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
 231,234Pa
 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
5-2
5-3
5-4
5-5
5-6
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
5-7
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
5-8
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
5-9
5-10
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
H2 O2
 Di-isopropylketone
* Used to examine Pa, Nb, Db
 Concentrated HBr
 Pa>Nb>Db
 Dimethyl sulfoxide
5-11
Pa purification
• 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
5-12
Pa purification
• Aliquat 336
 Methyltrioctylammonium
chloride
 Extraction from
HF, HCl, and HBr
5-13
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
5-14
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
5-15
Pa atomic properties
X-ray energy in eV
5-16
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
5-17
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
5-18
5-19
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
5-20
5-21
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
 fcc NaCl type structure
 At 2200 ºC new lines from XRD attributed to PaC2
 5f electrons calculated to be important in bonding
5-22
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
5-23
Rhombohedral
(trigonal)
orthorhombic
hexagonal
5-24
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 b-UF5

PaF5. 2H2O
 Evaporation of Pa in 30% HF solution
• PaCl5

Pa2O5 with Cl2 and CCl4 (300 ºC), reduction at 400 ºC
5-25
5-26
5-27
5-28
5-29
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
5-30
5-31
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
* Calculations based on relativistic calculations
5-32
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
5-33
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
5-34
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
5-35
5-36
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
5-37
5-38
5-39
5-40
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
5-41
5-42
5-43
5-44
5-45
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
5-46
5-47
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
5-48
5-49
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
5-50
Solution chemistry
•
•
Pa(IV)

Precipitates in acidic solutions
 i.e., HF
Spectroscopy

6d15f1
 Peak at 460 nm
5-51
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
5-52