J. C. SHEPPARD
1190
of the two solvents. Thus the A constant of the Debye-Huckel equation is 0.5115 mole-l’z 1.’’’ for ordinary water a t 25’ and 0.5151 for heavy water,
whereas Ado”z is 0.5108 for ordina-y water and 0.5413 for heavy water. The value of for DC1 in DzO is about 30% higher than for HC1 in H20.9v12
z,
The Kinetics of Reduction of Neptunium(V1) by Vanadium (111) in Perchloric Acid’
by J. C. Sheppard Hanford Laboratories, General Electric Company, Richland, Washington (Received September 13,1963)
The rate law for the reduction of neptunium(V1) by vanadium(II1) in perchloric acid obeys the expression, rate = [Np(VI)][V(III)]fk k’/[H+]]. At 25’ and a t an ionic strength of 2.0, the values for k and k‘ are 6.3 f 1.2 M-l sec.-l and 20.3 f 0.5 sec.-l, respectively. The corresponding energies and entropies of activation are 18.6 f 1 kcal./ mole and 5 e.u. for the hydrogen ion concentration independent path and 22 f 4 kcal./ mole and 19 e.u. for the hydrogen ion concentration dependent path.
+
The study of the kinetics of reduction of neptunium(VI) by vanadium(II1) in perchloric acid provides an opportunity to compare results obtained for other reactions involving the reduction of neptunium(V1) . 2 , 3 Since neptunium(V1) is isostructural with plutonium(VI), it is also of interest to compare the rates and kinetics of reduction of these ions with a common reducing agent, vanadium(II1). Preparation of Reagents. All reagents used in this investigation, unless specified otherwise, were reagent grade. Sodium perchlorate solutions were prepared by addition of an equivalent amount of perchloric acid to a sodium carbonate slurry. Perchloric acid solutions of vanadium(II1) and vanadium(1V) were prepared in the following ways. Vanadium pentoxide, formed by the ignition of ammonium metavanadate, was suspended in 1 M perchloric acid and electrolyzed until vanadium(II1) was f ~ r m e d . A ~ second preparation involved the dissolution of ammonium metavanadate in hot, dilute sodium hydroxide. The centrifuged solution was acidified with perchloric acid to precipitate the hydrated vanadium pentoxide. The Journal of Physical Chemistry
The oxide was washed several times with dilute perchloric acid and then suspended in 1 M perchloric acid where it was electrolyzed to form vanadium(III).5 Neptunium(V1) stock solutions were prepared by ozone or electrolytic oxidation of neptunium(V) in 1 M perchloric acid. The results of the kinetic experiments were not dependent on the mode of preparation of the reactants. Analytical. The concentrations of vanadium(II1) and vanadium(1V) in perchloric acid solutions were determined spectrophotometrically using a Cary Model 14 spectrophotometer and the molar extinction coefficients found by Appelman and S ~ l l i v a n . ~Total neptunium concentrations were determined by a(1) Work performed under contract AT-(45-1)-1350 between the General Electric Co. and the U. S.Atomic Energy Commission. (2) (a) J. C. Sullivan, A. J. Zielen, and J. C. Hindman, J . Am. Chem. Soc., 82, 5288 (1960); (b) A. J. Zielen, J. C. Sullivan, and J. C. Hindman, ibid., 80, 5632 (1958). (3) D. Cohen, J. C. Sullivan, and J. C. Hindman, ibid., 76, 352 (1954). (4) S.W. Rabideau, J . Phya. Chem., 62, 414 (1958). (5) E. H. Appelman and J. C. Sullivan, ibid., 66, 442 (1962).
KINETICSO F REDUCTION OF NEPTUNIUM(VI) BY VANADIUM(III) I N
assay as well as by coulometric analysisS6 The composition of the neptunium(V1) stock solutions were established spectrophotometrically and were found to contain less than 1.5% neptunium(V). Radiochemical purity of the neptunium was determined by a-pulse analysis. Apparatus. Reaction vessels for these experiments were 2.5-cm. silica absorption cells, each of which had a 10-cm. piece of plastic tubing connected to its neck. This allowed these cells to be immersed in the thermostated cell with a provision for the introduction of one of the reactants as well as providing for a needed air vent. The flow of water through copper coils which were inserted in the cell compartment maintained the temperature a t *0.2' of that desired. By the use of a syringe, the reaction was initiated by forcing one reactant solution fromi a storage vessel, through a Tygon tube, and into the absorption cell containing the second reactant where mixing took place. Experirnents involving the neutralization of sodium hydroxide solutions indicated that the mixing time was less than 2 sec. Another experiment in which an equivalent amount of iron(I1) in 1 M perchloric acid was injected into a neptunium(V1) solution showed that mixing and the reaction were complete in less than 2 sec. The reactant concentrations M after mixing. Assuming that the were 5 X iron(I1)-neptunium(V1) reaction obeys second-order kinetics and has a half-time of less than 1 sec. at 25', the specific rat>e constant is calculated to be greater than 2 X loa M-l see.-'. Additional checks of this experimental setup involved the determination of the rates of the Hz02-U(IV)7and Np(V1)-U(1V) reactions. Within an experimental error of about 3%, the rate constants found for these reactions agreed with those reported. l r 7 The progress of the reaction was followed by continuously monitoring the growth of the 980 mp neptu-, nium(V) peak with a Cary Model 14 spectrophotometer. To ensure that the vanadium(II1) was not air oxidizedl during the course of the reaction, all solutions, previously purged of oxygen, were purged again by bubbling; nitrogen through them for 5 to 10 imin. just prior tal their use.
