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prior to the issuance date of this Micro-
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ORN - ACC - OFFICIAL
ORNA-p-ang
Dries
FEB 11 965 ASTER COPY
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FEB 1 1 1965
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STUDIES OF THE BEHAVIOR OF PROTACTINIUM IN SULFURIC ACID
D. O. Campbell
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wwmaran ACTION,
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January 8, 1965


APPROVED FOR PUBLIC RELEASE
This paper was submittod for publication
in the open Literature at least months
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OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
Operated by
UNION CARBIDE NUCLEAR CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION
*Research sponsored by the U. S. Atomic Energy Commission under contract with the
Union Carbide Corporation.
A paper to be presented at the Colloquium on The Physical Chemistry of Protactinium
sponsored by Centre National de la Recherche Scientifique.
ORNI - AEC - OFFICIAL

Summary
ORNI - ACC - OFFICIAL
• ORNI - AEC
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Our studies of the chemistry of protactinium in sulfuric acid solutions have
continued, utilizing primarily tho solvent extraction method. Emphasis has been
placed on the behavior at relatively high protactinium concentrations, up to several
mg/ml. Data is reported for extractions into 0.03 M trilaurylamine from 2.5, 3.2,
and 4.17 M sulfuric acid. In all cases a reasonably constant extraction coefficient is
observed if the protactinium concentration in the aquaous phase is below about 0.004
mg/ml, but at higher concentrations extraction increases sharply. Above 0.01 mg/ml
consistont extraction coefficionts are observed for the higher acid concentrations, but
for 2.5 M acid results are not reproducible. kates of extraction are very slow for the
higlier protactinium concentrations. The change in extraction behavior with protac-
tinium concentration is interpreted in terms of the existence of several species which
are probably polymeric in protactinium. At least two of these are extractable, but
the species formed at higher protactinium concentrations are inextractable although ~
they reach equilibrium with the eretractable species vary slowly.
Résumé
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Nous avons continué nos études de la chimie du protectinium dans les solutions
d'acide sulfurique en utilisant principalement la méthode d'extraction par les solvents.
Nous avons prêté une attention particulière au comportement des solutions à concen-
trations de protactinium relativement élevées (jusqu'à plusieurs mg/ml). Les resultats
indiqués se rapportent aux extractions par une solution 0,03 M. de triaurylamine à
partir de solutions 2,5, 3,2 et 4,17 M. d'acide sulfurique. Dans tous les cas on remarque
que le coefficient d'extraction est à peu près constant quand la concentration de la
phase aqueuse est inférieure à environ 0,004 mg/ml. L'extraction s'accroit rapidement
pour les concentrations plus élevées. Au dessus de 0,01 mg/ml or remarque que les co-
efficients d'extraction se maintiennent quand les concentrations d'acide sont élevées;
mais quand la concentration d'acide est 2,5 M les résultats ne peuvent pas être
reproduits. Les vitesses d'extraction sont très lentes pour les concentrations élévees en
protactinium. Le changement du caractère de l'extraction en fonction de la concen-
tration en protectinium est expliqué par l'existence de plusieurs composes polymeres du
protactinium. Au moins deux d'entre eux peuvent être extraits; mais les composes formes
aux concentrations élevées en protactinium sont pratiquement impossibles à séparer bien
qu'ils atteignant tres lentement un equilibre avec les composés séparables.
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'STUDIES OF THE BEHAVIOR OF PROTACTINIUM IN SULFURIC ACID
ORNL - AEC - OFFICIAL
Introduction
ORNI - AEC - OFFICIAL
Our studies of the chemistry of pentavalent protactinium in sulfuric acid solutions have
continued, utilizing primarily the solvent extraction method. Sulfuric acid was chosen for the
initial study because, aside from fluoride and organic complexes, sulfate complexes appear to
be the only ones that might be resistant to hydrolysis. Emphasis has been placed on behavior
at relatively high protactinium concentrations, up to a few milligrams per milliliter (mg/ml).
