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
3-1
Actinium
Isotopes
3-2
Actinium Isotopes and fundamental data
•
•
•
•
Ac is trivalent

Very similar chemistry to lanthanides and Y

Trivalent chemistry
• Atomic radius

electropositive
227Ac

195 pm
235

From U decay
• Ionic radius

Beta emitter
225Ac has been proposed for nuclear medicine

126 pm

Ac from 229Th parent

6-coordinate
 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
3-3
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
3-4
• 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
Ac Metal
• Density

10.07 g/mL
• Melting point

1050 °C
• Boiling Point

3300 °C
• Crystal structure

fcc
3-5
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
3-6
Ac purification: solvent extraction
• Extration with 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
O
O
O
CF3
S
CF3
S
Keto
O
OH
Enol
HO
OH
CF3
S
Hydrate
3-7
Ion exchange
• Cation exchange
 Separation of 227Ac from
227Th and 223Ra
 Strong cation exchange
 DOWEX 50
• Organic stationary phases
 Trioctylamine
 Bis(2-ethylhexyl)phosphoric acid
(HDEHP)
 TBP
 TTA
• Inorganic
 MnO2
3-8
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
3-9
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
3-10
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
3-11
•
•
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
Ac salts
3-12
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
 Ksp around 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
3-13
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
3-14
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
3-15
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
3-16
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?
3-17
Pop Quiz
• Describe a method for separating Ac from the
lanthanides.
3-18