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 is necessary which results in small losses in the course of many crystallizations. As the bromides are finally ignited, small quantities are apt to adhere to the dish. ‘Then too, as all crystallizations were made in open dishes, small losses some- times result from ‘ ‘crawling. ’ ’ In the following table several complete sets of results are given for each sample of bromides used in the experi- ments. The crystallization factor, Cc/Co, is given in the fifth column. Similar tabulations were made for all crystalliza- tions made, but inasmuch as we are concerned chiefly with but two items in Table I, namely crystallization factor and percentage of crystals corresponding thereto, the other results have not been included in the tables that follow. Concentration of Radizdm and Mesothorium, Etc. 197 TABLEI-SELECTED SINGLE FRACTIONATIONS

II Crystals Mother Liquor __ Wt. of Conc. Concentra - Percent Original Percent Weight tion of Salt Mesth. of cc 3f Crys of ' "eso- Cc, co bxeso. tals Salt thorium per g 3erthoriuz g of S 1 - l- j .OI 2.35 33.4 3.819 0.69 21.4 4.08 1.95 46.6 3.143 0.46 11.8 --2.13 3.269 3.45 -1.56 61.9 2.469 0.30 7.7 0.19 2.804 0.52 2.64 28.8 6.906 0.06 23.9 --0.19 3.804 0.43 -2.26 38.7 6.029 0.048 14.9 0.54 1.579 I .06 I .96 43.4 2.054 0.14 15.1 0.54 11,759 0.99 I .83 48.2 I.883 0.12 12.1

TABLE11-CRYSTALLIZATION FACTORS FOR NEUTRALSOLUTIONS

Percent of Salt as I ~rysta~ization Percent of Salt Crystallization Crystals I Factor li as Crystals Factor

24.3 I .gs 27.9 I .96 33.4 I .83 35.1 2.34 53.6 1.71 38.3 2.23 56.9 1.62 38.7 2.26 62.3 1.47 43.4 I .96 69.0 1.44

TABLE111-CRYSTALLIZATIONS FROM ACID SOLUTIONS

Percent Crystalliza- ~~~~1~~ Percent Crystalliza- Normality of of Salt, tion 01 of Salt, 1 tion Acid in Crystals Factor Crystals Factor Mother ?-..---_I Liquor

~ 28.0 2 .79 1,33 55.3 1.68 2 * 93 28.8 2.64 1.16 29.0 , 2.60 4. I2 29.6 2.66 1.44 34.2 I 2.34 * 4.23 36.4 2.19 I .56 48.8 1.81 4.32 38.8 2.22 1.75 52.8 I.70 4.58 56.9 1.59 I.14 61.7 1.56 4.34 66.3 I .36 I. 14 68.2 1.35 4. IO 32.2 2.45 2.31 51.5 1.74 6.91 37. I 2,19 2.36 65.8 I .41 5.52

43.4 2.00 2.55 83.6 1.16 6.88 ~ 198 John L . Nierman

Having shown that the crystallization factor is inde- pendent of the acid concentration of the solution it is of interest to point out that the enrichment is apparently not influenced by the differences in concentration of mesothorium of our solutions. In the course of the crystallizations with the three samples of mesothorium-bearing bromides almost the same percentage of crystals separated out. As shown in the following table the crystallization factor remains constant.

Conc. Mesth. Acidity of Conc. of Percent of . Crystallization per gram Solution Crystals Crystals Factor

0. rg Neutral 0.43 38.7 2.26 0.55 Neutral 1.23 38.3 2.23 2.23 I.75 N.HBr 4.96 38.8 2.22

