s. Jv. R s B I D E A U *&XI)B. J. ~IASTERS
318
Armstrong and W. G. Schlaffer for supplying many of the gels studied here. Additional thanks are due W. G.
Vol. 67
Schlaffer, H. H. Voge, and J. N. Wilson for stimulating discussions on these results.
OXYGEN EXCHANGE REACTIONS OF PLUTONIUM IONS IX SOLUTION' BY S. W. RABIDEAU ASD B. J. MASTERS Cniversity of Calijornia, Los Alamos Scientific Laboratory, Los Alamos, S e w Mexico Received J u l y 27, 1962 -4n acceleration of the rate of exchange of solvent water oxygen with that of the plutonyl(V1) ion is observed in solutions in which the lower valent oxidation states of plutonium are present. The rates of several previously measured reactions of plutonium together with the PUOZ+-HzO reaction can satisfactorily account for the observed exchange measurements. The upper limit of the specific rate constant for the PuOzf-HzO reaction, obtained by the differences between the observed and calculated exchange rates, has been estimated t o be 6.4 x 10-5 sec.-l in molar perchloric acid a t 23". It has been shown that the fluoride ion can IabiIize the plutonyl(V1) oxygen with respect to exchange. A half-time of about 17 hr. ww observed in 6.5 M HF at 23"; this is in contrast to the half-time in excess of lo4hr. found for the plutonyl(V1)-water intrinsic exchange reaction. Measurements of the oxygen-18 enrichment of plutonyl(V1) ions have been made after reacidification of solutions made alkaline with sodium, ammonium, or barium hydroxides. Only with barium hydroxide, which produced an insoluble product in the alkaline medium, was exchange observed; the other bases did not form precipitates. In the formation of the plutonyl(V1) ion by the ozonization of Pu(III), it has been noted that one of the oxygens is derived from the water and one from the ozone.
Introduction The intrinsic exchange of oxygen atoms between PuOz++ and solvent water is exceedingly slow in acid solution; the observed half-time exceeded lo4 hr. at 83°.2 In the absence of a scavenger such as chlorine, however, the exchange was found to progress at a continuously accelerating rate. This effect was attributed to catalysis of the exchange reaction by lower oxidation states of plutonium which are formed in the solutions as a result of self-radiolysis. The present work is primarily concerned with continued studies of the influence of lower valency plutonium species upon the plutonyl(V1)-water exchange system. In addition, several limited investigations of relevance to the general subject of metal-oxygen bonding in the actinyl ions are reported. These studies include: the influence of pH and of complexing anions upon the Pu02++-water exchange; the exchange between PuOz+ and water in acid solution, and the oxidation of Pu(II1) with ozone. Experimental The details of the experimental procedures have been described previously.2 Water enriched in the oxygen-18 isotope t o the extents of 1.6 and 5.170 normalized to the natural abundance of hydrogen was obtained from Yeda Research and Development Co. Acids were standardized against mercuric oxide. All chemicals were of reagent grade quality. Cncertainties in halftimes, where given, correspond t o a single standard deviation.
Results
PuOz+ +-HzO* Exchange in Absence of Scavenger.The rate of the plutonyl(V1)-water oxygen exchange reaction in the absence of chlorine or other scavenger to maintain the plutonium in the Pu(V1) oxidation state appears to be autocatalytic. A plutonyl(V1) solution initially 0.1396 M was prepared in 0.236 M perchloric acid with 1.6% H2Ol8. The ionic strength of the solution was adjusted to 1.42 with sodium perchlorate and the temperature was maintained at 23'. Samples were removed periodically and ozonized to oxidize the lower oxidation states of plutonium which are formed by (1) T h h work wan done under the auspices of the U, S. Atomic Energy Commi'Bhion. (24 *k\J. &Iw.tersand s6 W. Rabideau, Inoro. Cham., 3, I (1963).
the a-reduction process. Corrections were applied for the introduction of normal oxygen into the plutonyl(V1) ion by ozone (vide infra). The isotope enrichment in the plutonyl(V1) fraction was determined as usual by precipitation as the ferricyanide and conversion to carbon dioxide followed by mass spectrometric analysis.2 The results are given in Table I, in which the carbon dioxide mass ratios are given in column 2 as 46/(44 45). These exchange results also are presented in Fig. 1 in a non-linear plot of log (1 - P ) us. time, where F is the fraction exchanged. The complete exchange sample was determined after reduction of the plutonium with hydrogen in the presence of platinized platinum to Pu(II1) followed b y conversion to the Pu(V1) oxidation state by heating the sample at 83' with chlorine in a sealed tube.
