Lecture 4: Thorium
Chemistry
• Chemistry of actinides
 Nuclear properties
 Th purification
 Metal
 Compounds
 Solution chemistry
4-1
Thorium isotopes
•
232Th




main isotope of Th
228Th from 232Th
decay
Other isotopes from
decay of U isotopes
 227,231Th (from
235U decay)
 230,234Th (from
238U decay)
Isotopes can be
isolated from U ore
 Free from 232Th
Other isotopes from
nuclear reactions with
Pb and Bi targets
4-2
Th ore processing
•
•
Main Th bearing mineral is monazite

Phosphate mineral
 strong acid for dissolution
results in water soluble
salts
 Strong base converts
phosphates to hydroxides
* Dissolve hydroxides in
acid
Th goes with lanthanides

Separate by precipitation

Lower Th solubility based on
difference in oxidation state
 precipitate at pH 1
* A number of different
precipitation steps can
be used
 Hydroxide
 Phosphate
 Peroxide
 Carbonate
(lanthanides
from U and
Th)
 U from Th by
solvent
extraction
4-3
Th atomic spectroscopy
• Electronic states of Th can provide information on higher actinide states

Neutral atom has available valence orbitals
 5f, 6d, 7s, 7p
 Stable 6d27s2 (3F2)
• Term symbol review

abbreviated description of angular momentum quantum numbers
2S+1L

J

S from unpaired electrons
 2 d electrons, S=1, 2S+1=3

L from orbital occupied by of unpaired electrons
 2 d electrons (5 orbitals; 2,1,0,-1,-2): 3
 3 is F
* S=0, P=1, D=2, F=3

J has some rules
 Less than half filled, J=|S-L|
 J=|1-3|=2
4-4
Th atomic spectroscopy
• Wide range of values based on configurations
• Singly ionized states

d2s, ds2, fs2, fds, d3, fd2
 Energy range from 1859 cm-1 to 12485 cm-1

p orbital occupation starts at 23372 cm-1
 dsp

Double f occupation at 24381 cm-1
 f2s
• Increase in ionic charge increases f orbital stabilization, decreases p
orbitals
• Odd or even electron parity

sum of p and f electrons defines parity

Strong spectral lines result only from transitions between
configurations of unlike parity
• Actinide data

http://www.lac.u-psud.fr/Database/Introduction/Table1-dir.html
4-5
Th levels (cm-1)
4-6
Th levels (cm-1)
4-7
Thorium metal synthesis
• Reduction of ThO2 with Ca
• Electrolysis of anhydrous ThCl4 in a fused
mixture of sodium and potassium chlorides
• Ca reduction of ThCl4 mixed with anhydrous
ZnCl2
 Formation of Th2Zn17
Distillation of Zn
• reduction of ThCl4 with an alkali metal
• Reduction of ThCl4 by DyCl2
• Decomposition of ThI4 on hot W surface
4-8
Th metal properties
• silvery-white metal which is air-stable

Oxide slowly forms, to gray and
finally black.
• Changes structure with temperature

ffc to bcc at 1360 ºC
 High pressure forms body
centered tetragonal
• Metal is paramagnetic (2 d electrons)
4-9
Th metal reactivity
• Attacked by oxygen, hydrogen, nitrogen, halogens, and
sulfur at elevated temperatures
• Dissolved by HCl
 Can form ThOClH
• Numerous alloys
 Mag-Thor magnesium alloys containing thorium
 magnesium-thorium-zirconium
 magnesium-thorium-zinc-zirconium
 magnesium-silver-thorium-rare earth metalzirconium
* Alloys have high strength, creep resistance
at high temperatures, and light weight
4-10
Th compounds
•
Hydrides

Formed by reaction with H2
 Powdered Th at room temperature

ThH2 and Th4H15
 ThH2 tetragonal
 Th4H15 cubic
* Th in center of 12 H
* 1st metal hydride superconductor

Hydride forms oxide

Range of ternary hydrides
 Fe, Zr, Mn, Al
• Borides

Formed from chlorides with MgB2

ThB6 (octahedra), ThB4, ThB12
 A few higher borides reported
 Ternary borides are known
• Carbides

Formed from oxide with carbon
 ThC, ThC2, and Th2C3
 Boride-carbides also formed
4-11
Th silicides
• Four Th-Si compounds

Th3Si5

Th3Si2
 Si bond distance 2.33 Å

ThSi
 Zig-zag structure

ThSi2
 Hexagonal and tetragonal
 Th in 12 fold coordination
with Si
• Numerous ternary compounds

