Page 260 - Radiochemistry and nuclear chemistry

244 Radiochemistry and Nuclear Chemistry

interference of polymerization reactions. However, one then must be aware of the
adsorption risks.
An additional complication can arise in solution if the radioactive species in trace amounts
react with trace concentrations of impurities. For example, in an investigation of the
properties of pentavalent protactinium, Pa(V), it was found that the protactinium was
extracted into pure xylene from 1 M HCIO 4 solutions. Further experimentation showed that
this extraction was due to the presence in the xylene of organic impurities at concentrations
below the detectable limit of 0.01%. Support for this interpretation was provided when the
solution was made 10 -4 M in thorium, which was expected to form complexes with the
probable impurity, thereby preventing the reaction with the protactinium. In fact, no
protactinium was extracted into xylene from this solution. The thorium in this case acts as
a hoM-back carrier.

9.2.4. Precipitation and crystallization

Due to the low concentration of radioactive tracers in solution the solubility product for
an "insoluble" salt is not always exceeded upon the addition of macro concentrations of a
counter ion. Let us as example take the insoluble lanthanum hydroxides. The solubility
product for the reaction La(OH)3 (s) ~ La 3+ + 3 OH- is Kso ~- 1019; in 1 mM NaOH the
concentration of La 3+ in the solution is only 10 -10 M in equilibrium with the La(OH)3
precipitate. If 100 MBq (2.7 mCi) 14~ obtained as a fission product or by milking from
14~ (Table 4.1), is dissolved in 1 1, the solution will have [La 3+] ~-. 3.5 • 10 -11M. In this
case the solubility product is not exceeded in 1 mM NaOH. To precipitate the La(OH) 3
quantitatively the NaOH concentration must be raised to 10 - 100 raM; however, at these
concentrations the precipitate is formed as a colloid in solution and even upon centrifugation
the amount of precipitate is so small as to be unweighable by present techniques. With the
addition of a carrier for 140La, the precipitation can be carried out without any difficulty.
It is possible to remove ions at tracer level concentrations from solutions by precipitation
using adsorption or coprecipitation. Coprecipitation occurs if the compound of the tracer
and the oppositely charged ion of the precipitate is isomorphous with the precipitate. In
these cases the active ion may be included in the crystal lattice of the precipitate at a lattice
point, particularly if the tracer ion is close in size to the ion which it displaces. However,
at trace level concentrations exceptions are found to this requirement of similarity in size
as well as to the requirement of isomorphism. When the distribution of the tracer is found
to be uniform throughout the precipitate it can be described by the Berthelot-Nernst
homogeneous distribution law which is expressed as

xly = D' (a - x)/(b - y) (9.1)

where x and y are the amounts of A z+ and B z+ in the precipitate, a and b are the initial
amounts of these ions, and D' is the "distribution coefficient'. A more "true" distribution
constant (D = concentration of tracer in solid/concentration of tracer in solution) can be
obtained by using a conversion factor, e.g. C = gram solute per ml of saturated carrier
solution divided by the density of the solid