690
WALTERE. NERVIK
termining species, i t is suggested that a physicochemical technique such as measurement of optical absorbance in the ultraviolet range32might be employed to determine the concentration of NzOsand perhaps NOa- present in “Os. The effects of the various additives used in this investigation were emphasized by introducing them at relatively high concentrations of about 0.1 mole (32)
R. N. Jones and G . D. Thorn, Con. J . Res., 27B,580 (1949).
VOl. 59
fraction. If further studies.in this field are undertaken, it is suggested that more dilute solutions be investigated. Such a procedure permits the use of additives such as KC104 which, by virtue of their low solubility in HN03, could not be investigated in the present work. Moreover, in dilute solutions the neutral salt effect of nitrates may be diminished t o the extent that the expected acceleration of the rate of decomposition by NOs- could be observed.
AN IMPROVED METHOD FOR OPERATING ION-EXCHANGE RESIN COLUMNS IN SEPARATING THE RARE-EARTH ELEMENTS1 BY WALTERE. NERVIK University of California, Radiation Laboratory Livermore Site, Livermore, CaE. Received January 14, 1066
A ‘ I radient elution” method has been adapted for operating an ion-exchange resin column using an eluting agent in which the p f I is changed continuously. The method shortens the column operating time required for separation of the rare earth elements without adversely affecting their purity.
I. Introduction the individual rare earths in relatively short column Procedures for separating individual members of operating times. Although the author was not aware of it a t the the rareearth group of elements through the application of synthetic cation exchange resins have time, the elution technique which was finally develbeen described by many investigators.2-6 The ex- oped was quite similar to the “gradient elution” act experimental conditions in each case depended chromatographic method originally described by on the number of elements to be separated and the Alm, Williams and Tiselius.6 In addition, the degree of separation desired. I n general, the rare physical arrangement of the various reservoirs and earth elements were first adsorbed on the top por- column equipment components was similar to that tion of a long narrow column of cation-exchange used by Busch, Hurlbert and Potter.’ resin, then selectively eluted by passing a solution Experimental containing a suitable complexing agent through the Materials.-Commercial 12% cross-linked Dowex-50 resin column and collecting portions of the eluent. cation-exchange resin of “minus 400” mesh size was graded The degree of separation for a given experimental to obtain that portion which settled between 1.0 and 1.5 arrangement depends on variables such as tempera- cm./min. in distilled water. The resin was washed with 6 ture, type of cation-exchange resin, size of the resin M ammonium thiocyanate until the red ferric thiocyanate particles, dimensions of the column, eluting agent color was no longer visible, then washed in turn with diswater, 6 N hydrochloric acid and distilled water a ain. used, concentration and p H of the eluting agent, tilled Finally, the resin was converted to the ammonium form flow rate of the eluting agent, etc., but for a given with 1 M ammonium lactate and stored in distilled water eluting agent the relative position of the various until loaded on the column. The eluting agents were prepared from conductivity water, rare earths remains approximately constant.8 An Baker and Adamson reagent grade 85% lactic acid, and conexperimental consequence of this phenomenon is centrated ammonium hydroxide. All solutions were 1 M the fact that if operating conditions are adjusted in total lactate concentration with the p H adjusted with conto effect separation of the heavy rare earths, the centrated ammonium hydroxide and measured on a Becklight rare earths require excessively long column man Model G pH meter. The solutions were also made 0.01 M in phenol t o prevent deterioration operating times before being eluted. If conditions approximately of the lactate. are adjusted to give separation of the light rare Rare earth activities were obtained by bombarding a earths in reasonable operating times, the heavy rare target consisting of alternate layers of pure uranium and tantalum metal foil with 340 MeV. protons in the 184-inch earths are usually eluted too quickly to give high Berkeley synchrocyclotron. target foils were dissolved purity. Ordinarily the chemist has to make some in a solution of concentratedThe hydrofluoric acid and nitric compromise between purity of individual rare acid to which 5 mg. of lanthanum carrier and 1 mg. each of earths and column operating time. This series of strontium, zirconium, niobium, molybdenum, ruthenium, palladium, tin, antimony, tellurium and barium experiments has been undertaken to determine rhodium, carriers had been added. The mixed rare earth activities whether introduction of an additional variable, were then purified as a group by fairly well-known chemical Le., continuous change in pH of the eluting solution, steps which included a fluoride precipitation in the presence could be used to effect complete separation of all crf of dichromate, a barium sulfate “scavenge” in dilute acid (1) This work was performed under the auspices of the U. S. Atomic Energy Commission. (2) B. H. Ketelle and G . E. Boyd, J . Am. Chem. Sac., 69, 2800
(1947). (3) 9. W. Mayer and E. C . Freiling, ibid., 76, 5647 (1953). (4) F. H. Spedding, J. E. Powell and E. J. Wheelwright, ibid., 76, 012 (1954). (5) E. C. Freiling and L. R. Bunney, i b i d . , 76, 1021 (1954).
solution, precipitation of the rare earth hydroxides with ammonium hydroxide, assage of the rare earth group through a small Dowex 1-1 anion-exchange resin column in concentrated hydrochloric acid (to remove those elements (6) R. 1.A h , R . J. P . Williams and A. Tiselius, Acta Chem. Scand.1 6, 825 (1952).
