THEKINETICS OF
THE
OXIDATION OF PLUTONIUM(III) BY NEPTUNIU~M(YI)
1661
pared with those for similar actinide(II1)-actinide(V1) reactions in Table VI11 of ref 7 .
the data nearly as well as those given above; the mean and maximum deviations are 2.4% and lO.S%, respectively. It is important to note that the activation parameters for the predominant path for reaction are essentially independent of the form assumed for the minor term in the rate law. The activation parameters for reaction 1 are corn-
Acknowledgments. I gratefully acknowledge many helpful discussions with Dr. R. B. Pulton and with Dr. C. E. Holley, Jr., under whose general direction this work was done.
The Kinetics of the Oxidation of Plutonium(II1) by Neptunium(V1) by R. B. Fulton2and T. W. Newton Uniuersity of California, Los Alamos Scientific Laboratory, Los Alamos, New (Recehed September 89, 1968)
+
Mexico 87644
+
The reaction Pu(II1) Np(V1) = Pu(1V) Np(V) has been studied at unit ionic strength in acid perchlorate solutions from 0.06 to 1.00 A l HClO4 and temperatures from 0.8 to 53.2'. At low acid concentrations and high temperatures the rate constants were corrected for the effects of disproportionation and further NpOZ2+ [Pu.NpO#+]* oxidation of Pu(1V). Two net activation processes are important: (1) Pus+ for which AF* = 15.34 kcal/mol (kap = 35.5 M-l seew1), AH* = 3.49 =k 0.05 kcal/mol, and AS* = -39.73 0.16 cal/mol deg; (2) Pu3+ + NpOz2+ + HzO = [Pu.0H-NpO24+]* H+ for which AF* = 16.78 kcal/mol (k160 = 3.1 M-l sec-l), AH* = 12.82 f . 0.12 kcal/mol, and AX* = -13,28 Jr 0.39 cal/mol deg. Chloride ion was found to cause a moderate increase in rate. The effect of ionic strength was studied from 0.24 to 3.7 M and is found to be in agreement with the extended Debye-Huckel theory. The results are compared with other analogous actinide(II1)-actinide(V1) reactions.
+
-
+
Introduction The importance of studying the kinetics of the oxidation of Pu(II1) by Np(V1) is twofold. First, it extends the chemistry of actinide-actinide reactions; none of the possible plutonium-neptunium reactions has been examined quantitatively and only a few other reactions in which Pu(II1) is oxidized have been studied.3 Second, this reaction is of interest for comparing the kinetic results with those for analogous actinide (11I)-ac tinide (VI) reactions. 4 ~ 5 The published heat and entropy data6 indicate that the equilibrium quotient, Ql, for reaction 1 a t 25" and
+
P u ~ + n'p0z2+
+ NpOz+
Pu*+
(1)
1 M HCIOl is approximately 350. However, a t higher temperatures Q1is much smaller, about 75 at 45O, and the effects of the back-reaction, reaction 2, must be considered.
+
P u ~ + i"';pOz+
Pu3+
+ Np0Z2+
(2)
I n addition to the back-reaction, tu70 other reactions become significant at high temperatures arid low hydrogen ion concentrations. They are the disproportiona-
tion of Pu(IV), reaction 3, and the further oxidation of Pu(1V) by Np(VI), reaction 4.
+ Pu4+ + 2H20 = PuOz+ + Pu3+ + 4H" Pu4+ + Np0i2+ + 2Hp0 =
Pu4+
PuOp+
+ NpOz+ + 4H'
(3)
(4)
Rabideauv found that the rate constant for reaction 3 has an inverse third power dependence on the [H+] and therefore its effects in low acid and high temperatures should be considered. Reaction 4 has not been previously studied, but separate experiments showed that a t low [H+] and high temperature its rate was large enough to be significant. Finally, reactions 5 and (1) Work done under the auspices of the U. S. Atomic Energy Commission. (2) Los Alamos Scientific Laboratory Post-Doctoral Fellow. (3) T. W. Newton and F. B. Baker, Advances in Chemistry Series, No. 71, American Chemical Society, Washington, D. C., 1967, p 278. (4) T. W. Newton, J . Phys. Chem., 74, 1655 (1970). (5) S. W. Rabideau and R. J. Kline, ibid., 62, 617 (1958). (6) J. J. Katz and G. T. Seaborg, "The Chemistry of the Actinide Elements," Methuen and Co., Ltd., London, 1957 (also John Wley and Sons, New York, N. Y.), pp 429-430. (7) S. W. Rabideau, J . Amer. Chem. Soc., 75, 798 (1953).
Volume 74! Number 8
April 16, 1970
R,B. FULTQN AND T, W. NEWTON
1662
6, which determine the fate of the relatively unstable
Pu4+
+ PuOz+
+ PuOe2f NpOz+ + PuOZ'+
= PUS+
+
N P O ~ ~ +PuOz+
(5) (6)
Pu(V) produced in reactions 3 and 4,are included to complete the mechanism, I n the present study, the mechanism represented by reactions 1 through 6 was found to be adequate. The reaction of primary interest was reaction I, and therefore, the observed rates were corrected, when necessary, for the combined effects of reactions 2 through 6.
