The radiochemistry of polonium

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But the designation "polonium" had a provocative significance, since as a state Poland had virtually disappeared in , being parcelled out between Prussia, the Austrian Empire and Russia.

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At the time when Marie Curie had left her homeland, a Grand Dukedom of Warszaw had been constituted, but politically it was a mere dependence of Russia. The announcement of the existence of a new element that remained invisible and could be identified solely on the basis of its emission of "uranic rays" was unique in the history of chemistry. It had become customary that no such claim was considered valid until a pure substance had been isolated, the atomic weight of the element determined and its spectral lines measured.

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Eugene Demarcay , who was a recognized expert in spectroscopy, examined the spectrum of the new element but, to the disappointment of the Curies, could not distinguish any new characteristic line apart from those of impurities. The authors admitted that " The mass equilibrium ratio U to Po is 1. Accordingly, the g sample used by the Curies may have contained of the order of 6 ng of Po. One of the largest preparations of polonium in the early history of radioactivity was carried out by Marie Curie and Andre Debierne , who started with several tons of the residue from the sulfuric acid extraction of uranium ore.

Their final product weighed about 2 mg and probably contained about 0. The spark spectrum of this material revealed for the first time a few lines characteristic of the element, 12 years after its discovery [13]. The largest reported amount of natural Po is about 40 Ci. It was extracted from a residue of 37 tons of lead oxide from the Port Hope radium refinery [15].

The isolation of polonium from uranium had thus been accomplished, although the Curies were unaware of the genetic relationship between the two elements. They considered the whole material as a mixture. They also knew nothing at the time of the phenomenon of radioactive decay. In this sense, it was a matter of chance since the separations were performed within 3 months, which was a short time with respect to the day half-life of polonium. It was only a few years later that the Curies noticed with astonishment and great perplexity that polonium was progressively disappearing.

Moreover, the Curies could not imagine that they were dealing with tracer amounts of the new element. By simple reasoning, the substance in the ore that was more active than uranium itself must obviously be present in very small amount, since it had not yet been found.

However, it was not evident at first that this amount was below the limit of any chemical means of detection. Without knowing it, the Curies had invented the use of carriers; in the final step the polonium was effectively carried by bismuth. The words "tracers", "chemistry at the tracer level", "carriers", are terms that arose from the pioneer work of the Curies and that introduced the classical vocabulary of radiochemistry.

The assignment of polonium in the Periodic Table of the elements does not seem to have preoccupied the discoverers. At the time, the terminal part of the Table was unoccupied beyond 83 Bi, with the exceptions of 90 Th and 92 U. In principle, the new element could have fitted into any of the vacant positions.

It appeared remarkable that the polonium was concentrated in the bismuth fraction. This affinity led Marie Curie to consider that the two elements had similar chemical properties. Radionuclides are, however, often carried physically by any kind of solid.


As pointed out by Wolke [7], polonium hydroxide, PoO OH 2 , and polonium sulfide, PoS, are co-precipitated at trace concentrations with Bi OH 3 and insoluble sulfides, but this behaviour does not necessarily imply a strong chemical resemblance. Nevertheless, it seemed reasonable to assume that polonium should be situated not too far from Bi in the Periodic Table. Rare earths are not carried by sulfides, and this further excludes the 89 Eka-La position.

Nevertheless, Mendeleev had made a mistake: the element which he anticipated was dvi-selenium or eka-tellurium. As will be pointed out in the Epilogue, the persistence of Marie Curie in considering that the chemistry of Po should be closely similar to that of Bi turned out to be detrimental to the discoverers of the element. The second Note of the Curies also ended the short story of polonium for several years. Now the Curies were on the verge of another, much more important, discovery. All the same, Marie Curie conserved a sentiment of ownership for polonium, which she defended with emotion, vehemence, and even with aggressiveness.

In one sense, she was right, owing to the role of her polonium in the discoveries of the atomic nucleus, the neutron, artificial radioactivity and fission. The title of the third Note, like the second one, announced the discovery of a radioactive element and bore one additional word: "On a new strongly radioactive substance contained in pitchblende".

The notebook entries of the Curies were interrupted from July to November 11th Polonium had been found in the sulfide fraction precipitated from the soluble part of the pitchblende. Even before their announcement of the discovery of polonium, the Curies probably pursued the analysis of the residual solution, suspecting that the latter might contain still another radioactive substance. They focused their attention on the sulfides because it was easier to search for the activity concentrated in this material.

Bismuth and other metals whose sulfides were insoluble were the carriers of Po. This role was now ensured by barium, which fortunately was present in sufficient amount in the mineral. Few details on the procedure appear in the Note. The Curies began with a recapitulation of the properties of Po in order to emphasize the strikingly different nature of the "new radio-active substance".

