August, 1957
ISOTOPIC EXCHANGE BETWEEN Pu(IJ.1) AND Pu(IV)
1117
ISOTOPIC EXCHANGE BETWEEN Pu(II1) AND Pu(1V)' BY THOMAS K. KEENAN Los Alamos ScientiJic Laboratory, Los Alamos, New Mexico Received April 80, 1967
The rate of exchange between Pu(II1) and Pu(IV) in aqueous perchlorate media has been investigated. The exchange is rapid so that it was necessary t o work in the concentration range of 10-8 to 10-6ftotal plutonium. Satisfactory separation techniques were developed for this concentration region. Over the temperature range 0 to 25', a t ionic strength 2.00, the rate law may be approximated: rate (mole-liter-'-rnin.-') = 1.8 X 10LO[ P u + + + [] P u + + + + ]exp( -7700/RT) 1.3 X 108 [Pu+++][Pu(OH)+++]exp(-2800/RT). The rate increases with increasing ionic strength.
+
Introduction
Any trivalent actinide impurities were not absorbed and
5.150 Mev.,l3 t i / * = .24,360 years," was used for the nontracer isotope in this study. The PuZ39 used contained small amounts of Pu238, Puzro and Pu24*. However, appropriate isotopic mass analyses were carried out following the purification procedures described below. A specific activity value (a-disintegrations/minute/microgram total plutonium) waR then calculated. Tracer Isotope.-The plutonium isotope PuZ38, Ea = 5.495 R!Iev.,l6 = 86.4 years,16 was used as the tracer isoLope. The energy difference between the alphas of the tracer and non-tracer plut,onium was detected readily on the instrument used i n this study. Plutonium Purification.-All plutonium (both tracer and lion-tracer) was separately purified by absorption of Pu(IV) from 12 f IICl onto the anion-exchange resin Dowex A-l.I7
Some of the purified PuzsS was combined with the purified Pu238 to form a tracer solution of suitable a-energy ratio. The tracer solution was 97.1% Pu239 by weight but was only 16.7% Puza9by activity, the remainder of the activity being due to Pu238. The plutonium of the tracer solution always entered the exchange reaction in the trivalent state. This was desired because tetravalent plutonium was separated from trivalent plutonium to follow the course of the react,iori. Therefore, it was more advantageous to watch the Pues8 tracer grow int,o the initially Pu288-free tetravalent' plut,onium. Trivalent plutonium was prepared hy hydrogen reduction of the stock d u t i o n in the presence of plat~iniim black19 before each exchange run. Completeness of redunLion was tested by a trial extraction of an aliquot, from 6 f HCl into 40% TBP/hexane analogous t,o the Pu(IV) test. Other Reagents.-Perchloric acid of "Analytical Reagent" grade was used without further purification. Lithium Derchlorate trihydrate was prepared by neutralization of AR lithium carbonate with perchloric acid. The compound was recrystallized from water, washed with alcohol and ether and dried in vacuo. Both chemical and microsconic analvses established the stoichiometrv of the comporhd as LkY04.3H20. Ordinary distilled water was redistilled from alkaline permanganate for use in all solutions. Following the usual cleaning with detergents, all glassware was boiled in aqua regia p6or t o use. T h i s treatment was necessary to remove final traces of detergents, which, if present even in trace quantities, interfered with the separation of reactants. Following the treatment with aqua regia, the glassware was rinsed very thoroughly with the redistilled water and dried. Analysis.-The activity ratios of Puass and Puzaewere determined on a 20-channel alpha energy analyzer with an energy discrimination of ca. 50 kev./channel. This allowed