Experimental Section
Reagents. The neptunium used was from a lot of Np02 for which spectrographic analysis showed the transition metals t o be less than 0.1 ppm and the rare earths to be less than 0.5 ppm. Radio assay showed the presence of about 200 ppm of 2asPu.s In addition, some thorium contamination was suspected so an ionexchange methodg was used for further purification. The NpO, was put into the form of a Np(V1) solution, reduced to Np(V) with NaN02, precipitated with NaOH, washed with water, and dissolved in HC1O4. This solution was placed on a column of Dowex-50 cationexchange resin and eluted with 1 M HClO,; only the middle fraction was retained. This solution was taken to strong fumes with HC1O4to oxidize any organic contaminants and to produce Np(VI), then diluted and placed on a column similar to the first. The Np(V1) was eluted with 1.5 M HCI;again only the middle fraction was retained. Finally, the solution was fumed with HC104to remove organics and chloride. The Np(V1) Bolutions were prepared from the above neptunium stock as described below, A portion of the stock solution was taken nearly to dryness and weighed. The 71% HC104 remaining was determined by subtracting the weight of the Np02(C10&. The solution was diluted with the appropriate amount of standardized HC104to give Np(V1) in the desired acid concentration. Separate experiments showed that fuming IIC1O4 oxidizes neptunium quantitatively to the (VI) state and that no substance capable of oxidizing Pu(111), other than the Np(V1) itself, was found in the resulting solutions. A portion of the Np(V1) solution was reduced t o Np(II1) with zinc amalgam and the total neptunium content of ths completely reduced solution was determined by a spectrophotometric titration using standard Ce(1V).lo Stock solutions of plutonium were prepared by dissolving high-purit,y plutonium wire in concentrated HCIOI. The resulting solutions were filtered and then fumed with HC104to remove any chloride formed in the dissolution process. A portion of this plutonium stock solution was taken nearly t o dryness and weighed to determine the 71% HC104 remaining. It was diluted with the appropriate amount of standardized HC104 The Journal of Physical Chemistry
Table I: Reactant Dependences, 0.8" and 0.98 M HC104 lO'[Nplo,
10~[Pula,
M
M
1,05 1.05
2.58 5.15 0.86 8.59 5.76 5.55 5,54 0.96 0.96 0.96 0.96 0.96 0.99 1.29 1.29 1.29 1.29 1.41 1.41 1.41 1.41 1.41 1.36 1.18 1.19
1.05 1,05 1.08 1.08 1.08 0.95 1.51 3.15 4.71 6.25 4.79 6.70 8.12 1.02 1.23 1.11 6.68 3.34 4.46
5.57 1.33 1.34 6.57 Total or av
No. of detns
1 1 2 2 2 2 2 2 1 1 1 1 1 3 3 2 2 2 2 1 1 1 2 2 2 42
-
kl, seo-I
M-1
l9,9 19.5 20.1 19,l 19.9 19.7 20.4 19.7 20.2 20.4 20.8 20.8 20.7 19.7 19.7 19.7 19.7 19.6 19.6 19.8 19.6 19.4 19,8 19.4 i9.6 19.8
-
Mean d ev I , .
... 0.5 0,l
0.2 0.0 0,2 0.1
... . . I
.. ...
...
0.1 0.1 0.0 0.1 0.1 0.0 1 , .
...
... 0.0 0.1 0.2 0.3
to give the desired acid concentration. The resulting solution mas completely reduced to Pu(TI1) with zinc amalgam just prior to use. The Pu(II1) content of this solution was determined by a spectrophotometric titration using standard Ce(IV). In 0.5 M HzS04,the oxidation of Pu(II1) to Pu(IV) by Ce(IV) is quantitative. Separate experiments, using solutions prepared from ultrahigh-purity electrolytic plutonium metal" and plutonium wire solutions that had been purified by an ion-exchange procedure similar to that used for the neptunium, gave the same results as the solutions of plutonium described above. Stock solutions of HC10, (-4 M ) were prepared by diluting commercial 71% acid which had been boiled and cooled in a stream of argon. The diluted solutions were boiled again and analyzed by titration with standard NaOH before use. Solutions of I,iC104, nTaC104, and I,a(C104)3were prepared by neutralizing analytical reagent grade carbonates (or oxide for La) with HClO,. These were purified by two or three recrystallizations from water. The concentrations of the stock solutions were determined by passing aliquots through ion-exchange colunins filled with Dowex-50 (8) These results were provided by the Analytical Chemistry Group of this Laboratory. (9) J. C. Sullivan, personal communication. (10) T. W. Newton and N. A. Daugherty, J . P h y s . Chem., 71, 3768 (1967). (11) We thank Dr. L. J. Mullins of this laboratory for providing this sample. Reported analyses showed 99.99% Pu with the greatest impurities being 11ppni U, 6 ppm Am, and 2 ppm Fe.