The results reported here are part of a continuing study, and they represent a progress
report more than a definitive or final statement. We hesitate to draw many conclusions at
this time, especially quantitative ones, because as the history of protactinium well illustrates,
unexpected and even unknown factors may exert a controlling influence on experimental be-
havior. Therefore, the proposals relative to species are qualitative and tentative, and they
are presented for the primary purpose of guiding further work. We expect to have more posi-
tive evidence in the future, either supporting or contradicting the proposals in this report.
At the previous protactinium symposium we reported distribution measurements between
sulfuric acid and both Dowex-1 anion exchange resin and trilaurylamine, which showed a
change in distribution behavior as the protactinium concentration was varied with all other
parameters held constant.' These observations led to the suggestion that at least one small
polymeric species of protactinium was formed reversibly in these solutions under appropriate
conditions, and this polymer is more extractable than the simpler species. The present paper
is concerned with further study of this phenomenon.
Experimental
Protactinium was purified by extraction from HCI-HF solution containing either aluminum
or boric acid to complex the fluoride. Dowex-1 resin and solvents such as di-isobutylcarbinol,
TTA, andamines have all been used as extractants. The extracted protactinium was stripped in-
to HCI-HF solution which was then scrubbed with diethylbenzene. It was then repeatedly pre-
cipitated with ammonium hydroxide and dissolved in sulfuric acid and water. The final solutions
were analyzed for protactinium and acid. Various dilutions with sulfuric acid of the same con-
centration were used for the extraction experiments. No significant difference in extraction
behavior of several such solutions has been observed. in all distribution experiments the organic
phase was 0.03 M trilaurylamine dissolved in diethylbenzene, and it was scrubbed before use
with sodium carbonate and sulfuric acid solutions.
The most extensive measurements were made with a stock solution containing protactinium
at 3.6 mg/ml (0.016 M) in 2.5 M H2SO4. After purification by extraction as described above,
niobium (if present) was removed by extraction into 6 M H2SO,-4 M HF, and boric acid was
added to complex fluoride before the first two precipitations. Finally, the hydroxide was washed
carefully with water, dissolved in 5 M H2SO4 and diluted to 2.50 M. The resulting solution was
somewhat turbid, but it was clear by the next day. During approximately four months there was
no evidence of precipitation in this stock solution, and extraction experiments started at different
times showed no effect of aging.
.
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For extractions from 3.2 and 4.17 M H2SO, the solutions were contained in 1/2-oz.-.
glass bottles with plastic caps containing a polyethylene disc liner to prevent contamination
of the solutions. They were mixed with a tumbler turning at 15 rpm. Some extraction ex-
periments with 2.5 Macid were mixed by hand shaking for a few minutes several times a day.
When it became obvious that rates were slow in some cases, with experiment extending for
months, these procedures were changed. For the rest of the experiments with 2.5 M acid, a
tumbler turning at 60 rpm was used to mix the phases and 15-ml glass-stoppered, glass centri-
fuge tubes were used to contain them. Glass-stoppered tubes were used because the organic
diluent, diethylbenzene, evaporates through polyethylene (used in the bottle caps), resulting
in a slow decrease in volume and increase in amine concentration in the organic phase. All
experiments were at room temperature.
r
i
Stirniekstrinum vinteresting v
Analyses were made by withdrawing a sample with a micropipet, placing the sample di-
rectly on a counting plate (stainless steel for organic samples, tantalum for aqueous), drying
and "flaming" the plate, and finally determining protectinium by alpha counting and alpha
pulse analysis. For high-activity samples the pulse analyzer was used as a low geon:erty counter.
All protactinium solutions after purification analyzed greater than 98% 23 1 Pa by alpha pulse
analysis. Activity in the organic solutions was always essentially, 100% 231 Pa, but the aqueous
solutions generally had variable amounts of daughter activity, depending on the age and the
magnitude of the extraction coefficient.
Results and Discussion
Neunkielineet.