through the resin column. The plutonium was deExchange reactions between simple cations have passed sorbed from the resin by elution with 0.1 f HCl. Following been the subject of considerable investigation in this treatment, both eluents were fumed repeatedly with recent years. Among reactions which have been concentrated HC10, to destroy all chloride. Preparation of Tetravalent Plutonium (Pu239).-Following studied are Fe(IIl-Fe(IIIl.2 V(IIl-V(IIIl.3 Cr- conversion to the perchlorate as described above, a portion (11)- cr(111) , 4 cb(iI)-cb(rii) ,6 Ce(rI1) ' - C ~ ( I V ) , ~ ~ ~ of the Pu239was reserved for use as the tetravalent or nonTl(1)-TI(111),*-lo and E u (11)-Eu (111). It tracer stock solution. The tetravalent plutonium was seemed interesting to extend such studies to the ele- freshly purified before each exchange run by extraction of a few micrograms of plut,onium from 0.5 f HCIOa into 0.l.f ment plutonium which has well-defined tri- and thenoyltrifluoroacetone (TTA) in benzene.'* The organic tetravalent states. The author has reported pre- fraction was scrubbed with fresh 0.5 f HC104 and the plutoviously12 the rapid but measurable exchange be- nium back extracted into 6fHCIOI. This aqueousphase was tween Pu(II1) and Pu(1V) a t 0" in 0.5 f HC104. then scrubbed with benzene. From this perchlorate solution, aliquots were removed to form the tetravalent PlutoA bimolecular mechanism was postulated for the nium stock solutions. reaction with a specific rate constant of ca. lo4 The presence of tetravalent plutonium in the exchange f-'-min.-' a t the stated conditions. I n the pres- solution was tested immediately before the exchange exent paper, the reaction is described in more detail periment. A trial estraction was carried out from 6 f HCl as the parameters of hydrogen-ion concentration into 40% TBPjhexane (see below). If the KD for the SYStern was in approxiinate agreement with that determined and temperature have been experimentally ex- previously for Pu(IV), the exchange experiment was comamined. pleted. The exchange stock solutions were so dilute, Le., 10-6 to 10-6fplutonium, that no other method was adequate Experimental for rapid verification of a valence state. Non-tracer Isotope.-The plutonium isotope PtP9,Ea = Preparation of Trivalent (Tracer) Plutonium (PP8>.-
( 1 ) (a) This work was sponsored by the United States Atomic Energy Commission. (b) Presented a t the 131at meeting of the American Chemical Society, Miami, Florida, April 9, 1957. (2) J. Silverman and R . W. Dodson, T H I SJOURNAL, 66, 846 (1952). (31 W. R. King and C . 6. Garner, J . A m . Chem. Soc., 74, 3709 (1952). (4) (a) A. Anderson and N . A. Bonner, ibid., 76, 3826 (1954); (b) R. A. Plane and H. Taube, THISJOURNAL, 66, 33 (1952). (5) N. A. Bonner and J. P . Hunt, J . A m . Chem. Soc., 7 4 , 1866 (1952). (6) J. W. Gryder and R. W. Dodaon, ibid., 71, 1894 (1949). (7) F. R. Duke and F. R. Parchen, ibid., 78, 1540 (1956). (8) R. Prestwood and A. C . Wahl, ibid., 71, 3137 (1949). (9) G. Harbottle and R. W. Dodson. ibid., 73, 2442 (1951). ( 1 0 ) F. J. C. Rossotti, J . Inorg. NwE. Chem., 1, 159 (1955). 66, 853 (1952). (11) D. J. Meier and C . S. Garner, THISJOURNAL, (12) T. K. Keenan, J. A m . Chem. Soc.. 78, 2339 (1956). (13) F. Asaro and I. Perlman, Phys. Rev., 88, 828 (1952). (14) J. C . Wallman, UCRL-1255 (1951). (15) A. H. Jaffe and J. Lerner, ANL-4411 (1950). (16) D. C . Hoffman, G. P. Ford and F. 0. Lawrence, J . Inorg. Nuel. Chem., in press. ( 1 7 ) E. K. Hyde, Paper P/728, Proceedings of the International Clonference on the Peaceful Uses of Atomic Energy, Geneva, Switeerand, 1955, Vol. VII, p. 299.
