KINETICS AND REACTION OF NEPTUNIUM(IV), (V)
Aug. 5, 1957
trast, lithium appears t o behave more like calcium in that no trimethylboron has been found among the reaction products, and the dimethylborohydrides of both of these metals have been isolated. The explanation of the differences in the apparent stabilities of these salts is not obvious, although the lattice energies of the calcium and lithium salts would certainly be greater than those of the sodium and potassium salts, and the lattice energy may well serve to stabilize the dimethylborohydride
[CONTRIBUTION FROM
THE
AND
4029
(VI) IONS
ion in the solid state. I n solution it could be postulated that ion aggregation, also dependent upon change density, would serve to stabilize the calcium and lithium salts. Acknowledgments.-This work was sponsored by the Office of Ordnance Research, U. S. Army. The author gratefully acknowledges the many helpful comments and suggestions offered by Professor Anton B. Burg, of the University of Southern California, with whom this work was started. HOUSTON 4, TEXAS
CHEMISTRY DIVISION,ARGONNE NATIONAL LABORATORY]
Kinetics of Reactions Involving Neptunium(1V), Neptunium(V) and Neptunium(V1) Ions in Sulfate Medial BY
J.
c. SULLIVAN, DONALD COHENAND J. c. HINDMAN RECEIVED APRIL5, 1957
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The rates of the forward and reverse reactions involved in the equilibrium Np(1V) Np(V1) $2Np(V) have been measured in sulfate solution. Both forward and reverse reactions involve multiple parallel reaction paths. The rate of the forward reaction can be expressed in the form R = k ~ ' [ N p +[NpOl++] ~] [ H + I d 2 (k![NpS04++][NpOz++] k ~ [ N p S 0 4 + + ] [Np02S04])(ka[H+]-2 kr[H+]-3]. At 25' and p = 2.2, ko' = 2.69 mole 1.-' min.-l, kl = 4.27 mole-'1. min.-l, k2 = 7.12 mole-' I. rnim-1, ka = 3.6 mole2 I.-* and kc = 1.7 mole3 The rate of the disproportionation reaction may be represepted by R = [ks[NpO~+]~[HS04-] Ks[NpOz+]2[HSO~-]2) (k7 ka[H+]}. At 25" and p = 2.2, ks = 0.30 mole-* min.-', ks = 0.090 mole-3 La min.-I, k7 = 0.74, and ks = 0.14 mole I.-'. Apparent activation energies are given. Probable restrictions on the geometrical configurations for the transition state complexes and the mechanism of the electron transfer processes are considered. Relationshim of the reactions in sulfate solution to similar reactions occurring in perchlorate media are discussed.
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Three prior investigations of the kinetics of the neptunium(1V)-neptunium(V1) reaction have been r e p ~ r t e d . ~ -I~n perchloric acid solution the mechanism of the reaction appears to involve hydrolysis products of neptunium(1V) .2 I n mixed ethylene glycol-water media the mechanism is altered, ethylene glycol apparently being present in the activated complex.s I n sulfate solution the only measurements made were in one molar sulfuric acid.4 The present research has been carried out to see if alternate paths involving complex.ion species are available. The sulfate system has been selected for further investigation for two reasons. First, data on the complexity constants of neptunium(1V) and sulfate are now available.s Second, measurement of the reverse reaction, the disproportionation of neptunium(V), is possible in this medium. Interest in the latter reaction is due to the apparent difference in reaction mechanism deduced from kinetic and equilibrium data in perchlorate solution2 and the reaction which would be expected if the behavior of neptunium(V) paralleled that of uranium(V). ( 1 ) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) J. C. Hindman, J. C. Sullivan and D . Cohen, THISJOURNAL, 76, 3278 (1954). (3) D . Cohen, E. 5. Amis, J. C. Sullivan and J. C. Hindman, J . P h y s . Chcm., 60, 701 (1956). (4) L. B. Magnusson, J. C. Hindman and T. J. LaChapelle, "The Transuranium Elements," edited by G. T. Seaborg, J. J. Katz and W. M. Manning (McGraw-Hill Book Co.,New York. N. Y., 19491, National Nuclear Energy Series, Plutonium Project Record, Div. IV, Vol. 14B,paper 15.11, p. 1134. (5) J. C . Sullivan and J. C . Hindman, THISJOURNAL, 76, 5931 (1054).
