The extraction chromatography of californium, einsteinium, and fermium with di(2-ethylhexyl)orthophosphoric acid

J. inorg,nucl.Chem., 1969,Vol. 3I, pp. 1149to 1166. PergamonPress. Printedin Great Britain

THE EXTRACTION CHROMATOGRAPHY OF CALIFORNIUM, EINSTEINIUM, AND FERMIUM WITH DI(2-ETHYLHEXYL)ORTHOPHOSPHORIC ACID* E. P. H O R W I T Z , C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N Chemistry Division, Argonne National Laboratory, Argonne, I11. 60439

(First received 28June 1968; in revised form 23 August 1968) A b a t r a c t - T h e extraction chromatography of Cf(III), Es(III), and Fm(IIl) was studied using di(2ethylhexyl)orthophosphoric acid ( H D E H P ) adsorbed on hydrophobic diatomaceous earth. Dilute nitric and hydrochloric acid solutions were used as the elutriants. Column performance was studied as a function of particle size, capacity, flow rate, and temperature by measuring the number of theoretical plates obtained for CffIll) and Es(llI) elutions. The distribution coefficients of Cf(III), Es(lll), and Fm(lII) and the separation factors, Es/Cf and Fm/Es, were studied as a function of temperature with H D E H P adsorbed on Celite, pure undiluted H D E H P , and H D E H P in beptane. The extraction chromatography method was applied to the separation of Es(IIl) and Fm(III). INTRODUCTION

THE advantages of di(2-ethylhexyl)phosphoric acid (HDEHP) for the separation of certain tripositive transplutonium ions by means of the extraction chromatography technique has been described by Kooi et al.[1-3]. Their work involved the separation ofCm, Bk, and Cf from each other and from certain fission products using columns containing H D E H P supported on hydrophobic kieselguhr. The elutriant was dilute hydrochloric acid. Moore and Jurriaance [4] have reported additional data on the Cm-Cf and Ce-Bk separations using HDEHP-teflon columns and dilute nitric acid elutriants. The extension of HDEHP-extraction chromatography to the transcalifornium elements has been handicapped by the lack of availability of sufficient quantities of longqived isotopes. Gavrilov et al.[5] have reported on the separation of Cf, Fm, and Md. These workers found a Fm/Cf separation factor of 4.5 for the toluene-HDEHP-HCI system and 1-5-1.9 for the extraction chromatography system. However, the short half-lives, small quantities of the isotopes available, and the absence of Es made accurate Ko and column parameter measurements very difficult to perform. The recent availability of microgram quantities of ZSSEs,enriched in 254,Z~Es and daughter 25SFm,has made it possible to study in more detail the extraction of Cf, Es, and Fm with HDEHP. In addition, the various column parameters involved in extraction chromatography could be measured in order to obtain the optimum conditions for separations. The results of such a study are presented in this paper. 1. 2. 3. 4. 5.

* Based on work performed under the auspices of the U.S. Atomic Energy Commission. J. Kooi, R. Boden and J. Wijkstra, J. inorg, nucl. Chem. 26, 2300 (1964). J. Kooi and R. Boden, Radiochim. A cta 3, 226 (I 964). J. Kooi, Radiochim. A eta 5, 91 (1966). F. L. Moore and A. J urriaanse, A nalyt. Chem. 39, 733 (1967). K.A. Gavrilov, E. G vuzdz, J. Stary and Wang Tung Seng, Talanta 13, 471 (1966). 1149