(18) G. T. Seaborg and J. J. Kate (editors), "The Actinide Elements," NNES-IV-I4B, McGraw-Hill Book Co.. New York, N. Y., 1954, p. 210. (19) G. T. Seaborg and J. J. Katz (editors), "The Transuranium Elements," NNES-IV-l4B, McGraw-Hill Book Co., New York, N. Y., 1949, p. 168.
THOMAS K. KEENAN
1118
about 5 channels between energy eaks of Pu238 and PuaaO. The “vdley” between the two peafs was 51%of total peak heights. Reproducibility of the energy analyses was ca. 2%. This instrument has been described in more detail in another publication.*O The trivalent plutonium was initially 83.3% P U *as~noted ~ above, If diluted by an equal weight of tetravalent Puz39 during the course of an exchange reaction, the tetravalent plutonium grew from essentially 0%OB1 Puza8to an e uilibrium value of 71% Pusas. The actual equilibrium v&e varied with different metal ion concentrations. Total plutonium was determined radiometrically by evaporation and counting aliquots of each (111) and (IV) solution after an exchange run. Counting statistics were 5 1%. In the runs where lithium perchlorate was present, it was necessary to apply an empirically determined correction factor to the observed counting rate because of self-absorption of alphas in the lithium salt. Hydrogen-ion concentration was determined by neutralization of aliquots of the exchange stock solutions with standard base. Total perchlorate was determined by precipitation and weighing of (Ct.H&AsClOa from another aliquota2a Separation.-Separation of reactants was carried out by preferential extraction of Pu(1V) from 6 f HCl into 40 volume yo tributyl phosphate (TBP) in n-hexane. It was found that the K D for such a system was ca. 4 to 5 for Pu(IV) and ea. 0.002 for Pu(II1). Equilibration time for extraction was 15 seconds a t 0’. This separation procedure led to values of ca. 20% induced exchange. However, this amount was reproducible and was included in the calculations in the usual way. Direct evaporation of suitable aliquots of the T B P solutions on stainless steel plates gave no visible residue. Excellent resolution of the alpha energy peaks was obtained from such treatment. Procedure.-A constant temperature bath was used t o maintain temperatures a t 25 or 12.5 The experiments at 0” were carried out in an ice-bath. All pipets, flasks, etc., used in the exchange experiments were also maintained at the desired temperature. The separate Pu(II1) and Pu(1V) exchange solutions in the appropriate media were prepared in 5-ml. volumetric flasks and brought t o thermal equilibrium. The testing procedures for trivalent and tetravalent plutonium described above were then carried out. Aliquots of 200 microliters each of Pu(II1) and Pu(IV) were combined raDidls in a 3-ml. centrifuge tube bv means of micropipets and syringes. A timing-clock was activated when mixing was complete-usually ca. 1 second from the time mixing was begun. Fifty microliter samples were removed a t timed intervals from this mixture during the next few minutes. Every sample was discharged into a 1-ml. volumetric flask containing 0.5 ml. each 6 f HCl and 40% TBP/hexane. The flask containing the separation mixture was then shaken vigorously for 15 seconds and briefly centrifuged to separate the phases. The T B P layer was withdrawn and rapidly transferred to a clean tube. The dilution followed by immediate separation effectively quenched the exchange. This separation cycle lasted about 40 seconds for each sample and was completed before another sample was withdrawn. Five such samples were taken during one exchange run. Following the five samples, the exchanging solution was allowed to stand for ca. 1 hour a t which time aliquots were withdrawn to determine the isotopic ratio a t exchange equilibrium or “infinite-time.’’ Samples for a-energy analysis were prepared in duplicate from each of the “timed” samples and in triplicate from the “infinite-time” solution. Counting rates for the energy analysis samples were ca. 100 to 200 c./min. Treatment of Data.-The usual equation for evaluating for exchange data isZa
.