Distribution Measurements with 2.5 M Sulfuric Acid
in
Numerous measurements of the distribution of protactinium between 2.5 M sulfuric acid
and 0.03 M trilaurylamine are summarized in Fig. l; all points represent equilibrium measure-
ments. Points for lower concentrations (open circles) were reported at the previous protactinium
chemistry symposium.' The study has been extended to higher protactinium concentrations; and
the data include results of stripping experiments as well as extractions, and some kinetic studies
for cases in which reactions were slow.
The phase volume ratio was varied in the first set of experiments to (o/a in ml) 0.5/1,
1/1, 2/1, and 5/1 (v points, Fig. 1). Two facts became obvious during these measurements:
the extraction coefficients increased for long periods, at least several weeks, and the total
amount of protactinium extracted during the first few weeks was nearly the same in the four ex-
periments. The fraction of protactinium extracted tended to be constant at 30 to 50% after a
month, but solvent evaporation made the results at lower phase ratios of little use after 40 days.
Subsequently, extraction increased very slowly to a little over 50% for the highest phase ratio.
These results suggest a slow equilibrium between extractable and relatively inextractable
species, and that most of the protactinium in these aqueous solutions was initially in an inex-
tractable form which is slowly converted to extractable species. Throughout this report the term
"inextractable" will mean having an extraction coefficient small compared to that of other
species in the same solution.
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Extraction from dilutions of the stock solution was studied, using the original stock solu-
tion and two successive five-fold dilutions, with a phase volume ratio (o/a) of 3/2 (A points,
Fig. 1). The stock solution gave results in substantial agreeme.. with those discussed above.
The first five-fold dilution similarly gave a slo: increase in extraction. At equilibrium the
protactinium concentration in the organic phase is less than in the first case by a factor of 2,
but in the aqueous it is less by a factor of 12. With the second dilution, 0.14 mg/ml, all the
protactinium is converted to the extractable form in a much shorter time. At equilibrium the
concentration of extracted protactinium varied only a factor of 5 for the three experiments,
but the aqueous concentration varied a factor of nearly 500. Since the amount extracted and
the rate of extraction are both dependent on the protactinium concentration, it is likely that
the inextractable species contain polymerized protectinium, and that equilibria between the
polymers are slow.
The rate of extraction was investigated more carefully, using 2 ml of the stock solution
and 2 ml of organic (Fig. 4). The phases were repeatedly mixed for specified periods of time,
centrifuged and sampled. After 1 minute of mixing (and 1 minute for sampling) 0.08 mg/ml
had extracted. After a second minute of mixing (total elapsed time 7 minutes) the extracted
protactinium concentration increased to 0.12 mg/ml; after 4 minutes total mixing time (14
after 3 hours; 0.8 mg/ml after 5 days; and finally about 1.1 mg/ml (vs 2.9 in the aqueous)
after 3 to 6 weeks (v point, Fig. 1). After the first hour, only intermittent mixing was used.
These results permit estimation of the concentrations of the protactinium species in the
stock solution. The 0.08 mg/ml extracted after 1 minute represents an upper limit to the con-
centration of protactinium initially in the extractable form. The data cannot be extrapolated
back to zero time with any confidence, but one might estimate a value of 0.02 to 0.04 mg/ml
for the extraction at zero time, which might represent the concentration of protactinium in the
extractable form in the stock solution. The increase in extraction with time is probably a
measure of the rate at which inextractable species are converted to extractable ones. Thus,
approximately 0.1 mg/ml additional protactinium (0.15 mg/mi total) is extracted in the first
few minutes, so this represents the amount present in rapid equilibrium (minutes) with the ex-
tractable species. An additional 0.2 mg/ml is in equilibrium in hours, 0.5 mg/ml in a few
days, and so forth.
In the preceding experiment, after six weeks there was some indication that extraction
increased again. The material balance decreased slightly, and a precipitate that resembled
protactinium hydroxide appeared. After 60 days, both the organic and aqueous phases were
diluted by about 50% (with 2.5 M acid and 0.03 M amine). The extraction increased sharply
to 1.8 mg/ml in the organic phase vs 0.65 mg/ml in the aqueous. Subsequently, extraction
decreased very slowly. This increase in extraction upon dilution of one or both phases has
been observed several times. An explanation for this is not obvious, but perhaps dilution
could cause polymers or perhaps even colloidal or precipitated material to break down into
simpler species.