+
TABLE I EXCHANGE OF PLUTONYL(VI)-WATER OXYGEKIK 0.235 il/l PERCHLORIC ACIDAT 23" IN ABSENCE OF SCAVENGER Time, hr
0 46 102 125 148 191 215 m
00 25 47 47 45 57 39
46 (14
+ 46)
0 004396 004500 006135 005920 006960 008940 009880 016355
iipparent
% exchange
0 0 6 12 21 38 45 100
0 9 2 8 4 0 9
Coir. 70 exchange
0 0 0 1 4 7 11 1 19 9 37 4 45 5 100
Influence of Pu(II1) on PuOz++-HzO* Exchange.An exchange solution which contained 1,45'% H2018 together with initial concentrations of 0.1158 M unlabeled PuOz+* and 3.86 x 10-3 M Pu(II1) in molar perchloric acid was prepared. The solution was stored at 23' and samples were periodically analyzed to determine the oxygen-18 content of the plutonyl(V1) ion. I n this series of measurements, the individual aliquots of the exchange solution were not ozonized prior to precipitation of the plutonyl(V1) ferricyanide. However, the small amounts of lower oxidation states present should have produced a constant effect upon the enrichment measurements; an effect which would tend to
OXYGENEXCHAXGE REACTIOXS
Feb., 1963
OF
PLCTONIUM IONS IN SOLUTIOS
319
be canceled in the calculations o€ the percentage exchange. In Table [I and in Fig. 2a are given the results of the exchange measurements. From the least squares slope of a linear plot of In (1 - F) us. time, a half-time for the PuOz++-HzO* exchange is computed to be 11737 :k 90 hr. at 23' in molar perchloric acid. This half-time corresponds to a rate of exchange of 4.51 X 10-5JB/hr. TABLE I1 THE P u O ~ + + - I ~ EXCHASGE ~O~~ IN THE PRESENCE OB ADDED PU(IX1) IN MOLAR PERCHLORIC ACID AT 23' T i m e , hr.
0.00 1.oo 18 40 90 32 1%.54 m
46/(44
+ 45)
0 006102 .006130 .006193 .006400 006608 014970
% Exchange
0.00 0.32 1.03 3 36 5 71 100
Effectof Added Pu(1V) onPu02++-H20*Exchange.The rate of exchange of PuOz++ and HzO* was examined in a 1.03M perchloric acid solution to which was added Pu(1V) in perchloric acid. The concentrations of unlabeled PuOz++and Pu(1V) initially were 0.121 PI and 5.29 X 10-3 M , respectively. The Pu(1V) was prepared by the dichromate oxidation of Pu(II1) and the exchange solution contained oxygen-18 of 1.3101, enrichment. Over a 30-hr. period there was no measured exchange between PuOz++and HzO". It appears that there is no catalysis of the PuOz++-H20* exchange by the presence of added Pu(1V) in solutions in which the disproportionation rate is very small. Exchange of Oxygen between PuOz+ and HzO*.-The specific rate constant for the exchange of oxygen between PuOz+ and water has been computed from measurements of the acceleration of the rate of oxygen exchange between Pu02++and H%O*in the presence of added Pu02+. An unlabeled 0.106 M PuOz++solution in 0.457 M perchloric acid which contained 5.28 x M PuOa+ in 1.60% HzOls was prepared. The PuO:~+ was prepared by the iodide reduction of the plutonyl(VI) ion. The exchange solution was held a t room temperature and a t various timed intervals samples were removed for isotopic analysis of the plutonyl(V1) fraction. AIeasurements are given in Table 111, and in Fig. 2b.
0.3
O - 1
0.2 0
100
200 300 HOURS, Fig. 1.-Exchange of oxygen between PuOz++and HzO in 0.:!35 M HC10, in the absence of Clz scavenger a t 23". 1.0
0.9 0.e
0.7
\- \ \
\
0 . E 0.5
* 0.4
CI
LL
I
Y
0.:
TABLE I11 EFFECTO F PuOz' Time hr.
