The Nobel Prize
in Chemistry 1943The Nobel Prize
in Chemistry 1943In regard to de Hevesy's work, Professor A. Westgren, member of the Nobel Committee for Chemistry of the Royal Swedish Academy of Sciences, made the following statement*
When, in 1913, de Hevesy was working with Rutherford in
Manchester, this young scientist had been commissioned to isolate radium D from
radioactive lead. His efforts were unsuccessful. It had in fact become apparent
that radioactive radium D differed so little from inactive radium G, the last
of the series of descendants of radium, that all attempts to isolate them from
each other seemed destined to failure. The reason for this was at the same time
discovered. Radium D and radium G are isotopes and constitute different species
of lead. They differ in their atomic weight whilst their atoms have the same
nuclear charge. The shells of their electrons, shells which determine their
chemical properties, are therefore more or less identical.
Although unsuccessful, de Hevesy's efforts were not wasted. They gave him the
idea for a new method of chemical research.
If it is impossible to isolate chemically a radioactive isotope from an element
of which it is part, it must be possible to use this peculiarity to follow in
its details the behaviour of this element during chemical reactions and physical
processes of different kinds. The active atoms are recognized by their radiation
and, being faithful companions of the inactive atoms of an element, they serve
as markers for them. Since the intensity of radiation can be determined with
such precision that imponderable quantities can be measured in this way, extremely
small quantities of a marker of this kind are sufficient.
By using radium D as a marker, de Hevesy determined the solubility of highly
insoluble lead compounds. He succeeded in determining exactly the quantity of
lead sulphide or of lead chromate taken up under different conditions from solvents
of different types. He studied the exchangeability of lead atoms into the dissolved
substances and was able to confirm that it corresponded to the behaviour of
the lead atoms as ions. The movements of the atoms in solid lead, i.e. the self-diffusion
which occurs in this metal, would be determined; it had previously been impossible
to measure this process. By precipitating thorium B, a very active isotope of
lead, on the surface of a lead crystal and by following the reduction in radiation
intensity brought about by the changes in place of the active atoms with the
inactive lead atoms of the lower layer, and hence with the penetrations which
took place in the crystal, he was able to measure the energy needed to liberate
an atom from the crystallised part of the lead, in other words the dissociation
energy of the crystal lattice. This energy was found to be of the same order
of magnitude as the heat of vaporisation of lead. This latter research is particularly
interesting from the physico-chemical point of view.
The new method has also enabled biological processes to be studied. Beans placed
in solutions containing lead salts with a mixture of active lead atoms absorbed
a part of these salts but the distribution of the metal was not the same in
the root, the stem and the leaves. Most of the lead, which does not favour natural
biological development but on the contrary acts as a poison, stays in the root.
Relatively more lead was extracted from dilute than from more concentrated solutions.
Absorption and elimination of lead, bismuth and thallium salts by animal organisms
was studied in this way. A knowledge of the distribution of bismuth compounds
introduced into an animal organism is valuable from the medical point of view,
since some of these compounds, as we know, are used therapeutically.
So long as natural radioactive elements only were used as markers, use of the
new method was inevitably very limited. In fact the method could be applied
only in the case of heavy metals - lead, thorium, bismuth and thallium - and
their compounds. The situation was to be very different when Frédéric
and Irène Joliot-Curie, and Fermi succeeded in producing radioactive
isotopes from any element by bombarding it with particles. This discovery was
made some ten years ago and the study of chemical processes by means of radioactive
markers has since then been carried to such a point that it is now widely used
in laboratories throughout the world. De Hevesy has remained the prime mover
in this new field of activity and much first-class and important research has
been carried out by him and his co-workers.
Exceptionally valuable results have thus been obtained in biology. An isotope
of radioactive phosphorus, which can be obtained by exposing sulphur to neutron
radiation or ordinary phosphorus to radiation from nuclei of heavy hydrogen,
has mostly been used. This radioactive phosphorus is sufficiently long-lasting
for tests of this nature. It has a half-life of approximately 14.8 days. De
Hevesy produced physiological solutions of sodium phosphate containing this
marker and injected them into animals and humans. The distribution of the phosphorus
was determined at certain intervals. A study of blood samples showed that the
phosphorus thus introduced quickly left the blood. In human blood the radio-phosphorus
content had fallen after only 2 hours to a mere 2% of its initial value. It
diffuses into the extra-cellular body fluid and gradually changes places with
the phosphorus atoms of the tissues, organs and skeleton. After some time it
can even be found, though in very small quantities, in the enamel of the teeth.
Exchanges small and slow as they may be, therefore occur between the outer hard
parts of the teeth and the inner tissues of the bones and the lymph. Most of
the phosphorus introduced, finds its way into the skeleton, muscles, liver and
gastro-intestinal organs. Elimination of phosphorus from living organisms has
also been studied by this method.
Phosphorus is an extremely important element in biological processes. The knowledge
of its functions in living organisms which has been acquired thanks to the use
of radioactive markers is therefore of the very greatest interest. De Hevesy
succeeded in detecting where and at what speed the various organic compounds
of phosphorus are able to form and the paths which they take in the animal organism.
In order to form from a phosphate which has been injected into the blood they
must first penetrate into the cells. Acid-soluble compounds of phosphorus form
rapidly, whereas phosphatides closely related to fatty substances are slower-forming.
These latter form mainly in the liver, whence they are carried by the blood
plasma to the places where they will be consumed. De Hevesy showed that the
phosphatides of the chicken embryo are produced in the embryo itself and that
they cannot be extracted from the egg yolk.
De Hevesy also carried out several investigations with radioactive sodium and
potassium. He studied how physiological saline containing radioactive sodium
which was injected into a human subject first spread into the blood and then
slowly penetrated into the cells; he also studied the manner in which it is
excreted. After 24 hours the blood corpuscles had lost approximately half their
sodium content.
In addition to the above-mentioned markers, several other active isotopes, such
as magnesium, sulphur, calcium, chlorine, manganese, iron, copper and zinc,
have been used for this type of research. In the case of the lighter elements
it has also been possible to use inactive isotopes such as heavy hydrogen, with
an atomic weight of 2, nitrogen, with an atomic weight of 15, and oxygen, with
an atomic weight of 18. It is of course less easy to determine the content of
an inactive than of an active marker, but this can be done by determinations
of density or mass-spectrographically. To determine the concentration of deuterium,
or heavy hydrogen, which is twice as heavy as ordinary hydrogen, is a relatively
easy matter. De Hevesy used deuterium as marker in many tests. He then noticed
that a person who has drunk water containing heavy hydrogen excretes deuterium
in the urine after only 26 minutes. Frogs and fishes swimming in water containing
deuterium absorb it and, after about 4 hours, are in equilibrium with the medium
as far as the deuterium is concerned. Heavy nitrogen and heavy oxygen have also
been used in many investigations.
* Talk given on the radio on 10th December, 1944, with amendments and additions. The Nobel Prize in Chemistry 1943 was announced on November 9, 1944.
From Nobel Lectures, Chemistry 1942-1962, Elsevier Publishing Company, Amsterdam, 1964