column, thus requiring more 0 . 1 9 phosphoric acid elution. The results of experiments with single isotopes of tellurium, uranium, and molybdenum are given in Table 11. All samples were loaded onto a chloride resin column from concentrated hydrochloric acid solution, this being the usual practice in the sequential procedure. Therefore, it was necessary to convert the resin to the phosphate form for the tellurium separation. The t\vo methods tried were as follows: Air was drawn through the column until it was dry; this was followed by an ether wash and elution with 1 5 phosphoric acid; and alcohol 1 S in phosphoric acid (made from 45N phosphoric acid and absolute alcohol) was passed through the column until the resin was in the phosphate form; 3 to 5 ml. \\-ere sufficient. The air-drying method was tried in runs 1 and 2 but residual chloride caused 18.7 and 13.291, of the uranium-237 to elute with the tellurium. The alcohol-phosphoric acid method was used in the remainder of the runs (results given in the fourth column). Less than 0.1% of the uranium in runs 3,4, and 5, and 0.2% of the molybdenum in runs 10 and 11, appeared in the tellurium fraction. Figure 1 shows a typical colunin
separation of telluriuni(1V) from uranium(V1). The tellurium elution tails and requires 25 ml. of the 1K phosphoric acid to elute, whereas the uranium is removed with only 8 ml. of the hydrochloric-hydrofluoric acid niixture. Because the conversion of the resin from one form to another using alcohol mas satisfactory, alcohol saturated with hydrogen chloride gas instead of aqueous concentrated hydrochloric acid was used to change from the phosphate to the chloride form. The results in the fifth column of Table I1 showed that the uranium-237 iyas quantitatively eluted after the double conversion of the resin, chloride to phosphate to chloride form. Table 111 gives the results of the sequential separations of mixtures of uranium, tellurium, and molybdenuni: in the first three runs, of uranium from telluriuni; in the last two, all these isotopes from one another. The contamination of uranium in tellurium was determined from decay nieasurements and n-as less than lye. Similarly that of tellurium in uranium was less than 0.5%. I n all cases but one a t least 99% of the radionuclides were recovered, the molybdenum-99 recovery in run 4 being 98.87& Also, for run?
4 and 5> ?-ray pulse height distributions of all the separated fractions were made. S o significant contaniination was detected. Three aliquots of a fission product mixture containing added uranium-237 were analyzed for tellurium-132 and uranium-237 by the complete sequential ion exchange procedure, and their relative activities are given in Table IT’. The precision of the three samples is Tithin 1.5%. -211 the fractions n-ere allowed to decay for several half lives and no cross contamination 1%-as found. The tellurium decay curves showed a longer lived activity with a half life of 34 to 36 days and this was identified as tellurium-129. LITERATURE CITED
(1) Bunney, L. R., Ballou, N. E , PaecuaI, Jaun, Foti, Stephen, A N ~ L .CHEJI.
31,324 (1959). E. C., Pascual, Jaun, Delucchi, -4.A,, Ibid., 31, 330 (1959). (3) Lee, TS’illiam, Ibid., 31,800 (1959). (4) TS’ish, Leon, Zbid., 31, 326 (1959). ( 2 ) Freiling,
RECEIVED for review January 25, 1059. Accepted lpril 11, 1960. Division of Analytical Chemistry, 136th Meeting, ACS, .4tlantic City, S. J., September 1959.
