Determination of Protactinium-233 F. L. MOORE and S. A. REYNOLDS Analytical Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn.
b Protactinium-233 is an important radioactive constituent of reactorirradiated thorium, and its analytical determination is frequently required. In investigating various methods for determination of the nuclide, the technique selected was extraction with diisobutylcarbinol. Data on elimination of interference of certain chemical and radioactive species are given, and the half life and gamma spectrum are also discussed. The method has been used successfully in a process for separation of uranium-233 from irradiated thorium. Other applications are suggested.
I
in protactinium is increasing, as indicated by the recent appearance of several papers on its analytical measurement and radioactive characteristics. Protactinium-233 is a beta-gamma emitter with a half life of approximately 27 days. It is of importance in the atomic energy program because it is formed from thorium232 by neutron irradiation, and it is the precursor of fissile uranium-233, the desired product of thorium bombardment. It constitutes most of the beta-gamma activity in thorium !slugs at the beginning of the uranium-233 recovery process previously described NTEREST
(4).
The chemistry of traces of protactinium has been reasonably familiar for some time, and the chemistry of macro amounts has been studied recently. Descriptions of the properties of the element have been given (11, 10). As the concentration of protactinium233 in solution of counting level is about 10-13M, its trace chemistry is of primary interest to the analyst. Protactinium is a member of the same family as niobium and tantalum, and its behavior is similar to that of those elements. I t s normal valence is +5, and it is very hydrolytic, being carried by many precipitates unless a complexing agent is present. It forms complex species with many common ions, notably chloride, fluoride, and oxalate. The purpose of the investigation of the liquid-liquid extraction system was to develop a simple, rapid, controltype method for the determination of protactinium, especially the determination of protactinium-233 in Thorex (uranium-233 recovery) process (4) solutions. Previous analytical methods 1596
ANALYTICAL CHEMISTRY
for the determination of protactinium233 at the Oak Ridge National Laboratory were found unsuitable. The older method (9), which involved the extraction of protactinium-233 from aqueous nitric acid solutions into diisopropylcarbinol (2,4-dimethy1-3-pentanol), followed by carrying the protactinium-233 on zirconium iodate and finally on hydrated niobium oxide, was timeconsuming and subject to the erratic hydrolytic behavior of protactinium in nitric acid solution. The more recent method (17) consisted of the extraction of protactinium-233 tracer from aqueous hydrochloric acid solutions into diisopropylcarbinol. The main disadvantage of this method was the poor decontamination from niobium and uranium. Because diisopropylcarbinol was available commercially only in very impure form, it was necessary t o purify it before use. For this reason diisobutylcarbinol (2,6-dimethyl-4-heptanol) was chosen as the. extractant because it is readily available in high purity and has been found to be superior to diisopropylcarbinol by several workers ( 6 , 14). It was realized that the major problem would be to effect good decontamination of the protactinium-233 from niobium-95. An effective separation (14) of these two elements is possible in a hydrofluoric acid medium, but it was desired to use a nonfluoride system if possible. So, a system was sought in which niobium could be complexed and held nonextractable without sacrificing the yield of protactinium-233. METHOD FOR DETERMINATION OF PROTACTINIUM-233
Exploratory experiments indicated that -90% of the niobium-95 tracer would extract from 6M hydrochloric acid into diisobutylcarbinol saturated with 6M hydrochloric acid. The extraction of the protactinium-233 tracer under these conditions is ordinarily -99.5%. Also, it was observed that the use of oxalic acid in the aqueous phase readily inhibited the extraction of the niobium-95 tracer without reducing the recovery of the protactinium233 tracer. The conditions finally selected for the standard method were as follows. The sample draTvn from the process