( 7 ) H. Busch, R. B. Hurlbert and V. R . Potter, J . Bid. Chem., 196, 717 (1952).
August, 1955
AN IMPROVED METHOD FOR OPERATING ION-EXCHANGE RESINCOLUMNS
which form anionic chloride complexes under these conditions), and a zirconium phosphate “scavenge” in 4 N hydrochloric acid. Each of these steps was performed at least twice before the rare earth group was considered pure enough to be used on the column. Previous experience had shown that activities of all of the rare earths were formed in this type of bombardment in sufficient quantities for easy detection and that no measurable amounts of fission product activities other than the rare earths were present after the initial chemical purification. Apparatus.-Preliminary experiments indicated that “gravity feed” of the eluting agent, L e . , controlling the flow rate of the eluting agent through the resin by adjusting the height of the reservoir containing the eluting agent, did not give sufficiently high flow rates for the long column dimensions and fine resin particles that were to be used in the current experiments. Therefore, an apparatus (a schematic diagram of which is shown in Fig. 1) was designed which would ermit a wide range of accurate external pressures to be appEed to the solution passing through the resin bed and which, in addition, would be relatively simple in operation and maintenance. The ion-exchange resin bed dimensions were 7 mm. i.d. X 60 cm.; this column was surrounded by a 7 cm. 0.d. reservoir filled with water, and around the reservoir was wound a 3 inch by 60 inch “Electrothermal” heating ta e. The temperatore in this entire apparatus was contro\ed py a “Variac” variable transformer connected to the heating tape and measured by means of a thermometer in a well immersed in the water reservoir. Since normal operating temperatures were above go”, a small water-cooled condenser was installed above the water level in the reservoir to minimize losses due to evaporation. Elution characteristics of rare earths on columns using this type of heating arrangement were found not to differ in any way from those which used the conventional heating systems with either trichloroethylene or steam vapor jacketing. The eluting agent reservoir system consisted of two 2,000ml. flasks arran ed in such a manner that the solution in the upper flask coufd be made to drop into the lower flask at any desired rate by means of a stopcock control. Both of these units were connected to a “Nullmatic” pressure regulator which was found to maintain the desired pressures to fl inch of water. This regulator was then connected to the laboratory air pressure system through a small air filtering unit. Procedure.-When preparing for a run, the equipment was assembled and the column unit brought up to a stable temperature of approximately 90”. The resin was boiled in distilled water to remove dissolved gases and transferred to the column unit while hot. The new resin bed was preconditioned by passing through a t least 100 ml. of the eluting agent which was to be used in the run. The rare earth activities were adsorbed on a small amount of resin by equilibrating in a hot solution of low ionic strength for several minutes. The resin was then washed twice with boiling distilled water and transferred while still hot to the top of the resin bed. Since the purpose of the experiments was to determine the effect of various continuous rates of change of pH on the elution characteristics of the rare earths, an attempt was made to keep all experimental conditions other than the rate of change of pH constant. A single resin bed was used throughout, the resin being reconditioned between runs by passing suitable quantities of pH 7, 1 M lactate and pH 3.19, 1 M lactate solutions through the column. A pressure of 5 p.s.i., which gave a flow rate of approximately 0.4 ml./cm.Z/min., was used on all runs. The initial pH of the eluting agent in the lower reservoir was 3.19. The upper reservoir contained a solution of the same total lactate concentration at a pH of 7.0, and the flow rate of this solution into the lower reservoir was adjusted during each run to give the desired rate of change of pH. I n most cases the initial volume of pH 3.19 solution in the lower reservoir was adjusted so that the flow rate of the pH 7 solution was approximately the same as the flow rate of the solution through the column. I n order to ensure complete and continuous mixing of the solution in the lower reservoir, a small magnetic stirring device was kept in o eration during each run. After a run had begun, sampre, of the eluent were collected in the collecting tubes over three-minute intervals by means of an automatic sampling turntable. For the activity assay, s drop of the eluent was obtained on an aluminum plat,e, evaporated to dryness under a heat lamp, and the activity in each sample counted in a proportional
G9 1
w LOWpH SOl”t,O”
Moqnitlc Magnet Stlrnr
Fig. 1.-A schematic diagram of an ion-exchange column apparatus in which the pH of the eluting agent may be changed continuously. counter. Unequivocal identification of the individual peaks could be made through the known decay characteristics of the active nuclides present in each element. Determination of the pH of the eluting agent was made by direct measurement with the Becknian pH meter on the solution in the collecting tubes.