Radiometric Extraction Method for Fluoride Pyrolysis-Ion Exchange Separation WILLIAM J. MAECK, GLENN L. BOOMAN, MAXINE C. ELLIOTT, and JAMES E. REIN Afomic Energy Divirion, Phillips Petroleum Co., ldaho Falls, Idaho
b In many liquid-liquid extraction systems, complexing anions cause lowered extraction of metal ions. The potentiality of using this as the basis for determining anions has been investigated with the system hafniumfluoride-trioctylphosphine oxide. An inverse linear response was obtained for hafnium extraction vs. fluoride concentration for the range of 2 to 10 pmoles of fluoride. Hafnium distribution is rapidly determined using hafnium-1 81 tracer and gamma counting the liquid phases. Pretreatments by pyrolysis to separate fluoride from metal ions and anion exchange to separate it from interfering anions are described. At the 250-pmole fluoride level in the original sample, the coefficient of variation for the over-all procedure, including pyrolysis, ion exchange, and extraction, is 3.3%. 922
ANALYTICAL CHEMISTRY
I
liquid-liquid extraction systems, metal ion extraction coefficients are decreased by the complexing action of anions. Although well recognized, this phenomenon has received little study as a means of determining anions. K i t h suitable systems the extraction coefficient or a derived function of the extraction coefficient can be proportional to the amount of the anion present. Fluoride mas chosen to evaluate this scheme because it forms strong complexes with numerous nietal ions. Radiotracer techniques, simple, rapid, and accurate for microconcentrations, were selected for measuring the cation distributions. Desirable characteristics of a radiotracer for this application are: half life greater than 1 day, ready availability, gamma emitter to allow direct counting N MANY
of liquid phases in a well-type scintillation crystal, and absence of actix-e daughters. Of the estraction systemsinvestigated, the extraction of hafnium by trioctxlphosphine oxide (TOPO) v a s selected for evaluation. Conditions were established that gave an inverse linear response for hafnium estraction z’s. fluoride concentration for the range of 2 to 10 pmoles of fluoride. Like other methods for fluoride analysis, this one is subject to diverse ion effects. Separations based on pyrolysis and ion exchange provide specificity. For a given application, the proposed method is not necessarily superior to existing methods: The main purpose of this paper is to present a new approach to the determination of anions.
APPARATUS
The pyrolysis apparatus was similar to that described by Powell and RIenis (5) except that a platinum-lined quartz combustion tube was substituted for a plain quartz tube and water-saturated air for oxygen as the sweep gas. Extractions were made in 15 X 125 mm. test tubes with polyethylene stoppers using a rotating wheel (3). A sodium iodide (Ti) well-type crystal and Kuclear Chicago gamma scaler were used for gross gamma counting of hafnium-181 activity. Onemilliliter aliquots were placed in 10 X 7 5 mm. test tubes for counting. The ion exchange column was a 75em. length of 1-em. diameter borosilicate glass a-ith a 50-ml. reservoir at the top and a stopcock and glass wool plug a t the bottom to which was added a m t e r slurry of 20- to 50-mesh Amberlite IR-400 resin to a settled height of ci a cm. The column was washed with tliree column volumes of 6N hydrocliloric acid, three column volumes of O.li\- hydrocliloric acid to decrease impurity metal (especially iron) contamination: and finally, with three column volumes of 0.5.U ammonium chloride. Columns may be re-used by passing 100 ml. of elutaiit through after each sample.
maximum temperature of 200' C. Collect the distillate in excess sodium hydroxide, adjust (with a p H meter) to p H 9 with hydrochloric acid, and dilute to 50 ml. in a volumetric flask with distilled water. Proceed to the extraction unless nitrate is present and the ion exchange separation must be used. I o n Exchange. Transfer the 50-
ml. pyrolysis distillate to the ion exchange column. Maintain the flow a t i t o 10 ml. per minute. Start eluting, maintaining the same flow rate, with 0 . 5 X ammonium chloride adjusted t o p H 9 when the sample nears the ion exchanger level. Discard a volume equal to column holdup (about 20 ml.) and collect the next 100 ml. in a volumetric flask.
CONDITIONS: AQUEOUS: 4 m l , 2N HcS04, IpMOLE HI ORGANIC: 4ml.
DIELECTRIC CONSTANT HEXANE 1.87 E . CYCLOHEXANE 2.05 C. CARBON TETRACHLORIDE 2 . 2 4
A.
n W
.
Figure 1 Hafnium extraction as function of trioctylphosphine oxide concentration and diluent
REAGENTS CONDITIONS: AOUEOUS; 4 m l . , l p M O L E H f
100
All reagents were prepared with analytical reagent or Eastman Kodak White Label chemicals without purification. Sodium fluoride, dissolved in dist'illed water and stored in polyethylene bottles, seryed for calibration. Hafnium-181 tracer was obtained from the Isotopes Division, Oak Ridge National Laboratory. Hafnium Solution. Dissolve 500 mg. of hafnium tetrachloride and aliout 0.5 m c . of hafnium-181 tracer in 9 nil. of w t r r in a 50-ml. beaker. Add 5 mi. of concentrated sulfuric acid and heat t o sulfur t'rioside fumes. Cool, dilute t o 50 nil. with 1 X sulfuric acmid, and transfer to a 900-ml. separator>- funnel. Extract for 3 minutes with 50 ml. of 0.05M trioctylphosphine oxide-hexane. Discard the aqueous phase. iidd 25 nil. of I N sulfuric acid and 250 ml. of hexane and strip the hafnium back into the aqueous phase for 3 minutes. St'ore in a glass or plastic bottle. Determine the hafnium concentration by gravimetric ignition to HfO, at 1000" C. in platinum. Determine acid by a standard base titration (the small error caused by hafnium can lie neglected). .idjust the solution to 5 x 10-4X hafnium and 4 N sulfuric acitl by dilution with sulfuric acid.
