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Lecture 5: Protactinium Chemistry

• From: Chemistry of actinides Nuclear properties Pa purification Atomic properties Metallic state Compounds Solution chemistry Analytical Chemistry

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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 =211 b Small fission branch (t1/2=1.1E16 a) Complex alpha and gamma spectra

Photopeak at 27.35 keV• 234Pa

Metastable state

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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• 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 carriers Pa

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Pa purification• Difficult to separate from Zr, Ta, and Nb with macro amounts of

Pa• 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

Reduction in presence of fluorides Zn amalgam in 2 M HF PaF4 precipitates

* Redissolve with H2O2 or air current H2O2 precipitation

No Nb, Ta, and Ti precipitates Silicates

K, Na silicates with alumina

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Pa purification

• Ion exchange Anion exchange with HCl

Adhere 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

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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

Dimethyl sulfoxide

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Pa purification• TTA

10 M HCl PaOCl6

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

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Pa purification

• Aliquat 336 Methyl-

trioctylammonium chloride

Extraction from HF, HCl, and HBr

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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

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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 kev Use of Pa2O5 and PaO2

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Pa atomic properties

X-ray energy in eV

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Metallic Pa

• Preparation Bombarding Pa2O5 for several hours with 35 kV

electrons at 5-10 mA Pentahalide heated on W filament at 10-6 torr 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

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Metallic Pa• Preparation

Pa precipitated with dilute H2SO4, HF solution on metal plate (Zn, Al, Mn)

Electrolytic reduction from HN4F 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

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Metallic Pa• 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

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Pa compounds

• Pa hydrides (PaH3) H2 with Pa at 250 ºC at 600 torr Black flaky, isostructural with -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

fcc NaCl type structure At 2200 ºC new lines from XRD attributed to PaC2

5f electrons calculated to be important in bonding

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Pa oxide

• 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

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orthorhombic hexagonal

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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) of PaCl5 (295 K)* PaC used for formation of other halides

PaI5 with I2 (400 ºC) PaI4 from PaI5 and PaC (600 ºC)

Isostructural with -UF5

PaF5. 2H2O

Evaporation of Pa in 30% HF solution• PaCl5

Pa2O5 with Cl2 and CCl4 (300 ºC), reduction at 400 ºC

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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

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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 PaX6

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5f16d1 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* Calculations based on relativistic calculations

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Pa Pnictides

• 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

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Various compounds

• 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

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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

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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

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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->SO42->Cl->Br->I->NO3

-≥ClO4-

• Nitric acid Pa(V) stabilized in [HNO3]M>1 Transition to anionic at 4 M HNO3

• 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

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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

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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

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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

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Solution chemistry• Pa(IV)

Precipitates in acidic solutions i.e., HF

• Spectroscopy 6d15f1

Peak at 460 nm

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Analytical methods• Radiochemical

Alpha and gamma spectroscopy for 231Pa Beta spectroscopy for 234Pa

Overlap with 234Th• Activation analysis

231Pa(n,)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