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Experimental The stock solutions of the neptunium ions in perchloric acid were prepared according to previously described methods. The sulfuric acid was reagent grade standardized by titration with sodium hydroxide. The experimental technique also has been previously described.* The only major modification of that technique used in this investigation was the preparation of calibration curves necessary t o determine the concentration of Np( IV) in the sulfuric acid-perchloric acid mixtures. The interaction between Np(1V) and bisulfate is reflected in the lowering and slight shift in wave length of the band at 724 mp which is most useful for analytical purposes.
Results I. The Np(1V) Np(V1) +2Np(V) Reaction Effect of Metal Ion.-The stoichiometry of the reaction with respect to each of the metal ions was determined by experiments in which the concentration of Np(1V) and Np(V1) were varied in the reaction mixture. Data were obtained a t total sulfuric acid concentrations of 0.0865 and 1.00 molar. At zero bisulfate the reaction is bimolecular.2 I n the bisulfate solutions the data were also found to be consistent for a bimolecular process. The rate constants, Kobsd, were computed from the experimental data both by graphical evaluation of the initial rateB and by means of the integrated equation for a bimolecular reaction
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kabad
=
2.303 ([Np(IV)IO lNp(V1)Iqt log [NP(VI)lO [NP(VI)lO [ N P ( W I ~ [NP(IV)I' - x
+
(
")
(1)
( 6 ) Cf. R . Livingston, "Technique of Organic Chemistry," Vol. VIII, Interscience Publishers, lnc.. New York, N. Y . , 1953, p 182,
-1030
J. C. SULLIVAN, D. COHENAND J. C . HIXDAIAS
where [ N P ( I V ) ] and ~ [Np(VI)]O represent the initial total concentrations of reactants, and x the concentration of Np(1V) a t time t. In the calculations involving equation 1 only the data for the first lOyoof the reaction were used in order to minimize the correction for the back reaction. The data are summarized in Table I . 1)ETEKMISATIOK
KEACTION
Initial LHzS041, mole 1. - 1
TABLEI I f E T A L I O S DEPENDESCE FOR Np(Iv) xp(L'1) + 2 s p ( l - ) t = z,jc, = 2.2
OF
+
10s x [;&/W~~,
Initial /H'l,-l mole 1.
0.0865
.OS65 0865
. OX65
1 .oo I .UO 1 . 00
1.96 1.95 1.92 1.94 1,04 1.04 1.04
108 x [SP(VI)~, mole 1.
2.64 5.26 13.36 10,70 9.82 13.60 5 27
18.78 15.65 6.26 9.08 9.27 6.26 15.65
THE
Apparent rate kobsd,
TABLE I1 EFFECT O F [HSOa-] O N THE RaTE O F THE REACTION Np(IV) Sp(V1) s 2Sp(V) t = %.lo, fi = 2.2, [H+li . . = 2.20 If
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Initial f HSOi -1. mole 1. '1
(L
0.00 0.00 1 . 0 2 x 10-3 :3.07 x 10-3 3 . 2 2 x 10-3 3 . 1 1 x 10-3 6.14 x 10-3 1 . 0 2 x 10-2 1.64 X 10+ 2.02 x 10-2 3 . 0 7 X IO-? 5.12 x 10-2 9 . 2 1 x 10-2 0.1228 ,168 ,2075 ,321 ,498 ,504 ,942 ,942 1. 93IZ 11' = 2.11.
6.86 , . .