It behaved like "nearly pure barium": it was not precipitated by hydrogen sulfide or ammonium sulfide, nor by ammonia. On the other hand, the sulfate and the carbonate were insoluble in water. The high solubility of barium chloride in water, and its insolubility in concentrated HCI and in alcohol proved to be of particular importance in the concentration of the radioactive sub stance.

Finally, the spectra showed the characteristic pattern of pure barium. The authors concluded that the new radioactive substance must be contained in the barium. They advanced three reasons in favour of this hypothesis First, Marie Curie verified that "normal" barium was inactive. For this purpose, she undertook fractional crystallizations on 50 kg of commercial barium chloride until a final amount of 10 g was obtained an which showed no activity.

Accordingly, the radioactive substance was not barium. The second proof was the progressive concentration of the radioactive substance in the course fractional crystallization of barium chloride. The first hydrated chloride was 60 times more active than uranium. Upon dissolution in water, followed by partial precipitation with alcohol, the solid was much more a active than the solution. The Curies pursued the progressive concentration until the activity of the chlorides was times higher than that of uranium.

At this point they had to cease the experiment because lack of material. They concluded that the solubility the chloride of the radioactive element must be low than that of barium chloride. Later, the precipitation of chlorides from water-alcohol mixtures was replaced by the more efficient procedure of crystallization of chlorides or bromides from water alone. The third and last argument was the decisive one. This time, the spectroscopic test was successful.

Demarcay observed in the spectrum of the radioactive barium chloride several lines which could not be assigned to any known element [17]. The wavelength the most intense line was The intensity of the line increased with the radioactivity of the sample from "very weak" the first sample up to "notable" for the sample with fold activity of that of uranium. The authors conclude "We think this is a very serious reason to attribute it [the line] to the radio-active part of our substance.

The various reasons which we have enumerated lead us to think that the new radioactive substance contains a new element, to which we propose to give the name radium ". This name, followed by a question mark, appeared in the notebook for the first time on December The chemical behaviour of the new substance was much easier to determine than that of polonium.

Clearly, it belonged to the family of the alkaline earths, for which the best fingerprint is the insolubility of the sulfates. Indeed, the statement "radium sulfate more soluble in concentrated H 2 S0 4 than barium sulfate" also with a question mark, was written by Pierre Curie. Besides the spectroscopic analysis, a second "official" proof for the existence of radium would have been the determination of the atomic mass.

Marie Curie was aware of this exigence.

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She determined the amount of chlorine present in the radiferous anhydrous barium chloride and compared it to that of a batch of ordinary barium chloride. The calculated atomic masses of the radioactive material were always slightly higher, but still within limits of errors. Obviously, the amount of radium was too low to change the apparent atomic mass of Ba, The determination of the atomic mass of radium became an obsession for Marie Curie that lasted for years.

In successive trials, as the radiferous barium became more and more enriched in radium, the measured atomic mass increased. This astonishingly close value was obtained with a self-luminous sample of 0. Now radium could be definitely placed in the 88 Eka-Ba position in the Periodic Table. The first preparation of radium, as noted by the Curies, "contains very likely a high proportion of barium; nevertheless, the radio-activity is considerable. Hence the radio-activity of radium must be enormous".

The authors now compared the radioactivity of the four known radio-elements: U, Th, Po and Ra.

The last two left an image on a photographic plate within half a minute, U and Th only after exposure times of several hours. When the Curies ran out of material, they were aware that the concentration procedure of radium in radiferous barium should be pursued. They were also aware that vast amounts of raw material would be necessary in order to prepare "visible" or at least much larger quantities of Po and Ra. In a footnote of the publication reporting the discovery of radium, the authors thanked Professor M. The latter offered to the Curies free of charge kg of a uranium-free residue from the treatment of the Joachimsthal pitchblende [14].

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The residue still contained Po and Ra. With the foregoing discovery of polonium, the Curies had oddly enough begun with the most difficult part of the work.

In its own right, radium had outstanding advantages; its concentration in the ore was about times greater than that of polonium; it is a true chemical analogue of barium, from which it can be separated and it could be readily assigned to its correct place in the Periodic Table. The Nobel Prize for Physics was attributed in to "H.

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Becquerel for his discovery of spontaneous radioactivity and to P. Curie for their joint researches on the radiation phenomena".

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At this time, no mention was made of the discoveries of polonium and radium. Thus it is perhaps significant that Becquerel travelled alone to Stockholm; in the meantime, the Curies were allegedly occupied with their teaching duties. In , Marie Curie received the Nobel Prize in chemistry "for her services to the advancement of chemistry by the discovery of the elements radium and polonium, by the isolation of radium, and for the study of the nature and compounds of this remarkable element". Badash stated that the "great discoveries of [of Po and Ra] The moribund subject was resurrected and interest now never flagged until radioactivity merged into atomic physics and evolved into nuclear physics" [19].

During the year , the discovery of uranic rays also stimulated research outside of France. Jump up to the previous page or down to the next one.