ThM2Si2
 Mn, Cr, Fe, Co, Ni, Cu, Tc

Th2MSi3
 Mn, Fe, Co, Ni, Cu, Rh, Rh,
Pd, Os, Ir, Pt, Au
 From modification of ThSi2
4-12
Oxides and hydroxides
• Oxides of ThO2 and ThO

ThO postulated as defect
 Surface of metal exposed to air
 fcc lattice

Dioxide can form colloids
 Sintered dioxides are extremely refractory
 Dissolves in nitric acid with HF
 Hot HF or gaseous HF converts oxide to tetrafluoride

Dioxide produces blue light when heated
• Hydroxide

Converted to oxide above 470 ºC

Absorbs atmospheric CO2

Environmentally important specie
• Peroxide formed by hydrogen peroxide and Th salts
4-13
Th halides
• Tetrahalides have been formed
 ThF4
 Precipitation with fluoride and dehydration
with HF or F2
 Th metal or carbide with F2
 Other Th halides, oxalates, or oxides with HF
 ThO2 with NH4HF2
* NH4ThF5 that decomposes to ThF4 above
300 ºC
* Requires excess NH4HF2 (8x)
 Structure is square antiprism
• Mixed fluorides are also formed
 Th(OH)F3, ThOF2
• Hydrate of Th6F24.H2O
 Water centered 6 Th
4-14
Th chlorides
• Crystallized from aqueous solution

Hydrated form, removal of water upon heating greater than
100 ºC

Reaction of ThH4 with HCl

Th metal or carbide with Cl2

Th metal with NH4Cl
• 2 phases

Transition at 405 ºC

Low temperature a-ThCl4

High temperature b-ThCl4 (metastable)
 Both dodecahedra, 8 fold coordination
 Difference due to relationship between dodecahedra
• Mixed chlorides

ThOCl2
4-15
Th bromides and iodides
•
•
•
•
Similar synthesis to the
chlorides

i.e., HBr instead of HCl
 Solution synthesis
yields hydrates and
mixed oxide
(ThOBr2)
Also dimorphic, similar to
chlorides

Transition temperature
at 426 ºC
ThI4 from the reactions of the
elements

No water or O2; (forms
ThOI2)

ThH4 with HI

Distorted square
antiprism
Lower valent ThI3 and ThI2
known

Formed from ThI4 with
Th
4-16
S, Se, and Te complexes
• Heavier analogs of the oxides
• All form compounds
 Some simple fluorite or NaCl structures
 Electronic properties of S, Se, and Te can yield
complex structures
• Synthesis
 H2S with metal, Th halide, or hydride
• Se form series similar to S
 Se on metal, halides for synthesis
• Te slightly different structures
 CsCl structure for TeTh
4-17
Nitrides, P, As, Sb
• Range of binary compounds

ThN, Th3N4, Th2N3

ThP, Th3P4, Th2P11, ThP7

ThAs, Th3As4, ThAs2

ThSb, Th3Sb4, ThSb2

ThBi2
 Heavier compounds form similar binary phases to
nitrides
 Bi blanket with ThBi2
• Th3N4

Heating of metal in N2

Under NH3, hydride intermediate forms

Heating nitrides under O2 produces oxides
• Reaction of binary compounds with Th halides leads to ThNX
4-18
Complex ions
•
•
•
Th(ClO4)4

Tetrahydrate, decomposes to
mixed oxide at 280 ºC, then
dioxide at 335 ºC

Prepare from ThCl4 and Cl2O6

Used-as starting material since
ClO4 weakly binds
Sulfates (Th(SO4)2)

Prepared from salts with
sulfuric acid
 Different hydration states
* Lower temperature 9
waters
 8 waters also
found
 Tetrahydrate
also stated to
form
* 10 coordinate to Th(IV)
 2 sulfates, 6
waters
 Distorted
bicapped
squared
antiprism
Mixed species formed

Dihydroxide

Monooxide

Dimer (Th2(OH)2(SO4)8
•
•
Wide range of sulfates

A2Th(SO4)3
 A=Na=Cs, NH4

Fluoride species
 Th(SO3F)4
Nitrates

Prepared from Th(OH)4 in
nitric acid

Soluble in water

Nitrate extracted into
tributylphosphate
 Nucleophilic
 Metal ion interaction
through oxygens on TBP
* 2-3 TBP per thorium
nitrate