1150

E. P. H O R W I T Z , C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N EXPERIMENTAL

Reagents All nitric and hydrochloric acid solutions used for column runs and for liquid-liquid extraction experiments were prepared from specially purified stock solutions. (The nitric and hydrochloric acid stock solutions were prepared by distillation in a quarz still and by saturation of triply distilled water with purified gas, respectively.) The normality of the acids was measured by conventional acid-base titration; however, the acid concentration was also calculated in terms of molality by the use of the published densities of the acid solutions. The H D E H P was purified following the procedure given by Peppard et al.[6]. The heptane, toluene and acetone used were reagent grade. Preparation of column material A commercial diatomaceous earth, Celite-545, was used as the solid support for the H D E H P . The Celite was graded by settling in an apparatus almost identical to the grading apparatus described by Aly and Latimer[7] The apparatus consisted of four different size columns connected together in the order of smaller to larger cross sectional areas. Ungraded acid-washed Celite was place in the smallest column and the apparatus was filled with 0.1 N HCI. A Beckman solution metering pump, Model 746, was used to maintain a constant volume flow rate. Cefite particles of diminishing size were collected in the successive columns. The following four particle size fractions (based on settling rate) were collected: > 3.8 cm/min, 3-8-1-3 cm/min, 1.3-1.0 cm/min, and < 1-0 cm/min. The graded Celite was dried at 100°C for 24 hours and then made hydrophobic by exposing to the vapors of dimethyldichlorosilane (DMCS) on a vacuum line. The mixture of Celite and DMCS vapors was mixed manually by means of a magnetic stirring bar. Each day the excess DMCS plus HCI vapors were pumped from the Celite and fresh DMCS vapors were allowed to enter the reaction chamber. After 3-4 days, the resultant hydrophobic Celite was removed from the vacuum line and dried in an oven at 100°C for 15 rain. The HDEHP-Celite column material was prepared by mixing the appropriate amounts of graded hydrophobic Celite and purified H D E H P with acetone and evaporating the excess solvent at room temperature. The quantities used in a typical preparation containing 88.2 rag of H D E H P per g of dry column material were the following: 193.4 nag of H D E H P , 10-15 rnl of acetone, and 2 g of Celite. Pyrex glass columns ( - 2.8 mm i.d.) were used for all experiments. The columns were exposed a minimum of 30 rain to dimethyldichlorosilane vapors in a desiccator, then washed with acetone, air dried, and weighed. The bed volume and cross sectional area of the columns were measured by weighiog the volume of water (while in the column) between two marks 10cm apart. After drying, the columns were filled by gently tamping the dried HDEHP-Celite material to form a bed 10 cm in length. The columns were again weighed to obtain the weight of column material. The bed density was calculated from the weight of column material and the bed volume. Tracer solutions The alpha-active nuclides used in this study were ~ssCf (2.65 yr), SUEs (20.47 days), and Z55Fm (20.1 hr). The Cf and Es were obtained from A N L stocks and purified by ion exchange using ahydroxyisobutyrate elutriant and by extraction chromatography using H D E H P with a nitric acid elutriant. (The extraction chromatography column was used to separate traces of N a +, K +, Mg +s, Ca +s, AI +a, SO4 =, and HaBOa. The Cf and Es tracer stock solutions were given a final purification step using a standard micro cation exchange column [8]. High purity 2 N and 7 N HCI were used as elutriants. (The final cation column was used to remove any traces of phosphate esters or phosphate ions.) The stock solutions were stored in specially cleaned volumetric flasks. The S55Fm was obtained by "milking" ~53.s54.2~Esand is described in the results and discussion. The radiocbemical purity of these tracers was established by alpha energy determinations from alpha pulse height analyses. 6. D. F. Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll, J. inorg, nucl. Chem. 4, 334 (1957). 7. H . F . Aiy and R. M. Latimer, J. inorg, nucl. Chem. 29, 2041 (1967). 8. K. Street, Jr., and G. T. Seaborg, J. Am. chem. Soc. 72, 2790 (1950).

Californium, einsteinium and fermium

1151

Column procedure and Kd measurement All columns were placed in a thermostated jacket which was connected to a Haake Model F E circulator. The columns were routinely preconditioned with 10 bed volumes ( - 6--7 ml) of preboiled 0.1 N HNO3 or HCI at 18-20°C and at a rapid flow rate. After the preconditioning the columns were thermostated to the proper temperature. This procedure removed all air pockets from the bed. The void volume of a given column was measured by determining the breakthrough of l~Cs. Tracer level mixtures of 252Cf, 253Es, and ~SFm were loaded on a column from a minimum volume (~ 100/~1.) of 0.1 N HNO3 or HCI and then eluted with HNO3 or HC1 solution. (The elutriant solutions were degassed with argon before placing in the column reservoir.) The reservoir was also backwashed three successive times with elutriant before starting the elution. During the elution the drop number was counted electronically. The drop volume of each elutriant at a given temperature was determined by measuring the volume of a counted number of drops. This drop volume was used to measure the flow rate and to correlate the number of drops collected with the volume of solution expended during the elution. Flow rates were adjusted and maintained by regulating the nitrogen pressure applied to the top of the column. The activity of the eluate fractions was determined by standard radiochemical techniques. When column runs were carried out with Cf-Es and Es-Fm mixtures, the fractions were also pulse height analyzed for the 6.1, 6.6 and 7-0 MeV alpha particles of 25~Cf, 2~Es, and 2SSFm, respectively. The number of free column volumes to peak maximum (C) and the separation factors (CEJCct) and (Cvm/CEs) were calculated from the position of the peak maxima using the following equation: C = Vmax- v~

(I)

Vm

where (Vmax) is the eluate volume to peak m a x i m u m (also called the retention volume) and (v,~)is the void volume or the volume of the mobile phase. The distributioncoefficient(Ka) was calculated from (C) by means of the following equation: Kd = C . v_~

(2)

I? s

where (C) and (vm) are the same as defined above, and (vs) is the volume of the stationary phase. The (vs) was obtained by dividing the weight in grams of H D E H P on a column by the density of H D E H P (0.975 g/mi at 25°C [9]). The maximum error in (v,) (as a result of) the change in density with temperature) was estimated at 1-2%. Distribution coefficients measured under the same experimental conditions but with different columns were reproducible within - 4%. The inaccuracy involved in measuring (Vmax - v~) is largely responsible for the error in the K~. The reproducibility of the Ka using the same column is_+ 1-5%. The height equivalent to a theoretical plate ( H E T P ) was calculated from the equation given by G lueckauf [ 10]:

N = 8vL~__ L W2 HETP

(3)

where (N) is the number of plates, (Vm~) is the volume of eluate to the peak maximum, (W) is the width of the elution peak at 1/e times the maximum solute concentration, and (L) is the length of the bed. Liquid-liquid extraction Ka measurements Liquid-liquid extraction measurements were carded out at different temperatures by vortexing the liquid phases in a centrifuge tube which was contained in a small thermostated jacket. After disengaging, the liquid phases were separated by means of a transfer pipet and assayed radiometrically as previously described[l 1]. The Ka and separation factor measurements involving Fro(Ill) were corrected for the decay of 20 hr. 255Fm which took place between counting the aqueous and organic phases. 9. D. F. Peppard, J. R. Ferraro and G. W. Mason, J. inorg, nucl. Chem. 7, 231 (195 8). 10. E. Glueckauf, Trans. Faraday Soc. Sl, 34 (1955). 11. E. P. Horwitz, C. A. A. Bloomquist, L. J. Sauro and D. J. Henderson, J. inorg, nucl. Chem. 28, 2313 (1966).

