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Lecture 3: Actinium Chemistry

Lecture 3: Actinium Chemistry. Lecture notes from Radiochemistry of the Rare Earths, Scandium, Yttrium, and Actinium Chemistry of actinides Metals Soluble and insoluble salts Complex ions and chelate compounds Organic compounds Separations. Actinium Isotopes.

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Lecture 3: Actinium Chemistry

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  1. Lecture 3: Actinium Chemistry • Lecture notes from Radiochemistry of the Rare Earths, Scandium, Yttrium, and Actinium • Chemistry of actinides • Metals • Soluble and insoluble salts • Complex ions and chelate compounds • Organic compounds • Separations

  2. Actinium Isotopes

  3. Actinium Isotopes and fundamental data • Ac is trivalent • Very similar chemistry to lanthanides and Y • Trivalent chemistry • electropositive • 227Ac • From 235U decay • Beta emitter • 225Ac has been proposed for nuclear medicine • Ac from 229Th parent • Alpha emitting therapeutic agent • Use of 225Ac to produce 213Bi • Chemistry of Ac basis of use as radiopharmaceutical • Use of Ac in delivery to tumor based on fundamental chemical interactions • Separation of Bi daughter from Ac parent • Atomic radius • 195 pm • Ionic radius • 126 pm • 6-coordinate

  4. Redox • Trivalent is predominant state • Divalent state postulated based on similarities to Ac behavior with divalent Eu and Sm • Reinfored with polarogram data • Two waves in HClO4, pH 1.9-3.1 • Ac2++2e-+HgAc(Hg) • Ac3++3e-+HgAc(Hg) • Other experiments failed to find divalent state

  5. Ac Metal • Density • 10.07 g/mL • Melting point • 1050 °C • Boiling Point • 3300 °C • Crystal structure • fcc • Preparation of metals difficult • Tendency to form oxides and hydroxides in water • Formation in electrolytic reduction • Molten salt systems • Reduction of fluoride salts by metallic Ca • Need to melt both CaF2 and resulting metal • Metal oxidizes in air • 4Ac + 3O2 → 2Ac2O3 • Useful starting material for synthesis • Ac2O3 + 3H2S Ac2S3 + 3H2O

  6. Ac preparation and purification • Separation from U ores • Ores also contain a fair amount of lanthanides, require separation • Nuclear reactions and generators • Irradiation of 226Ra • 226Ra(n,g)227Ra, beta decay to 227Ac • 227Ac sg=762 barns • 225Ac from 229Th generator • Start with 233U from neutron activation of 232Th

  7. Keto Hydrate Enol Ac purification: solvent extraction • Extrationwith TTA • pH control of extraction • Ac extracts well above pH 6 • Hydrolysis in this range • Synergist extraction with 0.1 TTA in 0.1 M TBP • Above pH 4

  8. Ion exchange • Cation exchange • Separation of 227Ac from 227Th and 223Ra • Strong cation exchange • DOWEX 50 • Organic stationary phases • Trioctylamine • Bis(2-ethyl-hexyl)phosphoric acid (HDEHP) • TBP • TTA • Inorganic • MnO2

  9. Preparation of gram quantities of Ac • Irradiation of multigram quantities of 226Ra • Forms both 227Ac and 228Th • 222Rn daughter from 226Ra • Irradiation of RaCO3 • Dissolved in dilute HNO3 • Ra(NO3)2 precipitated • Recycle for further Ac production • Th and Ac remain in solution • 5 M HNO3, anion exchange • Th strongly absorbed, Ac only slightly • Oxalate precipitation of Ac • Calcination after precipitation • Used to form Ac2O3

  10. Ac salts • Salts are soluble in most acids • Some salt are insoluble and used in separations based on precipitation • Most data from one study • Each compound prepared from less than 10 µg • Purified by TTA • Hydroxides • pK1h=9.4 • Described by electrostatic model of hydrolysis • Linearity of log K1h versus 1/ionic radius • For trivalent metal ions, actually related to charge density • Hydrolyzes less than trivalent lanthanides or actinides • More basic than lanthanides • Ac hydrolysis paper • http://www.ingentaconnect.com/content/klu/jrnc/2004/00000261/00000001/05379859;jsessionid=4dkgcbb4sv85c.alice

  11. Ac salts • Fluorides • AcF3 • Density 7.880 g/mL • Formation of AcF3 • Ac(OH)3 + 3HF AcF3 + 3H2O • Oxalate • Oxalates used in precipitation of metal ions • Oxalate salt of actinium used to form oxide • Ac precipitated as an oxalate by the addition of an oxalic acid • Oxalates are destroyed by boiling concentrated HNO3 or HClO4 • In 0.1 M HNO3-0.5 M oxalic acid • Ac oxalate solubility 24 mg/L

  12. Ac salts • Chloride • Melting point: 1051°C (sublimes) • Density: 4.810 g/mL • Formation reaction • 4Ac(OH)3 + 3CCl4 4AcCl3 + 3CO2 + 6H2O • Bromide • Melting point: 1051°C • Boiling point: 3198°C • Density: 5.850 g/mL • Formation reaction • Ac2O3 + 2AlBr32AcBr3 + Al2O3 • For Iodide • Ac2O3 + 2Al + 3I22AcI3 + Al2O3

  13. Solubility and complexation • Generally co-precipitation with insoluble salts from cation • Fluorides, hydroxides of metal ion • Ac precipitated by Pb sulfate • Solubility of oxalates evaluated • Effects of radiolysis • La solubility half of Ac solubility • Ksparound 5E-27 • Large decrease in pH due to radiolysis from 227Th • Purified Ac did not show large pH decrease • Resembles lanthanum in complexation • Generally lower for Ac • Determined in tracer experiments • Solvent extraction • Compared to ionic radius to some evaluated constants • HDEHP

  14. Radiocolloid • Separation of 227Ac from 227Th and 223Ra • Formation of colliods • Ac goes into solution, Th remains in solid phase • Sorption of Ac onto filter increases with pH and time • Above pH 5 filter separation of Ac by centrifuge • Analytical chemistry of Ac • Radiation detection • Neutron activation for 227Ac • 1E-17 g level • Activity used to determine to 1E-20 g

  15. Actinium uses • Heat sources • 227Ac multiCi amount • 5 alpha particles • Neutron sources • a,n source using 227Ac • Nuclear medicine • Medical use based on ligand • Bone treatments (polyphosphonate) • 225Ac suitable isotope • Decay series produces alpha and beta • No hard gammas • Ac bound by marcocyclic compounds • HEHA incorporates Ac • 1,4,7,10,13,16-hexaazacyclohexadecane-N,N′,N′,N′,N′,N′-hexaacetic acid • Geotracer • Compare 231Pa with 227Ac • Ac higher than Pa in deep seawater • Ac as a tracer for deep seawater circulation

  16. Review • Actinide isotopes • Lifetimes, production • Actinium data • X-ray, radii, density • Redox • Oxidation states • Preparation and purification • Solvent extraction and ion exchange • Compounds and properties • Uses of actinium

  17. Questions • What is the longest lived Ac isotope? • What are two different ways to obtain actinium • Provide the isotopes • What Ac oxidation state can be found in solution? • How is Ac separated from U • Which Ac species are insoluble? • What are some uses of Ac?

  18. Pop Quiz • Describe a method for separating Ac from the lanthanides.

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