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Following a number of experiments, the organic and aqueous phases were separated and
contacted with a new voluine of the 07::osile phase to measure second stage extraction in one
case and stripping in the other. Resuits show that the two phases contain protactinium that be-
haves in an entirely different manner. The second extraction was, in general, very similar to
the first, with extraction increasing slowly for some days to typical values of 0.8 mg/ml in the
organic vs 1.4 in the aqueous (x points, Fig. 1). This indicates that the protactinium species
remain in equilibrium.
ope
Upon stepwise dilution of the organic phase (with 0.03 M ino) the extracted protac-
tinium concentration decreased only slightly to, typically, 0.6 mg/ml, while at the same time
the aqueous protactinium concentration decrecsed siecdily to a final value of 0.02 to 0.03
mg/ml, a factor of about 50 ( points, Fig. 1). !n sclercl tests the organic phese protactinium
concentration was relatively constant at 0.3 to 0.3 mg/mol, while aqueous phase concer inations
varied anywhere from 0.01 to more than 1 mg/ml. As the solutions were diluted the distribution
was siow, and extracti 7 coefficients increased from about 0.5 to 2 until the protactinium con-
centration in the aqueous phase reached about 0.2 mg/ml. At this point the behavior changed;
the concentration in the aqueous phase continued to drop by an order of magnitude and the ex-
traction coefficient, E., increase to over 20. In addition, equilibrium was reached more rapidly-
in about one day.
In one test 2 ml of both solutions were vigorously mixed for several minutes and then the
aqueous phase was separated and mixed with anew organic phase. The concentration of protac-
tinium extracted in the first case was 0.15 mg/ml, and in the second it was 0.087 mg/ml. Arter
1 day with occasional shaking the concentration rose to 0.3 mg/ml for the second extraction.
The aqueous phase was again removed and contccted with a fresh organic, and the amount of
protactinium extracted was 0.10 mg/ml after 1 day, 0.19 mg/ml after 2 days, and 0.6 to 0.8
after several weeks (vs 2.4 in the aqueous phase, down from 3.6 mg/ml, initially). The initial
cmount extracted was smaller for the second extraction than the firsi, and extraction after 1 day
was less for the third than the second. This suggests that factors causing the slow extraction
exist in the aqueous phase rather than the organic. If reactions in the organic phase were con-
trolling, then the extraction rates for all three successive extractions should be approximately
the same; but they were not. Instead, the cumulative amount of protactinium extracted in-
creased with total exposure time in good agreement with Fig. 4. This supports the proposal that
slow equilibria between inextractable species and at least one extractable species in the aqueous
phase control the rate of extraction. However, it certainly does not rule out the possibility that
behavior is not simple in the organic phase.
Stripping experiments gave results entirely different from extractions (o points, Fig. 1).
Equilibrium aqueous phase concentrations were 100-fold less in. the strip than in the original
extractions. Equilibrium was not reached in a few minutes, but it appeared to be essentially
complete in a few hours. Dilution of the organic phase (constant amine concentration) resulted
in a re-extraction of part of the stripped protactinium, and most of the measurements fell on or
near a constant E, of 30 (see Fig. 1), at least for aqueous phase concentrations from 0.027 to
0.03 mg/ml. At both higher and lower concentrations where other species may be present, lower
extraction coefficients were observed.
ORNL - AEC - OFFICIAL
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Li
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Since these experiments gave a constant E. of 30 over a fairly large ccocenticrico
range, they may well represent the distribution of o single species, ihe one we have been
calling the "extrac:cble species". At higher concentrations, the concent.crien in inte
aqueous phase is high enough that the more complex species (inextractable) bugin to form,
so Eo decreasus. At the lower concentrations, E: also decreases because the aqueous con-
centration is low enough that the "simple species" becomes significant, and it has a lower
extraction coefficient (about 1).
it!;;!• 338 .30
Over the vide range of protactinium concentration (aqueous phase) freioa 0.0! :0 over
1 mg/ml, the equilibrium organic concentration is relatively constant at 0.3 ro 0.E ::.:/sné.