1663
THEKINETICSO F THE OXIDATION O F PLUTONIUM(II1) BY NEPTUNIUM(VI) Table 11: Effect of Initial Reactant Ratio and Wavelength on ki, 44.7" and Unit Ionic Strength W+I. M
[Pula. M X 103
M x 10'
0.060 0.060 0,060 0.071 0.071 0.071 0,100 0.100 0.100
1.66 1.66 1.11 1.86 1.86 1.25 1.66 1.66 1.11
1.15 1.15 1.73 1.20 1.20 1.80 1.15 1.15 1.73
[Nplo.
in the acid form and titrating the eluents with standardized NaOH. The HC1 stock solutions were prepared by diluting analytical reagent HC1 to about 6 M and distilling it in an all-glass still. The distillate was diluted and analyzed by titrating with standard NaOH. The LiCl stock was prepared by recrystallizing LiCl from water and was analyzed gravimetrically by evaporating portions of the solution to dryness. Doubly distilled water was used for preparing all solutions; the second distillation was made from alkaline permanganate in all-Pyrex still. All concentration units employed in this paper are moles per liter, M , a t 23". Procedure. The reaction rates were determined spectrophotometrically in rapidly stirred 10-cm absorption cells which have been described preyiously. l 2 The reactions were followed a t either 4696 A, where Pu(1V) is the principal absorbing species, or a t 6005 A, which is an absorption peak for Pu(II1). In a rate run the initial reactant, usually Pu(III), was added to a cell containing the appropriate amounts of acid and/or salt. The cell was then positioned in a thermostated water bath in the cell compartment of a Cary 14 recording spectrophotometer and the temperature was allowed to equilibrate for 20 min. Since the oxidation of Pu(II1) by oxygen in HC104 solutions is extremely no attempts were made to protect the solution from air during this period. The reaction was started by injecting the final reactant, usually Np(VI), into the stirred cell from a calibrated hypodermic syringe with a Teflon needle. No significant effects were observed when the order of addition of the reactants was reversed.
Results and Calculations For rate runs carried out under conditions of high hydrogen ion concentration and low temperature, the observed absorbance vs. time data can be described by the following rate equation d[Pu(IV)J/dt = ki[Pu(III) ][Np(VI)] (7) A typical example is shown by curve A in Figure 1. The validity of this rate equation was further established by experiments at 0.8" in 1 M HC104 in which
length
No. of detns
6005 4696 4696 6005 4696 4696 6005 4696 4696
2 2 2 2 2 2 1 3 2
Wave-
Mean
kl.
M
deviation
-1 800 -1
264.1 248.8 279.4 246 6 247.5 250.5 181.8 183.3 185.7
6.3 1.1 9.6 2.5 1.1 0.1
I
001 0
30
60
TIME (SEC)
,
I
,
2.9 6.2
120
90
I
Figure 1. Typical rate recorded &t4696 .& in a 10-cm cell. Curve A, [PU(III)IO= 2.6 X M, [Np(VI)lo = 1.0 X M, [H+] = 0.98 M, p = 1.00 M , and T = 0.8'. Curve B, [Pu(III)]O = 1.1 X 10-3 M , [Np(VI)]o = 1.7 X lo-* M , [H+] = 0.06 M , I.L = 1.00 M , and T = 44.7'.
the initial concentrations of Pu(II1) and Np(V1) were varied by factors of 10 and 8.5, respectively. The results are shown in Table I. The values of kl were fitted to the function kl = a b[Pu(III)Jo c[Np (VI)]O by a least-squares procedure and the parameters were found to be: a = 19.9 + 0.1, b = 44 30, and c = 7 f 25. The values of b and c are small enough and their uncertainties are large enough to indicate no significant effectof concentration on k l . When the disproportionation or further oxidation of Pu(1V) is appreciable, the absorbance vs. time data, illustrated by curve B in Figure 1, are more complex. However, these observations are consistent with reactions 1 through 6, which yield the following rate equations
+
+
dJ:/dt = k l ( A -
J:
(&l/kl)(J:)(J:
- y)(R - z
- 3 ~ -)
+ 3y) - 2k3(.)(B - - 3Y) + - - 3Y)l/ [1 + ke(B x - 3y)/k,(z)] (8) 2
[ka(Z)2
k4(Z)(B
-
dy/dt =
+
k 3 ( ~ ) ~k4(2)(B
-z-3
~ )
(9)
(12) T . W. Newton and F. E. Baker, 67, J . Phys. Chem., 1426 (1963). (13) T. W. Newton and F. B. Baker, ibid., 60, 1417 (1956). (14) R. E. Connick and W. H . McVey, Paper 3.9, and K. A. Kraus and J. R. Dam, Paper, 4.14, in "The Transuranium Elements," G . T. Seaborg, J. J. Kata, and W. M. Manning, Ed., National Nuclear Energy Series, Division IV, Vol. 14B, McGraw-Hill Book Co., Inc., New York, N. Y . , 1949.