1152

E. P. HORWITZ, C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N

In order to establish the fact that equilibrium had been obtained, the separated organic phases containing the radioactive nuclides were equilibrated with fresh aqueous phase and the Kd's redetermined. The following equilibration times were used to insure attainment of equilibrium: 15°C-10 min, 25°C--5 rain, 45°, 60°, and 750(2-3 rain. HDEHP-heptane solutions required no preconditioning before making a K~ measurement; however, the undiluted H D E H P did require two pre-equilibrations with the corresponding aqueous phase in order to obtain reproducible Kd measurements. RESULTS AND DISCUSSION

The effect o f particle size and capacity on the H E T P A good method for measuring column performance is the calculation of the height equivalent to a theoretical plate, HETP, (also referred to as the plate height) of a given peak. Figure 1 shows the effect of particle size or settling rate* on the H E T P for the elution of Cf(III) with dilute HC1. On the basis of previous investigations by the authors[12, 13] and by Sochacka and Siekierski[14], column material containing 96.7 mg of H D E H P (0.3 meq of H +) per gram of hydrophobic Celite was chosen for the study. (This ratio produces a material containing 88.2 mg of H D E H P (0.27 meq) per gram of dry bed.) The flow rate and temperature, which also affect the plate height, were maintained constant. The data in Fig. 1 show that both the plate height and the tailing decreases (column performance increases) with decreasing particle size, except in the case of the < 1-0 cm/min fraction. The variation in the H E T P and tailing of elution curves A, B, C and possibly D of Fig. 1 may be explained, at least qualitatively, by considering the structure of diatomaceous earth. Ottenstein [ 15] has described the structure of diatomaceous earth materials as consisting of silica granules perforated with many small holes or pores (called primary pores) which are about 1/~ dia. These primary pores are also perforated with many smaller holes, which are referred to as the secondary structure. Some of the secondary structure in turn have a tertiary structure. The stationary liquid phase, i.e., the H D E H P , is probably distributed throughout the multilevel pore structure. According to Ottenstein[15] the liquid phase on a diatomite surface selectively fills the smallest available pores first. Equilibration between the mobile and stationary phases would be slower in the secondary and tertiary pores (because of the longer and more inaccessible diffusion path) than in the primary pores and on the outside surface. Since the larger granules probably contain longer pores and more intricate micropore structure, band broadening a n d tailing of elution curves should be more pronounced with the coarser Celite fractions. Except in the case of the < 1 cm/min fraction, this is what the data in Fig. 1 shows. It is possible that the pore size and structure of the < 1 cm/min fraction is smaller and more inaccessible than the 1.3-1.0 cm/min fraction. In addition, the smallest particle fraction could be largely composed of fragments of *The approximate diameters of the granules in the four fractions are the following: > 3-8 cm/min --~ 100/t, 3-8-1.3 cm/min ~- 75/~, 1.3-1.0 cm/min ~ 50/t, and < 1.0 cm/min ~ 25it. 12. E. P. Horwitz, L. J. Sauro and C. A. A. Bloomquist, J. inorg, nucl. Chem. 29, 2033 (1967). 13. E. P. Horwitz, C. A. A. Bioomquist, K. A. Orlandini and D. J. Henderson, Radiochim. Acta 8, 127 (1967). 14. R.J. Sochacka and S. Siekierski, J. Chromatogr. 16, 376 (1964). 15. D, M. Ottenstein, The Chromatographic Support, in Advances in Chromatography. (Edited by J. Calvin Giddings and Roy A. Keller), Vol. 3, p. 137. Marcel Dekker, New York (1966).

Californium,einsteiniumand fermium 105

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Settling Rate

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1153

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15 19 25 31 37 Free Column Volume of Eluate Fig. 1. The effectof particle size on the HETP and tailingfor the elutionof Cf(lIl) from HDEHP on Celite: 88.2 mg HDEHP/gof dry bed, elutriant0"365N HCI,flowrate l ml/cm~/min,60°C. the larger Celite granules, and therefore could have a considerably different pore structure than the other celite fractions. The lack of homgeneity of the Celite pore structure is indicated by the tremendous difference in curves A and B as compared to B and C of Fig. 1. Table 1 shows the effect of the quantity of H D E H P per gram of dry bed (column capacity) on the fractional void volume (f) and on the plate height. The elutions were carried out using Cf(III) and dilute nitric acid elutriants. Column material was prepared from the 1.3-1-0 cm/min Celite. The change in plate height for the four largest capacity columns is very similar to the results reported by Sochacka and Siekierski [ 14] for the Eu-HDEHPkieselguhr system. Data on columns containing less than 5 per cent H D E H P have not been reported. The tailing of the elution curves was small for the two highest capacity columns (but still greater than Fig. 1C). The elution curve obtained with the lowest capacity column tailed very badly. The data in Table 1 may again be explained by the multilevel pore structure of Celite. The decrease in void volume with an increase in capacity is a result