0 9 17 26 91 114 m
00 00 78 08 19 49
PuOzf+-HzO*EXCHANGE IN 0.457 M PERCHLORIC ACIDAT 23" UPON THE
46/(44
4-
45)
0 006798 007300 007750 008253 01143.7 012801 017043
70Exchange 0 0 4 9 9 3 14 2 45 3 58 6 100
Again, the complete exchange sample was prepared by the hydrogen reduction of the plutonyl(V1) ion follomed by oxidation with chlorine in a sealed tube at 83'. Acceleration of PuOz++-H,O * Exchange by Fluoride, -An exchange solution was made up from freshly ozonized plutonium(V1) perchlorate stock of normal isotopic composition together with 48% hydrofluoric acid and 5% H$Y, The final composition of the solu-
0.2
0.1
-1
I
50
I 100
I
I
HOURS. Fig. 2.-Acceleration of PuOz++-H?O oxygen exchange at 23" with the addition of: (a) Pu(1II) in 1 M HC101, (b) PuOz+in 0.457 M HClOI; (c) 6.5 M HF.
tion was 0.111 M PuOZ++in 6.5 A4 H F and l.6yo enriched water. The formation of a glutonyl(V1) flue-,
::20 ride complex spccies undcr thesr conditions is indicatrd by the bright pink homopencous solution u hich is produced. No variation in thc :Lnalyiicd mclhod for ihc dctcrmination of the o\ygrn-l8 c*ontclntof the plutonyl(VI) ion \vas rcquircd in spite of the prtwncc of the fluoride ion. Thc rcsults arc' sr:mmarixcd in l'ablc 11and in Fig. 2c. The cxrhangr half-iimc of ahout 17 hr. indicates thc grrat dcgrce of Iahilizatjon whicah is brought ahout by the prcsrncc of the fllioridc. ion. TAUI,I~ I\' h l ( I ~ ) - ~ ~I*:~'.( ( l)l k*N ( t l C
I N Till; PRI AT
v .
lirrie h r .
4fi/(li
(1,
0 1
['1,{
ORII)1:
I()h
23"
+
I;)
'
1;v Iiungv
0 00607 0 0 .006!)2 7 6 .01239 50.3 .0 1360 84 8 .0 15!jX hx 2 co .01730" 100 a Calculated from known exchange solution cnrichmcnt. 0 2 17 40 63
0 0 2 0 3
The PuO?++-H20* Exchange in Basic Solution.Several experimeiits were pcrformcd in which acidic solutions of plutoiiyl(T'1) ions of kno\\-ii iiiii is1 cnrichmcnt w r c made strongly alkaliiic, rcaridificd, and analyzed for the oxygcn-18 content. This scqucncc \\-as conducted both with labclcd and with un1at:cled plutonyl(T'1) ion. With sodirim hydrohidc and ammonium hydroside, dark brown apparcnt 1y homogcncous basic solutions m r c prodriccd. X o cvidrncc of isotopic exchangc \\-asobserved in this acid-hsr-acid cycbling. Whenever barium hydroside was used to prcpare the alkalinc solution, a hron-n prwipitatc 11 as formrd which readily dissolvcd during the rewidifleation proccdurc. Partial exchangc, which varird from 18 to ri070 of eomplctc cxchangc, was found to have occurrcd during each of t hc barium plutonate precipitations. A typical series of cspcrimcnis is summarizcd in Table Jr. In this scrics the 0 . l X J l plutonyl(1-I) ion was of norma1 isotopic composition in a medium of 0.21 dl IIC104 in 1.73% HsO1*. Thc tiasrs uscd wcre 2.7 11 XaOII or 2.7 JI NII40H, and thr rcaridifications were performed with 2.7 11 IIC101. Each of Ihcsc solutions nas prepared in 1.73% I-I?018. To 0.20 ml. of thc appropriate h s c , 0.50 ml. of plutonyl(\-I) stork was added. Aftcr 30 sec. stirring, 0.20 ml. of perchloric acid was add6.d and thr plutonyl(T'1) fraction ivas analyzed in the usual manner. The barium plutonate cxpwimcnt and control were madc with KaOH to xhirh 0.15 mequiv. of barium pcrchlorate was addcd. In ordcr io climinatc a possihlc intcrfcrcncr in the analytical prowdure mused by the prcscncc of sodium, ammonium, or barium ions, a control was pcrformrd for cach cspcrimcnt by first mixing the acid and the hasc beforc adding thc plutonyl(\'I) solution. An additional csperimcnt was madc in this scrics with sodium hydroxide in 1he prcsenw of about 0.2 ml of fincly ground Pyrcx glass to examinc thc possibility of a hcicrogcncous catalysis contribution to thc cxehangc. Oxidation of Pu(II1) to Pu02++by Ozone in H2Ol8.A scrim of mcasurrmeiits was madc of the isotopic composition of a plut onyl(\'I) solution whivh )\-\-as prepared by the ozonization of plutonium(II1) pcrchlorat c ill about I 6% oxygen-18 ciiriched watcr undcr (Bonditions of varicd acidit ics. Inasmuch as slightly dilfc.-rl:t \\ c/.cr ecri:.>mcnt:; ww u v d at the
+
various acidilics employcd, thc isotopic ratio, 46 '(14 45), corresponding t o the complcte exchange value was mcasnrcd at, cach acidity by oxidizing the l'u(II1) with chlorinc in B scald lube at 83'. Cndcr thrw conditions, the plutonyl(V1) oxygens are solely dcrived from the watcr. Also at cwh acid conrentration used, ihc I'u(II1) stock was ozonizcd to form the plutonyl(l'1) ion and the isotopic ratio again was determined. As shown in Tahle YI, approximately onc oxygcn of the plutonyl(\-I) ion is supplied by the n-atrr and one by the ozonc in ihc 0.23 Allperchloric acid solution. With increased acidity, thc ratio of thc numbers of oxygcn atoms supplicd by thc water and by the ozone reaches 1.00.
