Page 262 - Radiochemistry and nuclear chemistry

246 Radiochemistry and Nuclear Chemistry

In this equation E ~ is the potential for a standard state of 1 M concentration; the species in
brackets relates to the chemical activities in the particular solution phase. This relationship
indicates that the redox potential E of a solution is independent of the total concentration
of the species and depends only on the ratio of the oxidized and reduced forms. This has
been confirmed since concentrations of trace amounts of ions show the same redox behavior
as macro concentrations. Reduction and oxidation reactions can, therefore, be carried out
in solutions with trace amounts of radioactive species.
Electrolysis of solutions can be used for electrodeposition of a trace metal on an electrode.
The selectivity and efficiency which would be present for electrolytic deposition of macro
amounts of ions at a controlled potential is not present, however, for trace amounts. The
activity of trace amounts of the species is an unknown quantity even if the concentration
is known, since the activity coefficient is dependent upon the behavior of the mixed
electrolyte system. Moreover, the concentration of the tracer in solution may not be known
accurately since there is always the possibility of some loss through adsorption, complex
formation with impurities, etc. Nevertheless, despite these uncertainties it has been found
that the Nemst equation can be used, with some caution, for calculating the conditions
necessary for electrolytic deposition of trace metals.
It is also possible to precipitate insoluble species on electrodes. For example, if a
fluorosilicate solution is electrolyzed, thereby freeing a high concentration of fluoride ion
at the electrode, a thin uniform layer of UF4, can be deposited. Similarly, trace amounts
of dements which form insoluble hydroxides can be deposited from solutions in which
water is being electrolyzed as a region of extremely high pH is present at the cathode.

9.2.6. Tracer separation methods

All the analytical techniques used in conventional chemistry may be used for the
separation and isolation of radioactive elements and compounds in macro or trace
concentrations. The precipitation method was amply demonstrated by the early radio-
chemists M. Curie, Debieme, Rutherford, Hahn, etc., for the separation, concentration and
identification of the naturally occurring radioactive elements. However, in w167
we have pointed out the many pitfalls in working with tracer concentrations in solutions
containing precipitates, etc, as well as in the use of electrochemical methods (w
Normally these separation methods require the addition of a macro amount of isotopic
carrier. However, in some cases analytical procedures are available for separation and
isolation of cartier free radiotracer concentrations. Solvent extraction (see w and App.
A), and various forms of partition chromatography (w methods have been found to
be particularly advantageous in this connection since they are selective, simple, and fast.
Liquid-liquid (or solvent) extraction is a technique for selectively transferring a species
between an aqueous solution and an organic phase (e.g. kerosene, benzene, chloroform, etc)
by equilibrating the aqueous phase with an organic solvent. Usually the organic phase
contains a reagent A (extractant) which forms a neutral compound MA N with the species
M to be transferred between the phases. The number of extractants applied are numerous
and the literature must be consulted to determine the most suitable ones for the system of
interest; typical extractants are organophosphates, amines and metal chelating agents
(usually weak organic acids). The fraction extracted at equal phase volumes is