Results From the addition of an excess of neptunium(V1) solution to a vanadium(II1) solution it was found that 1.96 f 0.04 equivalents of neptunium(V1) were consumed for each equivalent of vanadium(II1) ; consequently the net reaction is
V(II1)
+ 2Np(VI) = V(V) + 2Np(V)
PERCHLORIC
1191
ACID
In addition, the rate data did not fit the reaction sequence
+ Np(V1) --+ V(1V) + Kp(V) (slow and rate determining) V(1V) + Yp(V1) kz_ V(V) + Np(V) (rapid) kl
V(II1)
However, the data are consistent with the idea that two Competitive, consecutive second-order reactions take place and that the specific rate constants, lcl and le2, have comparable values. Second-order plots of the data showed pronounced deviations in the later stages due to the competing vanadium(1V)-neptunium(V1) reaction. In 2.0 M perchloric acid and a t 25", the average rate constant for the vanadium(II1)-neptunium(V1) reaction was found to be 17.5 f 1.6 M-I sec.-] while the specific rate constant for the vanadium (1V)-neptunium(V1) reaction was estimated to be 30 f 15 M-I sec.-l. By direct determination of the rate of the vanadium (IV )-neptunium (V I) reaction under these conditions, it was found to obey second-order kinetics and have a specific rate constant of 13.3 M-I sec.-l. Further investigation of this reaction will be the object of future work. In view of the difficulties associated with the analysis of competitive, consecutive secondorder reactions, only those points representing the initial 20-30y0 of the reaction were used to determine the rate constant for the vanadium(II1)-neptunium(V1) reaction., This was done in an effort to eliminate or hopefully to minimize, as much as possible, the interf erence of the vanadium (IV)-nep tunium (V I) react ion. Therefore, the data reported here should be regarded with some reserve, since the influence of the vanadium (1V)-neptunium(V1) reaction cannot be completely eliminated by this approach. The rate data for these experiments are tabulated in Table I and as shown by Fig. 1 give a good fit when the observed specific rate constant 'IC' is plotted against the reciprocal of the hydrogen ion concentration. Hence, over the range of hydrogen ion concentration studied these data are consistent with expression
'IC'
=
7c
+ IC'/[H+]
where IC is the specific rate constant for the reaction between the unhydrolyzed reactants and IC' is the specific rate constant for a path involving hydrolyzed species of either reactant. Possible reaction paths consistent with this equation are V
+ NpOzf2 --+ k
+3
(6) R. W. Stromatt, U. S.Atomic Energy Comm. Rept. HW-59447 (1959). (7) F. B. Baker and T. W. Newton, J . Phys. Chem., 65, 1897 (1961)
Volume 68, Number 6
M a y , 1.964
J. C. SHEPPARI)
1192
and
VOH+2
+ NpOzf2-+ products k”
or k’”
V+3 -I- Np020H++ or combination of the last two reactions to give k’. On the basis of the extent of hydrolysis of the reactants alone, the VOH +2--Np02+zpath would be expected to predominate. Table I1 includes the least-squares values for IC and k‘ for the three temperatures a t which thc reaction was studied. The errors reported are in terms of standard deviations. The activation energies for the hydrogen ion independent and dependent paths were calculated to be 18.6 f 1 kcal./mole and 22 f 4 kcal./mole, respectively. These values are close to activation energies found for other reactions involving the oxidation of ~ a n a d i u m ( I I I ) . ~ JThe ~ * corresponding entropies of activation are 5 and 19 e.u., respectively.
Table I : R.ate Data for the Ncptunium(V1) -Vanadium(III) Reaction at an Ionic: Strength of 2.0“ Temp.,
oc. 25
[I1 ‘1.