1154

E. P. H O R W I T Z , C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N Table 1. The effect of the quantity of H D E H P on the fractional void volume and on the H E T P of Cf(III) (I .3-1.0 cm/min Celite, 60°C, flow rate 1.2 ml/cm2/min) mg of H D E H P / g of dry bed

f(fractional void vol.)

H E T P (mm/plate)

279 162 88-2 46. I 25.1 4.81

0.65 0.70 0.72 0-76 0.80 0.81

1.0 0.60 0.34 0-33 0.50 0.58

of the gradual filling of the vacant pores with liquid H D E H P . The intermediate capacity columns probably contain just the proper quantity of H D E H P to fill the tertiary and secondary pores structure (which are filled first) and to coat the surface of the large primary pores and outer surface of the Celite. The elutriant or mobile phase can then percolate through the large primary pores as well as around the Celite particles. This flow pattern would achieve a faster attainment of equilibrium because of the efficient contact of mobile and stationary phases. The volume of H D E H P in the secondary and tertiary pores would cause a small amount of tailing which is exactly what is observed for the intermediate capacity columns; i.e., 88.2 to 46.1 mg of H D E H P per g of dry bed. The large capacity columns, on the other hand, probably contain sufficient H D E H P to completely coat the surface and plug all the pores, which would result in a reduction in void space and in inefficient contact of the mobile and stationary phases. With the small capacity columns, just enough H D E H P is present to fill the most inaccessible pores (the secondary and tertiary pores) and to only partially coat the surface and primary holes. Thus, only a portion of the Celite surface is covered. The void space of these columns would be large but a lot of H D E H P would be absent from the surface where mobile and stationary phase contact can readily take place. On the basis of the data in Fig. 1 and Table I all additional column parameter studies and separations in this investigation were carried out with column material prepared from the 1.3-1.0cm/min Celite fraction and containing 88.2 mg of H D E H P (0-27 mmol) per g of dry bed. The effect offlow rate and temperature on the H E T P The elution of einsteinium with dilute HNO3 was used to study the effect of flow rate and temperature on the HETP. (The selection of Es and HNO3 was based on preliminary studies which indicated that the HDEHP-Celite system may have some practical applications for separating Es and Fm.) Figure 2 shows the effect of flow rate on the plate height at four different temperatures. The data dearly show that the H E T P increases linearly with flow rate (within the experimental error) at each temperature studied. Similar results have been reported for certain lanthanides by Sickierski and Sochacka [ 16]. 16. S. Siekierski and R. J. Sochacka, J. Chromatogr. 16, 385 (1964).

Californium, einsteinium a n d f e r m i u m _

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Flow Rote ml/cm2/min Fig. 2. T h e effect of flow rate and t e m p e r a t u r e on the H E T P for the elution o f E s ( l l I ) from H D E H P on Celite: 88.2 mg o f H D E H P / g o f dry bed, elutriant 0.383 a n d 0.391 N (0.388 and 0 . 3 9 6 m ) H N O 3 . C o l u m n bed sizes 0-0635cmZx 1 0 c m and 0"0616cm2× 10 cm, bed density 0.383 g / m l , f = 0.72.

The relationship between the plate height and flow rate (or velocity of mobile phase) is given by the van Deemter equation: h = A + B/v + Cv

(4)

where h is the H E T P and v is the flow rate. The term A is a particle size and eddy diffusion parameter, the B term is a longitudinal diffusion (i.e., mixing in the interstitial volume) parameter, and C is a mass transfer parameter. Since the data in Fig. 2 shows a linear relationship between flow rate and plate height, the longitudinal diffusion term B/v is unimportant (at the flow rates studied) and drops out of the van Deemter equation. The slope of the straight lines gives the C parameter. Since the slope decreases with an increase in temperature, the mass transfer becomes less significant (and the A parameter becomes more significant) in contributing to the plate height as the temperature increases and the flow rate decreases. In other words, the temperature increases the rate of attainment of equilibrium of Es(llI) between the stationary phase and the nitric acid mobile phase. Extrapolation of the lines to zero flow rate gives the A parameter. Theoretically the A parameter should equal the particle size; however, it never does in practice because of irregular packing and irregular flow (channeling) in the column. The data in Fig. 2 gives an A parameter of 0.23 +--0.07 mm which is within a factor of 4.6 of the particle size of the Celite (0.050 mm). Figures 3 and 4 show the effect of flow rate and temperature on the tailing

1156

E. P. HORWITZ, C. A. A. BLOOMQUIST and D. J. HENDERSON iO 5

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HETP =

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Fig. 3. The effect of flow rate on the HETP and tailing for the elution of Es(III) from HDEHP on Celite; 88.2 mg HDEHP/g of dry bed, elutriant 0.383 N HNOa, 60"C. Column bed size 0.0636 cm2x 10 cm, bed density 0.381 g/mi, f = 0.72.

