1939
NOTES
Sept., 1963
TABLE I B
Reaotion vesisel
dn
Tm
Sphere, radius a
Positive
Sphere, radius a
Negative
Infinite slab, thickness 2a
Xegative
-[ RXo2 p,sinh(-g) -[E rsinh(7a)
26to&a*( - 1)”fl
R To2a sin ( y r ) E r sin (vu)
RT2 >[ E
Z(Ro&a*( - 1)” + l
cosh(7z) cosh (vu)
lar to a relaxation time. We note that tl of eq. 7 is greater than that of the case of Benson, which corresponds to the limit E + 0, or equivalently, y 0. We also note that the same restriction as in the steadystate solution, y a < T or pa2 < a2,must be imposed to avoid divergence of bl as t + a. For the following reasonable values of the parameters
-
20 kcal.
Q
=
E To
= 20 kcal. moleb1
3OO01K. 1 ca,l. ~ m . -deg.-l ~ (liquid) PC = cal. ~ m . -deg.-’ ~ (gas a t STP) 2.7 X cal. cm.-l sec.-l deg.-l (liquid) 1 X: K 7 >< cal. cm.-l sec.-l deg.-l (gas at STP) 2 X: mole 1.-l sec.-l (liquid) mole 1.-l sec.-l (gas at STP) a 0 1.3 X (10) =
{ =(
={
one must require that the radius of the vessel be less than 5 cm. It is of interest to look a t eq. 8 when ya is slightly less than T. Since d, is the Fourier coefficient of ym, me may write
gaseous (of the order of 0.5 hr. us. 10 sec.), invalidating the assumption of negligible convection for the liquids, and rendering the establishment times and final gradients only of qualitative use. However, the expressions for b, in Table I shorn that if r2a2 is large enough, the establishment time may be much less than in the case of Bennon, allowing us to apply the results in Table I. However, even in cases for which convection is negligible, me must expect our results to be modified by the effects of macroscopic motion of the reactants as they are introduced into the vessel at t = 0, by the effects of heat transfer by diffusion and radiation, and by the effect on the reaction rate of reactant depletion. In summary, we have seen that inclusion of a linea,rized temperature dependence of the rate of reaction can modify the establishment time of the temperature gradient significantly. For example, a value of y2u2 = n2/2 mill double the establishment time for an exothermic reaction, and a value of q2a2= a2will decrease it by a factor of oine-half for an endothermic reaction, for a spherical vessel. Acknowledgment.-The author is grateful to Professor Joseph E. Mayer for helpful discussions. OBSERVATION OF A URANIUM(V) SPECIES I N MOLTEN CHLORIDE SALT SOLUTIOXS1a
If y2a2 = a2 -- E, where E is small and positive, then dl is much larger than all other d,, and we can write
BYM. D. ADAMS,D. A. WENZ,APJD R. K. STEUNENBERG Chemical Engineering Division, Argonne National Laborafory,lb Argonne, Illznoie Received M a y 9, 1968
2aoQ~t sin (.?/a) Tpcr
(12)
if t is not too 1.arge. The same analysis can be carried out for other cases. Table I gives r m ,d,, and b, for four cases. For the infinite slab, EL cosine series is used.
where x is the distance from the plane bisecting the slab. In this case, we make the restriction ya < n/2. In the endothermic cases (p < 0), we let 172
=
-P
(14)
I n these cases, no restriction on +?a2is necessary, and in contrast to the exothermic cases, the steady state is approached more rapidly than in the case of Benson. As pointed out by Benson, the establishment time in this case is much longer for liquid systems than for
Solutions of a u.ranium(V) species which are stable in vacuo or inert atmosphere have been produced in this Laboratory during studies of reactions of uranium oxides in molten alkali and alkaline earth chloride mixtures. The pentavalent state of uranium is well known but it is seldom encountered in solution chemistry. I n inert atmosphere, solid compounds of uranium such as UC16, UF5, U0CLl2 and UOBr33 can be maintained in the pentavalent state. In aqueous solution, pentavalent uranium compounds disproportionate rapidly into uranium(1V) and uranium(V1). In acid solutions with pH about 1.5 to 2.5, the disproportionation becomes slow enough to allow some reactions to lbe studied. A number of complex compounds involving pentavalent uranium have been prepared which are moderately stable in the absence of water or oxidizing (1) (a) Work performed under the auspices of the U. S. Atomic Energy Commission; (b) operated by the University of Chicago under Contraot No. W-31-109-eng-38. (2) J. 3. Kats and E. Rabinowitch, “The Chemistry of Uranium,” National Nuclear Energy Series, Val. VIII-5, McGraw-Hill Book C o . , Ino., New York, N. Y., 1951,PI). 398, 488, 492. (a) G. Kaufman and R. Rohmer, Bull. 8oc. ohern. Francs, 1969 (1961). (4) K. W.Edwards and D. AM.Kern, Anal. Chem., 22, 1876 (1058).