Volume 74, Number 8
Ami1 16. 1970
1004
R. B. FULTON AND T. W. NEWTON
where Q1 and IC1 are the equilibrium and rate constant8 for reaction 1 and ICs through Ice are t’he forward rate constants corresponding to reactions 3 through 6, X: [Pu(IV)], y [Pu(VI)], A = [F’u(III)]~, B = [NP(VI) 10, and [Pu(IV) 10 = [NP(V>IO = [Pu(VI) IO = 0,
The absorbance at any time during the reaction, D,, is given by
E
~~~~~~
Table I11 : Hydrogen Ion Concentration and Temperature Dependences a t Unit Ionic Strength Temp
0.8
W+J,
No. of
?GI
1M
detns
M-1
0 029 0,032 0.037 0.053 0.061 0.077
2 1 1 1 1 1 1 1 42 1 1
I
0.110
0 174 0.980 0.059 0,071 0.083 0.100 0.125 0,167 0.250 0,980 0.034 0.051 0.059 0.071 0.084 0.101 0.117 0.184 0.250 0 544 0.986 0.059 0 066 0.071 0.083 0.100 0.125 0.146 0.167 0,250 0.362 0.980 0.973 0.060 0.071 0.085 0.100 0.125 0.167 0.250 0.500 0.977 0.973 0,973 I
15.2
24.8
I
36.2
I
40.3 44.7
48,5 53.2
1 1 1 1 1 1 1 2 4 5 2 4 3 2 3 1 11 2 2 2 2 5 2 1 2 2 1 5 2 6 6 2 6 6 3 3 3 7 2 2
(obsdj, seo-I
33.8 32,4 30.1 26.2 26.1 24.6 23.3 21,8 19.8 53.0 48.9 47.2 42.4 40.1 36.8 34.6 29.4 129,5 97.6 88.8 77.0 72.9 66.9 63,4 52.2 48.8 40.0 37.9 163.0 156,O 142,6 133.8 115.9 108.7 95.9 96.5 80.5 66.0 55.1 58.5 264.1 248.2 183.3 183.9 155.1 131.2 102.2 77.1 67.1 76.1 88.9
ki (oalcd),‘ M
-1
seo-1
34.2 32.8 31.0 27.5 26,4 24.9 23.3 21,8 19.8 52.9 48.7 45.7 42.7 39.8 36.9 33.9 29.6 126.7 96.2 88.0 79.1 72.3 66.1 61.9 52.2 47.8 41.1 38.5 166.7 155.5 146.2 131.7 117.1 102 8 94.6 88.4 74,2 65.4 53,O 59.7 267 7 234.7 205.1 182.5 156.9 131.2 105.8 80.3 67.8 76.0 87.8 I
I
a Calculated using the values of AHd*, AH,*, A s d * , and A&* given in the text.
The Journal of Physical Chemistry
where Do is the absorbance reading at time = 0, I is the cell path length, the CtI1sare the concentrations of the various species in solution [Pu(III), Pu(IV), Pu(VI), Xp(V), and Sp(VI)] at time = t , and the et’s are their respective extinction coefficients at the wavelength employed. Since eq 8 and 9 cannot readily be solved in closed form, numerical methods employing a computer program based on the Runge-Kutta method16 were used. This program solved the equations and calculated the concentrations of the various species, as well as the absorbance readings, as a function of time for given values of kl, &I, k 3 - k ~ ,A , 3,DO,I , and the extinction coefficients of the species in solution. The values of Icl, ~ P ~ ( I V andepu(vI) ), which best reproduced the observed absorbance us. time data were determined by coupling this program to the Los Alamos nonlinear least-squares program. l6 For all these calculations the root-meansquare deviations were 0.0026 or lese. The values of ~ P ~ ( I Vand ) EP,(VI) determined by the program were found to be in satisfactory agreement with the values determined directly in separate experiments. Values for Q1 and the other rate constants and extinction coefficient were either determined in separate experiments or taken from previously published works. A detailed summary of these values, as well as their [H +] and temperature dependences, is presented in the Appendix, It is important to note that although reactions 2 through 6 play an important part in the overall reaction, they are relatively unimportant in the initial stages where reaction 1 predominates. It is in this region that kl is determined, so the effects of the complicating reactions amount only to small corrections. The calculated kl values are relatively insensitive to the values for the rate constants for reactions 2 through 6, as shown in the Appendix. The adequacy of the proposed reaction scheme was further checked by showing that the value calculated for kl was independent of which reactant was in excess. Further, rates determined at 6005 8, where the reactant Pu(II1) absorks, were not significantly different from those at 4696 A, where the product Pu(IV) is the principal absorbing species. These results are summarized in Table 11. (15) (a) H. Margenau and G. M. Murphy, “The Mathematics of Physics and Chemistry,” Van Nostrand-Reinhold Co., Inc., Princeton, N. J., 1943, p 469; (b) H. R. Siewert, P. N. Tenney, and T. Vermeulen, University of California Radiation Laboratory Report UCRL-10575, 1962. (16) This program was written by R. H. Moore and R. K. Zeigler and is described in Los Alamos Scientific Laboratory Report LA2367, 1959, and Addenda.
1665
THEKINETICG OF THE OXIDATIONOF PLUTONIUM(III) BY NBPTUNIUM(VI) Table IV : Chloride Dependence, 24.8' and Unit Ionic Strength
marized in Table 111,indicate that ki1 has a pronounced inverse first-power dependence on the [H*]. Thus, the apparent second-order rate constant for reaction 1 can be represented by the relationship IC1 = lca Le [H+]-I. The effect of chloride on ICl was studied in solutions of 0.1 and 1.0 M HC104at unit ionic strength and 24.8". The results, presented in Table IV, indicate a moderate, nearly linear increase in rate with chloride ion. The ionic strength dependence was studied at 0.8" in solutions of LiC104, NaC1O4, and La(C10& that were 0.22 M in HC104. Also, a short series was done at 25.1' in solutions of LiC104 that were 1 M in HC104. Table V summarizes the results for IC', as well as for the d term of kl. The ionic strength dependence of lcl can be described in terms of the extended DebyeHuckel eq 11. The least-squares best values for the
+
0.000 0.047 0,094 0.156 0.234 0 391 0.000 0.040 0,121 0.161 0.241 0,322
1.00.
