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Lecture 4: Thorium Chemistry

• Chemistry of actinides Nuclear properties Th purification Metal Compounds Solution chemistry

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Nuclear Properties

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Thorium Isotopes

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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 oreFree from 232Th

Other isotopes from nuclear reactions with Pb and Bi targets

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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

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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 abbreviated description of angular momentum quantum numbers 2S+1LJ

S from unpaired electrons 2 d electrons, S=1, 2S+1=3

L from orbitals 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=|3-1|=2

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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

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Th levels (cm-1)

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Th levels (cm-1)

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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

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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)

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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 metal-

zirconium* Alloys have high strength, creep resistance

at high temperatures, and light weight

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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

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Borides, Carbides, Silicides

• 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

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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

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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

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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

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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 -ThCl4

High temperature -ThCl4 (metastable) Both dodecahedra, 8 fold coordination Difference due to relationship between dodecahedra

• Mixed chlorides ThOCl2

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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

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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

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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

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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

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Complex ions• 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

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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

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Complex ions

• 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

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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-)

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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

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Solution chemistry

• Thermodynamic data Eº= 1.828 V (Th4+/Th) ΔfHº= -769 kJ/mol ΔfGº= -705.5 kJ/mol Sº= -422.6 J/Kmol

• 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

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Solubility

• Large variation with preparation Average OH- 2.5 without delayed precipitation Polymerization

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Solution chemistry

• Complexing media Carbonate forms soluble species Mixed carbonate hydroxide species can

formTh(OH)3CO3

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1,5 Phosphate shown to form soluble species

Controlled by precipitation of Th2(PO4)2(HPO4).H2O* logKsp=-66.6

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Complexation

• Inorganic ligands Fluoride, chloride, sulfate, nitrate Data is lacking for complexing

Re-evaluation based pm semiemperical approach* Interligand repulsion

Decrease from 1,4 to 1,5 Strong decrease from 1,5 to 1,6

• Organic ligands Oxalate, citrate, EDTA, humic substance

Form strong complexes Determined by potentiometry and solvent extraction

Choice of data (i.e., hydrolysis constants) impacts evaluation

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Analytical

• Low concentrations Without complexing agent

• Indicator dyes Arzenazo-III

• ICP-MS• Radiometric methods

Alpha spectroscopy Liquid scintillation

May require preconcentrationNeed to include daughters in evaluation