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Lecture 4: Thorium Chemistry

Lecture 4: Thorium Chemistry. Chemistry of actinides Nuclear properties Th purification Metal Compounds Solution chemistry. Nuclear Properties. Thorium Isotopes. Thorium isotopes. 232 Th main isotope of Th 228 Th from 232 Th decay Other isotopes from decay of U isotopes

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Lecture 4: Thorium Chemistry

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  1. Lecture 4: Thorium Chemistry • Chemistry of actinides • Nuclear properties • Th purification • Metal • Compounds • Solution chemistry

  2. Nuclear Properties

  3. Thorium Isotopes

  4. 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

  5. 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

  6. 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

  7. 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

  8. Th levels (cm-1)

  9. Th levels (cm-1)

  10. 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

  11. 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)

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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) • ThH4with HI • Distorted square antiprism • Lower valent ThI3 and ThI2 known • Formed from ThI4 with Th

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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-)

  27. 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

  28. 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

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

  30. Solution chemistry • Complexing media • Carbonate forms soluble species • Mixed carbonate hydroxide species can form • Th(OH)3CO3- • 1,5 • Phosphate shown to form soluble species • Controlled by precipitation of Th2(PO4)2(HPO4).H2O • logKsp=-66.6

  31. 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

  32. 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

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