Table 1.
A
I3 C
D E F
Recovery
of PMP
Calcium PMP Added Grams/ Milliliters 100 ml. 10 0.020 50 20
10 10 10
0.002j
0.007, 0.030
0.050 0.040
Found, grams/ 100 ml. 0.024 0.002.g 0.0079 0 028 0.046 0.038
maximum at 540 mh (Figure 2). Corresponding derivatives of pival and diphacinon under the same conditions produce orange solutions which have their maximum absorbance a t about 350 mp. Treatment of these hydrazones with alkali-acetone, alkali-2-butanone, or iT,N-diniethylformamide did not produce any appreciable color differences. The observed difference in color reaction between the 2,4-dinitrophenylhydrazones of PlIP and pival or diphacinon is probably the result of steric influences of the indandione side chains. The crowding and bIocking
Figure 3. Reaction of m-dinitrophenyl compounds with KCN
effect of the pivalyl and diphenyl groups undoubtedly hinders reaction of the side chain carbonyl group of pival and diphacinon with the bulky 2,4-dinitrophenylhydrazine. I n the case of PMP, this hindrance is substantially reduced by a lengthening of the side chain. It would seem, therefore, that P h f P adds 3 moles of 2,4-dinitrophenylhydrazine, whereas pival and diphacinon add only 2 moles of this reagent. This difference could provide sufficient additional resonant energy to the PIUP derivative to cause a shift of the absorption maxima from the region of 350 mh, as ivith pival and diphacinon, to 540 mp in the case of P M P (Figure 2 ) . The reaction between potassium
cyanide and polynitrophenolic compounds to produce highly colored compounds was first described by Pfaundler and Oppenheimer ( 5 ) . Subsequent studies have shown that the basis of this color reaction is the formation of phenylhydroxylamines from m-dinitrophenyl compounds (1, 2) (Figure 3 ) . The developed color obeys Beer's law, and calculation has shown the molar absorptivity to be 1.6 X lo4. In our hands, recovery of PMP, using calcium PMP, has been good (Table I). LITERATURE CITED
(1) -Inger, I-., Jlzkrochim. Acta 2, 4
(1937).
( 2 ) Borsche, W., Bocker, E., Ber. deut. chem. Ges. 37, 1844 (1904). (3) Federal Specifications 0-R-00502,
December 20, 1954. (4) Meltzer, H., A-achrbl.
deut. Pflanzen-
schiitzdzenst 11, S o . 12, 233 (1957). ( 5 ) Pfaundler, L., Oppenheimer, A., 2. anal. Chem. 8, 469 (1865).
RECEIVED for reviex October 10, 1961. Accepted February 14, 1962. Division of Agricultural and Food Chemistry, 140th Meeting, .ICs, Chicago, Ill., September 1961.
Determination of Rare Earths in Fission Products by Ion Exchange at Room 1emperature KURT WOLFSBERG 10s Alamos Scientific laboratory, University of California, los Alamos,
b A method is described for the routine radiochemical determination of rare earths from fission of uranium. Separation of individual rare earths is achieved by eluting with a-hydroxyisobutyrate solutions through cationexchange resin columns operated a t room temperature. Titration with EDTA as a method of obtaining chemical yield is described.
A
for the radiochemical determination of rareearth activities with 5 to 10 mg. of inactive carrier for each element of interest are in current use ( I O ) . These methods rely on elution (generally with lactate solutions) through hot cation-exchange columns to separate individual rare earths from each other. Use of heated columns has disadvantages, however, in that specialized glassware must be constructed for each column, precautions must be observed in preparing and operating such columns to prevent formation of bubbles, and the resin in a DEQUATE PROCEDCRES