TABLE \'I IsoToiw COMI~OSITION OF P u O ; + - C I'REPAiZEI> BY OZONIZATION OF I'u(II1) IN 13,018 A S A FCSCTION OF ~ ~ C I D I T Y [11+],Jf
0.26 0.50 1 .OO
l.'rnr t ion exrhnaerl
0.5fi.i
,538 .50I
Oxypens derived from I120 Oaono
1.13 1.ox
1.00
0 .x7 .92 1 .OO
Oxygen Exchange between HzOi8and Acetone.-In the calculation of the initial isotopic ciirichmcnts of somc samples, it was of importaiicc to dctcrmhe whcther the water-acetone oxygen exchange proceeded rapidly undcr t hc txperimcntal conditions employed. To 0.30 ml. of 5% 1120'*,0.50 ml. of molar perchloric acid in normal watcr \\-as added together with 23 ml. of acetone. The solution was allowed to stand for 13 min. after mixing. About 1 ml. of the solution was placed in a tube, coolcd with liquid nitrogcn, and evacuated on a vacuum line. Thc liquid was thawed and held at the tcmpcrature of a Dry Ice-acctone bath. A small portion of the liquid, thc first fraction to distil, was collwted in a second tube hcld at liquid nitrogen tcmperaturc. This distillatc was refractionated at -12'; thc most volatile portion of which was condenscd into a combustion bomb with 100 mg. cach of dried Ilg(C?;)2 and ITgC12.3 The gas formrd by heating a t 400' with thew salts which was condcnsa?.de at liquid nitrogen iemperaturc was further purified by heating a t 200' with a saturated zinc amalgam. Thc final product wa$ again fractionated a t -80' and appeared to be csseniially only COz. Thc oxygcn-18 enrichment was O.SSO% as compared with a calculated complrte exchange value of 0.543yo. Thus, it appears that under thc conditions of this txpcrimcnt, the cxchangc was tssentially complctt within 15 min. (3) 11. -\ngir :in4 S G u t t m m n , I n t r m . J . .Ippl. I l n d i n f i o n I s o l o p ~ s ,I, 23 3 ( 1 ~ l j O )
OXYGE?;EXCHANGE RE.%CTIOXS OF PLUTONIUM IONS IN SOLUTION
Feb., 1963
Discussion Pu(V1)-H,O Unscavenged Exchange.-In an exainination of the rate of the plutonyl(V1)-water oxygen exchange in the absence of a scavenger, it was of interest to try to account for the observed exchange in terms of previously studied kinetic processes for reactions of plutonium ions in acid solution. Equilibria which lead to an exchange of plutonyl(V1) oxygens are
+ Pu02+ = PuOz++ + PuOz*+ (I) Pu02+ + HzO* = PuO2"+ + HZO (2) Pu(I1') + PuOz++ .f2HZO" = PuO2"+ + PuOz+ + 4H+ (3) 2Pu(IV) + 2Hz0" == PuOz*+ + Pu(II1) + 4H+ (4) PuOz*++
To this list of reactions could also be added the intrinaic exchange reaction between plutonyl(V1) ion and water; however, the rate of this reaction has been sho\m2 to be negligibly small in comparison with the rates of reaction under consideration. In the a-reduction of plutonyl(VI) solutions of moderate acidity, under which condition the equilibrium concentration of Pu(V) is small, it may be assumed that the net effect of the reduction is the formation of Pu(1V). I n any event, the concentration of Pu(1V) calculated in this manner represents an upper limiting value. The reactions which also occur
+ a-radiation = reduced Pu species I'uOz++ + Pu(II1) = Pu(1V) + PuOz+
PuOz++
(5) (6)
are considered to contribute an insignificant amount to the exchange of oxygen atoms between the plutonyl(VI) ion and the s o l ~ e n t . The ~ upper limit of Pu(1V) concentration is estimated from the rate of decrease of the mean oxidation number. Thus after 24 hr. the Pu(IV) concentration is 0.008-2 [Pu]. The equilibrium concentration of PuOz+ in the exchange solutions can be computed from the Pu(1V) and Pu(V1) concentrations together with a knowledge of the Pu(IV) disproportionation equilibrium quotient, K d .[Pu(III) l2 [Pu(VI)]/ [Pu(IV)I3, and Kw,, the equilibrium quotient for the reaction Pll(1V)
+ Pu02+ = PuOz++ + Pu(II1)
(7)
The expression which relates the PuOi+ concentration to these quantities is [PuOz+] =
d [Pu(IV)] [PuOz++]Kd/K23m ( 8 )