I
0.1OOd
38.1 52.1 67.3 88.6 116.3 171.7 69.8 82.9 111.4 126.3 166.6 186.9
38.35 51.4 67.15 89.0 118.2 169.9 70.2 81.8 113.1 126.2 155.5 187.15
a The [Cl-I was varied using either LiCl or HC1. b Calculated using eq 18 and the values on line 1 of Table VI1 for 1 M HClOl and on line 6 for 0.10 M HC104. 0 Sets of runs with excess Np(V1) agreed with those with excem Pu(1II) with an average deviation of 0.9%. Average reactant concentrations were 1.28 x 10-8 M Np(V1) and 1.24 x 10-3 M Pu(II1). d Reactant concentrations were 1.42 X 10-3 M Np(V1) and 1.08 X 10-8 M Pu(II1).
The effect of the hydrogen ion concentration and temperature on kl was studied over a wide range in solutions in which the ionic strength was maintained a t unity with LiC104. The results, which are sum-
Table V : Ionic Strength Dependence Ionic Temp, Salt
"C
LiC104
0.8
NaC104
0.8
strength, M
0.236 0.473 0.889 1.422 1.956 3,201 0.237 0.508 0.870 1.412 1.955 2.678 3 882 0.237 0.412 0.937 1.463 1.988 3.741 1.000 2.000 3.000 I
La(ClOa)s
LiCIOa
0.8
25.1
ki (obsd), M -1 aec - 1
8.lC 12.5 19.6 29.4c 39.2 70.2~ 8 25c 12.5 17.3 24.6c 31.5 41.8 61.5c 8.35" 10.1 15.5 I
20.8C
26.7 53.5c 37.2s 71.0~ 112.49
6.9 11.0 17.7 27.0 36.3 65.4 7.1 11.1 15.7 22,6 29.1 38.7 57.2
7.4 9.0 14.1 19.0 24.6 49.5 34.0 66.2 105.8
6.9 11.2 18.0 26.7 36.4 65.6 7.1 11.2 15.8 22.3 29.3 40.0 56.6 7.3 9.3 14.2 18.9 24.2 49.7 34.0 66.1 105.7
a kd = kl(obsd) - k,[H+]-1 as described in text. Calculated on the basis of the Debye-Huckel parameters in Table VI. Average of two determinations.
log k = log k0
~~'/~ + 1A+A ZBP''~ +
elk
(11)
parameters Lo, A AZ2, B, and C are summarized in Table VI. The ionic strength dependence of the k d term was obtained by subtracting the k,[H+]-' term from k1 at each ionic strength. The ionic strength dependence of L, was estimated from eq 11 by assuming that the values of B and C are the same as those determined for lq. The kd values determined in this way are also in accord with eq 11, as shown by the calculated values in Table V. The values of the various DebyeHuckel parameters are presented in Table VI. The data for the three salts agree fairly well; at unit ionic strength, 0.22 M HC104 and 0.8 the values of kl vary from 16.2 M - l sec-l for La to 21.6 M-Isec-' for Li with Na being in between at 19.1 fM-' sec-'. The data for 1Ja(C104)3also indicate that changing the overall [C104-] has little effect on the rate constant. Finally, th? divergent values obtained for ICo indicate the unreliability of extrapolating to zero ionic strength from a value as high as 0.22 M .
Discussion The data in Table 111were obtained under conditions where the hydrolysis constants for Pu(II1) and Np(V1) are much smaller than the [Hf]. They lead to the following rate law d [Pu(IV)] / d t =
This rate law implies two net activation processes: (13), which does not involve hydrogen ions, and (14), which Pu3+
+ Np0z2+ = [ P ~ * N p 0 z ~ + ] * "
(13)
(17) The composition, but not the structure of the activated complex is indicated. Volume 74, Number 8
April 16, 1970
1666
R,B.FULTON AND T.W. NEWTON
Table VI : Debye-Hbckel Parameters loQ,
Temp,"C
Salta
LiClOa
Constant
ki (obsd)
0.8
kd ki
(obsd)
kd
ki (obsd)
0.8
kd Ice
La(CIO,),
b (obsd)
0.8
0.491 0 376 0.096 0.489 0 394 0,610 0.628 0.494 0,096 0.943 0 774 0.096 I
ko
NaCIOl
-1 880 -1
I
IC, 25.1
M
kd
I
ke
M
B,
C,
-l/z
AM -1
2.91 2.71 (2.91) 2.47 2.36 (2.47) 3.34 3.14 (3.34) 4.42 4.20 (4.42)
0.145 0.140 (0.145) 0.118 0.116 (0.118) 0.132 0.129 (0.132) 0 * 152 0.161 (0.152)
AAZZ,
M -'/g
5.864 5.864 1 955 6.113 6.113 2.038 5 864 5.864 1.953 5,864 5.864 1.955 I
I
A t 0.8' the [H+] = 0.218 M , at 25.1' the [H+] = 0.968 M.