1940
NoTm
were employed. The cells were attached to supplementary equipment by means of a quartz tube which was sealed to the top of each cell. Molten salt solutions were prepared in uacuo in a quartz tube separated from the cell by a sintered quartz filter. The solutions were forced through the filter into the cell by applying a slight pressure of helium. Salt mixture8 used in these investigations include: (a) the LiCl-KCl eutectic, (b) a mixture of 50 mole 70 M@;C12,30mole % NaCl, and 20 mole % KCI, and (c) an equimolar LiC1-hlgClz mixture. For the mixtures in items (b) and (c), MgCL was prepared by sublimation of commercial anhydrous material in uaeuo a t 750 to 800". All salt mixtures were purified by a method similar to that described by Laitinen.18 Anhydrous UO2C12 was prepared by the reaction of dry oxygen with sublimed anhydrous UCl, a t 350". UOCla was prepared by the reaction of UCl, with U02C12at 370". Anal. Calcd. for UOCl,: U, 66.0; C1, 20.5. Found: U, 68.8; C1,29.1. To calculate molar absorptivities, i t i s necessary t o measure the amounts of chlorine evolved as shown in eq. 1. The pressure of chlorine in a vessel of known volume was measured by means of a Booth-Cromer pressure transmitter attached to a mercury manometer.
URANIUM MONOXYTRICHLORIDE. vOC13, I N PETROLATUM MULL AT 25.C.
71
6 5 .
-u
4
8 3
r 2
,z 0
I
w' I O x 9 a
E s
SI
9 7
1
UOz'
IN LICI-KCI,
Vol. 67
650.C.
4
Results and Discussion A spectrum which could not be attributed to uranium(III), uranium(IV), or uranium(V1) species14 was obtained with a solution of MgU04 dissolved in LiC1-MgClZ a t 650'. The same spectrum was later obtained with a solution of UOZCIZin the same solvent a t the same temperature. Similar spectra then were observed in solutions produced by dissolving U308, UO3, and UOzClz in each of the salt mixtures described in the preceding section. Typical spectra in each of these salt mixtures are shown in Fig. 1. This spectrum has been assigned to a uranium(") species which is believed to result from the equilibrium reaction
n IIO L '
500
I 700
I
I 900
' ' '
I
1
I
ti00 1300 1500 WAVE LENGTH ( m p ) .
UOZClZ e UOZCl
I
I 1700
'
1 1300
Fig. 1 . S p e c t r a of UOzCl in molten chloride salts compared with spectrum of UOCL in petrolatum mull.
agents.6-10 Evidence to support the existence of the uranium(V) species, IT0 +, has been found in electrolytic reduction studies of UOzClz in both aqueous4 and in molten salt solutions.]' Experimental .4 Cary Model 14 recording spectrophotometer was modified
for high-temperature use in a manner similar to that described by Young and White.12 Fused quartz cells (1 cm.a X 4 cm.) (5) R. E. Punaer and J. F. Suttle, J . Inorg. Nucl. Chem., 13, 244 (1960). ( 6 ) R.E.Punaer and J. F. Suttle, ibid.. 90, 229 (1961). (7) H. Hecht, G. Sander, and H. Schlapmann, 2. anorg. &em. Chem.. as4. 255 (1947). ( 8 ) D. C. Bradley, B. N . Chakravarti, and A. K. Chatterjee, J . Inoru.
