Page 256 - Radiochemistry and nuclear chemistry

240 Radiochemistry atut Nuclear Chemistry

earths. Despite the demonstration of the value of the tracer technique by these early studies
the technique did not come into common use until after World War II when relatively large
amounts of cheap radionuclides became available through the use of nuclear reactors.
While it is not necessary to use radioactive isotopes for tracer studies, in general, the use
of radioactivity is simpler and less expensive than the use of stable isotopes. Research with
the latter requires rather sophisticated and expensive measuring devices such as mass
spectrometers, cf. w We restrict our discussion to the use of radioactive tracers.
Among the advantages of using radiotracers we can list the following: (a) radiotracers are
easy to detect and measure with high precision to sensitivities of 10 -16 to 10 -6 g, (b) the
radioactivity is independent of pressure, temperature, chemical and physical state; (c)
radiotracers do not affect the system and can be used in nondestructive techniques; (d) if
the tracer is radiochemically pure, interference from other elements is of no concern
(common in ordinary chemical analyses); (e) for most radioisotopes the radiation can be
measured independently of the matrix, eliminating the need for calibration curves.

9.1. Basic assumptions for tracer use

In some experiments answers to scientific questions which require knowledge of the
presence and concentration of a specific element or compound at a certain place and at a
certain time can be obtained only through the use of a radioactive tracer. For example, self
diffusion of metal ions in solutions of their salts cannot easily be studied by any other
technique. However, in other cases the use of radioactive tracers is not necessary in
principle but is justified by the greater convenience. In either type of investigation there are
two assumptions implicit in such uses.
The primary assumption is that radioactive isotopes are chemically identical with stable
isotopes of the same element, i.e. the substitution of 14C for 12C in a compound of carbon
does not change the type or strength of the chemical bonds nor does it affect the physical
properties of the compound. The validity of this assumption depends on the precision of
measurement of the chemical and physical properties. The difference in mass between the
various isotopes does cause some change in these properties (w but even in the case of
14C and 12C, with a mass difference of approximately 15 %, the isotope effect is rather
small and difficult to detect. Normally only for systems involving hydrogen-deuterium-
tritium substitution must isotope effects be considered. For heavier elements it can be
neglected in almost every situation.
The second assumption in the use of tracer techniques is that the radioactive nature of the
isotope does not change the chemical and physical properties. Until the moment of its
disintegration the radioactive atom is indistinguishable from its stable isotope except for the
isotopic mass difference. When the radioactive disintegration of the atom has been observed
('counted'), the decay product is normally a different element and its subsequent chemical
behavior is usually of no interest. If the disintegration rate is very high, the energy released
by the radioactive decay can cause observable secondary radiolytic effects (Ch. 7).
However, in well-designed tracer experiments the level of radioactivity is high enough to
provide accurate data but normally small enough not to produce noticeable chemical effects.
While the radioactivity of the tracers is assumed not to affect the chemical systems, the
parent-daughter relationship of radioactive nuclides needs special consideration. For