shows the formation of the activated complex with the
Ward and Welch18 have found the value of the formation constant for PuC12+to be 1.1 LW--~. However, Pu3+ Np02" HZO = this value has been criticizedz1as having been measured [ P u * O H ~ j S p 0 2 ~ + ] *H+ (14) on solutions contaminated by Pu(IV), in which case the true value of PPuClzt would be smaller than 1.1 prior loss of one hydrogen ion. f!-l. The chloride complexing of Kp(V1) has been An interpretation of the chloride dependence restudied in connection with work on the kinetics of quires the formulation of a rate law in terms of the isotopic exchange reactions,19 but the interpretation species actually present in the solution of the results may be questioned since the A H o red [Pu(IV)]/dt = ported for the SpO2C1+complex is negative, whereas the data for UOzCl+22 and PuOzCl+2o indicate the AHO's [PU"]]K~OQ~+](IC, kb[Cl-] k0[Cl-]*) (15) to be positive. I n the case of U(VI), Day and PowemQ2reported that only the 1:1 complex, UOzCl+, The concentrations of the species indicated here can seemed important and found its formation constant, be estimated by considering the chloride complexing PUO~CI-, to be 0.88 M - l a t 25". Newton and BakePo of Pu(II1) and Np(V1). studied the chloride complexes of Pu(V1) and found Studies of the chloride complexing of P U ( I I I ) ~ ~ 25' the formation constants for Pu02Cl+ and that a t indicate that only the 1 : 1 complex, PuCl2+, is imporPuO2Clt were 1.25 M - land 0.35M - 2 , respectively. tant in solutions less than 1 M in chloride. However, Since the values of the formation constants for the studies of Np(VI)l0 and its analog Pu(VI)~Oindicate three complexes involved appear to be somewhat unthat two chloride complexes of Kp(VI), NpOzC1+ certain, the values of k,, k b , and IC, were calculated by and ?JpO&l2, are probably important. I n terms of a least-squares method using a range of values for the formation constants for the three complexes, the 61, P , I and P3. These values, summarized in Table VII, following relationships hold indicate that the values of k, and lcb are not significantly [Pu(III)] = [Pu"](l P1[Cl-]) affected by uncertainties in the formation constants, (16) whereas the values obtained for k, are quite dependent and on the values chosen for PI, Pz, and P3. [Np(VI)] = [Np02"+1(1 Pz[Cl-l 4- P3[C1-12) (17) The uncertainties associated with each calculation shown in Table VI1 are the standard deviations calwhere PI, Pz, and P 3 are the formation constants for culated by the least-squares program. In the cases of the PuC12+, KpO,Cl+, and Np02C1, complexes, respecka and lib these statistical uncertainties are larger than tively. those resulting from the different assumptions about I n view of the above, the rate law in terms of the stoichiometric concentrations becomes
+
+
+
+
+
+
+
d[Pu(IV)]/dt
=
[Pu(III)][Np(VI)] X
It should be noted that le,
= lcd
The Journal of Physical Chemistry
+ k,[H+J-'.
(18) ,M.Ward and G. A. Welch, J . Inorg. Nucl. Chem., 2, 395 (1956). (19) D. Cohen, J. C. Sullivan, and J. C . Hindman, J . Amer. Chem. SOC.,77, 4964 (1955). (20) T,W. Newton and F. B . Baker, J . Phys. Chem., 61, 934 (1957). (21) See ref 6, p 305. (22) R. A. Day, Jr. and R . M. Powers, J . AmeT. Chern. Soc., 76, 3895 (1954).
1667
THEKINETICS OF THE OXIDATION OF PLUTONIUM(III) BY NEPTUNIUM(VI) Table VII: Values of k,, kb, and k, a t 25" and Unit Ionic St'rength
1.o
1.10 1.10 0.82 0.82
0.80 1.25 0.88 1.25
0.00 0.35 0.00 0.35
0.1
1.10 1,lO 0.82 0.82
0.80 1.25 0.88 1.25
0.00 0.35 0.00 0.35
38.1 =!= 0 . 3 38.2 z t 0.2 38.1 1 0 . 4 38.2 jz 0 . 3 Av 38.2 f 0 . 3 69,8 I 1 . 0 69.9 f 1 . 2 69.8 f 1 . 0 69.9 1 . O Av 6 9 , 8 & 1.0
*
953 34 1314 Et 26 849 f 35 1137 3Z 28 1063 i 251 1010 3z 90 1402 f.62 898 zk 88 1213 =!= 92 1131 f 271
355 A 9 353 6 351 f 9 350 =k 7 352 =t9 447 =k 25 460 =k 65 436 & 25 447 =k 25 447 =t25
*
Table VI11 : Thermodynamic and Activation Parameters for Similar Reactions@ Reaction
Np(W
+U(W
+ Pu(V1) Pu(II1) + Np(V1) Pu(II1) U(II1)
+ U(V1)
Np(II1)
+ Np(V1)
AFO
AH0
ASo
k(26')
AF*
AH*
AS*
Ref
-42.6 3Z0.6 -40.4 f 0 . 3 -39.73 i 0.16 -22.3 f 0 . 6 -26.1 f.0.25 (at p = 0.1) - 32* -35.7 f . 0 . 4 ( a t p = 0.1 M )
4 5 This work
2.06 1.51 -3.59 -16.0
-8.7 -9.3 -14.5 -26.8
-36.0 -36.1 -36.6 -36
39 2.7 35.5 5 . 5 x 104 1.14 X l o 4
15.39 16.86 15.34 11.0 11.92
2.60f0.16 4.83A0.09 3.49 & 0.05 4.33 f 0 . 1 8 4.15 zk 0.07
-22.6
-33.8
-38
1.05 x 106 2.18 X lo4
10.6 11.54
1.0 O.9O=kOO.11
C
C
Conditions: 25' and 1.0 M HCIOa,unless otherwise indicated. Heats and free energies are in kcal/mol, entropies are in cal/mol deg, and rate constants are in M - 1 sec-1. b The values were estimated for 1 M HClOd from those observed in 0.1 M €IC104 under the assumption that the ionic strength dependence is the same as for the U(II1)-U(V1) reaction. c T. W. Newton and R. B. Fulton, J . Phys. Chem., in press.
