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Introduetion THE CONCENTFL4TION OF RADIUM AND MESO- THORIUM BY FRACTIONAL CRYSTALLIZATION* BY JOHN I,. NIERMAN Introduetion MarkwaldlO and Soddyl' have shown independently that mesothorium is absolutely identical in chemical nature with radium and cannot be separated therefrom.** In consequence all radium separated from uranium minerals containing thorium, contains also the mesothorium in the mineral, and all preparations of mesothorium contain the radium that is present in the mineral from which the thorium is derived. In the extraction and recovery of the minute quantities of mesothorium and radium present in radioactive minerals, these elements become associated with barium and follow the barium throughout the process. The refining of mesothorium and radium then consists in separating these elements from barium, the method generally followed being fractional crystallization of the barium solution, first as chloride, and later as bromide. The mesothorium and radium continue to be enriched in the crystal fractions, and reduced in the succes- sive mother liquors. t In practice, l2 a fair concentration of acid is maintained throughout the chloride and bromide systems, for the reason ' that the factor of enrichment of mesothorium radium chloride from barium chloride and also of mesothorium radium bromide from barium bromide is regarded as more favorable in acid than in neutral solutions. While it has been shown3 that the crystallization factor is higher for bromides than for chlorides, the effect of the acidity of the solutions on the progress of * Abstract of a thesis submitted in partial fulfilment of the requirements for the degree of Master of Arts in the Graduate, School of the University of Missouri, August, 1919. * * The reference numbers refer to the appended bibliography. t The method is fully described in Bulletin No. 104, U. S. Bureau of Mines (1915). Concentration of Radium and Mesothorium, Etc. 193 concentration has not been clearly established. This in- vestigation is confined to the effect of hydrobromic acid on the crystallization factor in bromide solutions of barium, mesothorium and radium. Our experiments show definitely that the crystallization factor is independent of the acid con- centration. In other words, the enrichment of mesothorium and radium is the same in neutral as in acid solution when equal fractions of crystals are separated. The terms “crystallization factor,” “concentration factor, ” ‘‘factor of enrichment,” “fractionation coefficient’’ appear to be used synonymously in the refining of radium and mesothorium, but not aways with the same meaning. Evi- dently these terms always refer to a ratio, but the ratio may be expressed in several related ways, as (I) the concenlration of mesothorium in the crystals separated to the concentration of mesothorium in original crystals, used in preparing the solutidn, concentrations ’ being given as milligrams of meso- thorium (expressed in terms of radium) per gram or kilogram of anhydrous crystals; (2) total mesothorium in crystals to total mesothorium in mother liquors; (3) concentration of mesothorium in crystals separated to concentration of meso- thorium in crystals from mother liquor, where the ratio is used in the sense of a partition or distribution coefficient. Throughout this paper the term crystallization factor wilt be used with the first meaning: the ratio, concentration of meso- thorium in crystals separated to concentration of meso- thorium in original crystals. This factor serves directly in computing the radium or mesothorium content of the head crystals in “stepping up” as this factor raised to the nth power, n being the number of crystallizations, gives the de- sired multiplier. Since mesothorium has never been obtained pure, its amount is expressed by its activity compared with radium. One milligram of mesothorium is the quantity of mesothorium- one, with its equilibrium amount of mesothorium-two, whose gamma ray activity is equal to that of one milligram of radium containing the equilibrium amounts of its short-lived products. I94 John L. Niervnaw The radioactive bromides used in these experiments were loaned by the Welsbach Company through the courtesy of Dr. H. S. Miner. Before beginning determinations of the crystallization factor, two lots of high grade bromides each containing mesothorium and radium equivalent to about 150 mg of radium in 50 grams were refined by fractional crystallization so as to gain familiarity with the technique of fractionation in acid and neutral solutions. Purification of Bromides Before undertaking the determination of crystallization factors the bromides selected for the single fractionations were purified. The three samples of anhydrous bromides weighed approximately 5.7, 3.6, and 9.8 g and