Tritiated water is a radioactive form of water in which the usual protium atoms are replaced with tritium atoms. In its pure form it may be called tritium oxide (T2O or 3H2O) or super-heavy water. Pure T2O is a colorless liquid, and it is corrosive due to self-radiolysis. Diluted, tritiated water is mainly H2O plus some HTO (3HOH). It is also used as a tracer for water transport studies in life-science research. Furthermore, since it naturally occurs in minute quantities, it can be used to determine the age of various water-based liquids, such as vintage wines. The name super-heavy water helps distinguish the tritiated material from heavy water, which contains deuterium instead.
Self-radiolysis Tritiated water is primarily studied as a dilute solution within light water. Here, the proportion of the light, hydrogen tritium oxide is strongly favoured versus the more negligible heavy, double tritium oxide, as the conversion reaction has an equilibrium constant of 3.42 at room temperature.
H 2 O + T 2 O ↽ − ⇀ 2 HTO {\displaystyle {\ce {H2O + T2O <=>> 2HTO}}}
The molecules then experience beta decay and formation of the hydroxyl or tritoxyl radical via:
HTO ⟶ 3 He + + β + − ν ¯ + OH ⋅ {\displaystyle {\ce {HTO->\ ^{3}He^{+}\ +\ \beta ^{-}+\ {\bar {\nu }}\ +\ OH^{.}}}}
T 2 O ⟶ 3 He + + β + − ν ¯ + OT ⋅ {\displaystyle {\ce {T2O->\ ^{3}He^{+}\ +\ \beta ^{-}+\ {\bar {\nu }}\ +\ OT^{.}}}}
The average electron energy of the beta decay is 5.7 keV. The energy required to break hydrogen-oxygen bonds in water is three orders of magnitude lower at 5.2 eV. This leads to many radiolysis events:
H 2 O → beta rays e aq − , HO ⋅ , H ⋅ , HO 2 ⋅ , H 3 O + , OH − , H 2 O 2 , H 2 {\displaystyle {\ce {H2O\;->[{\text{beta rays}}]\;e_{aq}^{-},HO*,H*,HO2*,H3O^{+},OH^{-},H2O2,H2}}}
Many subsequent reactions occur, but primarily result in recombination to water, or the escape of molecular hydrogen and oxygen gas, alongside the helium-3. Studies of tritiated water often prefer to describe the concentration by the measurable radiation level in curies per liter (Ci/L) or terabecquerels per liter (TBq/L), rather than the species proportion. In one CEA study, relatively highly tritiated water at 1,800 Ci/L or 74 TBq/L (0.12% HTO, negligible T2O) was left to self-radiolyze for 56 days in three volumes. In the 300 mL volume, the primary gases collected were H2 at 2.54 mmol, O2 at 1.31 mmol, and 3He at 0.13 mmol. Thus in this geometry, for each tritium decay, roughly twenty water molecules were permanently dissociated.
Applications Tritiated water can be used to measure an organism's total body water (TBW). Unlike doubly labeled water this method relies on scintillation counting. Tritiated water distributes itself into all body compartments relatively quickly. The concentration of tritiated water in urine is assumed to be similar to the concentration of tritiated water in the body. TBW is determined from the following relation:
… excerpt ends here. Continue reading the full article.
