Studies on protactinium (V) in sulphuric acid solution—I

J. Inorg. Nu¢I. Chem., 1965, Vol. 27, pp. 405 to 409. PergamonPress Ltd. Printedin NorthernIreland

STUDIES ON PROTACTINIUM (V) IN SULPHURIC ACID SOLUTION--I CENTRIFUGATION STUDY J. TAKAGI and H. SHIMOJIMA NAIG Nuclear Research Laboratory, Suehirocho, Kawasaki, Japan

(Recewed 10 May 1964)

Abstract--Centrifugation experiments were carried out to investigate the hydrolytic behaviour of protactinium in sulphuric acid solution. While protactinium at a concentration of l0 -e M was stable against hydrolysis for at least two weeks in 0.2-0.5 M sulphuric acid solution; polymeric species, possibly formed by hydrolysis, were observed for tracer concentration (10-11 M) protactinium in 0-015-0-5 M sulphuric acid. For protactinium at tracer concentration the results suggest that polymeric species with an approximate radius of 10-15/~ are in equilibrium with ionic or molecular species. IT IS well known that protactinium does not exist in the form of ionic or molecular monomeric species even in moderately acidic solutions of ordinary inorganic acids except hydrofluoric acid which is known to give stable solutions.~l, 2~ MADDOCK and MILES(3) reported that a considerable amount of protactinium is removed on centrifuging tracer concentration solutions in hydrochloric or nitric acid less than 3 M. Although sulphuric acid seems to be a good solvent of protactinium, comparatively little data has been reported on the hydrolytic behaviour of thin solutions. BROWN et al. ~4~ studied the properties of protactinium in sulphuric acid solution by spectrophotomeric, ion exchange and solvent extraction methods, and it was concluded that there is a very strong tendency for protactinium (V) to form anionic complexes in thin solutions. Their results suggested that for 10-4 ~ , 10-5 M protactinium solution even at 0.1 , ~ 0.5 M sulphuric acid hydrolysis occurs to only a small extent, if at all. Such behaviour seems to be, as was pointed out in their paper, in remarkable contrast to the behaviour of protactinium in hydrochloric,~a, 5) nitric ~a,6) or perchloric acid/7) The present work was carried out to investigate by the centrifugation method the hydrolytic behaviour of protactinium in sulphuric acid solutions. EXPERIMENTAL

Purification of protactinium I n m o s t of the experimenls protactinium --231 at a concentration of about 10 -6 M and protactinium --233 at a concentration of about 10-11 M were used. ~atPa205 purchased from Amersham or neutron irradiated thorium nitrate (~3sPa) was first dissolved in concentrated nitric acid with a ~1~ G. BOUISSIERESand M. HAISSINSKY,Bull. Soe. Chim. France 18, 557 (1952). c~ K. A. KRAUS,Proceedings of the International Conference on the Peaceful Uses of Atomic Energy Geneva, 1955. Vol. 7, 245. United Nations (1956). ca~ A. G. MADDOeK and G. L. MILES, J. Chem. Soc. 248, Supple. Issue No. 2 (1949). ~ D. BROWN; T. SATO, A. J. SMITH and R. G. WILKINS, J. Inorg. Nuel. Chem. 23, 91 (1961). ~5~A. G. GOBLE and A. G. MADDOCK,J. Inorg. Nuel. Chem. 7, 94 (1958). ~6~C. J. HARDY; D. SCARGILL and J. M. FLETCHER, J. lnorg. Nuel. Chem. 7, 257 (1958). c7~ G. A. WELCH, Nature, Lond. 172, 458 (1953). 405

406

J. TAKAGIand H. SHIMOJIMA

few drops o f hydrofluoric acid and was then coprecipitated with MnO2c*~in the presence of excess AP +. The precipitation was dissolved in concentrated hydrochloric acid and protactinium was extracted from 8N HC1 solution into di-isobutylketone. ~9~ The extracted protactinium was stripped back into 2N HCI and another two cycles of extraction and stripping were performed. Then protactinium hydroxide was precipitated by making the final stripping solution slightly alkaline with redistilled ammonium hydroxide and was centrifuged out. This precipitate was dissolved in 5 M sulphuric acid, from which the hydroxide was precipitated again, centrifuged and washed carefully with hot redistilled water. The precipitate was finally dissolved in 5 M H~SOo which was kept in a polyethylene bottle. Analytical grade reagents were used except di-isobutylketone, which was available only in technical grade and was used after careful washing with 8N HCI. Redistilled water was used throughout the work. The puttied mPa and 2ssPa were proved to be radiochemically pure by ~- and ~,-s.pectrometry for the former, and by ~,-spectrometry and half life measurement for the latter.

