Chapter 2
GENERAL RADIOCHEMISTRY
I- ISOTOPIC EXCHANGE
I- ISOTOPIC EXCHANGE
1- Basic concepts
Isotopic exchange is the redistribution of atoms
of isotopes of a given element within the
molecule, between different molecules or phases,
which does not lead to other changes in the
qualitative and quantitative molecular
composition of the system.
XR + X*R’ === X*R + XR’
XR
X*R’
Isotopic exchange involves the substitution of one
isotope of an element by another isotope of the
element in the molecules of a given substance
without a change of their element composition.
For example,
if hydrogen chloride HCl, enriched by the heavy isotope
of chlorine 37Cl, is mixed with chlorine Cl2 with the
normal isotope composition (75.53 percent 35Cl and
24.47 percent 37Cl), then as a result of the isotopic
exchange reactions
H37Cl + 35Cl2 = H35Cl + 35Cl37Cl
(H-37Cl + 35Cl- 35Cl  H-35Cl + 35Cl-37Cl)
H37Cl + 35Cl37Cl = H35Cl +
(H-37Cl + 35Cl-37Cl  H-35Cl +
37Cl
2
37Cl-37Cl)
Isotopic exchange reactions may take place under
various conditions:
• Under homogeneous conditions
For example, between solute and solvent, isotopic
exchange reactions is interchange of iodine atom
between alkyl iodide and potassium iodide in alcohol
solution.
R127I + K131I == R131I + K127I
• Under heterogeneous conditions
For example, between solids or liquids, the
interchange of silver atoms between metallic silver
and its ions in solution
110Ag
+
107,109Ag
(solid) ==
107,109Ag
+
110Ag
(solid)
Isotopic exchange reactions play
an important role in,
• The synthesis of labeled compounds.
H2O
R-MgI + *CO2  R-*COOH
• The investigation of redox process of C = C
CH3 – CH = CH2
KMnO4/H+
CH3 – COOH + *CO2
KMnO4/OH-
COOH – *COOH + CO2
2- Mechanism of isotopic exchange
– Isotopic exchange by dissociation
– Isotopic exchange by association
– Isotopic exchange through other reversible
chemical processes.
– Electron exchange reactions
Mechanism of isotopic exchange
• Isotopic exchange by dissociation
The scheme of an exchange reaction through
dissociation is as follows:
AX == A
+ X
BX* == X* + B
AX*
BX
For example
• In the heterogeneous of halide ions between solid
silver halide and metal halides in solution
AgBr + NaBr* == Ag+ + Br - + Na+ + Br*
== AgBr* + NaBr
• In the homogeneous of halide ions in solution
between metal halides and compounds containing
halogen atoms as ligands:
K2[PtBr6] + KBr* == 2K+ + [PtBr5]- + Br - + K+ + *Br == K2[PtBr5Br*] + KBr
• The thermal dissociation mechanism operates in the
exchange of sulfur atoms between sulfur
monochloride and elemental sulfur in vapors:
S2Cl2 + *S8 == S2 + Cl2 + *S2 + S6
== *S2Cl2 + S8
• Isotopic exchange by association
The exchange reaction may be represented
schematically as follow:
AX + BX* == ABXX* == AX* + BX
Example:
The inter-exchange of bromine atoms between
hydrogen bromide and molecular bromine.
HBr + *BrBr == HBr2Br* == HBr* + Br2
• Isotopic exchange through other reversible
chemical processes.
Example
The intramolecular exchange of carbon atoms in
methylcyclohexane between the side chain and the
ring.
C6H11 - *CH3 == CH3 – C5H8 – *CH3 == CH3 - C5H11C*
- *CH3 
CH3 -
– *CH3
 CH3 -
*
• Electron exchange reactions.
The transfer of electrons from the isotopic atoms
which are contained in compounds of a given element
in different oxidation states leads to a redistribution of
the isotopes, the atoms being not actually transferred
from one compound into another.
FeCl2 + Fe*Cl3 === Fe*Cl2 + FeCl3
Fe2+ + *Fe3+ === Fe3+ + *Fe2+
e-
II- ADSORPTION OF RADIOACTIVE
ISOTOPES
1- DEFINITION OF ADSORPTION
Adsorption is the transfer of a material from
one liquid or gaseous state to a surface.
