method thus depends on the system being studied. -4t any rate, the same general equipment is used in both methods.
Acknowledgment.-The investigation was supported in part by the Office of Naval Research and by the National Science Foundation.
KISETICS OF THE REACTION BETIYEEX Pu(V1) AND Sn(I1) I N CHLORIDEPERCHLORATE SOLUTION' BY S. W. RABIDEAU AND B. J. MASTERS Universzty of California,Los Alamos Scientific Laboratory, Los Alamos, New Mexico Received January 26* 1961
+
+
A study has been made of the kinetics of the reaction, Pu(V1) Sn(I1) + Pu(1V) Sn(1V). In mixed chloride-perchlorate media, the rate of Pu(V1) reduction is firsborder dependent upon the concentration of each reactant, strongly dependent upon the chloride ion concentration and independent of the hydrogen ion concentration. These observations are consistent, with a parallel path mechanism in which the reaction proceeds primarily through activated complexes having the formulas (PuO*.dn.Cls) and (PuO2&.Cl4)0. Thermodynamic quantities for the activation processes have been obtained for these two reaction paths, and the correlation between charge and entropy has been found to a ply to these activated complexes of charge + 1 and zero. The experimental observation t h a t the reduction of Pu(V1) i y Sn(I1) occurs much more rapidly than does the reduction of Pu(V) by Sn(I1) has been interpreted as evidence t h a t the Pu(V1)-Sn(I1) reaction proceeds through a single step mechanism which involves the transfer of two equivalents of charge. +
Introduction Kinetic studies of the reactions of plutonyl ion with various reducing agents such as, for example, 'I'i(III),2aT'(III)2b and U(IV)3 have been made not) only because of the variety of mechanistic possibilities offered by these systems and the desire to provide a 'background of chemical information for this actinide element,, but also because there appear to be (opportunities provided by the study of these reactions to focus attention upon factors which may aid in the correlation of the relative rates of the chemical reactions. In the reductions of Fe(III)4 and P U ( I V ) with ~ Sn(II), it has been observed that although the reaction rate is very slow in perchlorate solution, in the presence of chloride ion a marked acceleration in rat'e is found. This observation has aleo been found to be applicable to the Pu(V1)-Sn(I1) system in the present work. It appears that the introduction of chloride ions into the activated complex provides a much more effective path for the oxidation-reduction process than is available in perchlorate solution even under conditions of low acidity. Experimental
chloride and the standardization of the hydrochloric and perchloric acids followed procedures described p r e v i ~ u s l y . ~ Analyses.-Solutions of Fe(II1) in hydrochloric acid were prepared from C.P. grade Fe2O3 and were standardized volumetrically against potassium permanganate with the use of the Zimmermann-Reinhardt procedure. Standard iron wire was carried through the analysis procedure as a control. Stock solutions of Sn(I1) were analyzed by the addition of the sample to R slight excess of an argon-flushed 2 M hydrochloric acid solution of Fe(II1). The stannous ion concentration in the stock solution was computed from the spectrophotometrically observed decrease in ferric ion concentration. At the reagent concentrations used, a significant time was required for essentially complete reaction. The kinetics of the reaction between Fe(II1) and Sn(I1) in hydrochloric acid solution has been studied .6 All spectrophotometric measurements were made with the Cary Modcl 14 instrument. A molar extinction coefficient of 2,350 w ~ s measured for Fe(II1) in 2 M hydrochloric acid a t 3350 A. with a slit width of 0.10 mm. The concentration of Pu(V1) was followed in the rate measurements as a function of time from optical density measurements at a wave length of 8298 A. and a slit width of 0.10 mm. Molar extinction coefficients of standard Pu(VI) solutions were measured in each experiment under varied conditions of solution composition and tempersture. The contribution to the optical densities a t 8298 A. by Sn(II), Sn(IV), Pu(II1) and Pu(1V) was considered to be negligible a t the concentrations of metal ions used In these experiments. In experiments in which the concentration of Pu(V) was Materials.-Stocksolutionsof 1.3 X 10-3 to 1.1 X 10-*M followed as a function of time, the sharp absorption peak a t stannous chloride in hydrochloric acid were prepared from 5674 b. in dilute hydrochloric-perchloric acid solution was Baker and Adamson reagent grade crystalline SnClp.2H20. used. Procedure and Apparatus.-It was necessary to flush the These solutions were protected from atmospheric oxygen with the use of a vessel fitted with stopcocks through which spectrophotometer cell and all reagent solutions with puriwas directed a stream of Oxsorbent-scrubbed argon. Pluto- fied argon to reduce the atmospheric oxidation of the Sn(I1) M nium(V1) perchlorat,e solutions were prepared from weighed to negligible proportions. Wlth this precaution, samples of high-purity metal. After dissolution of the metal solutions of stannous chloride could be used without loss of in a weighed quantity of standardized 70% perchloric acid, titer because of air oxidation. Weight aliquots of the Puthe solution w a s ozonized overnight. Prior to use, this (VI) stock solution were added to one leg of the doublechambered spectrophotometer cell. The Sn(I1) solution s! ock solution was subjected to an additional period of ozonization to ensure quantitative conversion of the plutonium to together with appropriate quantities of acid and salt soluthe pilltony1 form. The residue of dissolved ozone was re- tions were added to the second leg. Prior to mixing the moved b y prolonged flushing with purified argon. The solutions in the two compartments, thermal equilibrium was purificatio.n of tho wnter, sodium perchlorate and the sodium attained by the solutions in a water thermostat. The cell . . -. was then placed in a water-thermostated compartment (1) This w o r k WEIS done under the auspices of the United States within the Cary instrument. The total volume of mixed Atomic Energy Commission. solutions was computed for each rate run from the measured ( 2 ) (a) 5. W. Rabideau and R Kline, J . Phys. Chem., 68, 1502 solution densities and the weights of the component sol~i(1959); (b) 62, 414 (1928). tions. ( 3 ) 1'. K. Nerytori, ibid., 62, 943 (1958). (4) 11. H. Gorin, J . A m . Chem. Soc., 58, 1787 (1936). ( 5 ) S. W. Rabidenu, J . Phys. Cheni., 64, 1491 (1960).