Centrifugation Aliquots of the stock solution were centrifuged for 30 min at a velocity of 14,000 rev/min in polyethylene tubes and the supernatants were diluted to the desired protactinium and acid concentrations. These solutions which served as the sample solutions were stocked in polyethylene bottles. Each 10 ml of the sample solutions was taken in a polyethylene centrifuge tube and centrifuged at a velocity of 14,000 rev/min (~25,000 g). The centrifugation was continued for 20 min, unless otherwise mentioned. The per cent removal on centrifuging (or the sedimentation per cent) was determined from the difference between the activities of the supernatants of the centrifuged samples and those of the reference solutions, which had been taken from the same sample solutions and kept standing, without centrifugation, in the same centrifuge tubes and for the same length of time as the centrifuged samples. The activity of 2sxPa samples was counted by 2*r gas flow counter accompanied by ~t-spectrometry with a silicon detector, while that of ~33Pa samples was counted by a well-typed scintillation detector. Each point in the figure or each value in the tables is an average of at least three experiments. While the reproducibility was :l:1"5 per cent for the experiments where the pH of the solutions was below two, the results became less reproducible when the pH exceeded two. RESULTS AND DISCUSSION

A. Variation o f sedimentation with acid concentration and storage time Sample solutions o f various sulphuric acid concentrations were centrifuged after various periods o f storage after the preparation o f the solutions. Fig. 1 shows the results. Since the diffusion-back o f the centrifuged particle does n o t seem entirely negligible, the results obtained in the present w o r k m a y by no means give the absolute values for the a m o u n t o f centrifugable particles i n the solutions. It is believed, however, that qualitative data on the polymeric species can be obtained. Generally, the a m o u n t o f sedimentation decreased with increase in the sulphuric acid concentration. Remarkable differences, however, can be seen between the samples at the two different protactinium concentrations. (i) While a few per cent o f protactinium was removed f r o m even 0.5 M sulphuric acid solution o f 10-11 M protactinium, no removal was observed from 10-s M protactinium solution in sulphuric acid above 0.2 M. (ii) Sedimentation o f tracer concentration protactinium (~*Pa 10-11 M) reached equilibrium in a b o u t three days in contrast to that o f protactinium at a much higher concentration (sSlpa 10-6 M) where no such equilibrium was observed. ~a, L. I. KATZ1Nand R. W. STOUGHTON,J. Inorg. NucL Chem. 3, 299 (1956). ~9~C. FERmERADE MIRANDAand A. G. MADDOCK,d. Inorff. NucLChem. 44, 1623 (1962).

Studies on protactinium (V) in sulphuric acid solution--I

407

B. Variation of sedimentation with centrifugation time The variation of sedimentation with centrifugation time for the solutions of protactinium at a tracer concentration in 0.015 M and 0-15 M sulphuric acid was examined, and it was found that the sedimentation reached constant values after 20 ~-~ 30 min centrifugation, in both cases. This result in in good agreement with that of MADDOCK and MILES{a) for hydrochloric and nitric acid solutions. The radius 25

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F~. l.--Variation of sedimentation with the concentration of sulphuric acid and storage time. solid lines, 23aPa0"7 × 10-11 M dotted lines, *31Pa 1.0 × 10 -e M (O 0.015 M, /~ 0.05, M, O 0.15M, × 0-25 M, • 0-50 M) R of the particles of density p centrifuged from a solution of viscosity B and density P0 at a rotation angular velocity o~ in the course of time t is given by R2 _. 9~7 In

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2~°2t(P -- Po) where x 1 and xz are the distances of the particles from the centre of the rotation before and after centrifugation respectively. For the present centrifugation conditions, if we assume p - P0 ~ 10 g/cm ~, R is calculated to be al~proximately 10A ~ R ~ 15A. Varying the centrifugation velocity gave the similar results, this value of R will give the approximate radius of the polymeric species formed when the protactinium concentration is low. Similar experiments with respect to 10-6 M

408

J. TAKAGIand H. SHIMOJIMA

sa1pa in 0.015 M sulphuric acid did not give reproducible results. But the analysis of the data suggested that somewhat larger aggregates were formed. C. Dependence o f sedimentation on protactinium concentration