• The substance that is transferred to the surafce
is the adsorbate.
• The material on which the adsorbate deposits is
the adsorbent.
Example: Silica gel, Activated carbon, Alumina, Zeolites and
molecular sieves, Polymers.
adsorbate: material being adsorbed
adsorbent: material doing the adsorbing.
(examples are activated carbon or ion
exchange resin).
A (gas, liquid)
adsorbent
adsorbate
ADSORPTION.
Adsorption is the concentration of a substance at
a phase boundary
Adsorption
Absorption
M
Diffusion
> 99% of surface for
removal is internal
ABSORPTION
ADSORPTION
Adsorption
PHASE I
‘PHASE’ 2
Absorption (“partitioning”)
PHASE I
PHASE 2
Causes of Adsorption
• Attraction to the Sorbent Surface
– Van der Waals forces: physical attraction
– Electrostatic forces (surface charge
interaction)
– Chemical forces (e.g., - and hydrogen
bonding)
Van der Waals Forces
A polar molecule that has two poles.
Van der Waals Forces
Johannes Diderik van der Waals
Van der Waals Forces
Two electrically neutral, closed-shell atoms
dTemporary dipole resulting
from quantum fluctuation
d+
Gives net
attraction
d-
d+
Induced dipole, due to
presence of other dipole
• Although Van der Waals forces are weak, they are
often the only attractive force between molecules.
Electrostatic Interactions
Interaction of charged side chain with the opposide
charged side chain.
O
+
NH3
-
O
H2C C
O
C CH2
O
+
NH3
(CH2)4
Charges
Two charges of the same type repel one another
+
+
Two charges of the opposite type attract one another
-
+
The two charges will experience a FORCE pushing them apart
or pulling them together
Adsorbents
Characteristics and general requirement
Most adsorbents fall into one of three classes:
- Oxygen-containing compounds – Are typically
hydrophilic and polar, including materials such as silica
gel and zeolites.
- Carbon-based compounds – Are typically hydrophobic
and non-polar, including materials such as activated
carbon and graphite.
- Polymer-based compounds - Are polar or non-polar
functional groups in a porous polymer matrix.
Zeolite
Graphite is a crystalline form of carbon.
Graphite is the most stable form of
carbon under standard conditions
Adsorbents
Characteristics and general requirement:
* Adsorbents are used usually in the form of
spherical pellets, rods, moldings, or monoliths with
hydrodynamic diameters between 0.5 and 10 mm.
* The adsorbents must also have a distinct
pore structure which enables fast transport of the
gaseous vapors.
Different physical forms of activated carbon
Increasing magnification
Adsorbent
• Activated Carbon
• Activated Alumina
• Silica Gel
• Molecular Sieves (Zeolites)
• Polar and Non-polar adsorbents
activated carbons
Activated alumina is manufactured from aluminium hydroxide
by dehydroxylating it in a way that produces a highly porous
material; this material can have a surface area significantly
over 200 m²/g. It is made of aluminium oxide (alumina; Al2O3)
silica gel
Silica gel
synthetic polymers
variety of functional groups
Adsorption Process
Classified as:
– Physical adsorption
– Chemical adsorption
• Physical adsorption occurs when the bonding
forces are dispersion and coulombic type.
• The amount of heat released during this process is
equal to the heat of condensation.
Physical adsorption:
Van der Waals attraction between adsorbate and
adsorbent.
The attraction is not fixed to a specific site and the
adsorbate is relatively free to move on the surface.
This is relatively weak, reversible, adsorption capable
of multilayer adsorption.
A (gas, liquid)
adsorbent
adsorbate
Chemical adsorption:
Some degree of chemical bonding between
adsorbate and adsorbent characterized by
strong attractiveness.
Adsorbed molecules are not free to move on
the surface. There is a high degree of specificity
and typically a monolayer is formed. The process
is seldom reversible.
Chemical adsorption
• Chemical adsorption occurs when there is sharing
of electrons between adsorbent and adsorbate.
• The amount of heat released during this process is equal to
the heat of reaction.