51 8
ANALYTICAL CHEMISTRY
N. M.
column must be either replaced or decontaminated between analyses. Smith and Hoffman (9) have shown that elution of the rare earths on a tracer scale with a-hydroxyisobutyrate (a-HIB) solutions at room temperature from Dowex 50-X4 resin columns is comparable to elution from Dowex 50-X12 columns with hot a-HIB. Since under similar conditions, a-HIB is more efficient than lactate for the separation of rare earths, an investigation of the use of a-HIB for the separation of 5- to 10-mg. amounts of individual rare earths, on columns operated a t room temperature, as undertaken. The procedure developed and described in detail here was developed for the routine determination of Y, Eu, Sm, Pm, Nd, Pr, Ce, and La actirities from thermal-neutron fission of uranium. Other rare-earth separations may be made by making changes in eluting condition, column size, or preliminary purification, depending on the elements to be separated, the macro quantities involved,
the relative activities, and the source of the activities. The construction and use of columns operated a t room temperature is simple. Many columns may be prepared prior to use and discarded afterwards. The titration of rare earths with EDTA (4) was adapted as a method of determining chemical yield alternative to the normal method of the weighing of ignited oxides. B y this method, yield determination is delayed until after the sample is counted. This is particularly advantageous when shortlived activities are measured or when a large number of samples are processed on the same day. Also, this method circunivents the care that has to be taken when the -7-mg. samples are weighed to determine chemical yield. EXPERIMENTAL
Apparatus. GRADIENT-ELUTION EQUIP~IENT.The gradient-elution equipment shown in Figure 1 is similar to that described by Bock and Ling (2) and Bunney et al. (S), but it is
i'
HIGH pH SOLUTION
\
i
,/
niodified so that the system may be operated under pressure. Several columns may be operated from one set of flasks by delivering the eluent to the columns through Y connecting tubes. A pair of 500-ml. flasks is used for the operation of one or two columns; a pair of 1000-ml. flasks, for the operation of three or four columns. Elution is started with the levels of the two solutions a t the same height. One half of the volume that is removed from the flask containing the solution of low p H is continuously replaced by solution of high p H by gravitational leveling. Thus, the p H of eluent (and, hence, the concentration of a-hydroxyisobutyrate ion) passing through the columns increases continuously from that of the solution of low p H a t the beginning of elution to that of the solution of high p H a t the end of elution. CATION-EXCHANGE COLUMSS. A column is constructed by drawing the end of a 27-inch length of 8-mm. I.D. glass tubing to a tip (I.D., 0.8 to 1.2 nim.). A slight constriction is made inch from the other end for attachment of a flexible tubing connection. A small plug of glass wool is placed in the tip, and the column is loaded with a mater slurry of cation resin. Resin should be added to a height of about 25 inches. The columns may be loaded long before use, but the resin should always be kept moist. Reagents. RARE-EARTH CARRIERS. Five grams of the desired rare-earth ovide are dissolved in 6 M HCl. The solution is filtered, diluted to 1 liter, and the HC1 concentration is adjusted to 2 or 3 M . The carrier solutions may be standardized by gravimetric (8) or volumetric (4) methods. PROMETHIUM TRACER. Promethium145 was employed as a chemical-yield tracer in this investigation. However, other Pm isotopes may be used. The tracer is standardized by adding a known amount of Xd or Sm carrier to a n aliquot of tracer. The carrier is mounted as the oxide (See Procedure). The recovery yield of the tracer is assumed to be the same as t h a t of the carrier. ELUEKTS.Five-tenths molar a-HIB is prepared by dissolving 208 grams of
LOW pH SOLUTION
\
