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Lecture 5: Protactinium Chemistry

Lecture 5: Protactinium Chemistry. From: Chemistry of actinides Nuclear properties Pa purification Atomic properties Metallic state Compounds Solution chemistry Analytical Chemistry. Pa Nuclear Properties. 29 known isotopes 2 naturally occurring 231,234 Pa

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Lecture 5: Protactinium Chemistry

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  1. Lecture 5: Protactinium Chemistry • From: Chemistry of actinides • Nuclear properties • Pa purification • Atomic properties • Metallic state • Compounds • Solution chemistry • Analytical Chemistry

  2. Pa Nuclear Properties • 29 known isotopes • 2 naturally occurring • 231,234Pa • Reactor produced 233Pa • From irradiation of 232Th • 231Pa • Longest lived Pa isotopes • Large thermal capture s=211 b • Small fission branch (t1/2=1.1E16 a) • Complex alpha and gamma spectra • Photopeak at 27.35 keV • 234Pa • Metastable state

  3. 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 H2O2 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 Al3+ forms precipitate that carries Pa

  4. Pa purification • Difficult to separate macro amounts of Pa from Zr, Ta, and Nb • Precipitation • Addition of KF • K2PaF7 • Separates Pa from Zr, Nb, Ti, and Ta • NH4+ double salt • Pa crystallizes before Zr but after Ti and Ta • Selective precipitation • Reduction in presence of fluorides • Zn amalgam in 2 M HF • PaF4 precipitates • Redissolve with H2O2 or air • H2O2 precipitation • No Nb, Ta, and Ti precipitates • Selective formation of peroxide actinide species • Silicates • Pa for K, Na silicates with alumina

  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 Kd • Addition of NH4SCN • Numerous species formed, including mixed oxide and fluoride thiocyanates

  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 H2SO4 and 6 M HCl • Removal from organic by 9 M H2SO4 and H2O2 • Di-isopropylketone • Used to examine Pa, Nb, Db • Concentrated HBr • Pa>Nb>Db • TTA • 10 M HCl • PaOCl63- • 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

  7. Pa purification • Aliquat 336 • Methyl-trioctylammonium chloride • Extraction from HF, HCl, and HBr

  8. Application of Pa • Scintillator for x-ray detection • Oxides of Gd, Pa, Cs, and lanthanides • Cathode ray • Green fluorescence • Dating • 231Pa/235U • Use of gamma spectroscopy • Range of 100K a • Geology • 231Pa/235U ratios related to formation conditions

  9. Atomic properties • Pa ground state [Rn] 5f26d17s2 • Relativistic calculations favor [Rn] 5f16d27s2 by 0.9 eV • Pa+ [Rn] 5f27s2 • Confirmed by experiment and calculations • Calculation for other ions • Pa2+ [Rn] 5f26d1 • Pa3+ [Rn] 5f2 • Pa4+ [Rn]5f1 • Emission spectra of Pa • 231Pa • Numerous lines, hyperfine splitting • 3/2 nuclear spin • Moessbauer effect • Beta decay of 231Th produces 84.2 kevexcited state in 231Pa • Use of Pa2O5 and PaO2

  10. Pa atomic properties X-ray energy in eV

  11. Metallic Pa: Preparation • Bombarding Pa2O5 for several hours with 35 kV electrons at 5-10 mA • Pentahalide heated on W filament at 10-6torr • PaF4 treated with Ba, Ca, or Li vapors • In crucible of single fluoride crystal supported by Ta foil • i.e., Ba with BaF2 of LiF • About 15 mg of metal • Larger amounts (500 mg) • PaC from Pa2O5 with C • Heating PaC with I2 form volatile PaI5 • PaI5 decomposed on W filament

  12. Metallic Pa • Preparation • Pa precipitated with dilute H2SO4, HF solution on metal plate (Zn, Al, Mn) • Electrolytic reduction from HN4Fsolution 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

  13. Metallic Pa reactions • Metal attacked by 8 M HCl, 12 M HF, 2.5 M H2SO4 • Reaction starts quickly, slows due to formation of protective hydrolysis layer on Pa(IV) or Pa(V) • Does not react with 8 M HNO3:0.01 M HF • Very slow oxidation of metal • Formation of Pa2O5 from reaction with O2, H2O, or CO2 from 300-500 ºC • Metal with NH3 forms PaN2 • Metal with H2 yields PaH3 • Formation of PaI5 from metal with I2 above 400 ºC • Alloys with noble metal from reduction with Pa2O5