Polymeric

Th4(OH)10(NO3)6TBP4

A2Th(NO3)4
 A=monovalent
* 12 coordination by O

Also with divalent cations
4-19
Complex ions
•
•
Carbonate

From the hydroxide
 ThO(CO3)2 then
dicarbonate under high
CO2

Numerous mixed species
 Metal ion with extra
carbonate
* MTh(CO3)x
Phosphate

ThO2/P2O5
 Range of sulfates
* 3,4 (may not exist, as
Th4(PO4)4P2O7
 4 monodentate,
one chelating
* ThO3(PO4)2
* (ThO)2P2O7
* ThP2O7

Range of MTh2(PO4)3
 M monovalent
• Range of metal oxides
with Th
 Vanadates
 M2Th2(VO4)3
 Th(VO4)(VO)3
 Molybdates
 Th(MoO4)2
 Chromates
 Th(CrO4)2
• Prepared from salts
• Range of hydrates
 Higher
temperature, lower
hydrates
4-20
Coordination compounds
• Range of compounds examined
 TBP for extraction
 Ligands with
C-O, N-O, P=O, As=O,
S=O
• Th tetrakis(acetylacetone)
[Th(acac)4]
• 8-hydroxyquinoline
• Thorocene
 2 cyclo-octatetraene
• Cyclopentadienyl (Cp-)
4-21
Solution chemistry
• Only one oxidation state in solution
• Th(III) is claimed
 Th4+ + HN3  Th3+ +1.5N2 + H+
 IV/III greater than 3.0 V
* Unlikely based on reduction by HN3
 Claimed by spectroscopy
* 460 nm, 392 nm, 190 nm, below 185 nm
* Th(IV) azido chloride species
• Structure of Th4+
 Around 11 coordination
 Ionic radius 1.178 Å
 Th-O distance 2.45 Å
 O from H2O
• Thermodynamic data
 Eº= 1.828 V (Th4+/Th)
 ΔfHº= -769 kJ/mol
 ΔfGº= -705.5 kJ/mol
 Sº= -422.6 J/Kmol
4-22
•
Hydrolysis

Largest tetravalent actinide ion
 Least hydrolyzable tetravalent
 Can be examined at higher pH, up
to 4
 Tends to form colloids
* Discrepancies in oxide and
hydroxide solubility

Range of data
 Different measurement conditions
 Normalize by evaluation at zero
ionic strength
Solution
chemistry
4-23
Solubility
• Large variation with
preparation
 Average OH2.5 without
delayed
precipitation
 Polymerization
4-24
4-25
Solution chemistry
• Inorganic ligands
 Fluoride, chloride, sulfate,
nitrate
Carbonate forms
 Data is lacking for complexing
soluble species
 Re-evaluation based pm
Mixed carbonate
semiemperical approach
hydroxide species can
* Interligand repulsion
form
 Decrease from 1,4
to 1,5
 Th(OH)3CO3 Strong decrease
 1,5
from 1,5 to 1,6
Phosphate shown to
• Organic ligands
form soluble species
 Oxalate, citrate, EDTA, humic
substance
 Controlled by
 Form strong complexes
precipitation of
 Determined by potentiometry
Th2(PO4)2(HPO4).H2O
and solvent extraction
* logKsp=-66.6
 Choice of data (i.e.,
hydrolysis constants)
impacts evaluation
• Complexing media



4-26
4-27
Analytical
• Low concentrations
 Without complexing agent
• Indicator dyes
 Arzenazo-III
• ICP-MS
• Radiometric methods
 Alpha spectroscopy
 Liquid scintillation
May require preconcentration
Need to include daughters in evaluation
4-28
Review
•
•
•
•
•
•
•
•
•
Long lived isotopes
Production of Th isotopes
Electronic properties
Methods for the purification of Th metal
General properties of Th metal
Trends in Th Compounds
Methods of compound synthesis
Solution chemistry
Methods for analytical analysis of Th
4-29
Questions
• What is the general production route of low A Th
isotopes?
• What is the predominant decay mode of Th
isotopes?
• How can Th be separated from U?
• What are the different phases of Th metal?
• How are Th halides prepared?
• What Th hydroxide species can be found at high Th
concentration?
• Why do differences exist for the Ksp of Th
hydroxide?
4-30
Pop Quiz
• Why does Th solution chemistry have limited
UV-Visible absorbance? What conditions are
needed to have UV-Visible absorbance of Th
solution compounds?
4-31