• Heat liberated during chemisorption is in the range
of 20-400 kj/g mole
• Results from a chemical interaction between
the adsorbate and adsorbent. Therefore formed
bond is much stronger than that for physical
adsorption
Adsorptive Equilibration in a Porous Adsorbent
Pore
Early
Later
Laminar
Boundary
Layer
GAC Particle
Adsorbed Molecule
Diffusing Molecule
Equilibrium
ADSORPTION EQUILIBRIA
If the adsorbent and adsorbate are contacted long
enough an equilibrium will be established between
the amount of adsorbate adsorbed and the amount
of adsorbate in solution.
The equilibrium relationship is described by
isotherms.
Isotherm models:
• Langmuir Isotherm
• Freundlich Isotherm
Langmuir isotherm model
In Langmuir isotherm assuming a unimolecular layer
can be obtained by a kinetic approach:
At equilibium knowing that the rate of adsorption is
equal to the rate of desorption:
ra=CaP(1-f)
rd=Cdf
Ca P
f 
Ca P + C d
ra: rate of adsorption
Ca, Cd: constant
P: partial pressure of the adsorbate
f: is the occupied fraction of the total solid surface
Freundlich isotherm model:
For the special case of heterogeneous surface
energies (particularly good for mixed wastes) in
which the energy term, “KF”, varies as a function of
surface coverage we use the Freundlich model.
1
q e  K F Ce
n
n and KF are system specific constants.
Application of Adsorption
In ordinary chemistry, adsorption is important when a
gas or solution is in contact with a material of high
surface area, usually either a porous or a very finely
powered solid.
• In clarification of sugar
• In gas masks
• In catalysis
• In adsorption indicators
• In chromatographic analysis
• In softening of hard water
• In paint industry
• In removing moisture from air in the
storage of delicate instruments
2- ADSORPTION OF RADIOACTIVE ISOTOPES
The adsorption of radioactive isotopes from solution
plays an important role in radiochemistry. Radioactive
isotopes present in solution in micro-concentrations
may be lost during the work as a result of:
• their adsorption on the walls of the vessel,
• contamination of solution,
• precipitation process,
• or due to adsorption on the previously formed
precipitates,
• on filters,
• etc.
• Ionic adsorption
Ionic adsorption occurs on finely crystalline
precipitates, bulky precipitates of such types as
hydroxides, silica gel, alumosilic gel, etc., on particles
of suspensions, colloidal particles, carbon, ionexchange materials, filter paper.
In a number of cases ionic adsorption also obeys
the equations of molecular adsorption.
For examples:
• The adsorption of Ra2+ on glass over a wide range of
concentration
• The adsorption of Ba2+ and Sr2+ at concentration of 5.10-3 to
10-5 M takes place according to the Freundlich equation
III- DISTRIBUTION OF MICROCONCENTRATIONS OF RADIOACTIVE
ISOTOPES BETWEEN TWO PHASE
• Specific and most important to radiochemistry are the
distribution of micro-concentrations of radioactive isotopes
between a solution and a solid phase.
• The distribution of micro-concentrations of radioactive
isotopes between a gaseous and a liquid phase is governed
by the solubility of radioactive gases in liquids and obeys
Henry’s law.
• If a radioactive isotope is in solution in the form of ions in an
ultrasmall concentration so that when there are added
substances capable of forming with a given element slightly
soluble compounds it cannot form an independent solid
phase, then it can be separated from solution by
coprecipitation with a so-call carrier, whose concentration in
the solution is sufficient for precipitation.
1- General Properties of Aqueous
Solutions
General Properties of Aqueous Solutions
• Solution - a homogeneous mixture
– Solute: the component that is dissolved
– Solvent: the component that does the
dissolving
Generally, the component present in the
greatest quantity is considered to be the
solvent.
Aqueous solutions are those in which
water is the solvent.
Concentration of Solutions
• Concentration is the amount of solute dissolved in a
given amount of solvent.
• Qualitative expressions of concentration
– Concentrated – higher ratio of solute to solvent
– Dilute - smaller ratio of solute to solvent
Solubility is the maximum amount of a solid that can dissolve in a given
amount of solvent at a specified temperature
Comparison of a Concentrated and Dilute Solution
• Quantitative concentration term
– Molarity is the ratio of moles solute
per liter of solution
– Symbols: M or [ ]
– Different forms of molarity equation
mol
M
L
mol
L
M
mol  M  L
Calculate the molarity of a solution
prepared by dissolving 45.00 grams of
KI into a total volume of 500.0 mL.