If the extractable species is predominant over this range, then its conceni:ctica ia ine coue-
ous phase increases only from 0.01 to about 0.03 mg/ml (based on E: = 30). This uspuf:
ure, 0.03 mg/ml, represents the approximate uppe: limit of the conceniration of the cäirccicble
species in aqueous solutions containing up to 1 ng/ml of protactinium. This value is in scod
agreement with the figure derived from kinetic data. The rest of the prciccrinium (abova 0.03
mg/ml) exists as inextractable species in slow equilibrium with the extraciebie one, cnc, since
their formation is dependent on protactinium concentration, ihey are probably polymeric in
protactinium.
In some experime...s with smoll organic/aqueous phase ratios extracted proccinium
concentrations have exceeded 1 mg/ml, but these have always been asscciaied with such
aqueous phase protactinium concentrations, generally greater than 2 mg/mol at equilis.iu.in,
or they have occurred after dilution of one of the phases, which sometimes caused odd be-
havior. Extraction results in this concentration range were not consistent (see Fig. 1), with
organic concentrations varying anywhere from 0.4 to 1.2 mg/ml. Two strip experiments are
shown for high organic concentrations.
The scatter in the points of Fig. I for high protactinium cor.ceniictions and is cservo-
tion that the orgoric phase concentration varies much less than the aqueous could cisc be
explained by looding the amine solution to near its capacity for protactinivin insteca of by
formation of inextractable polymers as proposed here. The organic phase proiectinic.in con-
centrations were about 0.003 M, only about one-tenth the amine concentration. In some
cases protactinium concentrations up to 0.01 M have been observed in the organic phase, in-
dicating the amine does have more capacity. However, the strongest evidence for the ex-
planation based upon polymer formation is derived from the kinetic experiments; it is difficult
to explain how solvent loading could cause the very slow extractions that were observed.
Initiclly no solids were present in any of the solutions, but the acuecus preses conicin-
ing more protactinium than about 1 mg/ml tended to break slowly arier mixing, anä с..:-
fugation was necessary to clarify them. After several days small amounts of piccieiinia ve:
visible after centrifuge: .. in most of the experiments, the precipitate having insin.iscic.ga
pearance of protactinium hydroxide. Samples containing some of the precipiicies showed that
they did contcin protactinium. In general, solutions having concentrations below 0.01 mg/ml
did not develop the precipitate, while those above 1 mg/ml ultimately contained suósicorial
amounts and, in some cases, a white interfacial materici also appeared. Precipitates also
isii ..:.:: 1:"05
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formed in strip tests in which none of the original stock solurion was presen:. Nonetheless,
in most cases, material balance were within ihre linii of volume measurement ar.d sampling
and counting errors, normally !0 iv 20%, so any loss of protectinium to precipitation was
relatively small.
O*NI-AIC-0!!!11!
In a few very long time tests at protectinium concentrations of several mg/ml there
appeared to be a consistent decrease in material balance after 6 to 8 weeks. This has general-
ly been followed by a recovery of the original bulance if the phases were diluted. The de-
crease amounted to perhaps 20%, and it occurred almost entirely as a drop in aqueous phase
concentration, the organic phase conceniration remaining nearly constant.
