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Lecture 4: Thorium Chemistry

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

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Thorium isotopes

• 232Th main isotope of Th§ 228Th from 232Th decay§ Other isotopes from

decay of U isotopesà 227,231Th (from 235U

decay)à 230,234Th (from 238U

decay)§ Isotopes can be

isolated from U oreà Free from 232Th

§ Other isotopes from nuclear reactions with Pb and Bi targets

4-3

Th ore processing• Main Th bearing mineral is monazite

§ Phosphate mineral à strong acid for dissolution

results in water soluble saltsà 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 stateà precipitate at pH 1

* A number of different precipitation steps can be usedØ HydroxideØ PhosphateØ PeroxideØ Carbonate

(lanthanides from U and Th)

Ø U from Th by solvent extraction

4-4

Th atomic spectroscopy• Electronic states of Th can provide information on higher actinide states

§ Neutral atom has available valence orbitalsà 5f, 6d, 7s, 7pà Stable 6d27s2 (3F2)

• Term symbol review§ abbreviated description of angular momentum quantum numbers§ 2S+1LJ

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

§ L from orbital occupied by of unpaired electronsà 2 d electrons (5 orbitals; 2,1,0,-1,-2): 3à 3 is F

* S=0, P=1, D=2, F=3§ J has some rules

à Less than half filled, J=|S-L|à J=|1-3|=2

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Th atomic spectroscopy• Wide range of values based on configurations• Singly ionized states

§ d2s, ds2, fs2, fds, d3, fd2

à Energy range from 1859 cm-1 to 12485 cm-1

§ p orbital occupation starts at 23372 cm-1

à dsp§ Double f occupation at 24381 cm-1

à 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

à Distillation of Zn• reduction of ThCl4 with an alkali metal• Reduction of ThCl4 by DyCl2

• Decomposition of ThI4 on hot W surface

4-9

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

à 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

à magnesium-thorium-zirconiumà magnesium-thorium-zinc-zirconium    à 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

à Powdered Th at room temperature§ ThH2 and Th4H15

à ThH2 tetragonal à Th4H15 cubic

* Th in center of 12 H* 1st metal hydride superconductor

§ Hydride forms oxide § Range of ternary hydrides

à Fe, Zr, Mn, Al• Borides

§ Formed from chlorides with MgB2

§ ThB6 (octahedra), ThB4, ThB12

à A few higher borides reportedà Ternary borides are known

• Carbides§ Formed from oxide with carbon

à ThC, ThC2, and Th2C3

à Boride-carbides also formed

4-12

Th silicides• Four Th-Si compounds

§ Th3Si5

§ Th3Si2

à Si bond distance 2.33 ŧ ThSi

à Zig-zag structure§ ThSi2

à Hexagonal and tetragonal à Th in 12 fold coordination with Si

• Numerous ternary compounds§ ThM2Si2

à Mn, Cr, Fe, Co, Ni, Cu, Tc§ Th2MSi3

à Mn, Fe, Co, Ni, Cu, Rh, Rh, Pd, Os, Ir, Pt, Au

à From modification of ThSi2

4-13

Oxides and hydroxides• Oxides of ThO2 and ThO

§ ThO postulated as defectà Surface of metal exposed to airà fcc lattice

§ Dioxide can form colloidsà Sintered dioxides are extremely refractoryà Dissolves in nitric acid with HFà 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

§ ThF4à Precipitation with fluoride and dehydration

with HF or F2à Th metal or carbide with F2à Other Th halides, oxalates, or oxides with HFà ThO2 with NH4HF2* NH4ThF5 that decomposes to ThF4 above

300 ºC* Requires excess NH4HF2 (8x)

à Structure is square antiprism• Mixed fluorides are also formed

§ Th(OH)F3, ThOF2• Hydrate of Th6F24

.H2O§ Water centered 6 Th

4-15

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)à Both dodecahedra, 8 fold coordinationà 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

à 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

à Heavier compounds form similar binary phases to nitrides

à 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

acidà Different hydration states

* Lower temperature 9 watersØ 8 waters also foundØ Tetrahydrate also

stated to form* 10 coordinate to Th(IV)

Ø 2 sulfates, 6 watersØ Distorted bicapped

squared antiprism• Mixed species formed

§ Dihydroxide§ Monooxide§ Dimer (Th2(OH)2(SO4)8

• Wide range of sulfates§ A2Th(SO4)3

à A=Na=Cs, NH4§ Fluoride species

à Th(SO3F)4• Nitrates

§ Prepared from Th(OH)4 in nitric acid

§ Soluble in water§ Nitrate extracted into

tributylphosphateà Nucleophilic à Metal ion interaction

through oxygens on TBP* 2-3 TBP per thorium

nitrate§ Polymeric§ Th4(OH)10(NO3)6TBP4§ A2Th(NO3)4

à A=monovalent * 12 coordination by O

§ Also with divalent cations

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Complex ions• Carbonate

§ From the hydroxideà ThO(CO3)2 then dicarbonate

under high CO2§ Numerous mixed species

à Metal ion with extra carbonate* MTh(CO3)x

• Phosphate§ ThO2/P2O5

à Range of sulfates * 3,4 (may not exist, as

Th4(PO4)4P2O7Ø 4 monodentate,

one chelating* ThO3(PO4)2* (ThO)2P2O7* ThP2O7

§ Range of MTh2(PO4)3à M monovalent

• Range of metal oxides with Th§ Vanadates

à M2Th2(VO4)3

à Th(VO4)(VO)3

§ Molybdatesà Th(MoO4)2

§ Chromatesà 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

à 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+

à IV/III greater than 3.0 V* Unlikely based on reduction by HN3

à 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 ŧ Th-O distance 2.45 Å

à 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

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Solution chemistry

• Hydrolysis§ Largest tetravalent actinide ion

à Least hydrolyzable tetravalentà Can be examined at higher pH, up to 4à Tends to form colloids

* Discrepancies in oxide and hydroxide solubility

§ Range of dataà Different measurement conditionsà 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 formà Th(OH)3CO3

-

à 1,5 § Phosphate shown to

form soluble speciesà Controlled by

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

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

à Re-evaluation based pm semiemperical approach* Interligand repulsion

Ø Decrease from 1,4 to 1,5

Ø Strong decrease from 1,5 to 1,6

• Organic ligands§ Oxalate, citrate, EDTA, humic

substanceà Form strong complexes

§ Determined by potentiometry and solvent extractionà 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

à May require preconcentrationà Need to include daughters in evaluation

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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

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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?

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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?