A value of 2.0 was computed for Kd in 0.235 M perchloric acid at 23' from the previously measured5 molar perchloric acid value and the acid dependence of this quantity. The acid-independent equilibrium quotient, can be taken to be equal to 13 in this sohtion.6 An instantaneous half-time of 96 hr. is obtained from the non-linear McKay' plot of the data of Table I (4) The absence of an exchange contribution from reaction 6 may be demonstrated from the results of Table 11. The equilibrium rate of reaction 6 may be estimated to he 0 13 M hr.-i with the aid of the specific rate constant reported in ref. 6. Since the observed exchange rate is smaller by a factor of about 104, I t may be concluded t h a t reaction 6 is a simple electroin transfer process and does not involve the breakage of plutonyl(VI)-oxygeq bonds. (5) 5. W. Rabideau. J , A m . Chem. Soc., 7 6 , 798 (1953). (6) S.W. Rabideau and R. J. Kline, J . Phys. Chem., 62, 617 (1958). (7) H. A. C. hlcXay, Nature, 142, 997 (1938).
221
at a time of 200 hr. The concentrations of Pu(JV), Pu02+, and I'uO2 f + at this time have been computed to 3.8 X and 0.130 M , respectively. be 9.3 X The observed rate of exchange for the plutonyl(V1)water system is related to the instantaneous half-time, by the relation
R,, (obsd.)
=:
0.693 [P~Oz++]/ti/~~
= 9.4 X
(9)
M/hr. (at 200 hr.)
In the computation of the expected rate of exchange of oxygen between plutonyl(V1) ion and water in the absence of a scavenger, it is assumed that eq. 1, the Pu02+-PuO~++exchange, is very rapid. The ratedetermining steps then should be equal to the sum of the exchange contributions of reactions 2, 3, and 4. The specific rate constants for the forward reactions of eq. 3 and 4 under the experimental exchange conditions have been computed to be 0.036 and 9.4 M-' hr.-l, respectively, from previously studied system^.^,^ The calculated contributions of the reactions in eq. 3 and 4 equal 8.5 X 10-4M hr.-l= O.O36([Pu(IV)][Pu(VI)]) 9.4( [PU(IV)])~.From the difference between the observed and the computed exchange rates, a value of 0.66 X see.-' is computed for the specific rate constant for the Pu02+-H20 exchange in 0.235 M perchloric acid a t 23'. This result is in good accord with the values derived from instantaneous half-times of 238 and 144 hr. at exchange times of 120 and 160 hr., respectively, in this same exchange experiment. The very great difference in the specific rate constants for the U02+-H209(a value 2 180 sec.-l was inferred from observations in molar perchloric acid) and the Pu02+-H20 exchange systems is somewhat unexpected especially since both the uranyl(V1) and plutonyl(V1) exchanges are very slow. From the measurements of exchange in molar perchloric acid between plutonyl(V1) and water in the presence of Pu(II1) which are given in Table 11, it is possible to make another calculation of the PuOz+HzO exchange rate constant. The equilibrium concentrations of the plutonium species after alsout 48 hr. were calculated to be: [PuOz++]= 0.113 M ; [ P u o ~ + ] = 1.8 X d; [Pu(IV)] = 5.8 X M ; and M . The calculated contribu[Pu(III)] = 1.2 X tion to the exchange between plutonyl(V1) ion and water as a result of the disproportionation of Pu(1V) is found to be 0.33 X 10-6 M hr.-I; while that produced by the Pu(1V)-Pu(V1) reaction is only 0.05 X 10-5 M hr.-'. These rates are to be compared with the observed rate of 4.51 X M hr.-I. Thus, it appears that in this experiment, the major fraction of the observed exchange is attributable to the PuOz+-H20 exchange. If the difference between the observed and calculated rates, 4.13 X 10-5 M hr.-l, is ascribed to set.-' is obtained for the eq. 2, a value of 6.4 X specific rate constant in molar perchloric acid a t 23O, assuming a bimolecular exchange reaction between PuOz+ and water. The initial slope of Fig. 2b yields a specific rate constant for exchange of this same order of magnitude. It is recognized, however, that reactions other than those represented by eq. 1-4 may contribute to the observed exchange, particularly reactions which involve radiolysis products of water. Con(8)s. W. Rabideau, J . Am. Chem. Soc., 79, 6350 (1957).
