Scientists at Vemork first observed the curious heavy-water in 1934 when it appeared as a by-product of their revised ammonia production process. Physically and chemically the substance is similar to ordinary water, but while the hydrogen atoms in normal H
2O consist of one proton and one electron, many of the hydrogen atoms in heavy-water have the added weight of a neutron– an isotope known as
deuterium. This deuterium oxide (D
2O) does exist in water naturally, though its ratio is normally only about one part in 41 million, so it had not been previously observed in significant quantities. For eight years Vemork’s scientist had been collecting the exotic liquid for scientific scrutiny, supplying samples to the world’s researchers for basic experiments.
Heavy water itself is not radioactive, and has physical properties similar to water save for being about 11% more dense. However, as commercially made, heavy water contains whatever tritium was present in the water from which it was isolated. When the water in eukaryotic organisms is replaced by more than about 25 to 50% heavy water, they experience toxicity due to interference by the deuterium with the mitotic apparatus of these cells. Higher organisms, including mammals, if given only heavy water, soon become ill and die at the point that about half their body water has been replaced. Bacteria, however, are able to grow slowly in pure heavy water.
Small concentrations of heavy water are nontoxic. The adult human body naturally contains deuterium equivalent to the amount in about 5 grams of heavy water, and comparable doses of heavy water are still used as safe non-radioactive tracers for metabolic experiments in humans and other animals.
Heavy water is 10.6% denser than ordinary water, a difference which is difficult to notice in a sample of it (although it looks like water, it reportedly tastes slightly sweet[3]). One of the few ways to demonstrate heavy water's physically different properties without equipment, is to freeze a sample and drop it into normal water. Ice made from heavy water sinks in normal water. If the normal water is ice-cold this phenomenon may be observed long enough for a good demonstration, since heavy-water ice has a slightly higher melting-temperature (3.8 °C) than normal ice, and thus holds up very well in ice-cold normal water.
Heavy water is the only known chemical substance that affects the period of circadian oscillations, consistently increasing them. The effect is seen in unicellular organisms, green plants, isopods, insects, birds, mice, and hamsters. The mechanism is unknown.[7]Circadian rhythms are endogenously generated, and can be entrained by external cues, called Zeitgebers, the primary one of which is daylight. These rhythms allow organisms to anticipate and prepare for precise and regular environmental changes.
Circadian Rhythms
A great deal of research on biological clocks was done in the latter half of the 20th century. It is now known that the molecular circadian clock can function within a single cell; i.e., it is cell-autonomous.[7] At the same time, different cells may communicate with each other resulting in a synchronized output of electrical signaling. These may interface with endocrine glands of the brain to result in periodic release of hormones. The receptors for these hormones may be located far across the body and synchronize the peripheral clocks of various organs. Thus, the information of the time of the day as relayed by the eyes travels to the clock in the brain, and, through that, clocks in the rest of the body may be synchronized. This is how the timing of, for example, sleep/wake, body temperature, thirst, and appetite are coordinately controlled by the biological clock.
Deuterium has a different
magnetic moment from
hydrogen and therefore does not contribute to the NMR signal at the hydrogen resonance frequency.
Even though there is very little commercial use for heavy water beyond it's use in nuclear reactors, it is produced by several countries in the world, including India, the worlds largest producer.