A. 4 1 H Z S 0 4 B. 2M H z S D 4 C. I N H z S O i
90
a BO W
I-
2K 70
-
ORGANIC; 4ml.,CYCLOHEXANE
60
W
I
50
$40 30 20 IO
0
2
4
6
810 MOLE
Figure 2. 100
-
80
-
CONDITIONS
W 0
I 60
-
20 4 0 60 80 100 R A T I O TOPO/Hf
200
400 600
1000
Hafnium extraction as function of acid
AQUEOUS; 4 m l , Ip M O L E ORGANIC ; 4 ml. H E X A N E A : IN H o S O 4 , 200/1 MOLE B IN H z S O i , 300/1 MOLE C : 2N H z S O i , 200/1 M O L E D : 3& H z S O i , 200/1 MOLE
Hf RATIO RATIO RATIO RATIO
TOPOlHf TOPOlHf tOPO/Hf TOPO/Hf
E
z
40
-
PROCEDURE
Pyrolysis. Half fill a 1 x 10 cm. platinum boat with tungstic anhydride. Pipet a sample aliquot of 2 nil. or less, containing from 50 t o 250 pmoles of fluoride (100 to 500 kmoles if nitrate is present), dispersing it evenly on the tungstic anhydride. Pyrolyze for 10 minutes a t 900" C. starting a t a
20
p MOLES
Figure 3. oxide
F-
Hafnium retention as function of fluoride, acid, and trioctylphosphine
VOL. 32, NO. 8, JULY 1960
923
Extraction. Add a 2-ml. aliquot of t h e pyrolyzed distillate or of t h e ion exchange eluate t o a test tube containing 2 ml. of the hafnium solution.
Add 4 nil. of 0.05M trioctylphosphine oxide-hexane, stopper, and mix for 15 minutes. Centrifuge t o facilitate phase separation. Pipet 1-ml. ali-
quots of each phase into 10 X 75 mm. test tubes and gamma count in the well crystal. Calculate the per cent hafnium retained in the aqueous phase. Determine the fluoride by comparing to a standard curve or regression equation prepared from standards processed through the procedure. EXPERIMENTAL A N D D I S C U S S I O N
Many stable metal-fluoride complexes are known. Table I is a summary of the extraction systems either considered or investigated. The hafnium-trioctylphosphine oxide system appeared most promising. Hafnium-181 is a gamma emitter \vith a half life of 46 days. I t s daughter activities are short lived, so
that no error is introduced as is the case [Zrg6 Figure 4.
Fluoride and nitrate elution as function of chloride concentration
Table I.
Evaluation of Extraction Systems
Metal
Extraction Conditions
'41, B, Be, Th
...