111. Results and Discussion An initial pH of 3.19 was used since it was found to effect satisfactory separation of the three heaviest rare earths in a reasonable column operating time under these experimea tal conditions. A “standard” elution curve which was obtained by passage of pH 3.19 solution through the columii without any change in pH is shown in Fig. 2. While the curve shows some evidence of excessive “tailing” on several of the peaks, it has the general characteristics of a “normal” elutioii curve. The heavy elements (lutetium, ytterbium and thulium) are quite closely spaced but reasonably pure with a column operating time of two hours and thirty-five minutes for lutetium. The lighter elements are also separated but the column operation time rapidly becomes excessive, samarium requiring more than 38 hours for elution. Figure 3 shows the elution curve which was obtained with an initial pH of 3.19 and an average rate of change of pH (dpH/dt) of 0.107 pH unit/ hour. Actually, this run was made before that shown in Fig. 2, so that the short-lived activities of erbium and holmium could still be seen. Comparison of the curves of Fig. 2 and Fig. 3 shows several striking differences. The heavy elements are eluted slightly sooner with the changing pH (1 hour and 55 minutes versus 2 hours and 35 minutes for lutetium) but the separation between adjacent elements is still satisfactory. However, lighter rare earths are eluted much more rapidly in the second case than in the first without any adverse effect on their purity. Thus, with this rate of change of pH, samarium was eluted in 4 hours and 58 minutes as against 38 hours and 25 minutes for the constant p H elution. Lanthanum was eluted in 8 hours and 20 minutes while, with no change in pH, it may reasonably have been expected to take more than 100 hours to elute. Additional elution curves similar to that show1 in Fig. 3 have been obtained with average dpH/dt
692
WALTERE. NERVIK
VOl. 59
Elution Time (Hours).
Fig. 2.-Elution
curve of carrier-free rare earth tracer activities at a constant p H of 3.19. Carrier: 5mq La Pr
Fig. 3.-Elution
curve of rare earth tracer activities plus 5 mg. of lanthanum; initial p H 3.19; p H changed continuously at 0.107 pH unit/hour.
rates of 0.007, 0.015, 0.031 and 0.067 pH unit/ hour. Since the elution times for the light rare earths under constant pH condition are extremely sensitive to the experimental conditions, several elution curves were run a t n constant pH of 3.19.
Data for all runs are summarized in Fig. 4,where the column operation time required for elution of a given element is plotted versus (dpH/dt). From Fig. 4 it may be seen that the elution time for heavier rare earths, the so called “yttrium” group, is
August, 1956
AN
IMPROvED
METHOD FOR
OI‘l?R.iTING
not affected greatly by any of the dpH/dt values used. However, elution of the lighter rare earths is affected markedly even by a slowly changing pH. Thus the elution time for neodymium is halved for a cYpH/dt of 0.01 pH unit/hour. It may be noted also that the speed of elution is not increased very much for any of the rare earths when the pH is changed faster than approximately 0.07 pH unit/ hour. The elution curve shown in Fig. 3 represents separation of a mixture of rare earth activities in which the only carrier present was 5 mg. of lanthanum. It is well known that the presence of weighable amounts of carrier may easily distort the shape of an olution peak and affect the separation of adjacent elements. In order to determine the kind and extent of such distortions with a changing pH elution, a series of runs were made in which known amounts of various rare earths were added to the mixed tracer activities. Two elution curves which show the typical effects of massive amounts of carrier are reproduced in Figs. 5 and 6. Figure 5 shows the curve which was obtained when a mixture of 9 mg. of yttrium, 3 mg. of neodymium, 2 mg. of praseodymium and 5 mg. of lanthanum, with tracers of all of the rare earths, was run with an average dpH/dt of approximately 0.1 pH unit/hour. Peaks for those elements of which only tracers were present (lutetium, ytterbium, thulium, gadolinium, europium, samarium, promethium and cerium) show the same symmetrical shape as those in Fig. 3. However, the peaks for neodymium, praseodymium and lanthanum are definitely