the formation constants. However, the lack of precise values for p', p2, and ps causes an uncertainty in k, much larger than the calculated standard deviations. The uncertainties listed with the average values in Table VI1 are our estimates of the actual ones. The values of k b a t 0.1 M and 1.0 M HClOl indicate that it, like k,, has an inverse [H+] dependence. This dependence can be described by kb = (kb)O (kb)-l' [H+]-' with (kb)O = 342 M-' sec-' and (kb)-1 = 10.5 M-' sec-'. The small apparent [H+] dependence shown for k, is probably not significant because of its large uncertainty. At 25' and 1 M HCI04, the ratio h / k a is about 9.2. The corresponding ratio for the known chloride bridged Cr(I1)-Fe(II1) reaction is about Therefore, it is reasonable to assume that the Pu(II1)-Np(V1) reaction takes place by either an outer-sphere mechanism or an inner-sphere mechanism in which the chloride does not occupy a bridging position. Assuming that the Eyring equation is applicable to the rate constants in eq 12, the data in Table I11 were treated by least squares to extract values of the activation parameters which best reproduce the observed values of kl. The results are M d * = 3.49 f 0.05 kcal/mol and ASd* = 39.73 f 0.16 cal/mol deg for the
+
hydrogen ion independent path and AHe* = 12.82 i 0.12 kcal/mol and AS,* = -13.3 0.4 cal/mol deg for the other path; the Uncertainties are the standard deviations computed by the least-squares program. The calculated values in Table 111 are based on these values of A H d * , AHe*, ASd*, and A&*. The definite but relatively small [H+] dependence found in the Pu(II1)-Np(V1) reaction is interesting for two reasons: first, no hydrogen ions are involved in the overall reaction, and second, no [H+]dependence was observed for the Pu(II1)-Pu(V1) reaction, while that observed for the Np(II1)-U(V1) reaction was small enough to be explained in terms of medium effects. Since both the Pu(II1)-Pu(IV) and Np(V)-Np(V1) exchange reactions have been studied, it is of interest t o see whether k d is consistent with eq 19, the Marcus
*
hi2
=
(knk2~Kizf)~'~;
log f = (log K d 2 / [4log ( k l l k 2 2 / 2 2 ) 1 (19) cross relation.24 At 0" the rate constant for the hydrogen ion independent part of the Pu(II1)-Pu(1V) ex(23) G. Dulz and N. Sutin, J . Amer. Chem. Soe., 86, 829 (1964). (24) R. A. Marcus, J . Phvs. Chem., 67, 853 (1963).
Volume 74, Number 8 April 16, 1970
1668
R. B. FULTON AND T. W. NEWTON
Table IX : Equilibrium and Rate Conxtants Used Con-
Reaction
+ H,O
PU4+ Pu3+ 2Pu4+
+ + + +
PuOHaf H+ Pu4+ NpOl;+ 2Hz0 = PuOz+ Pus+ 4H+ Np0z2+ 2Hz0 = PuOZ' NpOn+ PuOa' = P U S + PuOe*+ =
+ NpOaa+ +
+
+ + + + PuOzt + NpOp'+ = PuO$+ + NpOz+ Pu'+
Pu4+
--Values
stant
Kh k4
3.1 X 3.5 x 3.8 X 3.2 X
ks
4.0
Qi
+ 4H+
k3
of
x 2.4 x
constants----
Refer450
25O
10-2 102 10-5 10-8 10' 109
6.9 X
7 . 5 x 101 2 . 4 x io-*
3 . 4 x 10-2 1 . 7 X los 4 . 7 x 109
nn
em0
...
b,h,i
... -3 -2 0
c
d
e
f
ke 0 9 a Hydrogen ion dependence, [Hf]". References h and i, AH' a t p = 1 M taken to be the same as that determined at = 2 M . Reference 4. 25" value and n from ref 5 , 45' value from this work. e This work. Reference 3. 0 ks found to be 1.05 X loa nil-1 sec-' at 2') A S * and n assumed to be the same as for the analogous Np(V)-Np(V1) exchange reaction, ref 24. * S. W. Rabideau and J. I?. Lemons, J. Amer. Chem. floc,, 73,2895 (1961). S. W. Rabideau, ibid., 79,3675 (1937).