Ce
iYb
Zr
8-quinolinol in hexane; nitric acid, citrate buffer 0.05M to 0.50M thenoyltrifluoroacetone in xylene; acetate buffer pH 3-6 0.1M trioctylphosphine oxide in various diluents; nitric acid, citrate buffer Cupferron in various diluents 0.05M
0.02 to 0.10M trioctylphosphine oxide, in trimethylpentane; 0.1 to 1N sulfuric acid Monodibutyl phosphate in n-butyl ether; 2-47 sulfuric acid 0.05 to 0.50X thenoyltrifluoroacetone, in xylene; 0.5 to 5N
nitric acid
0.02 to O.1OM trioctylphosphine
oxide in trimethylpentane; 1 to
4 N sulfuric acid
Fe
HI
924
n-Amyl alcohol; 5 to 500 mole ratio thiocyanate to iron( 111), 0.25M perchlorate, pH 1-2 Acetylacetone 1 to 10070; 0.5 to 0.0005M HC1, 0.5M C1 0.05 to 0.5M thenoyltrifluoroacetone, in xylene; 1 to 4.V nitric acid Trioctylphosphine oxide in hexane; 4 to 800 mole ratio to hafnium, 1 to 4N sulfuric acid 0.05 to 0.5M dibutyl phosphate in n-butyl ether; 0.2147 sulfuric acid, I N sulfate
ANALYTICAL CHEMISTRY
Remarks Suitable isotopes not available Ce not extracted Pronounced fluoride effect, but not reproducible. Extraction very pH and time dependent Small fluoride effect, also hydrolysis problems Immiscible organic solvents were not found which dissolve the precipitated chelate Slight oxide in niobium extraction with increasing fluoride Insignificant fluoride effect Pronounced fluoride effect, radiocolloids of Nb95 daughter decreased precision System very acid dependent, Nb9j daughter decreased precision Decreasing color noted with increasing fluoride, no significant change in Fe extraction Insignificant fluoride effect Slow equilibrium Pronounced fluoride effect, system acid dependent Insignificant fluoride effect
Ps;& Nb95 -+ 1 1 0 ~ ~ 3.5d (3)
for zirconium-95. Also the colloidal nature of niobium and its erratic extraction behavior caused error in experiments with zirconium-95 tracer. Extraction Conditions. The eytraction of hafnium as a function of trioctylphosphine oxide concentration in various diluents is shown in Figuie 1. Hexane, the solvent with the lowest dielectric constant, gave complete extraction with the lon-est trioctylphosphine oxide concentration. Because fluoride effect mould be expected t o be greater under these conditions, hexane was selected as the diluent. As acidity is increased, less trioctylphosphine oxide is needed for complete extraction (Figure 2). However, with high acidity the sensitivity of the system to fluoride decreases (Figure 3). Sulfuric acid gave more reproducible extractions than did nitric acid. I n fact, as shown later, nitrate is a n interference. On the basis of the data presented in Figure 3, which shows the effect of fluoride as a function of acidity and trioctylphosphine oxide concentration, a 200 t o 1 mole ratio of the extractant to hafnium with a 221' sulfuric acid medium (curve C) was chosen for the method. The per cent hafnium retained was linear with fluoride concentration with a slope of 4.83 and a coefficient of variation of 1.1% for the range of 2 to 10 pmoles of fluoride. The acidity is sufficient to prevent hydrolysis of hafnium. Considerably higher slopes were obtained in some experiments covering a wide range of variables; however, the relationship was less linear and the extractions were less reproducible.
The effect of significant competing reactions was realized and was of some concern. Attempts were made to identify the species both extracting and remaining in the aqueous phase. Two series of extractions were made: one with hafnium-181 tracer and inactive fluoride and the other with fluorine18 tracer and inactive hafnium. The 1.87-hour fluorine-18 was prepared by neutron irradiation of lithium carbonate
Table 11.
Effect of Nitrate
Nitrate Fluoride Added, Meq. Found,O pmoles 0.001 4.94 0.01 5.05 0.02 5.17 0.04 5.74 0.06 6.21 0.08 6.71 0.10 7.40 5.00 @molesadded.
[Li6 (n,a),11:; 0'6 (H;, n ) F I S ]
in the Materials Testing Reactor. The resulting data mere insufficient to cstablish species; however, the following conclusions could be drawn: Hafnium sulfate species extract, hafnium fluoride species extract, fluoride reduces the hafnium extraction coefficient, and fluoride-containing species extract in the absence of hafnium. It is generally concluded that the linear response of hafnium retention M ith increasing fluoride that was obtained is the result of several competing reactions rather than the formation of a single hafniumfluoride species. Application to Analysis. It was immediately recognized t h a t metal ions significantly complexed by fluoride and anions which would significantly complex hafnium would interfere. The pyrolysis method of Powell and Menis (5) was selected for the quantitative separation of fluoride from metals. Inasmuch as Powell and Menis (5) have shown that fluoride is quantitatively separated from metals, only anions were considered as interferences. The effect of nitrate upon the extraction alone is shown in Table 11. The high bias is believed due to the formation of a stable nonextractable hafnium nitrate complex. Because high temperature volatilization can alter composition, a series of anions was carried through the pyrolysis before extraction. The results are shown in Table 111. The recommended procedure was followed. The sample consisted of 0.5 ml. of 0.5M sodium fluoride plus 2 ml. of concentrated acid for those anions available as such. Silicate and borate were added as solid salts a t a mole ratio to fluoride of 10 and 2.5, respectively. After pyrolysis, duplicate aliquots containing the equivalent of 5 pmoles of fluoride were processed through the extraction. A series of 12 samples without added diverse anion gave a per cent retention of hafnium of 23.2% with a coefficient of variation of 3.9%. Only nitrate and borate interfered, based on the allowable 957, confidence limits of 21.5 t o 25.0% retention for
Table 111.