contained, respectively, about 2.1,0.5, and 0.2 mg of mesothorium per g. The crude bromides were fused with a fusion mixture consisting of equal molecular weights of sodium and potassium carbonates and thus converted into carbonates. These carbonates, after being well washed, were converted into bromides by dissolving in pure hydrobromic acid. After adding about 5 mg of thorium bromide, the solutions were. made barely alkaline with ammo- nia, thus removing radiothorium, the isotope of thorium, in the precipitate of thorium hydroxide. The radiothorium was removed in order to have solutions practically free from thorium X and its final product thorium D, which emits a powerful gamma radiation which would interfere with our measurement of activity by the gamma ray method. Hy- drogen sulfide was then passed into the solution to precipitate lead and any other heavy metals as sulfides. After filtering the solutions were evaporated to dryness in silica dishes and the ammonium salts smoked off. The dry salts were then dissolved in water and the solution filtered. They are now ready for crystallization and addition of hydrobromic acid. Crystallization of Bromides The bromides were concentrated to the desired point by evaporation on an electrically heated water bath. Upon cooling groups of compact crystals form from both neutral Concentration of Radium and Mesotlzonkn, Etc. 195 and acid solutions. The formation of soft, mushy crystals from neutral solutions appears to be due to the presence of ammonium salts. The liquor was then decanted, and the crystals thoroughly dried, dehydrated, and finally sealed in weighed glass tubes, and their weight determined. The degree of acidity of the liquor from which the crystals sepa- rated was now determined by pipetting one cc of the liquor into a flask, diluting and titrating with a standard solution of ammonia, methyl orange being used as the indicator. After returning the titrated portion to the liquor originally poured off the crystals, the solution was evaporated to dryness, the ammonium bromide smoked off, and the anhydrous bromides sealed in a glass tube of the same dimensions as the one con- taining the crystals. All crystallizations were made in flat-bottomed silica dishes with nearly straight sides, and the same dishes were used as much as possible. After the activity of the crystals and the bromide obtained from mother liquor had been measured, the salts were combined and brought into solution. Other crystallizations were then made with the view of finally obtaining a series of different quantities of crystals separated ranging from 25 to 70 percent of the bromides in the solu- tion. Working first in neutral solutions the work was then extended to acid liquors. Different quantities of crystals were separated from solutions of different acidities ranging from normal to approximately six times normal. After each crystallization the entire procedure of separating crystals and mother liquor, titrating, drying, sealing, and measuring was repeated. In this way crystallization factors were obtained corresponding to different quantities of crystals from liquors whose acidity ranged from neutral to six times normal. The graphs obtained by plotting the crystallization factors on one axis and the corresponding percentages of crystals on the other axis of the ordinary system of rectangular co-ordinates are coincident within the limits of experimental error, and therefore show that the presence of acid has no effect on the value of crystallization factor. The experi- John L. Nierwan mental data upon which this conclusion is based are sum- marized in the tables below. About one-fifth of the total gamma ray activity of the bromides is due to radium. The values given are expressed I in terms of a radium standard. After each crystallization the activity of the crystals and of the salt obtained from the mother liquor was determined with a Lind2 lead-lined electro- scope. The glass tubing used in sealing the bromides was of uniform bore and thickness. At first the tubes were measured directly after sealing and every twelve hours thereafter up to 108 hours from the time of sealing. After a number of measurements had been made in this manner, it was found that after sixty hours the activity became practically con- stant, the equilibrium amount of mesothorium-two having accumulated, so that the activity at the time of sixty hours after sealing was taken as a measure of the mesothorium- radium content of the sample. Attention should be directed to some sources of error in the experiments. In the first place, as the anhydrous bromides are very hygroscopic it is difficult to keep out traces of moisture in sealing. Again, as a small amount of organic matter always enters the material from brushes used, filtering
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