These observations raise a question about the solubility of protectiriin in dilute sulfuric
acid-if, indeed, there is a single solubility. limigi cepend 0.7 inc particular assortment of
polymeric (or even colloidal) materiai preseni in a given soluiica; coa inis, because of the
very slow rates of equilibrium, could depend on the history of the solution. It also appears
that when the protactinium concentration recches that at which we propose polymer forma-
tion (a few hundredths mg/ml), a condensation reaction may progress until insoluble mate.
rial appears; the "inextractable species" may not be stable to precipitc:ion. Brown, et al.,
reported a loss after 15 weeks of protactiniu.. from solutions at a protoctinium concentration
of 1.11 x 104+ M (0.025 mg/ml) if the ccid concentration was below 2.8 M. Thus, the solu-
bility of protactinium in 2.5 M sulfuric acid may be as low as a few hundredths mg/ml, much
less than indicated previously 1,3 but equilibrium is obtained in such systems extremely slowly.
All the foregoing observations fit reasonably we!! an extension of the proposal made
earlier that at low concentrations of protactinium there is a simple species (possibly more
than orie) which extracis in a normal manner, but that at sovi 0.004 mg/ml there is reversi-
ble formation of a small polymer which is much more extrcciable. These suggestions resulted
from extraction experiments from an aqueous solution initially 100 times more dilute than the
stock solurion used in the present experimenis. The experiments reported here suggest that
after the extractable species reies a concentration about 0.03 mg/ml in the aqueous phase,
still other species, probably larger polymers, form at higher concentrations, and that as the
protactinium concentration increases these new species are in slower equilibrium with the
extractable species.
The present stripping experiments overlap the earlier measurements' in the range 0.01
to 0.1 mg/ml in the organic phase, and in this range the present experiments give aqueous
protactinium concentrations a factor of 2 to 3 smaller. The results of some of these extraction
experiments, however, are in reasonably close agreement with an extrapolation of the earlier
ccta, a factor of three higher in organic protactinium concentration. Thus, the two sets
of experiments are reasonably consistent.
(::::..!
Re-excmination of the earlier data for this sarrje system' is of interest. At los protoc-
tinium concenirations in the coveous phase (2 x 10m* to 7 x 10-3 mg/ml) the extraction co-
efficient was constant at 0.5 for measurements made 10 :0 20 minutes after mixing. However,
after both 6 and 13 days the extraction increased ivi all but the lowest point, Eo increasing
ORAL-nic-OLLICIA
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uniformly from 0.5 at 2 x 10°4 mg/ml to 1.0 at 2 x 100 mg/ml; Es increased with the 0.33
power of the aqueous phase protactinium concentration. This change in extraction with time
and the variation of Er with cor.centration suggest that more than one species may be ex- ..
tracting.
With equilibrium aqueous phase protactinium concentrations about 10 mg/ml Ein-
creased sharply with protactinium concentration, and it increased with time for several days
but was essentially constant after 9 days (possibly less). A kinetic measurement with 4 ml of
each phase, the aqueous containing initially 0.04 mg/ml, gave 48% extraction after 2 minutes
mixing, 76% after 2 days, and an equilibrium value of 80% (20 days). Thus, extraction was
much more rapid than it was from the solution containing 3.6 mg/ml initial protactinium con-
centration (see Fig. 4).
Published spectrophotometric data support the proposal that several protactinium species
whose concentrations depend on the gross protactinium concentration are present. The data
show that Beer's law is not obeyed in the protectinium concentration range of 10-3 to 10-2
mg/ml in 2.2 M sulfuric acid.4 No data is available for higher concentrations. This observa-
tion indicates that the species present in solution change with the protactinium concentration.
In view of the known properties of the neighboring elements of protactinium, in both group
V and the actinide series, formation of polyroric species is to be expected. However, protac-
tinium concentration has not been treated as a variable in most previous work, nor has it been
held constant when the effects of other factors were under study. In view of these considera-
tions, interpretation of data reporting extraction coefficient as a function of sulfuric acid
concentration alone is open to question.
Certain assumptions are implicit in the discussion presented here. One of the most
critical is that behavior of protactinium in the organic phase is simple and straightforward;
thus, all departures from simple distribution behavior are the result of phenomena occurring
in the aqueous phase. The primary support for this assumption results from the experiment in-
volving three successive extractions of the same aqueous phase, which indicated that more
than one protactinium species is certainly present in the aqueous phase but did not rule out
complications in the organic. Without this simplifying assumption we can only draw con-
clusions about the relative average complexity (polymerization) of protactinium species in
the two phases.