Effect of Anions
yo Retention of Hafnium After pyrolysis and ion exchange
After pyrolysis Mone 23.2~ Borate 18.0 16.6 Chloride 23.5 Nitrate 69.9 24.5 Perchlorate 25.0 Phosphate 22.5 Silicate 23.4 Sulfate 23.2 a Average of series of 12 standards. Anion
Table IV. Recovery of Fluoride with Varying Elutant Concentration'
Eluate Fluoride Recovered,* yo Volume, 1.OM 0.75M 0.5M M1. c1c1c1100 96.2 96.5 95.6 120 98.8 98.0 98.3 150 100.6 99.0 100.3 Ion exchange conditions, same as procedure. b 30 mmoles nitrate and 0.25 mmole fluoride (F18tracer) added. 0
the average of duplicates. The pyrolysis of nitrate was evident visually as nitrogen dioxide fumes. Borate undoubtedly volatilized as a complex fluoride rrhich was stronger than the hafnium complex causing low bias, Silicate did not volatilize. The other anions, although volatilized, had no effect upon the extraction. Because nitrate is commonly present in samples, various treatments for its destruction or separation were considered. Anion exchange was selected as most applicable. Atteberry and Boyd ( 1 ) reported separation of fluoride from other halides. The adsorption order of anions on strongly basic exchange resins is reported by Kunin and Myers (2) to be fluoride< chloride< phosphate< nitrate< hydroxyl< perchlorate. Chloride appeared to be the logical elutant for separating fluoride from nitrate. At high p H fluoride is dis-
sociated and elutes rapidly. Kewman (4) used a pH of 8.8 to separate fluoride from phosphate. Sodium chloride buffered with ammonia to pH 9 was selected. Fluoride recovery is independent of the chloride concentration over the range of 0.5 to 1M a t this pH (Table IV). However, the elution of nitrate is markedly affected by chloride concentration as shown in Figure 4. Fluoride elution was followed with fluorine-18 tracer and nitrate by spectrophotometric measurements a t 295 mp. With a total of 120 ml. of 0.5M chloride elutant, the recovery of fluoride is greater than 98% and the amount of nitrate eluted is less than significantly interferes, as shown in Table 11. The first 20 ml. (slightly less than column volume holdup) can be discarded, permitting convenient use of a volumetric flask for collection. A series of five 50-ml. samples (representing pyrolyzed distillates) was processed through the ion exchange and extraction steps. The fluoride concentration was such that a 2-ml. aliquot of the 100-ml. eluate contained from 2 to 10 pmoles in 2-pmole steps. The average recovery was 98.5%, which compares favorably to the 98.3% recovery of Figure 4. A regression analysis of the recovery data gave a precision of 1% coefficient of variation. Pyrolyzed samples containing nitric acid and boric acid a t the same levels as previously described were processed. As shown in Table 111, no interference from nitrate was now detectable; borate still interfered. The over-all precision of the methodpyrolysis, ion exchange, and extractionwas established by analyzing a series of 24 samples containing 250 pmoles of fluoride through the recommended procedure over 3 weeks. This level of fluoride was such that the 2-ml. aliquots used in the extractions contained 5 pmoles of fluoride. The coefficient of variation was 3.37,. LITERATURE CITED
(1) Atteberry, R. W., Boyd, G. E., J.Am. Chem. SOC.72, 4805 (1950). (2) Kunin, R., Myers, R. J., Zbid., 69, 2874 (1947). (3) Maeck, J., Booman, G. L., Elliott, M. C., Rein, J, E., ANAL. CHEM.30, 1612 (1958). (4)Newman, A. C. D., Anal. Chim. Acta 19,471 (1958). (5) Powell, R. H., Menis, Oscar, ANAL. CHEM.30,1546 (1958).
w.
RECEIVED for review January 25, 1960. Accepted May 2, 1960. Division of Analytical Chemistry, Radiochemistry Symposium, 136th Meeting, ACS, Atlantic City, Tu'. J., September 1959. Work supported by the U. S. Atomic Energy Commission under Contract AT-( 10-1) 205.
VOL. 32, NO 8. JULY 1960
925