Calculate the molarity of a solution
prepared by dissolving 45.00 grams of
KI into a total volume of 500.0 mL.
45.00 g KI 1 mol KI 1000 mL


 0.5422 M
500.0 mL 166.0 g KI
1L
How many milliliters of 3.50 M NaOH can
be prepared from 75.00 grams of the
solid?
How many milliliters of 3.50 M NaOH can
be prepared from 75.00 grams of the
solid?
1 mol NaOH
1L
1000 mL
75.00 g NaOH 


 536 mL
40.00 g NaOH 3.50 mol NaOH
1L
• Dilution
– Process of preparing a less concentrated
solution from a more concentrated one.
2- Precipitation – precipitation reaction
Precipitation – precipitation reaction
Some things (chemists use
term compounds) dissolve
in water – these are said to
be soluble.
Some things (compounds)
don’t dissolve in water –
these are said to be
insoluble.
For example
• Hydroxide Precipitation
pH is Raised by Addition of Hydroxide
Cu(NO3)2(aq)
+
2 NaOH(aq)
Cu(OH)2(s)
+ 2 NaNO3(aq)
Hydroxide Precipitation of soluble metals
Cu(NO3)2(aq) + 2 NaOH(aq) --> Cu(OH)2(s) + 2 NaNO3(aq)
Ni(NO3)2(aq) + 2 NaOH(aq) --> Ni(OH)2(s) + 2 NaNO3(aq)
2 AgNO3(aq) + 2 NaOH(aq) --> Ag2O(s) + 2 NaNO3(aq) + H2O(l)
• Sulfide Precipitation of Soluble Metals
Cu(NO3)2(aq) + Na2S(aq) --> CuS(s) + 2 NaNO3(aq)
CdCl2(aq) + Na2S(aq) --> CdS(s) + 2 NaCl(aq)
Ni(NO3)2(aq) + Na2S(aq) --> NiS(s) + 2 NaNO3(aq)
Agents to precipitate:
Ammonia/Hidroxide, NH3/OHSulfide, S2Phosphate, PO43Carbonate, CO32Cyanide, CN….
A precipitation reaction is a reaction
in which soluble ions in separate
solutions are mixed together to form
an insoluble compound that settles
out of solution as a solid. That
insoluble compound is called a
precipitate.
Precipitate Formation
soluble
insoluble
Precipitation (formation of a solid from two
aqueous solutions) occurs when product is
insoluble
Identify the Precipitate
A lead iodide precipitate.
(?) + PbI2 (?)
(s)
Pb(NO3)2(aq) + 2NaI(aq)  2NaNO3 (aq)
Mixing Solutions of Pb(NO3)2 and NaCl
Isolating the precipitate
The precipitate from a precipitation
reaction can be separated from the
reaction mixture by filtration.
Buchner
funnel
vacuum
pump
A Buchner funnel and
flask can be used to
accelerate the process.
filter paper
This apparatus uses a
vacuum pump to draw the
mixture through the filter.
Buchner
flask
The filtrate is finally
washed and dried.
The solubility product constant, Ksp
Precipitation is accomplished by combining a selected
ion(s) in solution with a suitable counter-ion in
sufficient concentrations to exceed the solubility of the
resulting compound and produce a supersaturated
solution
Sr2+ (soln) + CO32- (soln)  SrCO3 (s)
The solubility product constant, Ksp, is the
equilibrium constant for the former process, a
solid dissolving and forming ions in solution.
Ksp is defined in terms of the concentrations of Sr2+ and
CO32Ksp = [Sr2+][CO32-] = 1.610-9
In order for the carbonate to precipitate, the product
of the concentration of the ions in solution
representing the ions in the equilibrium expression,
the common ions, must exceed the value of the Ksp.
For example, if [Sr+2] is 1.10-6 molar, then the
carbonate ion concentration must be greater than
0.0016 molar for precipitation to occur because
(1.10-6) x (0.0016) = 1.6x10-9.
Uses of precipitation reactions
Most precipitation reactions are very
fast reactions that occur between ions.