Distribution Measurements with Higher Acid Concentrations
A series of extraction experiments was made with 4.17 M sulfuric acid solutions and
the same 0.03 Mamine solution (Fig. 2). The aqueous solutions differed from one another
only in the protactinium concentration, which varied from 2.8 x 10-4 to 0.27 mg/ml initially.
The phases were sampled after 1, 13, and 21 days. At the low end of the curve the points fall
on a straight line with slope 1.05. The extraction coefficient is very nearly constant at about
5. There is a sharp increase in slope at about 0.005 mg/ml, and equilibrium is reached rather
slowly because, for the points in this steep portion of the curve, the protactinium concentra-
tion of the aqueous phase decreased about a factor of 2 (extraction increased) for times longer
than one day. At higher protactinium concentrations a linear region with a slope of 2/3 is
observed. Es decreases with increasing protactinium concentration from 35 to 14. Here, there
is little change after 1 day.
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.
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A similar series of experiments with 3.2 M sulfuric acid and the same amine gave
somewhat different results (Fig. 3). At the low end...e points fit a straight line of slope
1.1; Es increased slowly with protactinium concentration from 13 to 17. Again, at about .
0.005 mg/ml in the aqueous phass there was a sharp increase in extraction, and this moved
to a lower aqueous phase protactinium concentration after 1 day. At higher concentrations
an apparently linear correlation is obtained with a slope slightly greater than 1; Es increases
from 80 to 100.
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Comparison of Results
w
The extraction behavior is similar in many respects for all three acid concentrations,
but the magnitude of the extractions differs in an unusual manner. At low concentrations, be-
low 0.004 mg/ml, Eo increases with protactinium concentration for 2.5 M acid, increases less
for 3.2 Macid, and is almost constant for 4.2 M acid. However, the values of E, are not at
all consistent, being about 1 for 2.5 Mucid, 15 for 3.2 M, and 5 for 4.2 M. Only the first
of these is even close to agreement with values in the literature, for a different but similar
amine;- the others are much higher.
For all three concentrations a sharp increase in extraction occurs at cbout 0.004 mg/ml
protactinium concentration in the aqueous phase. At higher protactinium concentrations there
is again no consistent trend. For 2.5 Macid reproducible extraction coefficients were not
obtained, but the value of 30 observed in stripping experiments might be appropriate. For
3.2 M acid a reasonably constant Eo of about 100 was obtained, with organic phase concen-
trations up to 2 mg/ml (30% of the cmine concentration). For 4.2 Macid, Es decreased with
the 1/3 power of the aqueous phase protactinium concentration, and the values ranged in the
vicinity of 25. The maximum concentration reached in this case was 1.4 mg/ml in the organic
vs 0.1 mg/ml in the aqueous. Thus, extraction coefficients were generally higher with 3.2 M
acid than with either 2.5 or 4.2 M.
The rather extreme contrast between ine scatter in the data for 2.5 M acid and the
linear correlation for 3.2 and 4.2 M suggests that a significant change in behavior may oc-
cur between 2.5 and 3.2 Min sulfuric acid concentration. However, aqueous phase protac-
tinium concentrations were not so high in the last two cases because of the high Eo. Brown,
et al., 2 reported changes in the absorption spectra and a slow loss of protactinium from solu-
tion at a protactinium concentration of 1.11 x 10~4 M (0.025 mg/ml) only if the sulfuric acid
concentration was below 2.8 M. Thus, between 2.5 and 3.2 M in sulfuric acid con
there may be a change in the protactinium complex ion species such that the species ....d
are more resistant to hydrolysis, which could eventually lead to condensation and precipi-
ration. The same change in species could cause the increased extraction from 3.2 Macid,
compared to 2.5, instead of the usual decrease in extraction at higher acid concentrations.