change reaction was found to be 183 M-l ~ e c - l , ~ ~kcal/mol. It appears that the ions involved show indiand that for the Np(V)-Np(V1) exchange reaction is vidual characteristics probably based on their elec30 M-' sec-1.26 Since these exchange reactions may tronic configurations. occur at least in part by inner-sphere mechanisms the Acknowledgment. The authors gratefully acknowlrate constants are upper limits for the outer-sphere conedge many helpful discussions with Dr. C. E. Holley, stants required in (19). The equilibrium quotient Jr. , under whose general direction this work was done. for the Pu(II1)-hTp(V1) reaction at 0" was calculated Appendix from the heat and entropy data,6 to be 3250. Substituting these values into (19) leads to a value of 5 Under some conditions reactions 2-6 contributed 3400 M-l sec-l for the outer-sphere rate constant. appreciably t o the observed absorbance changes, For This is not inconsistent with the observed value of these conditions the calculation of kl required values 19.4 M-l sec-', and indicates that one or both of the for &I, k8-k6, and the appropriate extinction coefficients, exchange reactions is predominantly inner sphere. The results for the five actinide(II1)-actinide(V1) Table X : Extinction Coefficients a t 45' reactions which have been studied are summarized in Table VIII. The overall reactions are formally idenIon f4698 fEOO5 tical and are similar in the sense that the AX" values Pu3 + 2.70 35.35 are the same within 2 cal/mol deg. Except for the 6.00 2.47 NpOa2+ Np(II1)-U(V1) reaction, the AF* values increase 3.15 3.41 NpO2+ smoothly with AP". This is qualitatively in accord Pu'+ ( E O ) 59.50 0.50 with the Marcus cross relation with the additional P u ( O H ) ~(€1) + 12.05 6.89 PuOn2+ 10.8 f O.O3/[H+] 1.28 assumption that the rates of the actinide M3+-M4+ and the MOz+--MQ~z+exchange reactions do not depend strongly on the atomic number. The agreement LIS previously described. I n all cases the effect was is not quantitative, however, since the average slope relatively small, so highly accurate values were not of observed AF" vs. AF" is about 0.29 compared with needed. Where the necessary rate constants could the value of 0.4 from the Marcus relation, corrected not be found in the literature they were determined forf. spectrophotometrically with sufficient precision for Quantitative agreement with (19) can be obtained these calculations. The hydrogen ion dependences only by accepting unreasonably large values for the for the reactions involving Pu(1V) were corrected for ratios of the rate constants for similar exchange reacits hydrolysis, which is particularly important in solutions. For example, the observed rate constants for tions at low acid and higher temperatures. Values the Pu(II1)-Np(V1) and the Kp(II1)-Np(V1) reacand sources of these various constants are listed in tions lead to a rate constant for the Pu(II1)-Pu(1V) Table IX. Values for other temperatures were obexchange which is about 3 X lo4 times as large as for tained by interpolation or extrapolation. the Np (111)-Xp (IV) exchange. Values for the various extinction coefficients were When the other activation parameters, AH* and determined at 45' and found to be in good agreement AS*, are examined, however, the regular dependence of AF* on AF' appears to be largely fortuitous. Thus (25) T. K. Keenan, J. Phys. Chem., 61, 1117 (1957). the AS* values range from -22 to -42.6 cal/mol deg (28) D.Cohen, J. C. Sullivan, and J. C. Hindman, J . Amer. Chem. and the AH* values vary irregularly from 1 to 4.8 SOC., 76,352 (1954). The Journal of Physical Chemistry
THEKINETICS OF
THE
OXIDATIONOF PLUTONIUM(III) BY NEPTUNIUM(VI)
Table XI: Effect of Changing the Values of the Parameters, 44.7' and 0.06 M HClOd Parameter
changed
% Change
% Change
in parameter
in kt
-0.5 -0.6
1.0 0.4 0.2 0.2
-5.9
-0.5 -1.5 -3.8 -33.0 -13.6 -7.3 200
0.2 0.1 1.1
0.6 0.4 1.3
with published values for 25' . 2 7 , 2 8 This agreement indicates that the temperature dependences of the extinction coefficients are quite small. The values chosen are shown in Table X. Calculations were made which show that the parameters involved in the corrections for reactions 2-6
1669
need not be known with high accuracy. The data from a typical run at low acid and high temperature were recalculated after changing each parameter in turn. The size of the change was chosen to represent the uncertainty in the parameter. The results, summarized in Table XI, show that reasonable uncertainties in the parameters introduce negligible uncertainties in the values for h. All of the calculations discussed above were made under the simplifying assumption that the occurrence of reactions 3 through 6 in the reverse directions could be ignored. However, these back-reactions were included in recalculations of eight runs representing several temperatures and [Hfl's. The effects of these back-reactions were found to be insignificant; in the least favorable case, low [H+] and high temperature, the recalculated rate constants differed from the originals by less than 2%. (27) R.Sjoblom and J. C . Hindman, J . Amer. Chem. Soc., 7 3 , 1744
(1951). (28) D. Cohen,
J. Inorg, Nucl, Chem., 18, 211 (1961).
Volume 74, Number 8 April 16,1970