(20) C.W.Johnstone, NucEeonica. 11, 36 (1953). (21) A very small amount of Pus88 by activity wa8 associated with the tetravalent Pu389. The exact activity ratio waa carefully determined and the value used as a constant correction factor in all calculations. (22) (a) H. H. Willard and L. R. Perkins, Anal. Chem., 26, 1653 (1953); (b) H. H.Willard and G. M. Smith, Ind. En& Chem., A n d . Ed., 11, 305 (1939). (23) A. C.Wahl and N . A. Bonner (editors), “Radioactivity Applied t o Chemistry,” John Wiley and Sons, Inc., New York, N. Y.. 1951,pp. 9-10.
Vol. 61
ln(1
(“AB)
- F ) = ln(1 - Fo) - - Rt
where
P FO
fraction exchange at time t fraction induced exchange B = concentration of reactants = rate of exchange t =i time where one plots ln(1 F ) us. time. The same equation in exponential form for the plutonium exchange under discussion is
i,
= =i
-
Since F and 1 are experimentally measured, it was desirable to determine the quantity -(R)[((III) (IV))/(III)(IV)l directly from the exponential form. Such treatment would eliminate any undesired weighting factor introduced from conversion of the data to logarithms.2‘826 Therefore, all exchange data were evaluated in the exponential form using an IBM-704 computer.26 The standard deviation of the uantity also was determined. These deviations represent %e fit of the initial data and they were used as explicit weighting factors in subsequent operations.
+
Results Metal Ion Dependencies.-The runs to determine metal ion dependence were done at 0” in 0.5 f HC1O4. Table I lists the values obtained from experiments where Pu(II1) or Pu(1V) was varied. These dependencies were obtained by calculating the relationship between log R and log [variable]. The data were appropriately weightedzs because of the log-log transformation. TABLE I Meta! ion varied
No. of expts.
Pu(II1)
9
Pu(IV)
15
Range of variation
2.54 X 10-6to 2.26 X lO-s! 4.36 X 10-5 to 2.20 x 10-5.j
Dependence confidence limits)
(*SO%
+1.05
f0.19
+0.825 i0.227
Based on these determinations, the Pu(II1) and Pu(JV) dependencies were taken as unity for each species. Hydrogen Ion Dependence.-The effects of hydrogen-ion dependence and temperature were determined in solutions of ionic strength 2.00. Lithium perchlorate was used to maintain constant ionic strength while hydrogen ion concentration was varied. Theoretical considerations by Robinson and Stokesz7indicate that the mean ionic activity coefficient of perchloric acid is essentially constant as lithium ions replace hydrogen ions in solutions of constant total perchlorate.28 Hydrogen ion concentration variance was investigated over the range 0.4 to 2.0 f HC104. This range of hydrogen ion concentration was studied a t each of three temperatures: 25, 12.5 and 0.0’. When the data were assembled, an inverse hydrogen ion effect was noted a t each tem(24) W.R. McBride and D. 5. Villars, Anal. Chem., a6, 901 (1954). (25) A. G. Worthing and J. Creffner, “Treatment of Experimental Data,” John Wiley and SOUS,Inc., New York, N. Y . , 1943, pp. 246248. (26) The author is indebted to R. K. Zeigler, LASL, for his assistance in this operation. (27) R. A. Robinson and R. H. Stokes, “Electrolytic Solutions,” Academic Press. Inc., New York, N. Y., 1955,p. 249. (28) The author is indebted to J. S. Coleman, LASL, for pointing out t h e Robinson‘and Stokes treatment.
.