of the elution curve. T h e data in these figures show that a column operated at a fast flow rate and high t e m p e r a t u r e can have approximately the same tailing (and H E T P ) as a column operated at a slow flow rate and lower temperature. F o r example, c o m p a r e the elution curves in Figs. 3A and 4A and Figs. 3D and 4D. (This p r o p e r t y was 'also noted in the case o f other elution curves which were used to calculate the data in Fig. 2.) T h e data in Fig. 3 also shows that the n u m b e r of free column volumes to p e a k m a x i m u m (C) is unaffected, within experimental error, b y flow rate. This was o b s e r v e d at other temperatures. Plate height data was also obtained for C f ( l l l ) and F m ( I l l ) using some of the same experimental conditions shown in Figs. 3, 4, and 5. Although the data is much less c o m p l e t e than in the case o f E s ( I l l ) , the three ions showed almost identical column behavior. T h e H E T P for F m ( I I I) was, however, approximately 15% higher than the plate height of C f ( l l l ) and E s ( I l l ) under the same experimental conditions. A p p a r e n t l y the mass transfer p a r a m e t e r is s o m e w h a t larger

Californium, einsteinium and fermium 105

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Fig. 4. The effect of temperature on the HETP and tailing for the elution of Es(III) from HDEHP on Celite: 88-2 mg HDEHP/g of dry bed, elutriant 0.383 N (0.388 m) HNOa, flow rate 1.2 ml/cm2/min. Column bed size 0.0635 cm2× 10 cm, bed density 0-383g/ml, f = 0-72. for F m ( l l l ) , indicating a slower rate of attainment of equilibrium with H D E H P than is the case with C f ( I I I ) and E s ( I I I ) .

Acid dependency of the Ka T h e extraction of lanthanide(III) and A m ( I l l ) ions by H D E H P - t o l u e n e solutions has b e e n s h o w n b y P e p p a r d et a/.[17] to be r e p r e s e n t e d b y the following equation: M~Sq)+ 3(HDEHP)z(org) ~ M[H(DEHP)2]n(org) +3H&q)

(5)

where ( H D E H P ) 2 is the dimer of di(2-ethylhexyl)orthophosphoric acid and the subscripts (aq) and (org) refer to the equilibrated aqueous and organic phases, 17. D. F. Peppard, G. W. Mason, W. J. Driscoll and R. J. Sironen, J. inorg, nucl. Chem. 7, 276 (1958).

1158

E. P. H O R W I T Z , C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N

10 3

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HCl Elutriant

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.

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o:1: or m of H + Fig. 5. Hydrogen ion dependency for the elution of Es(Ill) from H D E H P on Celite: 88-2 mg of H D E H P / g of dry bed, flow rate 1"2 ml/cmZ/min, 60°C. Column bed size 0.0638 cm z × 10 cm, bed density 0-384 g/m, f = 0-72.

respectively. The equilibrium constant expression for this extraction is as follows: K=

[Complex] [H+] 3 [M +3] [ ( H D E H P ) z ] 3

Ka = K

Y(¢omp~ex)" y~+ yM+3""y3HD~.HPh

[ ( H D E H P ) 2 ] "YM+3"Y~(HDEHPh [H+] a Y(complex)" Y~+)

(6)

(7)

where K is the equilibrium constant, Ka is the distribution coefficient, "Complex" is the species M[H(DEHP)~]3, and [ ] and y are the concentrations and activity coefficients on the molality scale, respectively, of the various species shown in the equation. Since the H D E H P present on the Celite columns is not dissolved in a diluent, there is some doubt as to whether the extraction of, e.g., Es(III) on H D E H P Celite columns involves dimer molecules of H D E H P or that the composition of the complex is as shown in Equation (5). Nevertheless, the Ka should still be inverse third power hydrogen ion dependent for extraction chromatography systems involving H D E H P unless the metal ions are extracting as, for example, nitrato or chloro complexes. The effect of the concentration of nitric and hydrochloric acid elutriants on the Ka of Es(III) is shown in Fig. 5. It is interesting to note that when the Ka is plotted against the molality of hydrogen ion (on a log-log scale), the points deviate slightly, but significantly, from a straight line of slope----- 3.00. (The