Assume we have 1000grams of barium bromide containing one mg of mesothorium per gram to be concentrated by fractional crystallization, the desired product to contain I 80 mg of mesothorium per gram. When 40 percent of the salt crystallizes out the factor of enrichment is 2.1. The number of series of fractionations, n, necessary to obtain head crystals whose mesothorium content is 180 times that of the original bromides will be given by the relation -(Z.I)~~ = 180. Solving we have ofl. = 7. Since 84 percent of the total meso- thorium in the head dish passes up in successive fractionations, the amount of mesothorium in the first head crystals removed from the crystallizing system after seven series’ of fractiona- tions will be given by -1000 X 0.84,~ or 2’94.5 mg. The weight of the first head crystals is expressed by 1000 X (04)~ = 1.636 g. If mother liquors are removed from the crystallizing system when their total mesothorium content is reduced to 0.1 mg, about 35 individual crystallizations will be included in the seven series of fractionations. When 60 percent of the salt is allowed to crystallize out the factor is 1.52. Similar calculations for this case will show Concentration of Radium and Mesothorium, Etc. 199

that I 2 series of fractionations involving about 80 single crystallizations will have to be made to obtain head crystals containing 180 mg of mesothorium per gram. Since 91.2 percent of the total mesothori.um in the head dish passes forward with successive fractionations the first head crystals will contain 331 mg of mesothorium and will weigh 2.178 g. By extending computations of this kind to include the second and third crops of head crystals of the desired meso- thorium content, it can be shown that in IO series of fractiona- tions embracing about 65 single crystallizations 534 mg of mesothorium will be obtained as high grade crystals weighing 2.97 g, when 40 percent of the salt is allowed to crystallize out in each dish. To obtain an equal quantity of high grade crystals, containing 180 mg of mesothorium per g, when 60 percent of salt crystallizes out, will involve more than twice as many individual crystallizations and about 17 series of fractionations. In making these computations, the crystals moving forward are combined with mother liquors backwards from dishes two places apart from each other in the systern.l2 Since the solubility of barium bromide falls off very rapidly with increase of hydrobromic acid in the solution, the tendency in crystallizing from solutions of fair acid con- centration is to separate larger crops of crystals than from nearly neutral solutions. As we have found in this work that the crystallization factor is independent of the acid concentra- tion we conclude that the concentration of radium and meso- thorium is effected more rapidly in very dilute acid or neutral solutions of the bromides. One advantage gained, as Mme. Curie pointed out, in crystallizing’from acid solutions of fair concentration consists in increasing the volume of the solu- tion when the quantity of high grade crystals becomes re- latively small. Summary

(I) The crystallization factor of mesothorium and radium in bromide solutions was found to be independent of+thecon- centration of hydrobromic acid. 200 John L. Nierwtan

(2) The crystallization factors corresponding to different percentages of crystals separating were determined. (3) For concentrations of mesothorium up to 2 mg per g of salt, the crystallization factor remains constant. (4) The separation of mesothorium and radium from barium can be conducted advantageously in neutral or dilute acid solutions of the bromides. This work was suggested by Dr. Herman Schlundt and was carried out under his direction. To him the writer is indebted for much valuable advice, assistance, and encourage- ment. BIBLIOGRAPHY I. Mme. Curie: Thesis, “Recherches sur la substances, radioactives,” Chemical News, 88, 146 (1903). 2. N. Giesel: Ber. deutsch. chem. Ges., 35, 3608 (1902). 3. Coehn and Kettembeil: Zeit. anorg. Chem., 38, 198 (1904). 4. E. Wedekind: Chem. Zeit., 28, 269-270 (1904). 5. Mme Curie: Nature, 84, 313, 356, 478. 6. Ebler and Fellner: Ber. deutsch. chem. Ges., 46, 1577-1586(1913). 7. Ebler and Bender: Ibid., 46, 1571-1573 (1913). 8. Erich Ebler : Eighth International Congress of Applied Chemistry, 2, 91-93 (1912). 9. McCoy: U. S. Patent No. 1,103,600;Jour. SOC.Chem. Ind., 33, 919 (1914). IO. Markwald: Ber. deutsch. chem. Ges., 43, 3420 (1910). 11. Soddy: Jour. Chem. SOC.,gg, 72 (1911). 12. Government Bulletin 104,U. S. Bureau of Mines (1915). Chemical Laboratory University of Missouri