ISOTOPIC EXCHANGE BETWEEN Pu(II1)
August, 1957
perature. Such an observation suggests competing reactions of the form k
+ P u + + + + exchange k? P u + + + + Pu(OH)+++e exchange Pu+++
(1) (2)
-with pa% (2) favored at lower hydrogen ion conaentratiow because of the equilibrium Pu++++ + H20 J _ PLI(OH)+++ H + (3)
+
[Pu(OH)+++][H+] = [PU++++]
(4) 2G There is evidence which indicates that Pu(1II) is not hydrolyzed a t the experimental hydrogen ion concentrations employedz9 and therefore Pu(II1) hydrolysis was disregarded. It was assumed that the unit dependencies observed for the metal ions at 0" in 0.5 f HClOl were also valid at othm temperatures and hydrogen ion concentrations. One may therefore write R = kG"(III) (IV) or k' = R/(III) (IV) and develop k' IciH+/(Ka H f ) kzKI/(Ki H + ) ( 5 ) wheTe ICl and kz are the rate constants for the equation
+
Rate
=
+
VALUESOF k'
+
AND
1119
Pu(1V)
TABLE 111 H + AT VARIOUS TEMPERATTREY
Tabulated values of k' include standard deviations and all are t o be multiplied by lo4. Total plutonium was ca. 7 X l0-6fin all experiments. 25' 'k 3.54 10.17 3.64 1 .43 3.45* .22 3 , 9 0 * .68 4 . 8 7 i .50 6 . 5 8 1 .49 5.575 .89 7 . 9 9 5 .OO 7.39f .44 9.36 5 1.14 13.3 1.29 12.1 10.76 13.0 rt 4.8 16.5 rt0.72
*
12.5' E' 4.0650.26 2.69f .23 3 . 3 2 5 .46 3 . 2 0 1 .34 2.27i2.11 5.04i0.41 2.94f .25 3 . 7 7 5 .48 4 . 8 4 5 .40 5.57 i . I 9 4.48 f .l9 3.15 i . 2 5 9.01 rt 1.25 ,405 7.36f0.82 10.7 5 3 . 1 9.34 1 2 . 0 9 9.97f1.56 9,1210.70 6.94 f 0.47
H' 1.99 1.99 1.58 1.58 1.08 1.08 1.08 0.718 .718 .718 ,509 ,509 ,405
+
kl[Pu+++][ P u ~ + + + ] ke[Pu+++][Pu(OH)+++] (1) (111 (6)
AND
0.0.
H+
'L
2.07 2.07 2.07 2.12 2.12 2.12 1.61 1.61 1.04 1.04 1.04 0.771 .771 ,771
0.586f0.570 2.81 1 .43 1.66 1 .53 1.23 1 .46 1.95 f .42 3.50 f .GO 1.96 1 .14 2.11 i .24 3.27 5 .35 3.79 rt .33 3.18 f .19 3.68 5 33 4.25 rt . 4 1 5.34 f .33 3.45 f .84 3.86 f .24 4.69 i .38 3.85 f .16
,492
,492 ,423 ,423 .423
H+ 1.96 1.96 1.96 1.56 1C6 1.56 1.04 1.04 1.04 0.709 ,702 ,702 ,454
,454 ,454
413 ,413
,413
TABLE IV RATE CONSTANTS FOR
THE
PLI(III)-Pu(IV) EXCHANGE
SYSTEM t,
No. of
kl (1. mole-l-min.-l)
kz (1. mole-l-min.-')
oc. expts. (fstd. dev.) ( *std. dev.) Various assumptions involving the relative im0.13 zt 0.53 x 104 1 . 2 f 0 . 1 x 106 25 14 portance of path (I) or path (11) and the mag2 . 3 f 0 . 5 X lo4 0.70 f 0 . 1 8 X lo8 19 nitude of K1 were used in preliminary treatment of 12.5 18 1 . 1 f 0 . 4 X lo4 0.80 f 0 . 1 4 X 10, the data. It became obvious that K1 could not be 0 . 0 disregarded in the denominator of the right-hand TABLE V side of eq. 5 , also it appeared that both path (I) ACTIVATION ENERGIES FOR Pu(II1)-Pu( IV) EXCHANGE and path (11) contributed to the over-all rate. SYSTEM Therefore, some estimate of Kl, the hydrolysis Activation energy (kcal./inole) constant of Pu(IV), at the experimental conditions Reaction ( *std. dev.) pu + t +-pu + + ++ was necessary. The constant was measured in 7 . 7 zt 6 . 6 such media30and has the values given in Table 11. Pu+++-Pu(OH)+++ 2.8 f1 . 4 TABLE I1 J7YDROLYSIS C O N S T A N T O F PU(1V) I N p =
2.00 l?ERCHLORATN
MEDIA
,oc.