Nucl. Chem.,3, 367 (1667). (9) R. A. Pennernan, L. B. Asprey, and G. Sturgeon, J . Am. Chem. Soc., 84, 4808 (1962). (10) D.C. Bradley, R. N. Kapoor, and B. C. Smith, J. Chetn. Soc., 1023 (1963). (11) S. Lawroski, at d.,Rsodor F d Pi'ooess., 6,46 (1962). (12) J. P. Young and J. C. White,And. Cham., 31,1892 (1959).
+ '/ZClZ
(1)
The reaction proweds to the right spontaneously at temperatures above 600' in inert atmosphere. It can be forced to the right also at temperatures as low as 450' by continuously evacuating the cell to remove chlorine. The reaotion can be reversed by addition of chlorine to the atmosphere over the solution. Oxidation of the uranium(V) to uranium(V1) was also demonstrated by addition of oxygen or ferric chloride to the molten salt solutions. Evidence that the spectrum is produced by a UOz+ ion is found by comparing it with the spectrum of neptuniuni(VI), N p 0 ~ +in ~ ,DC104.16 The two species are isoelectronic, the optical properties of each arising from transitions of a single 5f electron. The spectra bear a strong resemblance to each other with each peak in the NpOZ+2spectrum a t a higher energy than the corresponding UOz+ peak by a nearly constant factor. An interesting feature of the uranium(V) solutions is their color. Concentrated solutions are greenish yellow. Dilute solutions are yellow. As the solutions solidify, the color changes to black or gray depending on concentration. Addition of higher uranium oxides, UoOe and U03, to salts containing MgClz results in formation of RIgU04. (13) H. A. Laitinen, W. S. Ferguson, and R. A. Osteryoung, J . Electrochem. Soc., 104,516 (1957). (14) D.M.Gruen and R. L. McBeth, J . Inorg. Nucl. CAem., 9,290(1969). (15) D.Cohen and B. Traylor, i b d , a2, 151 (1961).
NOTES
Sept., 1963
1941
.+ MgClz +MgU04 + UO&lz
equal to 1.73 A., corresponding to a maximum radivs of 2.95 8.,the a-parameter is eel-tainly a t least 4 A. and one would expect negligible associations of this salt in water, on the basis of the sphere-in-continuum model. A possible explanation is that the potassiun ion is about the right size to fit into one of the triangular faces of the PFe- ion, thereby reducing significantly the center-to-center distances between cationic and anionw charges. With a larger cation, association to the PFsXlgUO4 MgCL +UOzClz 2MgO (3) ion should not occur. The purpose of this note is to and its subsequent decomposition by reaction 1. present some exploratory measurements which confirm Attempts to compare the new spectrum with those this expeatation. of known uranium(V) compounds, UC15 and u o c l ~ , Tetrabu ty lammonium and tetramethylammonium in molten chloride solutions were unsuccessful. At temhexafluorophosphates were prepared ; their solubility in peratures above the melting pointE of the chloride mater is very low, but they are sufficiently soluble in mixtures, UClli volatilized from the solution and coc13 acetonitrile to permit conductance measurements. was unstable. However, the spectrum of a petrolatum Acetonitrile has a dielectric constant of 36.01, about mull of UOCla was obtained which showed maxima corhalf that of water; therefore association is more probresponding to those of the uranium(V) species in soluable for a given electrolyte in acetonitrile than in water. tion. In addition, there were two major peaks in the Our data show that the tetramethyl salt is slightiy UOc13 spectral a t 1070 and 1270 mp which were not associated in acetonitrile, while association of the tetrapresent in the uranium(V) spectrum in molten salts. butyl salt is negligible. This spectrum is shown in Fig. 1. Experimental Molar absorptivities have been obtained for the Tetramethylammonium hexafluorophosphate