Californium, einsteinium and fermium

1159

diameter of the circles in Fig. 5 is the experimental error in the Kd values.) The best straight line through these points has a slope of approximately = - 3.5. However, acid dependency studies are usually carried out at constant ionic strength (/z) in order to maintain 3"M+3and Tn+ constant[17]. An increase in the 3'M+3, caused by a decrease in V,, would result in an increase in the Kd according to Equation (7). Therefore, the Kd's shown in Fig. 5 were corrected to the ionic strength of the highest acid concentration of HNO3 and HCI used as elutriants. This correction was made by multiplying a given Kd by Tkd3"~s, where Tks and T'~.s are the activity coefficients of Es(III) at the highest ionic strength and at the ionic strength of the elutriant used to measure the Ka, respectively. The TEs values were calculated using the equation of Davies [18], where A = 0.547 at 60°C. The corrected Kd could then be plotted against either the a± or the molality of the hydrogen ion, since the T-* of the hydrogen ion was essentially constant (within 1%) for the concentrations of HNO3 and HC1 used for the elutions [19, 20]. The a-* was chosen for the plot in order to shift the line away from the Ka vs. m of [H ÷] plot. (The 3'± for HC1 at 60°C and tz = 0.637 was 0.729 [19]. The 3'_*for HNO3 at 60°C and/z = 0.557 was estimated to be 0.677 using the equation of Davies [ 18] and the data of Davis and De Bruin[20].) The plot of the corrected Ka vs. the a , of the hydrogen ion on a log-log scale shown in Fig. 5 gives a slope exactly equal to --3.00. Thus, the einsteinium extracts as the tripositive ion; however, this conclusion only applies to tracer scale quantities of einsteinium.

Distribution coefficients and separation factors of Cf, Es, and Fm The distribution coefficients of Cf(III), Es(III), and Fm(III) with H D E H P Celite, pure undiluted H D E H P , and 0.4 F H D E H P in heptane were measured from 15 to 75°C. The latter two liquid-liquid systems were included in the study in order to determine if the H D E H P adsorbed on Celite is similar in behavior to the pure undiluted H D E H P or if the methyl groups on the Celite surface introduced a diluent effect. Plots of the Kd'S vs. 1/TQK on a semilogarithmic scale are shown in Figs. 6 and 7. In each case the data gave straight lines (within experimental error) over the temperature range studied. Approximate values of the enthalpy (heat of extraction) were calculated from the slopes of the straight lines by substituting the Kd for the thermodynamic equilibrium constant, K, and using the following equation [21 ]: 1

--AH °

A log Kd/A T = 2"303 R

(8)

It was assumed that AH ° was constant over the small temperature range studied. The slopes were determined from a computer programmed least-squares method. Even though the acidity of the elutriant and the concentration of the extractant 18. J. N. Butler, Ionic Equilibrium, a Mathematical Approach, Chap. 12. Addison-Wesley, Reading, Mass. (1964). 19. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions, 2nd Edn, p. 547. New York (1950). 20. W. Davis, Jr., and H. J. de Bruin, J. inorg, nucl. Chem. 26, 1069 (1964). 21. I. Fidelis and S. Siekierski, J. inorg, nucl. Chem. 29, 2629 (1967).

l l60

E.P.

HORWITZ, C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N iO 3 Undiluted H D E H P 1.28m HNO•

u z

- H O E H P on C e l i t e 0 . 3 8 8 m HNO 3

i0 s'

O; - -

_o m

Q

I i01 2.8

75"C I

60"C I 3.0

45"C I 31.2

25oC

I

o'1 3.4 2.8 I / T °K x 10 3

,5oC I

60"C

(

3.0

45°C

i

31,2

25"C I 3.4

Fig. 6. The effect of temperature on the distribution coefficient of Cf(III), Es(III), and Fm(III).

were fixed (see Figs. 6 and 7), the substitution of Ka for K in Equation (8) is not strictly valid due to the small influence of temperature on the activity coefficients, which in turn would affect the value of the Ka. The AH ° values obtained from Equation (8) can be used to compare the relative heats of extraction since the effect of temperature on the y's of Equation (7) would, at least partially, cancel in the activity quotient, especially in the case of the HDEHP-Celite and undiluted H D E H P systems. The values of AH ° for Cf(III), Es(III), and Fm(III) for the three systems are given in Table 2. The data in Table 2 show that the heat of extraction for HDEHP-Celite and for undiluted H D E H P are identical (within experimental error). This similarity was expected in view of the method of preparation of the column material and the quantity of H D E H P deposited on the Celite. Although the heats of extraction of all the systems are exothermic, the presence of heptane in the H D E H P causes a marked increase ( - 3 fold) in the AH °. The AH ° values for Cf, Es, and Fm with the HDEHP-heptane system are comparable to the A H ° of the Pm(III)- and Sm(III)-HEH [q~P]-heptane system [21 ]. The data in Figs. 6 and 7 also show that the selectivity of H D E H P for Fm(llI)

Californium, einsteinium and fermium 10 2

'0; 0 . 4 0 F HOEHP in Heptone0.427m HN03

/

I01

- 0 . 4 0 F HDEHP in Heptone- 0 . 4 3 5 m HCI

Y

L) U.I 0 0

1161

lIE

,o

Es

(:3

60*C I0

L

2.9

45°C I I

3.1

25aC 15°( I

3.3

I

I

60"C I

I0

3.5 2.9 I / T eK X 10 3

45"C I

3'.1

250C I

t5*C I

3'.3

3.5

Fig. 7. The effect of temperature on the distribution coefficient of Cf(III), Es(III), and Fm(IlI). i

Table 2. Enthalpy values for the extraction of Cf(III), Es(III), and Fm(llI) with HDEHP. - A H ° (kcal/mole) H D E H P-Celite Cf 1.08 +-0-10 Es 0-759 + - - 0 . 0 7 Fm 1"18+--0'11

Undiluted H D E H P 1.02+--0.34 0.761---0.25 1.11-----0.37

0-4 F H D E H P in heptane 3.52___0.28* 3.28+-0.26 3.52+-0.28

3.50--- 0.28t 3.31+-0.26 3.48+-0-28

*Nitric acid aqueous phase. "(Hydrochloric acid aqueous phase.

and Es(III) is relatively large and decreases with an increase in temperature; whereas, the selectivity of H D E H P for Es(III) and Cf(III) is very small and increases with an increase in temperature. The separation factors are shown in Tables 3 and 4 for the three extraction systems using the same nitric and hydrochloric acid solutions given in Figs. 6 and 7.