This makes them very useful for
identifying specific ions based on the
type of precipitate formed.
Precipitation reactions have a
number of other uses:
 production of coloured pigments
for paints and dyes
 removal of toxic chemicals
from water
 separation of reaction products.
A lead iodide precipitate.
Usually Used For Larger Flows
Addition of OH Controlled by pH Meter / Controller
NaOH
Storage
Clarifier
React
Flocculation
pH
Process Water
Chemical
Treatment
..
Equalization Tank
Treatment Tank
Treated Water
Back To
Equalization Tank
Clarifier
Filter Press
Sludge Tank
Sludge Bin
3- Coprecipitation
Radiochemical Precipitation
Coprecipitation
• Processes in which the microcomponent is
carried away from the solution by a
precipitating macrocomponent are called
coprecipitation.
• The term coprecipitation refers to the carrying
down of normally soluble impurities during
precipitation of an insoluble compound.
Coprecipitation
The simultaneous precipitation of a normally
soluble component with a macro-component
from the same solution by the formation of
mixed crystals, by adsorption, occlusion
or mechanical entrapment.
For example,
Radium in most environmental samples, for
example, is not present in sufficient concentration
to cause its very insoluble sulfate (RaSO4) to
precipitate.
The radionuclide can often be brought down
selectively and quantitatively from solution during
precipitation of an alternate insoluble compound
by a process called coprecipitation.
The insoluble compound commonly used to
coprecipitate radium isotopes is another insoluble
sulfate, BaSO4.
The co-precipitate (concentrated) ion –Ra2+:
Ra2+ (trace) + SO42- (soln) --/-> RaSO4 (s)
(Concentration of Ra is very low)
Ra2+ - the co-precipitate (concentrated) ion – the impurity ion
that comes down together with main precipitate.
BaSO4 - main precipitation
Ba2+, Ra2+(soln) + SO42- (soln)  (Ba,Ra)SO4 (s)
Ba2+: the ion doing precipitate (by adding Na2SO4
solution - precipitant)
Co-precipitation by mean of inorganic reagents
Main
precipitate
Zn(OH)2
LaPO4
BaSO4
Fe(OH)3
Precipitant
Zn(OH)2
Na2HPO4
Na2SO4
NH4OH
Element being
coprecipitated
Fe, Tl, Al
Cu, Mg
Pb
Be, Zn, Cu, Pb, Co, Cd
Co-precipitation with crystalline precipitates , in
which the micro-component is distributed
throughout the entire volume of the solid phase
and participates in the building-up of the crystal
lattice of the macro-component, is known as cocrystallization.
The processes of precipitation of the microcomponent on the surface of the solid phase of
the macro-component are referred to as
adsorption.
Coprecipitation may be:
- Internal coprecipitation
- External coprecipitation.
Internal coprecipitation may involve processe:
“The coprecipitated ions replace the ions in the crystal lattice of
the main precipitate. This may occur if the main precipitate and
the impurity ion together with the coprecipitating agent
crystallize in the same system”.
For example,
There takes place the coprecipitation of lead (Pb2+) with barium
sulphate since both BaSO4 and PbSO4 crystallize in the
orthorhombic crystal system.
Animated figure of orthorhombic crystal system
External co-precipitation
External co-precipitation takes place due to
adsorption processes. In this case, the ions that are
co-precipitate are absorbed on the surface of the
main precipitate. Such a co-precipitation is
especially effective with precipitates that have a
highly developed surface, as is the case when the
precipitate is a colloid .
4- Carrier
• Radiochemical analysis frequently requires the
radiochemist to separate and determine
radionuclides that are present at extremely small
quantities. The amount can be in the picomole
range or less, at concentrations in the order of
10-15 to 10-11 molar.
micro
10-6
mega (M)
106
nano (n)
10-9
giga (G)
109
pico (p)
10-12
tetra (T)
1012
femto (f)
10-15
peta (P)
1015
atto (a)
10-18
exa (E)
1018
If a radioactive isotope is in solution in the form
of ions in an ultrasmall concentration so that
when there are added substances capable of
forming with a given element slightly soluble
compounds it cannot form an independent solid
phase, then it can be separated from solution by
coprecipitation with a so-call carrier,.