Solutions of various protactinium concentrations in 0.015 and 0.15 M sulphuric acid were centrifuged after 3 days' storage. As Table 1 shows, the amount of sedimentation increased generally as the concentration of protactinium decreased, TABLE 1.---~EPENDENCE OF SEDIMENTATION(~o) ON PROTACTINIUM CONCENTRATION

Protactinium concentration

H~SO4 (M)

0"8 × 10-11M*

0"7 × 10 9 Mt

0.015 0"15

12'3 4'I

8'8 2"3

* ~33Pa

T ~31Pa % ~ssPa(tracer)

1'1

× 10 -s M~f

6'6 0

1 x 10 -~ M~:

11.5 0

;~23tPa

although an increase of sedimentation was observed for 10-6 M protactinium in 0.015 M sulphuric acid. From the results of A, B and C, it is believed that protactinium at tracer concentration exists in 0.015 ,-~ 0 . 5 M sulphuric acid solution partially as polynuclear species which may be formed by hydrolysis, and partially as ionic or molecular species. The result of CSuggests that the amount of these polymeric species increases as the protactinium concentration decreases. The estimated radius 10 ~ 15 A, implying the polymerization number of about 10, is almost as large as the reported radiocolloids of the other elements31°) Protactinium at higher concentrations (10-s ~ 10-6 M) is believed to be Stable for at least two weeks in sulphuric acid solution over 0.2 M in good coincidence with the observation by BROWN et al. <4> Polymerization may occur in solutions of still higher protactinium concentration ( > 10-5 M), as was recently pointed out by CAMPBELL, (11) who suggested the existence of polymeric molecules in sulphuric acid solutions over 0.5 M from the analysis of his solvent extraction data. It seems, however, that their properties differ much from those of the aforementioned hydrolytic species formed at tracer concentrations of protactinium. The observed increasing sedimentation with increasing storage time (Fig. 1) of rather large particles in the case of 10-6 M protactinium in sulphuric acid solution less than 0.05 M might be due to the precipitation caused by the hydrolytic polymerization of sulphate complexes. A similar tendency of protactinium in aged hydrochloric acid solution has been reported and discussed by CASEY and MADDOCKf1121 F r o m the knowledge so far available (13) the most probable species in dilute acidic solution of sulphuric acid (0.1 ~-~ 0.5 M) may be the anionic sulphate complex of charge -- 1 possibly of the form PaO~(SO4)- or Pa(SO4)2(OH)2-, which will on further dilution (0.015 ~ 0 - 0 5 M) slowly hydrolyse to give precipitation. The result of C also appears to be explained in this way. tl0) j. SCHUBERTand E. E. CONN,Nucleonics 4, No. 6, 2 (1949). till D. O. CAMPBELL,Proceeding Protactinium Chemistry Symposium, Gatlinbur,g, 1963. T.I.D.-7675, 87 (1964). ~1~)A. T. CASEYand A. G. MADDOCK,J. Inorff. NucL Chem. 10, 58 (1959). (1~)A. T. CASEYand A. G. MADDOCK,J. lnorg. NucL Chem. 10, 289 (1959).

Studies on protactinium (V) in sulphuric acid solution--I

409

On the other hand, for protactinium at tracer concentration it was found by the following experiment that fast precipitation occurs in more dilute acidic solutions. 0.7 x 10 -11 M protactinium solutions with 15H ranging 1.5-5 and with various sulphate concentrations were centrifuged immediately after preparation, the pH was adjusted by redistilled ammonium hydroxide. The results are shown in Table 2. TABLE 2.--SEDIMENTATION (~o) OF TRACER CONCENTRATION PROTACTINIUM FROM DILUTE ACIDIC SOLUTIONS

pH Sulphate concentration (M)

1-5

30

4.0

5-0

0"025 0"10 0"50

12 8 5

22 15 t0

50 42 13

60 60 24

A difference in the precipitation behaviour is seen between the solutions of low sulphate concentrations (0-025 and 0.10 M) and the higher one (0-50 M), indicating different types of precipitation. While the precipitation at lower sulphate concentrations seems similar to that observed for 10-6 M protactinium in sulphuric acid solutions less than 0.05 M, it might reasonably be postulated that the precipitate at higher sulphate concentration is some salt of sulphatoprotactinate, of which a sparingly soluble potassium salt is known. ~14~ The results suggest that protactinium (V) in sulphuric acid solution is more stable than in hydrochloric or nitric acid solution.

Acknowledgement--The authors wish to thank Mr. S. KEN~OCnIof this laboratory for assistance in experimental works. ~14~A. G. MADDOC~and C. MIRANDA. Unpublished work (1958). Cited from (13) and (3).