6.SU 6.86 10.52 6.80 6.86 6.S6 6.86 10.50 6.86 6.86 6.86 6.86 10.98 6.86 6.86 6.86 10.88 10.70 9.64 10.84 Reference 2.
x 103 7.30 ...
7.30 7.30 9.21 7.42 7.30 7.30 7.30 9.21 7.30 7.30 7.30 7.30 10.70 7.42 7.42 7.42 10.70 10,70 9.27 10.70
iSHSO,-],
=
+ iS0,
iIlS0
jt,ee
t
L-PG,CW,]
(4 where
3.87 4.01 3.5; 3.70 2.20 2.39 2.28
the reaction. Examination of the data in this table shows that the rate increases to a maximum and then decreases as the bisulfate concentration is further increased. The maximum in the rate oc-
Total [n'P(VC I, mole 1 . - 1
served the Np(V1) is presumably largely n'pOz++. Thus far successful measurements of the complex constants for the Xp(V1) sulfate system have not been The degree of complex formation was therefore estimated using the constants for the U(V1) system.* Calculations of the solution coiiiposition have been made using the above complexing data and a value of 0.080 for the dissociation constant of [HS04-] a t p = 2..2.9 These calculations have been made as
1. mole-' min. - 1
Effect of [HSOc-].-Table I1 summarizes the data on the effect of bisulfate on the apparent rate of
Total [NP(IV) I , mole 1 . - 1 x 103
\.ol. 70
Apparent r a t e kobsd..
1. mole-1
min. - 1
0.59' 0.61 0,776 1.26 1.64 1.13 1.61 2.12 2.58 2.21 3.11 3.38 3.36 3.20 2.51 2.65 2.17 1.57 1.36 0.716 0.760 0.320
curs when F Z I ~is close to one, that is, where the average number of sulfate groups attached to a neptunium(1V) ion is unity; (see Fig. 1). It would therefore appear that the rate is increased by the addition of a single sulfate ion to the Np(1V). At the point where the maximum in the rate is oh-
arid [H-'1 =
+ [so,.- . j f + [ Z v i C ~ r r i ]
[l€--]i
(6)
where [ H + ] i = initial hydrogen ion coricn.
and solving for [HS04-11, we get
(7)
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[HSOI-I~'- [ H S O I - ] ~ ( ~ [ Z H S O If- ; ~[H"]i 0.0SO [ Z n i C ~ ~ it l ) ([H"Ii[ZHS04-ji t [BHS04-]i2 [IIT]i[%L'ifii] - [ZHSO,-],[iliCiiiij) = 0 (SI
The quadratic is solved for [HSOd-If. T o obtain a final value for the bisulfate concentration i t is necessary to make successive approximations. This is done as follows. The fiiCmi terms are obtained using fi values taken from a plot of fi versus free bisulfate or by solution of equation 3. The plot is obtained using 81 = 270:/[H+] and P 2 = 29970 ' [H+I2for the constants of the Xp(1V) system a t a given acidity and 81= G.25j[H+] and 8 2 = i . G / [H+l2for approximating the constants of the N p (VI) sulfate system. In the first approximation the free [HSOd-] is set equal to the total sulfuric acid concentration and the fi values read from the plot. [HSOL-]~values are then calculated froin equation 8 and the process repeated. The degree of formation of the individual coinplex is computed from the relation a i.
[hIXi] - ___
-
CliZ
=
$.[ i . l i -a-I ! ) l J 1t Bi[.I]i i=l
whence concentrations of the individual complex species are calculated readily. The decrease in kobsd (Table 11) a t high bisulfate concentration is coincident with the formation of the complexes N P ( S O ~ )and ~ Np02(S04)2--, suggesting that these complexes do not contribute appreciably to the reaction. The general rate equation that would take into account the complex de-
c.
(7) 1. Sullivan, D. Cohen and J . C .Ilindman, T H I S J O O K N A I . , 77. 0 2 6 3 (1955). (8) R . A. Day, Jr., and K . R I . Pouer5, ibc