1162

E. P. HORWITZ, C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N Table 3. The effect of temperature on the separation factor, Es(lII)/Cf(III)

Temp. 15°C 25°C 45°C 60°C 75°C

HDEHP-Celite HNO3 HC1

0"989 1"02 1"03

0"993

UndilutedHDEHP HNOa HCI

0"945 0"970 1"01 1.02

0-995

HDEHPinheptene HNOa HCI 1"01 1"02 1"05 1-06

0-972 0"980 1"00 1"02

Table 4. The effect of temperature on the separation factor, Fm(III)/Es(III)

Temp. 15°C 25°C 45°C 60°C 75°C

HDEHP-Celite HNO3 HC1

2.24 2.20 2.13

2-04

Undiluted H D E H P HNOa HCI

2.27 2.22 2.21 2.12

2-06

H D E H P i n Heptane HNO3 HCI 2.5O 2-40 2.39 2.30

2.21 2.15 2.12 2.10

These separation factors were always measured with the two ions of interest present in the same solution. Thus, the error in the separation factors was considerably smaller (--+1.5-2.0%) than the error in the Kd measurements (__+4%). (See [11]). The trend in the variation of the separation factors with temperature is a result of the sign and relative magnitudes of the heat of extraction for these ions. For example, the Kd'S of both Fm(III) and Es(III) decrease with an increase in temperature. Since the AH ° for Fm(III) is slightly larger than the AH ° for Es(III), the Kd of Fm(III) decreases faster with temperature than the Kd of Es(III). Similar reasoning applies to the Cf(III)/Es(III) separation factor. All the separation factors change only slightly with temperature as a result of the very small values of AH °. The data in Tables 3 and 4 again point out the similarity of H D E H P adsorbed on Celite and the pure undiluted H D E H P liquid. Although the presence of the heptane diluent increases the selectivity slightly, the use of toluene as a diluent gave a Fm(III)/Es(III) value of 2.04_ 0.04 (average of forward and reverse measurements) at 25°C using 0-108 N HCI as the aqueous phase. This value is actually smaller than the corresponding factor for the undiluted H D E H P - H N O 3 system at room temperature. In view of the almost negligible separation factor for Es/Cf, the separation factor Fm/Cf is essentially the same as the Fm/Es factor. Our value for Fm/Cf with the HDEHP-toluene system is considerably smaller than the 4.5 value reported by Gavrilov e t a/.[5]. The data in Table 4 also shows the improved selectivity of H D E H P for Fm and Es when nitric acid is used as the elutriant or aqueous phase. Since the acid dependency data in Fig. 5 showed that the Es ÷3 ion was extracted in either nitric or hydrochloric acid media, the difference in separation factors for the two media

Californium, einsteinium and fermium

1163

could be due to the presence of a weak nitrate complex in the aqueous phase, which is slightly more stable in the case of Es(III) than Fm(llI). It is interesting to compare the magnitudes of the Ka's for Es(IlI) with HDEHP-Celite columns and with the pure undiluted H D E H P liquid when the measurements are made under the same experimental conditions. (This was not possible with the data in Fig. 6 due to the small number of counts of 25SFm.). According to the theory of liquid-liquid partition chromatography [22] the Ka in Equation (2) is identical with the Kd obtained for the corresponding liquid-liquid extraction system. The data in Table 5 show the results of such a study. Table 5. Comparison of the Ka of Es(III) measured by extraction chromatography and by liquid-liquid extraction at 60"C Molality ofHNO3 0.278 0-388 0.427 0.557

H D E H P on Celite* Pure undiluted H D E H P 9.03 × 2.83 × 2" 19 × 8.76 ×

102 102 10z 101

8-96 × 2.74 × 2.30 × 8.80 ×

102 102 102 101

*Column conditions same as given in Fig. 5.

For each acidity, the Ka's for the two systems are identical within the experimental error of_4%, which agrees with the liquid-liquid partition chromatography theory. An additional check of Equation (2) was obtained by measuring the number of free column volumes to peak maximum (C) as a function of the ratio (vm/v,). According to Equation (2), (C) is inversely proportional to (vm[vs) and, the product of the two quantities is a constant, Ka. The concentration of nitric acid elutriant was varied from 0.278 to 0.499 m in order to elute the Es(III) in a reasonable number of free column volumes. The values of (C) were then normalized to a constant acidity (0.427 m) assuming that (C) is inversely proportional to aa+ of the hydrogen ion. (See Fig. 5). The values of (C) were then corrected for the variation of yF.s with the change in elutriant concentration (change in/z) by the method described in the acid dependency section. The results are shown in Table 6. Table 6. Variation in (C) of Es(l I I) as a function of (Vm/Vs) (60°C, nitric acid elutriant)