+
(9) G. Gordon and H. Taube, J . Inorg. N u c l . Chem., 16,272 (1961).
322
S. JT. RABIDEAU ASD B. J. ~ I A S T E I ~ S
Vol. 67
sequently, the value for the PuO*+-H20 exchange rate constant in this work is to be considered an upper limit estimate. Influence of Fluoride Ion upon the Pu(VI)-H20 Exchange.-The pronounced acceleration of the Pu(VI)-H20 exchange reaction observed in the presence of fluoride ion (tl,2 = 17 hr.) compared with the rate of % lo4hr.) is of interest from the intrinsic exchange the standpoint of structural considerations. Connick and Hugus10 suggest a model for the U02(H20)O++ion in solution in which a puckered ring of six oxygen atoms surrounds the equator of the linear O-U-O++ group. It seems likely that a comparable structure prevails in the plutonyl(V1) ion and that the formation of fluoride complex ions may be represented by the successive substitution of fluoride ions for mater molecules in the equatorial positions. These equatorial ligands would be expected to undergo electrostatic interaction with the axial oxygen atoms. and, as a consequence, the ionic contribution to the axial bonding would be considerably weakened. The present results indicate that the isotopic exchange method provides a sensitive method of obtaining information relative to the effects of ligation upon the axial bonding of the aqueous actinyl ions. A systematic study of actinyl oxygen labilities as a function of the number and electronegative character of the ligands which replace the waters in the first coordination sphere would be valuable for purposes of comparison with bond strength data available for crystalline uranyl(V1) complexes.11 The Plutonate-Water Exchange.-The observed nonexchange of plutonyl(V1) oxygen atoms in the acidbase-acid cyclic sequence demonstrates that in the basic plutonates the axial oxygen groups retain their identity. This result is in contrast with that obtained for the uranyl(V1) systemI2; however, it appears possible that the uranium results may have been influenced by the presence of trace amounts of the exchange catalyst U02+. The non-exchange of plutonyl(V1) oxygens is consistent with structures previously inferred for hydrolyzed uranyl(V1) species and for crystalline uranate compounds. Independent experimental techniques have yielded evidence for the formation of polynuclear species in hydrolyzed uranyl(V1j solutions. l 3 , I 4 Ahrland, et al., suggest that polymerization results in the formation of a sheet-like structure in which the equatorial oxygen atoms are shared by adjacent uranium atoms while the uranyl(V1) oxygen bonds remain perpendicular to the plane of the lattice. Similar structures have been proposed15for the crystalline solids Ca(U02)02 and Sr(UOz)02, in which each uranium atom is surrounded by two oxygen atoms 1.9 A. distant and by six secondary oxygen atoms about 2.3 A. distant. The partial exchange observed in the barium plutonate precipitation experiments may arise entirely from induced exchange effects such as frequently are observed in heterogeneous systems. However, it is
noteworthy that the structure proposed by Sampson and Sil16n16for Ba(UO2)O2differs from that of the foregoing uranates in that only four secondary ozygen atoms are indicated a t a distance of about 2.2 A. from the uranium atom. It appears that in this coordination configuration the axial and equatorial oxygen atoms should be more nearly equivalent than in the case of the other uranates, a supposition which is in accord with the exchange results observed for the plutonium system. It is assumed that the sodium and ammonium plutonate structures are similar to those of the calcium and strontium uranates. Ozone Oxidation of Pu(III).