wider than in Fig. 3, althdugh there is still satisfactory separation between the neodymium and praseodymium peaks. The yttrium peak, with 9 mg. of carrier present, shows approximately a fivefold increase in width over the carrier-free yttrium peak in Fig. 3. The shape of the yttrium peak is also quite different from that of a carrier-free element, the activity rising slowly on the leading edge and falling abruptly after all of the yttrium had been eluted. This effect could be noticed most strikingly on the plates which were counted. As the yttrium came off the column, deposits of yttrium carrier could be seen on the plates after the lactate solution had been evaporated. These deposits became increasingly large up t o the point where the column had been operating for 6 hours and 19 minutes, after which no additional yttrium deposit was seen. The plate for 6 :19 had the heaviest of the visible carrier deposits and a considerable amount of act.ii.ity while the plate for 6 :22 had no visible deposit but approximately four times the activity of the 6 : 19 plate. From the curve of Fig. 5, it may be seen that the yttrium peak was so wide that it completely displaced the terbium activity. The terbium was probably eluted in a very narrow band after all of the yttrium had come off the column. If there had been only a small amount of terbium activity present, it would have been completely hidden under the yttrium peak instead of appearing as the sharp “spike” shown in Fig. 5. Figure 6 shows the elution curve which was obtained when a mixture of 8 mg. of yttrium, 10 mg. of europium, 8 mg. of neodymium, 2 mg. of praseodym-
TON-EXCHANGE ESIN
‘?OI,TTJIh‘S
GD3
7mm X 60 cm Column Dowex 50 Initial pH 3.19, I M Total Lactate Column Temp. >9O’C Flaw Rate -J 0,3&
h\P\
a dt Fig. 4.-“CoIumn
l ( p H Units), Hour
operation time required for elution”
versus “Average rate of change of pH” for several of the rare earths.
ium, and rare earth tracer activities were eluted with an average dpH/dt of 0.11 pH unit/hour. Here lutetium, ytterbium and thulium have the normal carrier-free peak shape. The yttrium peak is very wide and has the terbium “spike” as in Fig. 5. Europium also shows this saturation peak shape. The presence of relatively large amounts of europium carrier does not seem to affect the gadolinium-europium separation, but samarium is completely hidden under the back edge of the europium peak. Similarly, promethium is not affected by the presence of large amounts of neodymium carrier, but the neodymium-praseodymium peak separation is poor. IV. Conclusion The elution curve of Fig. 3 indicates that an eluting agent with a continuously changing pH may be used most effectively with a mixture of essentially carrier-free activities. The method is also of great aid when relatively large amounts of carrier are present, but greater caution must be taken to effect the desired separations. The curves shown here, however, suggest several general rules for operating an ion-exchange column with any given mixture of rare earth activities and carriers. 1. Any mixture of carrier-free rare-earth activities may be separated completely and quickly without difficulty. 2. Any essentially carrier-free rare earth may be easily separated from massive amounts of a rare earth of lower atomic number so long as there are no excessive massive amounts of a. rare earth of
WALTERE. NERVIIC
(is4 loop00
I
.-> ._
I
I
lop00
* c
I
Vol. 59 I
I
Carrlerr: 9mg Y 3mg Nd
5mg La Tb e:22
h 7
lP00
7:m Pm
1
I
f
.
.-ef
=e
100
IO
Elutlon Time (Hours).
Fig. 5.-Elution
curve of rare earth tracer activities plus 9 mg. of yttrium 3 mg. of neodymium, 2 mg. of praseodymium and 5 mg. of lanthanum carriers, initial p H 3.19; pH changed continuously at 0.1 pH unit/hour.
I poo,ooo
100,000
I
I
I
I
I
I
Corrlers, 8mg Y lOmg Eu 8mg Nd 2mg Pr
I
I
V
Elution
Time (Hours),
Fig. 6.-Elution curve of rare earth tracer activities plus 8 mg. of yttrium 10 mg. of europium, 8 mg. of neodymium and 2 mg. of praseodymium carriers; initial pH 3.10; pH changed continuously at 0.1 pH unit/hour.
heavier atomic number present, Le., in Fig. 6 lu- trium. Gadolinium was separated from europium tetium, ytterbium, thulium, erbium and holmium but would not have been pure if larger amounts of were separated from relatively large amounts of yt- yttrium carrier had been present.
