Polonium is named after Poland, the native country of Marie Curie. Marie Curie identified the element with her husband Pierre Curie in 1898, some days before they identified radium, whilst trying to discover the source of radioactivity in pitchblende.
Apart from this, there is little information about polonium in most standard textbooks because it was identified by its radioactivity and not by the isolation of a weighable and visible quantity of some compound. However, it is a Group 6 element and, despite its location within the group containing oxygen and sulphur in the periodic table, is quite metallic.
Polonium is said to have as many as 34 isotopes, more than any other element, and all of which are radioactive. The most stable isotope is polonium-209, with a half-life1 (T0.5) of about 103 years. Polonium-208 (T0.5, about 3 years) is the only other polonium isotope with a half-life over one year. The predominant naturally occurring isotope, and therefore most used, is polonium-210, historically known as radium F, which is part of the uranium-238 decay series and has a half-life of just over 138 days.
Isolation of Polonium
In any ore of uranium, polonium is present in 'radioactive equilibrium' (ie, steady state) to the extent of one part in 1.19 x 1010 (about 100 micrograms of polonium per metric tonne), compared to radium at one part in 3 x 106. This difference arises due to the enormous differences in half-lives of the other radioactive elements present: 238U, 4.51 x 109 years; 226Ra, 1590 years; 210Po, 138.4 days. Any attempt to isolate polonium results in the concentration of certain minor constituents in the source to a greater extent than of polonium itself. For this reason, the very first sample of pure polonium was not isolated until March 1944, at the forerunner of the Mound Laboratory in Dayton, Ohio. However, the extreme energy of the α-particles2 emitted by polonium (5.298MeV, 3.84 cm range in air) makes it possible to detect very minute quantities of the element.
Polonium is a 'natural' product of the radioactive decay of uranium-238. By loss of three α (alpha) particles and two β (beta) particles 238U yields:
226-Ra → 222-Rn → 210-Pb → 210-Bi → 210-Po → 206-Pb
These transitions involve sequentially: α emission, 3 α + 2 β emissions, β emission, β emission and α emission.
(Several other members of the series are not shown, including 218-Po and 214-Po, but their half-lives are very short: 3.05 minutes and 1.6 x 10-4 sec, respectively).
In the fractionation of uranium ores, the bulk of the polonium (90%) remains with a final residue of hydrated silica, from which it is difficult to isolate. Hence, it is better to obtain a reasonably pure sample of an earlier member of the disintegration series and allow this to decay to polonium. Thus, a solution containing 210-Pb can be 'milked' of its polonium content periodically. However, the sample would still contain all the inactive lead originally present in the uranium ore. (At an earlier time, Marie Curie had experimented by 'milking' radium salt solutions of their radon content, and small amounts of the gas sealed in small glass ampoules. These could be implanted in body tissue for radiotherapy, this being much safer than using radium itself. After the radon had decayed (T0.5 3.823 days) these 'seeds' contained only 210-Pb together with its disintegration products, and were an excellent source for very small amounts of polonium).
Preparation of Polonium-210
Nowadays, the most convenient and only practical way of obtaining 210-Po is by neutron irradiation of bismuth in an atomic pile.
209Bi (n, γ [gamma]) → 210Bi.
210Bi (Atomic number, 83) disintegrates with a T0.5 into 210Po, with emission of a β- particle.
Note, however, that 210Po, transmutes into the lead isotope 206Pb by the emission of an α-particle. The half life for this process is just over 138 days meaning that after 138 days one-half of the original 210Po has disappeared and after twice this period, 3/4 has gone.
The polonium is concentrated by dissolving it in aqua regia3, removing the last traces of nitric and nitrous acids, and agitating with bismuth powder. Repetition of these steps brings about a 100-fold concentration of 210Po.
The polonium then must be separated from the bismuth and any other impurities which have concentrated with the polonium. This is achieved by converting the bismuth to its oxide and volatilizing the polonium oxide through heating it in vacuo or by forming a black precipitate by treating a denitrated solution of HBiCl4 with 10% SnCl2. The principal contaminant still remaining is silver, and this is removed by dissolving the oxide or metal in nitric acid and precipitating hydrated polonium oxide. Pure polonium is obtained by electrodeposition onto a metal such as silver from 1.5M nitric acid at a controlled cathode potential.
Experimentation with polonium is difficult because of its intense radiation. Elaborate precautions need to be taken to protect workers from skin contact and ingestion, and a prerequisite of this is prevention of contamination of the work area.
Radioisotopes are often handled in glass capillary tubes, but the disruptive effect of the intense α-bombardment on the capillary walls can cause it to 'craze'4 and become very fragile. Furthermore, both helium and lead continuously increase their concentration in the sample and, after a week or so, the pressure of the former can cause the capillary to explode.