-The kinetics of the oxidation of Pu(II1) to PuOz++ by ozone does not seem to have been studied in detail. It appears that the rate of oxidation of Pu(II1) by ozone is inversely dependent upon the hydrogen ion concentration.17 In oxidations with ozone, usually only one of the oxygen atoms is reduced with the formation of molecular oxygen.ls I t is of interest to note that in the formation of both uranyl(V1) l 2 and plutonyl(V1) ions by ozonization of the lower unoxygenated valence states, one of the oxygens is derived from the ozone and one is supplied by the water. This suggests that the reaction proceeds through an inner sphere activated complex in which an oxygen atom from ozone is bonded directly to the plutonium ion rather than by an electron transfer mechanism through the intact hydration sphere of the metal ion. The process is accompanied by the loss of two protons from one of the hydration waters in the formation of the plutonyl(V1) ion. It seems that the disproportionation of Pu(IV), eq. 4, could be responsible for the introduction of additional water oxygen into the plutonyl(V1) ion in low acid solutions, since the disproportionation reaction is inverscly dependent upon the cube of the hydrogen ion concentration. Thus, in molar perchloric acid, in which epvironment the rate of Pu(IT7) disproportionation is small, the ratio of oxygens derived from the water and from the ozone is 1.00. Ozone Correction of Exchange Results.-In a few of the experiments in which Pu(V1) was initially unlabeled it was necessary to apply a correction for the amount of ordinary oxygen introduced into the sample as a consequence of the ozone oxidation of lower oxidation states of plutonium prior to precipitation as plutonyl(V1) ferricyanide, see, e.g., Table I. The fraction of Pu(1V) present is equal to 0.008 X time in days. Since the enrichment of COZ is equally derived from the water and from the ozone for the reduced portion of the sample, &)/a. It is assumed that its value is given by (S, a-reduction of the plutonyl(V1) solution leads primarily to the production of Pu(1V). The observed enrichment then can be corrected to give the true enrichment by the relation
(10) R. C Conniok and 2 Z . Hugus J r , J Am. Chem. S a c , 74, 6012 (1952). (11) L. H. Jones, Speetroehtm Acta, 16, 409 (1959) (12) G. Gordon and H. Taube, Inorn. Chem., 1, 69 (1962). (13) S..4hrland, S. Hietanen, and L. G. Sil16n, Arla Chem S c a d . , 8, 1907
where t is the time in days and Soand S , represent the initial and complete exchange enrichments, respectively.
(1964)
(14) R. M. Rush, J. 5. Johnson, a n d X. A. Kraus, Inorg. ChPm., 1, 378 (1962). ( 1 5 ) W. H . Zachanasen, Acta Crvst., 1, 281 (1@48)#
+
true enrichment = [obsd. enrichment 0.008t(S, S0)/2](1 - 0.008t) (10)
+
(16) 9. Sampson and L. G. SillBn, Aikrv Kemz, 26, No. 21 (1947). (17) S. W. Rabideau, unpublished observations. (18) W. L. Latimer, “The Oxidation States of the Elements and Their Potentials in Aqueous Solutions,” 2nd Ed , PrentirP-Hall, Inc., Kew Yorkt
N‘ Y . , 1952.
E'cb., 1963
DYNAMIC XEASUREMEKTS O F POLYISOBUTYLESE SOLUTIONS IS CETASE
Acknowledgment.-The authors wish to express their thanks to Professor Henry Taube and to Dr. Joe
323
Fred Lemons for their helpful discussions and continued interest in this work.
VISCOELASTIC PROPERTIES OF CONCENTRATED SOLUTIONS OF POLYISOBUTYLENE IN CETANE. I. DYNAMIC RTEASUREMEKTS' BY JOHX R. RICHARDS, KAZUHIKO ~\TINOMIYA,~ AND JOHN D. FERRY Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received July SO, 1962 The dynamic storage and loss compliances of polyisobutylene and five polyisobutylene-cetane solutions, ranging in concentration from 68.0 to 91.8y0 polymer, have been measured a t frequencies between 0.1 and 1000 cycles/ sec. and temperatures between 15 and 45", with the Fitegerald transducer and Plaeek torsion pendulum. The temperature dependence of the viscoelastic relaxation times is described in this narrow range by the Arrhenius relation. The concentration dependknce of the relaxation times is compared with that of the diffusion coefficient of cetane, obtained from earlier measurements. The translational friction coefficient of cetane is nearly equal to the friction coefficient per monomer unit of a polymer segment over the range of temperatures and concentrations studied. Composite plots against reduced frequency are obtained a t frequencies higher than that corresponding to the minimum in the loss tangent (tan 6 ) when the components of the complex modulus are reduced by the factor 0g-l ( 0 2 = volume fraction of polymer). At lower frequencies, divergences appear which are attributed to concentration dependence of the coupling entanglements. From the depth of the minimum in tan 6, the average spacing between entanglements can be estimated by the theory of Marvin, and it is found to be inversely proportional to 212.
