Heavy water is a unique form of the common substance that represents a significant milestone in 20th-century science. Its discovery and subsequent isolation were born from the theoretical possibility of a heavier hydrogen atom, which had previously remained undetected. This achievement provided scientists with an invaluable tool, altering the course of nuclear physics and chemistry. The successful concentration of this rare molecule transitioned from a laboratory curiosity to a substance of profound strategic importance. The history of heavy water’s isolation led to a new understanding of isotopes and their practical applications.
Defining Heavy Water
Heavy water, chemically known as deuterium oxide (D2O), is distinct from standard water (H2O) due to its atomic composition. The difference lies in the hydrogen component: standard water contains protium, the most common hydrogen isotope, whose nucleus has only a single proton. In contrast, heavy water contains deuterium, an isotope of hydrogen that possesses one proton and one neutron in its nucleus. The addition of this neutron makes a deuterium atom approximately twice as heavy as a protium atom, giving the entire water molecule a higher mass.
This heavier composition means that a molecule of deuterium oxide has a molecular weight of about 20, compared to about 18 for ordinary water. Although chemically similar to regular water, the added mass causes slightly different physical properties, such as a higher boiling point of 101.42°C and a higher melting point of 3.82°C. Heavy water is also about 10% denser than ordinary water, which is how it earned its name. Natural water sources contain only a minute amount of deuterium, roughly one deuterium atom for every 6,760 protium atoms.
The Key Figures in Isolation
The credit for the discovery of deuterium, the foundation of heavy water, belongs to American chemist Harold C. Urey. Working at Columbia University, Urey successfully identified the heavy hydrogen isotope in 1931. His detection was the culmination of theoretical work suggesting the existence of a heavier hydrogen isotope, which Urey then set out to find experimentally.
Urey’s breakthrough, which included collaborators George M. Murphy and Ferdinand Brickwedde, was quickly recognized as a major scientific achievement. Just three years after the discovery, Urey was awarded the 1934 Nobel Prize in Chemistry for his work. Following the identification of deuterium, the isolation of pure heavy water (D2O) became possible. Urey and his colleague Edward W. Washburn demonstrated the first effective concentration method a few months later, and Gilbert Newton Lewis isolated the first sample of pure heavy water by electrolysis in 1933.
The Scientific Breakthrough
The isolation of heavy water was a complex process necessitated by the minute concentration of deuterium in nature. The initial method for concentrating the heavy hydrogen isotope was a sophisticated technique involving fractional distillation of liquid hydrogen. Urey reasoned that the heavier isotope would have a slightly different boiling point and would concentrate in the liquid residue as the lighter hydrogen evaporated.
This demanding process began with the fractional distillation of about five liters of liquid hydrogen, which was ultimately reduced to just one cubic centimeter of concentrated residue. Urey then confirmed the presence of deuterium in this concentrated sample by observing its unique spectrum using a high-resolution spectrograph. Following the initial discovery of the isotope, a more practical method for isolating heavy water itself was developed, centering on the prolonged electrolysis of large volumes of ordinary water.
The electrolysis process leveraged the small difference in the rate at which protium and deuterium are released as gases from water. Since the lighter protium gas evolves slightly faster, the deuterium-containing water molecules gradually become concentrated in the residual liquid. Continued, exhaustive electrolysis of hundreds of liters of water was required to yield even a small volume of nearly pure deuterium oxide. This separation technique, while energy-intensive, was the only large-scale method available until more cost-effective chemical exchange processes were developed later.
Initial Impact and Application
The isolation of heavy water immediately provided a new tool for both physics and chemistry. One of its earliest and most significant applications was its use as an isotopic tracer in biological and chemical studies. Scientists could substitute heavy water into experiments and track the deuterium atoms to gain insights into metabolic pathways and reaction mechanisms.
The properties of heavy water also made it a substance of strategic interest in the burgeoning field of nuclear physics. It became a material for early nuclear research because of its ability to act as a neutron moderator. A moderator slows down the fast neutrons released during nuclear fission, making them more likely to cause further fission events and sustain a chain reaction.
Heavy water is effective as a moderator because, unlike ordinary water, its deuterium atoms are far less likely to absorb the neutrons. This unique property allowed for the use of natural, unenriched uranium fuel in reactors, simplifying the construction of early atomic piles. Heavy water was a component in the Manhattan Project, with the world’s first heavy water-moderated reactor, Chicago Pile-3, beginning operation in 1943.