was prepared by uranium(V) species in the LiC1-KC1 and in the ITaClmixing aqueous solutions of tetramethylammonium bromide and potassium hexafluorophosphate (Ozark-Mahoning Company). KCl-MgCl2 eutectics at 650'. These were obtained The precipitate was thoroughly washed with ice-water and was from absorbance measurements and the corresponding recrystallized from water. Tetrabutylammonium hexafluoroUOz+ concentrations calculated according to reaction phosphate was prepared in a similar way, starting with tetra1from the amount of chlorine remwed from the system. butylammonium bromide. I t s solubility in water is not over 5 X The values range from 5 to 20 depending 011 wave 10 -4; the butyl salt was recrystallized from 50-50 water-methanol. Acetonitrile was purified as described by Berns.4 The length and solvent. conductance cells used had constants equal to 0.3911 and 0.12448, Isosbestic points, at 590 mp in the NaC1-KCl-MgCl2 determined by comparison with a standard cell calibrated with system and at, 565 m p in the LiC1-KC1 system, were potassium chloride solutions.5 Electrical equipment and techgenerated when several spectra were taken of the same nique have already been described.6 The condqctance data are UOzClz solution at the same temperature after varying given in Table I. amounts of chlorine were removed. This behavior is TABLE I characteristic of concentration changes between only CONDUCTANCE IN ACETONITRILE two absorbing species in a solution, in this case uraA 104 e AA nium(V) and uranium(T'1). MedXPF6 Equilibrium constants for reaction 1 as calculated 14.010 181.85 2.44 from the preasures of chlorine in equilibrium with 10.533 183.84 1.86 various concentrations of UOnCl appear to be of the 7.703 185.72 1.36 order of atm a t 650'. 5.151 187.71 0.86 Acknowledgment.-The authors wish to express 2.680 190.36 0.53 their appreciation to Dr. D. M. Gruen for helpful 0.000 (196.75) 0.00 discussions on this problem. 2uo3
(2) Apparently UOzClz is also formed and is converted rapidly to the uranium(V) species as shown in reaction 1. MgU04 is slightly soluble in these salt mixtures, producing again uranium(V) solutions. This can be explained by formation of UOzClzas shown in reaction 3,
+
+
CONDl JCTANCE OF QUATERNARY AMMONIUM HEXAFLUOROPHOSPHATES I N ACETONITRILE BYJEHUDAH ELIASSAF,~ RAYMOND M. FUOSS, AND JOHNE. LISD, JR. Contrzbutzon X o . lY88 from the Steilzng Chemistry Laboratory of Yale Unzverszty, X e w Haven, Connecticut Receaved M a y 9, 1966
Potassium hexafluorophosphate has been shown to be associated ( K A = 2.42) in water, both by conductance and by vapor pressure measurements.2 Since the PFsion is octahedral, with P-F internuclear distances* (1) Grateful acknowledgment is made for a postdoctoral fellowship from a research grant made t o Yale University by the California Research Corporation. (2) R. A. Robinson, J. M. Stokes, and R. H. Stokes, J . Phgs. Cl*em., 65, 542 (1961). (3) H.Bode and G. Teufer, Acta CTgst, 8, 611 11955); H. Bode and H. Clausen, Z. a n o w C'lem ,266, 228 (1951);268, 20 (1952).
Bu~NPFB 154.40 154.62 155.83 158.93 158.31 159.52 (164.75)
9.497 8.876 6.840 5.137 3.412 2,213 0.000
Discussion For slightly associated electrolytes (KA conductance equation simplifies to the form A
= ho
- XC'"
2.38 2.15 1.70 1.26 0.86 0.56 0.00
>
5 ) the
+ EC log c + ( J - KAAO)C+ O(C"')
because y, the fraction of solute present as unpaired ions, becomes practically unity. A plot of
h' = A
+ SC'/' - Ec log c
(4) D.S. Berns and R. 3%.Fuoss, J , A m . Chem. Soc., 82, 5585 (1960). (5) J. E. Lind, Jr., J. J. Zwolenik, and R. & Fuoss, 'I. zbzd., 81,1557 (1959). (6) J. E.Lind, Jr., and 11. h1. Fuoss, J . Phys. Chem., 65 999 (1961).
