Thursday, 25 June 2015

physical chemistry - Are there any known chemical properties of tritium water that make it unusually different from protium water?


I suppose the first question supporting the main question is, has tritium water ever been synthesized in sufficient quantity to test chemical properties?



If so, and apart from the obvious radioactive nature of the molecule, and possibly self-heating nature, what unusual properties does tritium water have over protium water?


For example, deuterium oxide is not (easily) metabolized by living things; it cannot support life in the same manner as protium oxide. Also protium water is blue and deuterium oxide is clear. What color is tritium oxide?



Answer



Yes, TX2O has been prepared and is available in significant quantity. When relatively pure, the energy released by the radioactive decay process is so intense that TX2O will boil. It must be transported in a shielded, cryogenic dewar.


A significant difference between compounds containing an element bonded to protium, deuterium or tritium is the strength of those 3 bonds. The XH bond will be the weakest and XT bond the stongest. We can compare these bond strengths by measuring the relative rates at which the XH, XD and XT bonds are broken in a given isotopically substituted compound. These rate differences reflect what is known as a primary kinetic isotope effect. In carbon systems (X=C) the maximum primary isotope effects are roughly as follows: KHKD67 KHKT1314 In other words, a CH bond may break as much as 13 times faster than a CT bond.


These same effects will also be seen with OH, OD and OT bonds. The primary kinetic isotope effects will be slightly smaller here because the magnitude of the effect is mass dependent and oxygen has a larger mass than carbon. Still, as you pointed out, the effects can be disastrous in biological systems. Biological systems cannot survive if the rates for key reactions are slowed down by such large factors.


The blue color of protium water (HX2O) is due to red light absorption around 700 nm. The frequency of an absorption is given by the following equation


νe=12πkμ


Here μ is the reduced mass of the system (e.g. the bond involved in the vibration that is producing the light absorption) and is given as


μ=m1m2m1+m2



where m1 and m2 are the atomic masses located at both ends of the bond. We see that the reduced mass for an OH bond is 16/17, while it is 32/18 for an OD bond. Since, as shown above, the vibrational frequency is inversely related to the reduced mass, we would expect the OH vibration to occur at higher frequency (shorter wavelength) than the OD vibration. Indeed, while HX2O absorbs around 700 nm, DX2O absorbs at higher wavelength (~1000 nm) and is colorless (reference, see p. 82). Given the even larger reduced mass for TX2O (48/19), it's absorption should be shifted even further out of the visible range, so it too should be colorless.


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