(6) F. R. Duke and R. C. Pinkerton, J . A m . Chem. Soc., 78, 3045 (1951).
Iicaldensities oersus time a t $293 A,, initial reactant concentrations, molar extinction roefficients, stoichiammetric reaction coefficients and cell length, specific reaction rate constants were computed which corresponded to the least squares fit of the data. Input optical densities were used up to the point a t which it was calculated that lTo of the Sn(I1) had been consumed in the reduction of Pu(1V). This generally corresponded to about 507, reduction of the Pu(V1). .ilso, the associated uncertainties of the specilk rat,e constants as reflected by their standard devintions constituted a part of the program printout. If the equation for the formation of the activated complex is written in lrrnij of the uncomplexed reactant ions, then appropriate correction terms must be applied to the experimentally measured apparent rate constant obtained in chloride solutions to take into consideration the fact t,hat a fraction of each of the metal ion species is present in the form of chloro-complexes. For stannoris ion, the relationship between the uncomplpxed form, Sn ++,and the total bivalent tin, Sn(II), is :iven bj- the expression !Sn++]= [SlI(II)]/(l pl[cl-] p?[C1-]2 p3[C1-]3) (1) where the Bn values represmt, the successive formation quotients for the stannous chloro-complexes containing one, two and three chlorides, respectively, and molar concentrations are enclosed hv brackets. The general expression for the Sn(I1) formation quotient is 19, = [SnCl,] +2-./[Snf+] [CI-ln (2)
1257
Results Stoichiometry.-In chloride-perchlorate acid s+ lutions, Pu(V1) is reduced rapidly by Sn(I1). From measurements made a t a wave length of 8298 k. it was observed that the change in the concentration of Pu(V1) with the addition of known quantities of Sn(I1) corresponded to the reductio11 of one mole of Pu(V1) for each mole of Sn(I1). These stoichiometry measurements were made in solutions which contained chloride ion in the concentration of 0.5 111. Thus, it was concluded that the stoichiometry of the reaction can be represented by the equation Pu(V1)
+ Sn(I1) = Pu(TV) + Sn(1V)
(9)
In the presence of chloride ion concentrations of approximately 1.5-2.0 M , less than m e mole of Pu(V1) is reduced per mole of Sil(T1) hec:iiisr of the effective competition of Pu(1T') for the SiiiII). The Pu(1V) -Sn(II) reaction makeQ a greater + + + contribution to the observed stoichiometry at higher chloride ion concentrations berawe of i t larger chloride ion dependen~e.~ Pu(V)-Sn(T1) Reaction.-To determine whether Pu(V) is an intermediate product in the reduction of Pu(T'1) with Sn(I1). the rate of the Pu(\-)-Sn(II) reaction mas euamined. Pu(V) was prepared in 0.1 41 hydrochloric-O.1 M perchloric acid solution A report of t h e non-linear least squares procedure which was by the reduction of Pu(V1) with Pu(IT1). Evenu s 4 to obt>ainthe &values for the Sn!TI j chloro-complexes tially equimolar quantitiep of Pu(TrT) a i d Yu(I1I) hits been prepared.' A portion of the Pu(V1) is also present in the solutions in t,hc form of a chloro-complex or complexes were used in the initial solution. The equilibrium and the apparent rat,e constant, must be adjusted to reflect reaction
this fact if thc true chloride ion dependence of the reaction rate is to he evaluated. If it is considered that a single chloro-complex of Pu(V1) is present, the relationship bet mm the uncomplexed plutonyl ion, PuOz+.+, and t.he total plutonyl ion concentration, Pu(VI), is given by IPuOz++l = [Pu(VI?I/(l ~ l ' l c l - l ) (3) v+c~r.ethe formation quotient, p' , is defined by the expression
Pu(V1)
+ Pu(II1)
= Pu(1V)
+ Pu(6')
(10)
