Page 275 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 259

Photosynthesis ]
~"
Spinoch Cell
plus NH4*

Molote 3:
Alonine Ol~emte f~l~llb <['-~ ~

Citrate "~
...
Q.
~-
Jutonmte Q.
Glu c
ASl)Ortote " 0
3- PGA oO
Sucrose t
G6P
FrP ~ 6PG
UDPG ,~ ~,
Slorch Q
- Phenol' Acetic Acid: t420,24 hr
FIG. 9.8. Two-dimensional paper radiochromatogram of 14C-labeled products in photo-
synthesis after exposure to 14CO2 atmosphere. (Courtesy Calvin and Lemmon.)

9.4.2. Determination of chemical exchange rates

If two different chemical species with some element in common are mixed in solution,
exchange of this common component may occur. The chemical equation would have the
form

AX + BX* ~AX* + BX

Since the type and concentration of the chemical species remain unchanged, it is impossible
to observe the exchange unless the atoms in one reactant are labeled. By using X*, a
radioactive isotope of X, the reaction may be followed, and at equilibrium the activity
should be uniformly distributed between the two chemical species, i.e. the specific activity
of X* will be the same for both AX and BX. Of course, if AX and BX are both strong
electrolytes, uniform distribution is essentially immediate upon mixing. If at least one of
the reactants is an inorganic complex or an organic molecule, the exchange may be
measurably slow if it occurs at all.
Since the chemical form of the reactants is not altered by the isotopic exchange, there is
no change in heat content. However, the entropy of the total system is increasexl when
uniformity in the distribution of the isotopes of X is achieved throughout the system. This
entropy increase provides a decrease in the free energy, making isotopic exchange a
spontaneous reaction. Despite this spontaneity, the exchange may be prevented or made