+
Ki
25
0.054 031 ,017
12 5
0 0
Une m y rewrite eq. 5 in the form k'[l
+ KI/H+] = ki + kzKi/H'
(7)
Therefare a straight-line relationship should hold between the quantity k'[l Kl/H+] and 1/H+. Sample plots were made to verify eq. 7. Table I11 lists the experimental data for each of the three temperatures. The values of k'[l KI/H+] were weighted by the square of the reciprocal of their standard deviation for the calculation. Table IV lists the results of evaluating the slopes and intercepts of the lines for the three temperatures. The large deviations indicate the magnitude of the difficulty of measurement in this rapid system. When appropriate weighting factors were introducedZ6and the relationships between log kl or log ICz and ( 1 / T ) were calculated, the following activation energies were obtained.
+
+
(29) Ref. 17,p. 308. .430) 8.W. Rabideau, LASL, submitted for publication.
The final expression for the rate may now be written: rate (mole-liter-l-min.-l) = 1.8 X 10'0[Pu++f][Pu++++]exp(-7700/RT) 1.3 X 108[Pu+++][Pu(OH)+++]exp(- 2800/RT). Ionic Strength Effects.-A few experiments were carried out to determine the effect of varying ionic strength on the exchange. It was assumed t'hat the effect would be considerable because of the highly charged nature of the reactants. The data from such experiments are shown in Table Fr1. TABLE VI H + = 0.500 f HCI04, 25" ionic strength adjust8rd witjIi Li'C1O4
P
2.00 1.01
0.67
k'
=
R / ( I I I ) ( I V ) (istd. dev.)
13 0 f 1 . 0 x 103 6 2 f 0 . 5 x 104 5 . 1 0 . 6 x 104
As can be seen, the effect is not as great as would be expected by yigorous application of the Br@nsted theory. The ionic strengths are much greater than those usually considered in the classical approaich. Thermodynamic Activation Values.-One may calculate the values of the free energy of activation. AF*, and the entropy of activation, AS*, from tho
0. J. KLEPPAAND M.
1120
experimental activation energy.31 Table V1I lists these quantities calculated from the experimental data. Because of the uncertainties in the experimental data, the activation quantities are probably reliable to about a factor of 2. TABLE VI1 THERMODYNAMIC ACTIVATION VALUES Reaction
p,, + ++-pu + + + + PLI+++-Pu( OH)+++
AEew
AS**=,
(e.u.)
(kcal./mole)
7.7 2.8
-31
18
(kcal./iriole)
-32
AFfxp
12
Discussion It is felt that the competing bimolecular reactions indicated in eq. 1 and 2 are the best possible representation of the plutonium exchange system a t the stated conditions. Except that the rate of exchange is much greater in this system, i.e., some 100-1000 times as fast as the Fe(I1)-Fe(II1) exchange, the plutonium reaction generally is analogous to others involving simple cations. Although the experimental data were treated in a statistically rigorous manner, the discrete values of the rate constants and energy values are somewhat uncertain. This rapid exchange system did not lend itself t o precise measurements. For example, at plutonium concentrations of Pu(II1) = Pu(1V) = 5 X 10-Efin 1fHCIOl a t 25", the halftime for exchange is 0.68 minute. So-called (31) S. Glasstone, I