should immediately be adjusted to 6M hydrochloric acid (or greater). Add an aliquot of suitable counting rate to a separatory funnel (or 50-ml. Lusteroid tube). Adjust the aqueous phase to 6-11 hydrochloric acid-4% oxalic acid. (If the original sample contains thorium, omit the oxalic acid in the original aqueous phase and perform the extraction from 6M hydrochloric acid. Wash the diisobutylcarbinol phase for 1 to 2 minutes with an equal rolume of 6M hydrochloric acid before beginning the three scrubs of 6M hydrochloric acid4% oxalic acid.) Extract for 5 minutes n-ith an equal volume of diisobutylcarbinol (previously treated for 5 minutes n-ith an equal volume of 6JI hydrochloric acid). After the phases have disengaged, draw off and discard the aqueous phase. Scrub the organic phase for 5 minutes with an equal volume of wash solution (6J1 hydrochloric acid-47, oxalic acid). Repeat with two additional scrubs of the organic phase. Draiv the organic phase into a 50-ml. Lusteroid tube, centrifuge for 1 minute, and dran- off any aqueous phase which appears in the bottom of the tube. being careful not to lose any of the organic phase. (In many samples, an aliquot of the organic phase may be taken a t this stage for counting. Only if a substantial activity of niobium, antimony, or free iodine is present is it necessary to strip the organic phase.) Strip the organic phase by extracting for 3 minutes with an equal volume of 6M sulfuric acid-6-11 hydrofluoric acid. Allow the phases to disengage, centrifuge for 1 minute, and draw off most of the organic phase, being careful not to lose any of the aqueous phase. Add an equal volume of diisobutylcarbinol and extract for 3 minutes. Centrifuge for 1 minute and draw off most of the organic phase, being careful not to lose any of the aqueous phase. Centrifuge for 1 minute. Pipet suitable aliquots of the aqueous phase for protactinium233 counting (16). RECOVERY A N D DECONTAMINATION O F PROTACTINIUM-233
Table I shows the behavior of protactinuni-233 tracer, uranium-233 tracer. thorium, and the fission products in two systems. The results with an aqueous phase of hydrochloricoxalic acids indicate that the recovery and the general decontamination of protactinium-233 tracer are excellent. One stage is probably adequate but, obviously, additional stages may be used in special cases where additional
decontamination is required. The conditions selected for the standard procedure are considered the optimum conditions, as decreasing the hydrochloric acid concentration resulted in a decrease in the protactinium-233 tracer yield and increasing the hydrochloric acid concentration resulted in greater niobium-95 extraction. The data in Table I on the behavior of protactinium-233 tracer, uranium-233 tracer, thorium, and fission products in the 6M hydrochloric acid system without oxalic acid demonstrate the marked efficiency of oxalic acid as an inhibitor for elements often present with protactinium-233 tracer in process solutions. The uranium decontamination was found to be equally effective whether uranium-233 tracer or macro concentrations of uranium (27 mg. per ml.) were used. I n the thorium work 34.8 mg. of thorium was used in 10 ml. of the aqueous phase. The analysis of the three 6;M hydrochloric acid-4% oxalic acid scrubs was performed in two experiments. The diisobutylcarbinol contained 1.8 x lo5 gamma c.p.m. per ml. of protactinium-233 tracer before scrubbing according to the standard procedure. The following data indicate that the loss of protactinium-233 tracer by scrubbing the diisobutylcarbinol is small.
yo Pa233 Tracer in Scrub Scrub No.
Expt. 1
Expt. 2
1
0.05 0.07 0.03
0.06 0.04 0.06
2 3
While negligible radioiron is present in fission product solutions ordinarily, experiments were performed to determine the behavior of iron, as inactive iron may be present in some solutions. Iron(II1) (tracer or 1.4 mg. per ml.) was found to extract along with the protactinium-233 in amounts of 80 to 90%. Stannous chloride reduction of the iron(II1) was effective in rendering the iron inextractable. I n a typical experiment in aqueous solution consisting of 6M hydrochloric a ~ i d - 4 7 oxalic ~ acid-0.4M stannous chloride and 1.3 mg. per ml. of iron(II1) (originally) resulted in 0.1% extraction of iron into diisobutylcarbinol; in the absence of the stannous chloride, 90% of the iron was extracted along with the protactinium-233. The presence of stannous chloride did not inhibit the protactinium233 recovery. Under the conditions cited above, approximately 1% of the tin followed through the procedure. Because fission product antimony-125 has been found (7‘) in certain solutions, experiments were performed to study the behavior of antimony in the procedure. Approximately 97% of the antimony(V) tracer present extracted with the protactinium-233 into diisobutylcarbinol. Antimony(II1) extracted
Table 1.
Aqueous Phase 6M HC14770 H~C204 (standard procedure)
6M HC1; one 2-minute
wash with an equal volume of 6M HC1
Extraction Behavior of Various Elements
Yo Extracted EulS2-4 U233
pa238
Nb96
Zr95
99 5 99.6
0.4 0.5 0.3 0.3
0.07 0.13
0.003
0.12 0.11
0.06 13.6 14.1
99.5 99.7
90.0 90.3
Ru106
Th23Z4
0.01 0.04
0.2