  14. Pa compounds • Pa hydrides (PaH3) • H2 with Pa at 250 ºC at 600 torr • Black flaky, isostructural with b-UH3 • Cubic compound • Two different phases found • Prepared at 250 and 400 ºC • Pa carbide (PaC) • Reduction of Pa2O5 with C, reduced temperature at 1200 ºC • fccNaCl type structure • At 2200 ºC new lines from XRD attributed to PaC2 • 5f electrons calculated to be important in bonding • Pa2O5 common oxide form • Heat of formation 106 kJ/mol • PaO2 from the reduction of Pa2O5 with H2 at 1550 ºC • Did not dissolve in H2SO4, HNO3, or HCl • Reacts with HF • Pa2O9 from Pa(V) in 0.25 M H2SO4 with H2O2 • Ternary oxides • PaO2 or Pa2O5 with oxides of other elements orthorhombic hexagonal Rhombohedral (trigonal)

  15. Pa halides • Synthesis based on aqueous acidic solution of pentavalent Pa • Volatile at relatively low temperatures • Used in separation of Pa from Th • Pa fluorides • PaF5 • Fluorination of PaC (570 K) or PaCl5 (295 K) • PaC used for formation of other halides • PaI5 with I2 (400 ºC) • PaI4 from PaI5 and PaC (600 ºC) • Isostructural with b-UF5 • PaF5. 2H2O • Evaporation of Pa in 30% HF solution • PaCl5 • Pa2O5 with Cl2 and CCl4 (300 ºC), reduction at 400 ºC

  16. Pa halides • Number of alkali fluoro complexes formed • K2PaF7 • MPaF6 • M= group 1, Ag, NH4 • HF solutions equimolar Pa and M-fluorides • M2PaF7 • M=K, HN4, Rb, Cs • Precipitated from 17 M HF with Pa(V) by addition of acetone and excess fluoride • M3PaF8 from M2PaF7 and MF • 450 ºC

  17. Pa halides: Properties • Paramagnetic resonance of PaCl4 • Confirm 5f1 electronic structure • 231Pa nuclear spin of 3/2 • PaCl4 insoluble in SOCl2 • Electronic structures and optical properties calculated for PaX62- • 5f16d1 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

  18. Pa Pnictides and other compounds • PaP2 • Elemental P with PaH3 • Thermal dissociation forms Pa3P4 • PaAs2 • Tetragonal structure • PaH3 with elemental As at 400 ºC • Heating to 800 ºC yields Pa3As4 • Body centered • Electronic properties • PaN and PaAs have about 1 f electron • Paramagnetic • PaO(NO3)2 • Dissolved Pa(V) compounds in fuming nitric acid • Vacuum evaporation • Pa2O(NO3)4 • Pa(V) halides with N2O5 in CH3CN • Acetonitrile coordination to compound • MPa(NO3)6 from PaX5- in N2O5 • M=Cs, N(CH3)4, N(C2H5)4 • H3PaO(SO4)3 • Pa(V) in HF H2SO4 mixture evaporated to eliminate F- • Decomposes to HPaOSO4 at 375 ºC • Forms Pa2O5 at 750 ºC • SeO4 complex form HF H2SeO4 mixture

  19. Various compounds • Pa(IV) tropolone PaTrop4 • PaX4 (Br, Cl) with LiTrop in methylene chloride • Can form LiPa(Trop)5 • PaO(H2PO4)3.2H2O • From Pa(V) hydroxide or peroxide in 14 M H3PO4 • Heating to 300 ºC forms PaO(H2PO4)3 anhydrous • Heating to 900 ºC PaO(PO3)3 • Formation of (PaO)4(P2O7)3 at 1000 ºC

  20. 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 • 1st hydrolyzed species is PaOOH2+ • 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

  21. Solution chemistry • Pa(V) in mineral acid • Normally present as mixed species • Characterized by solvent extraction or anion exchange • Relative complexing tendencies • F->OH->SO42->Cl->Br->I->NO3-≥ClO4- • Nitric acid • Pa(V) stabilized in [HNO3]M>1 • Transition to anionic at 4 M HNO3or 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

  22. Solution chemistry • Sulfuric acid • Pa(V) hydroxide soluble in H2SO4 • At low acid (less than 1 M) formation of hydrated oxides or colloids • At high acid formation of H3PaO(SO4)3

  23. 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:C2O4

  24. Solution chemistry • Redox behavior • Reduction in Zn amalgam • Electrochemistry methods • Pt-H2 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 O2 and complexing ions • Pa(IV) • Precipitates in acidic solutions • i.e., HF • Spectroscopy • 6d15f1 • Peak at 460 nm

  25. Analytical methods • Radiochemical • Alpha and gamma spectroscopy for 231Pa • Beta spectroscopy for 234Pa • Overlap with 234Th • Activation analysis • 231Pa(n,g)232Pa, 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

  26. Overview • Nuclear properties • 231Pa 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

  27. Questions • Which Pa isotopes decay by b-? • 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

  28. Pop Quiz • Describe the range of data and issues related to Pa hydroxides.

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