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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

à 231,234Pa§ Reactor produced 233Pa

à 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

spectraà 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

à 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

à 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

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

§ Addition of KFà K2PaF7

* Separates Pa from Zr, Nb, Ti, and Taà NH4

+ double salt* Pa crystallizes before Zr but after Ti and Ta* Selective precipitation

§ Reduction in presence of fluoridesà Zn amalgam in 2 M HFà PaF4 precipitates

* Redissolve with H2O2 or air§ H2O2 precipitation

à No Nb, Ta, and Ti precipitates* Selective formation of peroxide actinide species

§ Silicatesà Pa for K, Na silicates with alumina

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Pa purification: Ion exchange

• Anion exchange with HCl§ Pa sorbs to column in 9-10

M HClà 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

acidà Suppresses dissociation

of HFà Lowers Kd

§ Addition of NH4SCNà 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-isobutylketoneà Pa extracted into organic from 4.5 M H2SO4 and 6 M HClà Removal from organic by 9 M H2SO4 and H2O2

§ Di-isopropylketoneà Used to examine Pa, Nb, Db

* Concentrated HBr* Pa>Nb>Db

• TTA§ 10 M HCl

à PaOCl63-

§ With TBP, Tri-n-octylphosphine oxide (TOPO), or triphenylphosphine oxide (TPPO)• Triisooctylamine

§ Mixture of HCl and HFà 0.5 M HCl and 0.01 M HF

* Used to examine the column extractionØ Sorbed with 12 M HCl and 0.02 M HFØ 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 HFØ 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/235Uà Use of gamma spectroscopyà 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

à Confirmed by experiment and calculationsà Calculation for other ions

* Pa2+ [Rn] 5f26d1

* Pa3+ [Rn] 5f2

* Pa4+ [Rn]5f1

• Emission spectra of Pa§ 231Pa

à Numerous lines, hyperfine splitting* 3/2 nuclear spin

• Moessbauer effect§ Beta decay of 231Th produces 84.2 kev excited state in 231Pa§ 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 foilà i.e., Ba with BaF2 of LiFà 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

à Valence electron transition spd to 5f* Similar to U

à Body-centered tetragonal

à High pressure fcc or bcc* As pressure

increases f electron band broadens

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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

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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

à 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

à 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

Rhombohedral(trigonal)

orthorhombic hexagonal

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• Synthesis based on aqueous acidic solution of pentavalent Pa§ Volatile at relatively low temperatures § Used in separation of Pa from Th

• Pa fluorides§ PaF5

à Fluorination of PaC (570 K) or PaCl5 (295 K)* PaC used for formation of

other halidesØ PaI5 with I2 (400 ºC)Ø PaI4 from PaI5 and PaC

(600 ºC)à Isostructural with b-UF5

§ PaF5. 2H2O

à Evaporation of Pa in 30% HF solution

• PaCl5

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

Pa halides

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Pa halides• Number of alkali fluoro complexes formed

§ K2PaF7

§ MPaF6

à M= group 1, Ag, NH4

* HF solutions equimolar Pa and M-fluorides§ M2PaF7

à M=K, HN4, Rb, Csà Precipitated from 17 M HF with Pa(V) by addition of acetone and excess fluoride

§ M3PaF8 from M2PaF7 and MF à 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 PaX62-

§ 5f16d1 transitionà 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

à f-f transitions separated from charge transfer bandsà based on relativistic calculations

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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

à Body centered § Electronic properties

à 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-

à Decomposes to HPaOSO4 at 375 ºCà 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 à Can form LiPa(Trop)5

• PaO(H2PO4)3.2H2O

§ From Pa(V) hydroxide or peroxide in 14 M H3PO4

à Heating to 300 ºC forms PaO(H2PO4)3 anhydrousà Heating to 900 ºC PaO(PO3)3

à 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, method to examine speciation

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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

à 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

à 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

à Low solubility due to mixed hydroxide species

à Higher solubility due to 1:2 Pa:C2O4

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Solution chemistry• Redox behavior

§ Reduction in Zn amalgam § Electrochemistry

methodsà Pt-H2 electrodeà Acidic solutionà Polarographic

methods* One wave

Ø 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 solutionsà i.e., HF

• Spectroscopy§ 6d15f1

à Peak at 460 nm

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Analytical methods

• Radiochemical§ Alpha and gamma spectroscopy for 231Pa§ Beta spectroscopy for 234Pa

à 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

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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 methodsà Radiochemical, spectroscopic

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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

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Pop Quiz

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