a-hydroxyisobutyric acid in 4 liters of water. If the solution is not clear, it should be filtered. Solutions are adjusted to the desired p H values (3.40 and 4.20) by the addition of concentrated KH40H. Measurements of p H are made with a meter having an accuracy of a t least h0.02 p H unit. Various commercial batches of the reagent give rise to somewhat different elution curves for -10 mg. quantities of rare earths as well as for trace amounts (9) a t a particular concentration and pH. I n gradient elution, the difference in elution curves for the various batches tested was not great enough to change elution positions by more than a few hours. However, i t is advisable to determine whether the batch of a-HIB used gives the desired elution curves. If it does not, the p H values of the solutions or the concentrations should be adjusted accordingly. CBTION-EXCHANGE RESIN. Dowex AG 50W--X4, dry mesh designation "minus 400 mesh" [actual rangeminus 200 mesh wet (U. s. Std.) to 0.27 cm. per second settling rate or nominal 62 to 23 microns], is treated in turn with 651 HC1, lJ1 NH4CNS, 6 X HCI, 1X ",OH, and mater. The specially processed resin mas obtained from Bio-Rad Laboratories, Richmond, Calif. BUFFERSOLUTION.The buffer solution (pH 10) is prepared by dissolving 61.5 grams of NH&l in 400 ml. of concentrated ",OH. PROCEDURE
Separation of Rare Earths as a Group. After the addition of P m tracer and -10 mg. (2 ml.) of each element to be determined, the rare earths as a group are purified from other fission products by routine radio-chemical operations, as for example in the method of Nervik (I?). These include, in order, fuming with perchloric acid, a zirconium phosphate scavenging precipitation, two precipitations of rare-earth fluorides, a barium sulfate scavenging precipitation, precipitation of rare-earth hydroxides, passage of the rare earths through a small anion-exchange column in concentrated hydrochloric acid, and re-
precipitation of rare-earth hydroxides. After each precipitation of rare-earth fluorides or hydroxides. the precipitate is washed with dilute hydrofluoric acid or ammonium hydroxide. Separation of Individual Rare Earths. The last hydroxide precipitate is dissolved in 2 to 4 drops of concentrated H N 0 3 , and the solution is diluted to 30 ml. One milliliter of a slurry of the cation-exchange resin is added, and the mixture is stirred or shaken for 1 minute. The resin with the absorbed rare earths is centrifuged, slurried in a little water, and added to the top of a previously prepared cation-exchange column. After the resin particles have settled, the water is removed, and the top of the column is rinsed with water. The flasks of the gradient-elution equipment are filled with O.5M a-HIB solutions of p H 4.20 and pH 3.40, respectively. About 144 ml. of each solution are used for each column operated from the setup. small additional volume of low p H eluent is added to compensate for the volume in the delivery tubing. After the columns are attached and the delivery tubing is filled, the screw clamp between flasks is opened, and the small air bubble in the capillary is pushed through by applying a little air pressure on one side. Air pressure is then applicd to both sides to adjust the average rate of elution to one drop every 17 to 19 seconds (-9 ml. per hour) from each column. Similar elution rates are also used for the operation of columns for other applications. Under these conditions, the p H of eluent passing through the columns is increased a t an aver:ge rate of -0.025 p H unit per hour. 1he eluent is collected in fractions of 10 or 15 minutes each in tubes in an automatic fraction collector. 1 few drops of saturated oxalic acid are added to each tube to precipitate and locate the individual rare earths. Promethium is located by monitoring the activity of the tubes between the Sm and S d locations. The rare earths are eluted from the columns in order from heavy t o light elements, and Y is eluted before Tb. Mounting Procedure. The mounting procedure may vary n i t h the particular application and with the particular counting geometry employed in each laboratory. The method described here gives samples similar to those described by Bayhurst and Prestwood (I)-Le., the samples are mounted on filter paper circles, and their thickness is -8 mg. per sq. cm. However, to obtain this thickness, the diameter of the samples had to be reduced from 17.5 to 11 mm. Fractions containing a specific rare earth are combined in a single centrifuge tube. Two milliliters of iYd or Sm carrier are added to the Pm fraction. Then 5 ml. of saturated oxalic acid are added to each centrifuge tube, and the oxalates are digested on a steam bath for 10 t o 15 minutes. The oxalates are centrifuged, suspended in water, and filtered on any convenient filtration assembly. The oxalates are ignited to VOL. 34, NO. 4, APRIL 1962
519
