Page 259 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 243

decreasing the specific activity by adding macro amounts of a nonisotopic element which
is easily adsorbed and may block the available surfaces from adsorbing the tracer.
In addition to adsorption on the walls of the container, radioactive species frequently
adsorb on precipitates present in the system. The nature of the precipitate as well as its
mode of precipitation are major factors in the amount of adsorption. If silver iodide is
precipitated in an excess of silver ion the precipitate has a positive surface layer due to the
excess concentration of silver ions on the surface. By contrast if the precipitation occurs in
excess iodide, there is a negative surface charge due to the excess iodide on the surface.
When trace amounts of radioactive lead ions are added to a suspension of two such
precipitates in water, the precipitate with the negative surface charge adsorbs > 70% of
the tracer lead ions from the solution, while the precipitate with the positive surface charge
adsorbs < 5 %. The amount of adsorption increases with the ionic charge of the radioactive
tracer, e.g. it has been found that with a precipitate of Ag2S about 7 % of Ra 2+, 75 % of
Ac 3 + and 100 % of Th 4 + is adsorbed.
The adsorption properties of trace elements have been used to advantage for the isolation
of the trace elements as well as for the separation of different trace elements with different
adsorption properties.

9.2.2. RadiocoUoids

Radioactive tracers adsorb not only on solid container surfaces and precipitates but on any
kind of solid material suspended or in contact with the solution. Dust, cellulose fibers, glass
fragments, organic materials, etc., are examples of substances that readily adsorb
radioactive tracers from solution. If the solution contains large molecules as, for example,
polymeric metal hydrolysis products, these also tend to adsorb trace elements. In addition
to sorption, the presence of such material in the solution can lead to the phenomenon of
radiocolloid formation, which is the attachment of radionuclides to semicolloidal aggregates
in solution. If the solution is kept at sufficiently low pH and extremely free from foreign
particles, sorption and radiocolloid formation are usually avoided as major problems.

9.2.3. Equilibrium reactions

The low concentration of radioactive tracers can lead to the formation of solute species
that are not observed at equilibrium with macro amounts. For example, the hydrolysis of
uranyl ions corresponds to the equilibrium

m UO22+ + p H20 ~ (UO2)m(OH)p 2m'p + p H +

With macro concentrations of uranium this equilibrium is shifted to the fight with the
observation of polymers with properties rather different than that of the uranyl ion. At a
uranium concentration of approximately 0.001 M, more than 50% of the uranium is
polymerizexi at pH 6, while for uranium concentrations less than 10 .6 M the polymerization
is negligible. This condition can be used to advantage: trace metal concentrations can be
used if one wishes to study the properties of a metal ion at relatively high pH's without