was driven to the right by the extraction of Pu(IV) with 0.2% dibutyl phosphate in benzene. The aqueous layer was washed with benzene to remom traces of dibutyl phosphate. The procedure of Markin and TciIcKay1O which accomplished the extrnction of Pu(IT-) from Pu(V) in nitric acid solution was found in the present work to be ap61' = [l'uOzC1+l/[PnO2++l [Cl-I (4) plicable in hydrochloric acid cr in perchloric acid The expression solutions also. Solution? of Pu(V) which were ( E O - z)/[Cl-] = PI'(; - €1) (5) pinkish-yiolet in color and which contained small can be derived with the assumption of a single Pu(V1) chloro- amounts of the other oxidation states of plutonium complex, where eo, ;s a d E, correspond t o the molar extinction as revealed by spectrophotcmetric exunminatioii coefficient values for PuOz++, Pu(V1) and PuO2CI+, re- were produced by this extraction procedure. If spectively. Turther, with the assumption of' the invariance of e l . eo and 8, , with chanKes in solution composition at con- Pu(V) were an intermediate stro in the reduction reaction of Pu(S'1) with Sn(II), then since the stant ionic strength, ,values of &" have been obtained fro? plots of (eo - ?)/[Cl-] vs. Z between 2.4 and 29.6" at 8298 A. over-all reaction is rapid, it would be expected that The spectrophotonietric data for Pu(T'1) in perchloratethe reduction of Pu(V) by Pn(1I) ~voiild also hr chloride media can also be explained by t,hP assumption of rapid. However, in 0.5 M HC1 solutions coiiiainiiig successive chloro-~omplexes.~~~ If the rc,lntionship between Af concentrations of earh reactant PuO*++and Pu(V1) .is written on the h i s of a two chloro- about 2 X and no added salt, the reduction of Pu(T') by Pn(I1) complex modei, it follows that [P110:,++1 !PiI(V1)1/(1 Pi"[C1--' p ~ ~ ~ [ C l - -(6) ] ~ ) is observed to proceed milch more s l o d v than docs the reaction between Pu(TvI) and SiriTI). At where the forrrmtion quotients are givcm by 2.4', if it is assumed that the disappeunrance vf Pn" = IPIIoZCln] +?-" '!l'llOz++l [CI-]" (7) Pu(J7) is attributable solely to the reactiori with In the present, work, both the single and the two chloro- Sn(II), an upper limit for the apparent hmolccular complex models of P,,i(VI) h n w been considered. The re- rate constant of about 0.15 M-i sec*.-l iq obtained. lationship between the succr,,ssive formation quotients for Rate Law.-The rate of the reaction between the two Pu(V1) chloro-complcxw, 81'' and &", an$ the formation quotient for the sin;le Pu(V1) complex, pi , can be Pu(V1) and Sn(I1) has been found to be first order shown8 to t w r~lrirtlto in each of the reactants. The initial nio1:i.r conc"~tration ranges used for the reactants were 1.6 X 82'' = Rl"" - pl') (8) lo-' to 3.1 X lo-.' A 1 for the Pu(VJ) and 1.6 X (7) S.IT. Itn'ii k a u a n d K. 11. hIorirc. J . P h p . Chem., 66, 371 (1961).
+
+
+
( 8 ) T. W. Newton a n d 1". R. Raker, ibid.. 61, 934 (1957). (9) R. K r u h , J . A m . Ctiem. S O C . , 76, 4866 (1954).
(10) T. 1,. Markin and H. A. C. JIcKak, J I?iorg Viirl C h e m 7 , 298 (1958).
to 1.1 x :l/r for the Sn(I1). Linear second-order plots were observed to approximately 30 to 80% of completion of reaction without taking into consideration the possibility of the reaction of the product, Pu(IT'), with the Sn(I1). With this coiisecutive reaction taken into consideration, the linear portion of the iecond-order plot increased to as much ,as SOCr,. In terms of the total concentration of the metal ion apecies, the rate law can be written as -~[PII(T'I'I][if
=
-d[Pu(VI)]/dt = -d[Sn(II)]/dt = kohsd[PuoZri] [Sn++I(l P~'[Cl-l)(l P,[Cl-I Pz[C1-12 !93[C1-13) (12)
+
+
+
+
With the substitution of k' =
kobsd(1
+ Pir[C1-])(1 + Pi[c1-]-k Pz[C1-1* f ~,[c1-13) (13)
in equatioii 12, the rate law can be simplified t o
-d[$r1lII~],df [Pu(yI)][sn(II)] (11)
kohsd
Pu(V1) and with the use of the formation quotients of Sn(I1) as derived from least sqiiares ralculations,' the rate law can be witten as
-d[Pii(VI'~]/dt = -d[Sn(TI)]/dt = k'IPuOr+-] [ S I I + ~ ]
in sol~tioiisof coii-taiit chloride ion (micentration. (14) In Table I are gi\'en a