For example,
To determine 90Sr in environmental samples, stable
strontium (containing no radioisotopes of strontium)
is added to increase the concentration of total
strontium to the point that the common ion effect
causes precipitation. The added ion that is present in
sufficient concentration to cause a precipitate to form
is called a carrier.
90Sr2+ (ultratrace)
+ SO42- (soln) --/-> 90SrSO4 (s)
(Concentration of 90Sr is very low
90SrSO don’t precipitate)
4
Addition of stable strontium (Sr2+) - to increase
the concentration of total strontium
Sr2+(soln) + SO42- (soln)  (90Sr, Sr)SO4 (s)
Sr2+ - carrier
• The use of a material that is different in isotopic
make-up to the analyte and that raises the
effective concentration of the material to the
macro level is referred to as a carrier.
Separations are most easily accomplished when
performed on a macro scale
• In many cases, the carrier is a
nonradioactive isotope of the analyte
(Isotopic Carriers).
• Some carriers are stable isotopes of
chemically similar elements (Nonisotopic
Carriers).
Isotopic Carriers
An isotopic carrier is usually a stable isotope
of the analyte. Stable strontium (consisting of
naturally occurring 84Sr, 86Sr, 87Sr, and 88Sr)
is frequently used as the carrier in the
analysis of 89Sr and 90Sr.
Nonisotopic Carriers
Nonisotopic carriers are materials that are similar
in chemical properties to the analyte being
separated, but do not have the same number of
protons in their nucleus.
• For several elements, nonisotopic carriers are chosen from
a different family of elements, but they have the same ionic
charge or similar crystalline morphology as the analyte.
For example, Lanthanum and neodymium as +3 ions are
frequently used as nonisotopic carriers for U4+ and Pu4+ in their
final separation as insoluble fluorides (LaF3) by the process of
coprecipitation
Holdback Carriers
• Coprecipitation of a radionuclide with ferric
hydroxide carries other ions in addition to the
analyte, because of its tendency to adsorb other
ions and occlude them in its crystal matrix.
• The addition of a holdback carrier, a highly charged
ion, such as Co3+, represses counter-ion exchange
and adsorption to minimize the attraction of foreign
ions.
• to add “holdback carriers” to analytical mixtures to
prevent unwanted radionuclides from being carried
in a chemical process.
• Hold-back carriers:
The role of hold-back carriers is to prevent the
unwanted coprecipitation of radionuclides or to
prevents the coprecipitation of impurities
• Highly charged ions, chemical homologs, and ions
isotopic with the radionuclide are among the most
efficient holdback carriers.
• Therefore, ions capable of displacing the
radionuclide ions (the hold-back carrier) are
added to prohibit the coprecipitation of the
radionuclide.
scavengers
• Another group of nonisotopic carriers can be
described as general scavengers. Substances with
high surface areas, or the ability to occlude
contaminants in their floc, can be used to effect
gross separation of all radionuclides from macro
quantities of interfering ions.
• Ferric hydroxide, manganese dioxide (MnO2) and
sulfides (MnS), and hydrated oxides [Mn(OH)x] are
examples of these nonspecific carriers that have
been used in many radiochemical separations to
eliminate gross quantities of interfering substances.
• Scavengers
The role of scavengers is to move impurities with
strongly adsorbing precipitation. The scavengers help
to remove miscellaneous traces of radioactivity,
leaving behind one or two chose radioelements for
which hold-back carriers have been added. Ferric
hydroxide and aluminium hydroxide have been widely
used as scavengers.
Yield of Isotopic Carriers
The use of an isotopic carrier to determine the
chemical yield (recovery) of the analyte is a critical
step in the plan of a radiochemical analysis.
The analytical method being used to determine the
final amount of carrier will govern the method of
separation. If a gravimetric method is to be used for
the final yield determination, the precipitate must
have all the characteristics that would be used for
macro gravimetric analysis.easily dried, definite
stoichiometry, nonhygroscopic, etc.
Similarly, the reagent used as source of carrier at the
beginning of the analysis must be of primary-standard
quality to ensure that the initial mass of carrier added
can be determined very accurately.
For a gravimetric yield determination, the aquation
would be the following:
mass of carrier in final separation step
Percent Yield = ------------------------------------------------- x 100
mass of carrier added
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