ISOTOPES OF HYDROGEN AND OXYGEN BASIC INFORMATION AND TUTORIALS

The heaviest water must fall first. Aristotle

In ordinary natural water the hydrogen and oxygen each consist of three different isotopic forms with different masses; this fact is of some geochemical significance.

The isotopes present in water are:
1H, 2Hor D, 3Hor T, 16O, 17O, 18O:

The first isotope of hydrogen has a nucleus consisting of only a single proton, and accordingly the mass number is 1. The second isotope has both one proton and one neutron in the nucleus, and the mass number is 2; this isotope has a special name: “deuterium,” hence the common use of the letter “D” to represent the isotope.

The third isotope has one proton and two neutrons, a mass number of 3, and also has a special name: “tritium.” Tritium is radioactive.

The nuclei of oxygen have 8 protons, and 8, 9, or 10 neutrons. These isotopes are present in all possible combinations, so that (not including molecules with tritium) all natural water contains nine kinds of water molecules.

The ratios of the different isotopes, one to another, can be measured with remarkably high precision using an isotope ratio mass spectrometer.

The different isotopic forms of water each have different vapor pressures and freezing points; these physical differences are important because they make it possible to use the isotopes as tracers of geochemical processes.

The differences in vapor pressure lead to a fractionation of the isotopes whenever water evaporates or condenses. The heavier isotopes are, in every case, concentrated into the liquid phase. This leads to differing isotopic compositions in water from different sources.

When water freezes, there is also a fractionation such that the heavier isotopes are concentrated into the solid phase, although the effect is less than in the
gas–liquid exchange processes.