August, 1955
SPECIFICADSORPTION
3. If a small amount of one rare earth is to be separated from massive amounts of a rare earth of higher atomic number, the most effective method seems to be one in which the column is run under saturation conditions; Le., the yttrium-terbium separation in Fig. 6 . The terbium fraction was not completely free of yttrium after the first run, but the amount of yttrium carrier had been great.ly reduced. A second column run on the terbium fraction should then give complete separation of the yttrium and terbium peaks without any significant loss in the total terbium activity. 4. When massive amounts of adjacent rare earth elements are present, the chemist has two choices if well-defined peaks are desired. He may either use a wider column to give an “unsaturated” rare earth carrier load per unit area of resin, or he may use a lower rate of change of pH (and a longer column operating time) to give total separation of the two peaks in question. These general rules have been used effectively in this Laboratory to determine the experimental conditions under which a mixture of 10 mg. of each of seven of the rare earths (yttrium, europium, samarium, neodymium, . praseodymium, cerium and lanthanum) containing uranium fission product activities could be separated completely on a routine basis. As an indication of how the curves of
695
Figs. 3 to 6 may be used to establish the operating conditions for optimum separation of a mixture of this type, the followinglineof reasoning was followed. (a) From the curves of Fig. 5 and Fig. 6 an estimate of the saturation peak width versus carrier mass could be made. Thus, for 10 mg. of carrier, the peak would be approximately 60 minutes wide. (b) Choosing the elements in the given mixture which were most difficult to separate, and allowing for the effects of carriers of these elements, the curves of Fig. 4 were used to estimate the highest dpH/dt which would give the desired separation of the most difficult pair. I n the given mixtures, the europium-samarium pair are hardest to separate because of the presence of carriers. Allowing for a generous error in the estimated carrier peak width ( i e . , using a 120-minute europium-samarium peak separation instead of 60 minutes), a value of 0.01 pH unit per hour was chosen from Fig. 4. This rate of change of pH has proven to give excellent separation of this mixture of carriers with no detectable cross contamination of the peaks. V. Acknowledgments.-The author wishes to thank Mr. D. Nethaway for his considerable assistance in assembling and operating the ion-exchange columns and Drs. P. C. Stevenson, H. G. Hicks and H . B. Levy for their aid in assaying the samples.
SPECIFIC ADSORPTION BY FRANK H. DICKEY’ Contribution from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, No. 1868; Biokemiska Institutionen, Uppsala Universitet; and the Department of Chemistry of the Univervzty of California at Berkeley Received February 1 , 1966
Experiments that demonstrate a method for preparing specific adsorbents for predetermined substances have been repeated under conditions that give a more reproducible and more pronounced effect. This method consists simply of allowing the solid structure of the adsorbent to take form in the presence of the particular compound for which n specific adsorbent is sought. Thus, a large part of the surface of a silica gel prepared in the presence of methyl orange displays a greater than normal affinity for that dye and a small part of the surface attracts the dye very strongly. Such specific adsorbents were obtained with silica gel for a large variety of organic compounds but the effect is not observed with compounds that are not appreciably adsorbed from aqueous solution onto ordinary silica gel. Low pH, low electrolyte concentration, and low temperature favor the development of the specific adsorption property and also retard the loss of this property on aging. A part of the organic compound present during the preparation of a silica gel specific adsorbent is so strongly adsorbed that it can not be extracted from the product. Compounds in this condition show remarkable resistance to destruction by chemical agents and light but will undergo certain reactions that involve only minor changes in their structure.
An announcement of the finding that silica gels prepared in the presence of certain dyes show an increased capacity for the adsorption of the particular dye present during their preparation was published in 1949.2 The technique of preparing these “specific adsorbents” consisted simply of acidifying a silicate solution containing methyl orange or one of its homologs, drying the resulting gel, and then washing it thoroughly to remove as much of the dye as possible. Each such product was found to be a better adsorbent than ordinary silica gel for any one of the dyes but the effect was greatest for the dye actually used in the preparation and greater (1) (a) A. A. Noyes, Postdootoral Fellow, 1949-1950: Guggenheim Fellow, 1950-1961. (b) Continental Oil Company, 135 Main Street, Seal Beaoh, California. (2) F. H. Dickey, Proc. N a l . Acad. Sci., 55, 227 (1949).
for near members of the homologous series than for distant ones. Subsequently there have been confirmations of this ~ o r k ~but - ~ the original description of the experiments did not emphasize certain details of procedure that are now known to be critical, and several private reports of disappointing results have been received. The present investigation has been directed toward obtaining a more substantial and more easily reproduced demonstration of the effect as well as answering certain questions about the processes involved. While the earlier work shows that the gels specifically attract the dyes that were present during their formation it gives no information about the strength of this attraction or about the amount (3) P. H. Emmett, private oommunioation. (4) R. Curti and U. Colombo, Oasz. chim. i t d , 84, 491 (1952). ( 5 ) S. Bernhard, J . Am. Chem. boc., 74, 4946 (1952).