M
2.00 2.00 2.00 2.00 2.00 2.00 1 .oo 0.67
IV(I1I)I X 104, M
’kl,
[Np(VI)I X 104,
M
M
-1
Bee. -1
0.29 0.20
12.0 9.35 9.35 9.35 4.68 5.30 4.68 4.65 4.65 4.65 4.65 5.38
12.0 10.0 4.50 13.0 4.86 9.95 4.86 5.40 5 32 5.40 5.50 5.43
18.3f 1.8” 20.3f 2.0 15.0f 1.6 17.1f 0.8 18.0f 0.8 19.2f 0.8 24.5f1.1 35.7f 3.5 4G.4f 2.8 50.5f 3.2 75.6f 7.6 118 f 11
17
2.00 1 .oo 0 60 0.40 0 30 0.25 0.20
4.65 4.65 6.51 6.51 6.51 5.38 6.51
5.15 5.48 5.43 5.43 5.43 5.44 5.48
9.0f 0.5 12.5f 0.7 24.0f 1.7 35.3f 2.0 41.3f 2.0 .52.3f 3.5 62.0f 6.0
34
2.00 1.40 1 00 0.70 0.60 0.40
5.38 5.38 5.38 5.38 5.38 5.38
5.08 4.98 4.98 5.37 5.26 5.38
41.5zk2.0 61.7f 5.7 80.5f 8.0 110 f 15 127 f 17 177 f 25
0 . t50 0 40
a The ionic strength was adjusted with sodium perchlorate. These estirtiated errors undoubtedly reflect the contribution of the coinpcting vanadium( 1V)-neptunium( VI) reaction.
Figure 1. Plot of the observed rate constant against the reciprocal of the hydrogen ion concentration.
Table 11: Summary of Rate Data for the V(II1)-Np(V1) Reaction in Perchloric Acid” M
OC.
17.0 25.0 34.0 a
p =
-1
k’.
see. -1
BCC. -1
2.4rt 1.4 6.3f 1.2 12 4 f 2 . 8
12.2f 0.5 20 3 f 0 . 5 67 l f 3 1
2.0,sdjuhted with sodium perchlorate.
It is of interest to compare the rate expressions for the neptunium(V1)--vanadium(III) and plutonium(VI)-vanadium (I I I)4 reactions, since plutonium(V1) and neptunium(V1) are isostructural and have a common reducing agent, vanadium(II1). I t seems likely that both reactions would obey the same rate law but have diffcrent specific rate constants. The rate laws for these reactions are
and rate
h
The Journal of Physical Chemistry
k.
Temp..
(8) W.
=
( + &I)
(Sp(VI)][V(TII)]k
C.E. Higginson and A. G. Sykes, J . Chem. Soc., 2841
(1962)
KINETICSOF REDUCTION OF NEPTUNIUM( VI)
BY
VANADIUM(III) IN PERCHLORIC ACID
The rate laws for these reactions differ in that the path involving the unhydrolyxed reactants appears to be negligible in the case of the plutonium(V1)-vanadium(II1) r e a ~ t i o n . ~But upon closer examination it was found that the data for the plutonium(V1)vanadium(II1) reaction could also be better fitted by the equation rate
=
(
11’-u(VI)][V(III)] IC
k’ IC” + [H+l $- m) I _
where the values for k , IC’, and k t t a t 2.5’ are 0.20 M-I set.-', 1.8 sec.-l, and 0.19 M set.-', respectively. Accepting these specific rate constants for the plutonium(V1)-vanadium(II1) reaction, the rate laws are the same except the missing third term for the neptunium(V1)-vaiiadium(II1) reaction and it may be accounted for by the fact that this reaction was studied over a smaller hydrogen ion concen1,ration range. Regardless of the rate law assumed for the plutonium(V1)vanadium(II1) reaction, the path involving the hydrolyzed species, the 1/ [H+] path, predominates as
1193
does the comparable path of the neptunium(V1)vanadium( 111)reaction. It is not possible to make a detailed comparison of the entropies and energies of activation between the vanadium(II1)-neptunium(V1) and vanadium(111)-plutonium(V1) reactions, since these activation parameters are not known for each path of the latter r e a ~ t i o n . ~Furthermore, the activation parameters for the former reaction, which have relatively large errors, indicate possible nonlinearity with respect to the Arrhenius plot. This may be due to a salt effect on the reaction and/or that the perturbing influence of the vanadium(1V)--neptunium(VI) reaction cannot be excluded. Comparison of the hydrogen ion concentration dependent paths of these reactions shows large differences in their activation parameters. Activation energies for these paths are 16 and 22 kcal./ mole for the reaction of vanadium(II1) with Plutonium(VI)4 and neptunium(VI), respectively, and the corresponding entropies of activation are -10 and 19 e.u. For very similar reactions these differences are larger than expected and are probably due to complications noted above.
Volume 68, Number 6 M a y , 1064