Introduction The detailed viscoelastic properties of very concentrated solutions of amorphous polymers, in the range from 50 to 100% polymer, have been given very little attention. A systematic study has been made314 of solutions of poly-n-butyl methacrylate in diethyl phthalate, yielding the concentration dependence of the monomeric friction coefficient, the average spacing between entanglements, and the shape of the relaxation spectrum. However, this system has some unexpected features, with what appears to be an anomalous dependence of the entanglement coupling density on both temperature and concentration, and may be a t y p i d . It therefore was considered desirable to study another polymer-solvent pair in which there would be no possible complications associated with polar groups or tacticity. Solutions of polyisobutylene in cetane (n-hexadecane) were chosen for this purpose, and measurements of dynamic shear properties, shear creep, and steady flow viscosity have been made. At the same time, the self-diffusion of the cetane has been studied in similar solutions by use of a radioactive tracer,6 so tha,t the frictional resistance of the diluent molecule can be compared with that of the polymer segments. The results of dynamic measurements of viscoelastic properties are reported here, and those of creep and steady flow will be given in a later communication.6 Materials and Methods The polyisobutylene and cetane were the same as those used in the diffusion studies,6 and the solutions were made up in the same manner, except that of course no radioactive cetane was used. The viscosity-average molecular weight of the polymer was 1.5 X lo6, and it was believed to have very little content of species with molecular weight less than 0.5 X lo6. Five solu(1) Part X L of a series on Mechanical Properties of Substances of High Molecular Weight. ( 2 ) Japan Synthetic Rubber Company, Yokkaichi, Japan. (3) P. R. Baunders, D. M. Stern, S.F. Kurath, C. Sakoonkim, and J. D. Ferry, J . CoEEozd Scz., 14, 222 (1959). (4) T. R. Nealin, S. E. Lovell, P. R. Saunders, and J. D. Ferry, zbzd., 17, 10 (1962). ( 5 ) R. S. Moore a n d J. D. Ferry, J . Phys. Chem., 66, 2699 (1962). (6) K, Ninomiya, J. R LRichards, and J, D LFerry, i b i d i , 67, 337 (t963),
tions were investigated, with concentrations ranging from 68.0 to 91.8% polymer by weight, as well as the pure polymer. The Fitzgerald transducer apparatus' was used in the frequency range from 25 to 1000 cycles/sec., and the Plazek torsion pendulum* between 0.1 and 5 cycles/sec. The temperature range was somewhat limited by the freezing point of cetane (16') and its volatility a t higher temperatures. The transducer measurements were made between 15 and 50'; the torsion pendulum measurements only at 25", except for the 91.8yGsolution, whil:h also was studied a t 15'. The dimensions of the disk-shaped samples for the transducer were 11/16 in. diameter by 3/64 in. thickness, except those of the 68% solution, which were 3/32 in. thick. After completion of measurements, each pair of samples (except those of the 68% solution) was removed and reinserted in the transducer apparatus with additional compression, and repeat measurements were made a t two temperatures to check the accuracy of the sample coefficient. Values of the storage and loss compliances, J' and J " , calculated from data from the two insertions agreed within 10% for the 82.3 and 91.8% solutions, and better for the others. The mean was taken in each case. The sample disks used in the torsion pendulum were those taken from the transducer apparatus, with a little more compression, except for the 68% solution, which was remolded to form a larger disk, 1.4 in. in diameter by 0.032 in. thick. After the transducer data had been reduced to 25' as described below, they overlapped slightly the torsion pendulum data on the reduced frequency scale for each solution and permitted comparison of the magnitudes of J' and J" as obtained by the two experimental methods. For each solution, both J' and J" measured in the torsion pendulum differed from those measured in the transducer by the same factor, indicating a sample coefficient error. Since the absolute magnitudes of the transducer data had been checked by reinsertions, and the sample coefficient in the torsion pendulutn is very sensitive t o the measurement of sample thickness, it was assumed that the transducer data were correct. The torsion pendulum data were adjusted empirically by adding to log J" and log J" the following corrections: 68.0%, -0.12; 86.7%, +0.13; 91.8%, 1-0.17; loo%, -0.20. Similar empirical corrections have been made in previous studies.* Aside from uncertainties in sample coefficients. the precision of the data is about 5%.
Results To save space, the original data are not given, but are represented graphically after reduction to a standard (7) E. R. Fitzgerald, J . Chem. P h y s . , 27, 1180 (1957). (8) D. J. Plazek, Ma Na Tlranaken, and J. W. Berge, Trans, Sor, Rheologir, 8 , 39 (1958)a
