa(V) compounds in fuming nitric acid Vacuum evaporation Pa 2 O(NO 3 ) 4 Pa(V) halides with N 2 O 5 in CH 3 CN Acetonitrile coordination to compound MPa(NO 3 ) 6 from PaX 5 - in N 2 O 5 M=Cs, N(CH 3 ) 4, N(C 2 H 5 ) 4 H 3 PaO(SO 4 ) 3 Pa(V) in HF H 2 SO 4 mixture evaporated to eliminate F - Decomposes to HPaOSO 4 at 375 ºC Forms Pa 2 O 5 at 750 ºC SeO 4 complex form HF H 2 SeO 4 mixture 5-20
Various compounds Pa(IV) tropolone PaTrop 4 PaX 4 (Br, Cl) with LiTrop in methylene chloride Can form LiPa(Trop) 5 PaO(H 2 PO 4 ) 3. 2H 2 O From Pa(V) hydroxide or peroxide in 14 M H 3 PO 4 Heating to 300 ºC forms PaO(H 2 PO 4 ) 3 anhydrous Heating to 900 ºC PaO(PO 3 ) 3 Formation of (PaO) 4 (P 2 O 7 ) 3 at 1000 ºC 5-21
Solution chemistry Both tetravalent and pentavalent states in solution No conclusive results on the formation of Pa(III) Solution states tend to hydrolyze Hydrolysis of Pa(V) Usually examined in perchlorate media 1 st hydrolyzed species is PaOOH 2+ PaO(OH) 2 + dominates around ph 3 Neutral Pa(OH) 5 form at higher ph Pa polymers form at higher concentrations Constants obtained from TTA extractions Evaluated at various TTA and proton concentrations and varied ionic strength Fit with specific ion interaction theory Absorption due to Pa=O, method to examine speciation 5-22
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Solution chemistry Pa(V) in mineral acid Normally present as mixed species Characterized by solvent extraction or anion exchange Relative complexing tendencies F - >OH - >SO 4 2- >Cl - >Br - >I - >NO 3- ClO 4 - Nitric acid Pa(V) stabilized in [HNO 3 ]M>1 Transition to anionic at 4 M HNO 3 or HCl Precipitation starts when Pa is above 1E-3 M Pa(V) stable between 1 and 3 M PaOOHCl + above 3 M HCl HF High solubility of Pa(V) with increasing HF concentration Up to 200 g/l in 20 M HF Range of species form, including anionic 5-24
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Solution chemistry Sulfuric acid Pa(V) hydroxide soluble in H 2 SO 4 At low acid (less than 1 M) formation of hydrated oxides or colloids At high acid formation of H 3 PaO(SO 4 ) 3 5-27
Organic complexes Use of ion exchange to determine stability constants Oxalic acid Low solubility in 0.05 M Increase solubility above 0.05 M Low solubility due to mixed hydroxide species Higher solubility due to 1:2 Pa:C 2 O 4 5-28
Solution chemistry Redox behavior Reduction in Zn amalgam Electrochemistry methods Pt-H 2 electrode Acidic solution Polarographic methods * One wave V to IV Calculation of divalent redox Pa(IV) solution Oxidized by air Rate decreases in absence of O 2 and complexing ions Pa(IV) Precipitates in acidic solutions i.e., HF Spectroscopy 6d 1 5f 1 Peak at 460 nm 5-29
Analytical methods Radiochemical Alpha and gamma spectroscopy for 231 Pa Beta spectroscopy for 234 Pa Overlap with 234 Th Activation analysis 231 Pa(n,γ) 232 Pa, 211 barns Spectral methods 263 lines from 264 nm to 437 nm Microgram levels Electrochemical methods Potentiometric oxidation of Pa(V) Absorbance Requires high concentrations Arsenazo-III Gravimetric methods Hydroxide from precipitation with ammonium hydroxide 5-30
Overview Nuclear properties 231 Pa for chemistry, limited availability Pa purification Purification from U Metallic state Properties and methods for formation Compounds Binary elements, routes for synthesis Solution chemistry Spectroscopy Analytical Chemistry Range of methods Radiochemical, spectroscopic 5-31
Questions Which Pa isotopes decay by β? Which is the longest lived Pa isotopes? Provide 2 ion exchange methods for the purification of Pa? Which organic soluble ligands can be used to separate Pa? What are 2 methods for the preparation of Pa metal? Which Pa oxide compounds can be formed? What routes can be used for the preparation of Pa halides? What is the general solution chemistry of Pa? Provide 3 methods to evaluate Pa concentrations 5-32
Pop Quiz Describe the range of data and issues related to Pa hydroxides. 5-33