Vs HDEHP (ml)

V,n nitric acid (ml)

C at 0.427 m HNOa

C corr. to /~ = 0.50

Vm/Vs

0.0443 0"0222 0.0120 0"00592

0.452 0-459 0.500 0.500

20.5 10.6 5.69 2-80

20.5 10" 1 4.96 2-36

10.2 20.7 41'7 84.7

22. A . J . P . Martin and R. L. M. Synge, Biochem. J. 35, 1358 ( 1941).

Kd 2.09 × 2"09 × 2.07 × 2.00 × av. 2.06 ×

102 102 102 102 102

1164

E . P . H O R W I T Z , C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N

The data in Table 6 are in excellent agreement with Equation (2), especially in view of the correction made for the change in YEs with/z.

Separation of Es and Fm and the 255Fm "'milking" Figure 8 shows the elution curves for the separation of Es(III) and Fm(III) at 60°C. The selection of the temperature, flow rate, and elutriant was based on the data in Fig. 2 and Table 4. The separation factor of 2-20 is higher than the factor of 1-84 for a-hydroxyisobutyrate, the elutriant currently used for this separation. The Es-Fm separation was applied to the "milking" of 20.1 hr 255Fm from 0.1 to 0.6/zg of 253Es, enriched in 25~'255Es. The per cent of 2S~Fm in the einsteinium sample increased with time (although the total quantity decreased) because of the relative half-lives of the alpha active 20.47 d 2S3Es isotope and the beta active 38.3 d 2~Es isotope. Initially the sample contained 0.025% of 255Fm by activity. Figure 9 shows a typical elution curve for the "milking" of 255Fm. The column

106

Es(1Tr)

~ Seporotion Factor = 2.20

I0 5 Fm(m)

1

t.-

3E

104

co U

103

L 1020 [

~ 50 100 150 200 250 300 ;550 Drop Number of Eluate fJ

Void 0

,,

, I , ,

, J J , p , , l , r J J l r

J,~]|

5 10 15 20 25 Free Column Volume of Eluote

Fig. 8. The elution o f E s ( l | l ) and F m ( i l l ) with 0-410 N HNO3 from H D E H P on Cclite:

88.2mg H D E H P / g of dry bed, flow rate l.lml/cm2/min, 60°C. Column bed size 0.0620 cm 2 x 10 cm, bed density 0"325 g/ml, drop volume 34.5/zl./drop,f= 0-72.

Californium, einsteinium and fermium

I0 9

-0.41N HNO 3

1165

•- I N HN03---~_~

i0 o o c

° ~

10 7 c

Fm Fraction

0 0

10 6

105

0

50

I 0 0 150 2 0 0 2 5 0 3 0 0 : 5 5 0 Drop Number of Eluate

llL[,,L,J,LI,,,,L~,LLI~LI

Void 0

i

5 10 15 20 25 Free Column Volume of Eluate

Fig. 9. The separation of :55Fro from - 0 - 4 p.g of 253":~4'25~Esusing H D E H P on Celite: 88-2mg H D E H P / g of dry bed, flow rate l-lml/cm2/min, 60°C. Column bedsize 0-0636 cm 2 × 10 cm, bed density 0.370 g/ml, drop volume 35.7/~l./drop,f= 0.73.

size and conditions were similar to those shown in Fig. 8. The decontamination factor of Es with respect to Fm was - 1 - 1.5 × 103 in the fraction containing - 9 9 % of the 2~Fm. In Fig. 9, the Fm fraction contained 30% 2~5Fm-70% 253Es (by activity), whereas the composition of the starting mixture was 0.027% 25~Fm. Ten "milking" experiments of the einsteinium sample were carried out during the course of this investigation. In a few of the column runs, the temperature and flow rate were increased to 75°C and 2.0 ml/cmZJmin, respectively, in order to decrease the time required for the particular "milking" experiment. The decontamination of the Fm was approximately the same as that obtained at the lower temperature. The exact position and shape of the Es elution curve was determined during a column run by evaporating every fifth drop on a tantalum plate and measuring its activity by means of a low geometry solid state detection device, mounted inside the hood. From a knowledge of the position of the Es peak, the void volume, and the separation factor, the exact point where the elutriant concentration should be increased in order to strip the Fm from the column was easily determined.

1166

E.P.

H O R W l T Z , C. A. A. B L O O M Q U I S T and D. J. H E N D E R S O N

The Es elution curve in Fig. 9 had an extremely sharp breakthrough curve, the first 90 drops containing only - 1 0 2 c/m. (Over 101°c/m of Es was loaded on the column.) When the same Es sample was eluted from a cation column with ot-hydroxyisobutyrate, the Fm fraction (which elutes before the Es) contained over 106 c/m of Es in only 50 per cent of the Fm. The Fm fractions were always put through an additional cation "clean-up" column (see experimental section on tracer solutions) to insure the absence of any traces of phosphate ion or H 2 M E H P which could affect the Kd and/or separation factor measurements. The Z53Es in the Z55Fm fractions was seldom separated since its presence served as a useful internal standard for measuring Kd's of Fm and Fm/Es separation factors.