Species in Solution
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The data reported here are consistent with the proposal that in the range of sulfuric
acid concentration studied, protactiniu.n exists ci a simple, perhaps monomeric species, and
a series of more complex species which are probably polymers, the identity and concentration
of which depend on the protactinium concentration.
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11291:0-lii. Ü
In equilibria involving a series of successively larger polymers the coacerirarion of
each is related to the concentration of the next smaller one raised to a power sucter than
1. Therefore, to a first approximation, they appear consecutively as the concertation is
increased. Each polymer increases in concentration to an approximate upper limit before
the next larger one i formed, and then increases littie as the total concentration continues
to increase. By comparison with such a simplified picture, it is possible to summarize some
properties of the protactinium species present in 2.5 M sulfuric acid containing about 1 .
mg/ml of protactinium with the following tabulation.
Rate of Equilibration
probably fast
Species
simple
extractable polymer
in rapid equilibrium
in intermediate equilibrium,
in slow equilibrium
Concentration (mg Po/ml)
0.074
0.03
0.1
30
«30
Small
small
minutes
hours
days
~0.2
> 1
Each category may represent several different polymers, and they are classified qucliictively
according to the rate at which equilibrium with the extractable species is reached. Thi .
species in slow equilibrium with the extractable one may also condense to yield precipitated
protactinium, but such a reaction is very slow. With 3.2 and 4.2 M sulfuric acid is co-
scription of the system is probably similar, but measurements did not extend to such high
Conclusions
We wish to emphasize that this is a continuing study, and more deriniiive rasviis will
be available in the future. There are discrepancies between our results and sore Iliercture
data, but we believe that the behavior of protactinium in sulfuric acid solution is not so
simple as indicated in the literature and, in particular, that conclusions must not be genera-
lized or observations extrapolated to conditions other than those of the experimerits. Furiherd
we believe, on the basis of our observations and the inapplicability of Beer's law,s iriat dif-
ferent species exist as a function of protactinium concentration, and iherefore that they are
probably polymeric in protactinium.
In 2.5 M sulfuric acid containing protactinium at less than 0.004 mg/ml extracrion
coefficients are very nearly constant, so the complexity of the protactinium species is iha
same in both phases; probably a simple, monomeric species is dominant. At siishily higher
protactinium concentrations the extraction coefficient increases sharply, indicating that
extracted species are polymeric relative to the average protactinium species in the aqueous
coefficients decrease as protactinium concentration increases, indicaring increasing poiy-
merization in the aqueous phase. Kinetic measurements support the proposal of exiensive
polymerization of protactinium species at high concentrations in the aqueous phase, and in-
dicate a slow equilibrium between these species.
0...!!...-0:71c!.il
The same general behavior is ob. ved at higher sulfuric acid concentrations, but
extractability is higher from 3.2 Macid than from either 2.5 or 4.17 Ni acid, iridicating
that there may be a change in the dominant protactinium - sulfate complex species.
täh
.
.
17101:::........:
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Reforences
1. D. Campbell, Proceedings of the Protactinium Chemistry Symposium, TID-7675,
p 87 (1963).
2. D. Brown, T. Sato, A. Smith, and R. Wilkins, J. Inorg. Nucl. Chem., 23, 91 (1961).
3. H. Kirby, The Radiochemistry of Protactinium, NAS-NS-3016, p 8 (1959).
4. R. Guillaumont, R. Muxart, G. Bouissions, and M. Haisinsky, 'Compt. Rond., 248,
3298 (1959).
:'(-0.71.1..1
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0:..:(
witsjo - 530-INIO
IVIDISJO-ON-IN
-
log
ORNL DWG 64-11364
'INITIAL
AQUEOUS PHASE
2 ml 0.05 M TLA
2 ml 2.5 M H2SO4
108
-al mg Po/mi
PROTACTINIUM CONCENTRATION (d/m/ml)
O MIXING TIME
• TOTAL TIME
ORGANIC PHASE
106
-
to
10
100
30
70

MINUTES
DAYS
Fig. 4. Rate of Extraction of Protactinium.
*.
4.
.
DATE FILMED
93 65