oxides in crucibles at -950" C. for 1 hour. After the oxides have cooled, 2 drops of absolute ethanol are added to each sample, and the oxide is ground to a fine powder with a stirring rod. The powder is suspended in ethanol and filtered on a circle of No. 42 paper using a 4/0 ground-off Hirsch funnel and a n 11-mm. I.D. glass chimney. The sample is dried a t 110" C. for 15 minutes. If chemical yield is to be determined from the weight of the oxide, normal mounting and weighing procedures (5) are followed. The chemical yield of P m is determined bv counting the P m tracer. I n this laboratory, tlhe Pm149 and Pm151 activities are counted with beta-proportional counters. After these activities have essentially decayed completely, the Pml45 activity is determined with a gamma-scintillation counter. If the samples are mounted on an aluminum plate with Scotch polyesterfilm tape (No. 850 type 2PTA), they can later be recovered for chemicalyield measurement by the alternative method described in the next paragraph. It is our experience t h a t the variation of thickness of the tape along the length of a roll and among several rolls produced from the same batch is quite uniform, the variation being about 1.5% of the thickness. However, there may be a larger variation between batches. The tapes of the two batches examined had thicknesses of 4.9 and 6.3 mg. per sq. em. Since a large number of rolls of tape can be obtained from one batch, this variation is not a serious problem. Yield Determination by EDTA Titration. After counting is completed, t h e filter paper, oxide, and polyester-tape cover are removed as a sandwich by cutting t h e tape around t h e paper Jvith a sharp blade. The rare-earth oxide is dissolved by treating t h e sandwich in a n Erlenmeyer flask n-ith -10 ml. of water and 2 ml. of concentrated HC1 and then heating the solution for -20 minutes. Disintegration of the filter paper does not interfere n i t h the analysis. The solution is diluted to -30 ml., and an excess of 0.01di EDTA (standardized against a known weight of a rare-earth oxide) is added from a 10-ml buret. For each milligram of rare-earth oxide, 0.6 to 0.7 ml. is added, and for each milligram of yttrium oxide, -0.9 ml. The solution is adjusted to p H 8 or 9 by adding 4 ml. of 257, S H & l solution, a drop of phenolphthalein indicator, and p H 10 buffer until the solution turns pink.
Table I.
Sample
i
4
1
1
'
Contaminant
Fract,ion of Total Contaminant in Sample
YQl
pmI49,Gl
3 x 10-5 4 x 10-6
Nd
NdI47 Pr14a
2 2
x x
-~
520
I
Decontamination Factors
Eu Sm
Pm
precipitate could be observed in a fraction when a drop of oxalic acid was added. Changing the p H a t a rate of -i 0.025 p H unit per hour results in separation of all the elements of interest in a reasonable length of time. Elution of -10 mg. of each element takes less Ce than 2 hours, and there is no overlap Pr If the rate of between elements. Nd , Pm 1 change of p H is 0.030 or 0.020 p H unit Sm per hour, there will not be much change in the elution curve. Gd J -1 rare-earth separation was perw 5formed on a sample containing 3-day zI- - n n h n old fission products from -2 X l O I 4 O Y fissions in uranium. Each fraction - (10 niinutcs) from the column was counted with a sodium iodide wellcrystal scintillation counter. The resulting elution curve is shown in Figure Ib 0.0, 0.62 0.63 0.04 3. The Eu, Sm, Pm, and Nd fractions AVERAGE RATE OF CHANGE OF pH (pH UNIT/ HOUR) were then treated as individual samples, appropriate carrier or tracer was added, Figure 2. Effect of rate of change of and a second separation was carried out pH on time of elution for the contaminants given in Table I. The activities of these contaminants mere compared rvith small aliquots The solution is brought almost to boil(3%) of the Y, Pin,Nd, and P r activities ing, and two drops of arsenazo indicator from the first separation. [0.05% 3-(2-arsonophenylazo)-4,5-diTo test the reproducibility of the hydroxy-2,7-naphthalenedisulfonic acid, procedure, several rare-earth detertrisodium salt in water] are added. minations were performed over an The excess E D T A is then back-titrated with 0.01M La+3 solution in -1-11 extended period. Each of three small HCl (standardized against, the EDTA samples (-ttrium, and Actinium," SAS-SS 3020, Office of l'echnical Services, Washington, 1961. (11) \Tolfsberg, K., Los Alamos Scientific Laboratory, Los Alanios, K . RI., unpublished results. RECEIWDfor revieiT December 1, 1961. Accepted Fehruarv 8, 1962. Work was performed under the auspices of the U. S. Atomic Energy Commission.
Correction
New Fire Assay for Iridium I n this article by G. G. Tertipis and F. E. Beamish [ANAL. CHEJI. 34, 108 (1962)], on page 110, column 1, under Discussion, line 1, Table I1 should read Table I. VOL. 34, NO. 4, APRIL 1962
521