In solution, the intense radiation can cause disruption of the solvent and thus an aqueous solution of polonium will slowly liberate small bubbles of oxygen gas. Hydrogen peroxide is also formed as a result of this decomposition, and this will oxidise the polonium from a lower to a higher oxidation state.
Health Hazards of Polonium
Being an α-emitter, polonium is a health hazard only if taken in to the body, as α -radiation cannot penetrate skin. The primary means of exposure therefore are ingestion with food or water, and inhalation. Between 50% and 90% of ingested polonium will leave the body in faeces, the remainder entering the bloodstream, from where it will come into intimate contact with all the cells of the body. However, the spleen and kidneys tend to concentrate polonium more than any other tissues, and it has been estimated that about 45% of ingested polonium will be deposited in the spleen, kidneys and liver, while 10% will be deposited in bone marrow. Thus polonium can cause tissue damage and potentially cancers in any of these organ tissues.
Polonium can also be inhaled, for example from cigarette smoke, and in this case, it is deposited on the mucuous lining of the respiratory tract. The high energy α-particles can then damage the cells lining the airways and lungs, potentially leading to cancers.
Properties and Uses of Polonium
The element is silvery grey in colour, similar to lead, and forms a bright mirror if pure. It is soft enough to be scratched by a needle, and is a good conductor of electricity. It is readily volatile in vacuo and can thus be moved about by heating and cooling. In the gaseous state it exists as Po2 molecules.
Polonium dissolves readily in dilute acids, but is only slightly soluble in alkalis.
Weight for weight it is about 2.5 x 1011 times as toxic as hydrocyanic acid (HCN). Polonium-210 has been found in tobacco as a contaminant originating from phosphate fertilisers. Studies have suggested that radioactive polonium may be the primary cause of smoking-related cancers.
Polonium has a few specific uses. It is a pure α-emitter (only 2 γ quanta per 105 α-emissions5) and, as such, it is useful for studying the effects of α-particles, and for calibration purposes.
The short half life of polonium-210 (138.4 days) and the heat generated by radioactive decay means that polonium metal is a small but concentrated source of power (heat liberated is 141W/g), which means that the metal and its compounds self-heat and 0.5g of polonium will reach a temperature of 500°C. Thus polonium can be used as a small (but expensive!) heat source and has been used for thermoelectric power sources in space satellites. It is especially suitable as there are no moving parts. Polonium-210 was also used in the lunar rovers to keep internal parts warm during the freezing lunar nights6.
The main use of polonium-210 is in 'static eliminators' - devices designed to remove static electricity in machinery, where it can be caused by such processes as paper-rolling, the manufacture of sheet plastics and the spinning of synthetic fibres. Alpha particles from the decaying polonium ionise the adjacent air molecules which, in turn, neutralise the static electricity on surfaces in contact with the air. Due to the very short half-life of polonium-210, these devices need to be replaced annually.
The disintegration of polonium-210 yields 206Pb, a pure isotope not easily obtained by other means.
Polonium metal is unique in that it is the only element whose structure (known as the α-form) is a simple cubic array of atoms in which each atom is surrounded by six other polonium atoms. Upon gentle warming to 36° C, this converts into an allotropic form known as the β-form, which is hexagonal.
Compounds of Polonium
Polonium forms a number of solid polonides: Na2Po, Ag2Po, BePo, CaPo, ZnPo, PbPo, NiPo and PtPo2.
The only known oxide is the pale yellow, feebly amphoteric7 PoO2, which decomposes to elemental polonium on heating in vacuo.The halides include yellow PoCl4, ruby red PoCl2, bright red PoBr4, purple brown PoBr2 and black PoI4, and these have the physical properties (volatility, solubility in organic solvents, etc) characteristic of the halides of non-metals; and also chemical properties such as formation of halosalts such as (NH4)2PoBr6.
Recent Interest in Polonium-210
It is thought that poisoning by polonium-210 may have caused the death of Alexander Litvinenko, a former Russian security officer, in November 2006. Following his death, traces of polonium were found at several places he had visited before becoming ill, including several venues in London, the British Embassy in Moscow and Russian and British Airways aircraft used on the Moscow to London route.
Experts say that as little as 0.1 micrograms of polonium-210 would constitute a lethal dose, an amount equivalent to one-ten millionth of a single aspirin tablet.
One of the few companies worldwide licensed to sell polonium-210 online is United Nuclear Scientific Supplies of New Mexico. They say that a single unit costs about $69, and so it would take at least 15,000 orders, costing more than $10 million, to kill someone. Anyone placing orders totalling 15,000 units within a short time-frame would be spotted. This means that there are only two plausible sources for the isotope: it was either obtained from a nuclear reactor or from very well connected black market smugglers.
A report on BBC's flagship current affairs programme Panorama said that the dose was massive - an estimated 4 billion becquerels8, whereas the normal level in the body is is just 20Bq. This quantity would only be obtainable by the ultra rich or by a government.
At the time of writing it is not clear who was responsible although the Russian authorities deny all involvement.