3353
HEATSOF FORMATION OF HEXAVALENT URANIUM COMPOUXDS
Heats of Formaition of Some Hexavalent Uranium Compounds
by E. H. P. Cordfunke Reactor Centrum 2Vederland,Petten, The etherlan lands
(Received iMay 18, 1364)’
The heat of formation of the 1J022+ion was determined by measuring the heat of solution of r-UO, in nitric acid calorimetrically. From the known heat of formation of r-UO3, the heat of forniation of the U022+ion in 6 N €€NO, was found to be: -AH298 = 242.2 f 1.0 kcal./mole. This value was used for the determination of the heats of formation of the modifications of uoo, the hydrates of UOs, the hydrates of uranyl nitrate, and the uranates of ammonium by heat of solution measurements in nitric acid.
Uranium trioxide is known to exist in a t least five crystalline modifications and an amorphous form. R,ecent studies by Hoekstra and Siegel’ have shown the coniplexity of this system. As most of the thermochemical data of UO, are prior to 1920, little characterization of the materials used was possible. Moreover, new compounds in these systems have been found since. Apparent inconsistencies in the data available, as pointed out by Rand2and by Rand and K~baschewski,~ and the absence of some essential data, have made it worthwhile to determine the heat of formation of the phases in the above-mentioned systems. The results obtained are presented in this paper.
Experimental The Calorimeter. A constant-temperature-environment calorimeter waEi used for the determination of the heats of solution. The calorimeter consisted of a dewar vessel, placed in a well-stirred, thermostated water bath, the temperature of which was fixed a t 25.0 f 0.1”. The dewar vessel was closed with a Perspex stopper (covered with a brass cap), through which a Ni-Cr heating element, a stirrer, a calibrated Beckmann thermometer (provided with a lens), and a thin-walled glass tube, containing the sample, were fitted. For further details we refer to a similar apparatus described by Skinner. When starting the experiments, the bath and the calorimeter were heated to the required temperatures, the latter being lower in temperature than the former by about half the expected temperature change in the calorimeter. The measurements were started when
a stationary condition was reached, which required 1 to 2 hr. The heats of solution of the uranium compounds were determined by dissolving 2.000 g. of uo3 (or equivalent molar amounts for the other compounds) in 200 ml. of 6.0 N nitric acid. The measurements were carried out a t least in triplicate. All samples investigated dissolved in less than 10 min., which is sufficiently fast for the heats of solution to be measured with good accuracy. The energy equivalent of the system was determined by electrical heating, the current being measured by the potential drop across a standard resistance with a potentiometer. The calorimetric quantities are expressed in terms of the defined calorie, which is equal to 4.1840 absolute joules. In the case of uranium trioxide, a series of 15 determinations of the energy equivalent gave a value of 197.1 f 0.4 cal./deg., the uncertainty being twice the standard deviation of the mean. The absolute reliability of the calorimeter was tested by the determination of the heat of solution of potassium chloride in water. The value found, -AH298 = -4.18 kcal./mole (7 X M ) , is in good agreement with data from literature, extrapolated to the same dilution.5 (1) H. R. Hoekstra and 8. Siegel, J. Inorg. Nucl. Chem., 18, 154 (1961). (2) M. H. Rand, Proceedings, Symposium on Thermodynamics of Nuclear Materials, International Atomic Energy Agency, Vienna, 1962, p. 71. (3) M. H. Rand and 0. Kubaschewski, “The Thermochemical Properties of Uranium Compounds,” Oliver and Boyd, Ltd., Edinburgh and London, 1963, p. 36. (4) H. A. Skinner, Ed., “Experimental Thermochemistry,” Vol. 11, Interscience Publishers, Inc., New York, N. Y., 1962, p. 189.
Volume 68, Number 11
November, 1964
E. H. P. CORDFUSKE
3354
Materials. The samples used were carefully prepared in order to obtain a high purity and a sufficient crystallinity. The purity was determined both by cheniical analysis (ignition to U308at 800") and by Xray analysis. A high degree of Crystallinity of the samples was ensured by prolonged heating in order to avoid corrections for the heat of crystallization. In some cases (e.g. , P-UOS prepared by decomposition of ammonium uranate at 500") heating times of about 1month were used. The polymorphic modifications of UO, were, in general, prepared as described by Hoekstra and Siegel, with the exception of a-UOa, the preparation of which was reported r e ~ e n t l y . ~The preparation of the hydrates of UO3 and of the urariates of ammonium has also been described elsemhere.'J The hydrates of uranyl nitrate were prepared as follows. UOz(N0d2.6H20 (nlerck, p.a.) was recrystallized from water several times and then placed over 40y0 H2S04 until a constant water content was obtained. Chemical analysis (with oxine) showed it to be of the correct composition. The trihydrate was prepared by recrystallization from a saturated solution of uranyl nitrate, which is about 20 N with respect to HN03. This hydrate was kept over 70% HzS04 according to the data given by Vdovenko, et aLg
Results Heat of Formation of the Uranyl Ion. The heat of formation of the uranyl ion was determined by Fontana'O in 0.5 M "2104. Recalculation of this data by Rand2 gave a value of -AH298 = 250 kcal./mole. Rand and Kubaschewski, a however, have shown that this value leads to inconsistencies in the data for the heats of formation of several uranium compounds. These authors suggest that the value for the uranyl ion may be too positive by about 6 kcal./mole. We have determined the heat of formation of y U O 3by dissociation pressure measurements." Using the known heat capacity of 7UO3, the heat of formation of y U O a at room teniperature was calculated to be: - AH298 = 293.5 f 1.0 kcal./ mole. This value is in good agreement with the value (- AH298= 293.0 kcal./mole) suggested by Rand and Kubaschewski. Samples of T-UOa of a high crystallinity were used for the determination of the heat of solution in 6 N HN03. A series of three different samples of y-UO3 (Table I) gave an average value of -AH298 = 17.04 f 0.03 kcal./niole for the reaction -y-UOa(s)
+ 2H+(aq) --+U022+(aq)+ HzO(1)
Using the heat of formation of HzO(l), -AH298 = 68.32 kcal./mole, the heat of formation of the U0t2+ T h e Journal of Physical Chemistry
~~~
Table I :
~
~
~.
Heat of Solution of y-liO8
Wt. of
?-uo8,g .
Purity, wt. % UOa
1.99975 2.00060 1.99955 1.99965 2,00000 1.99970 2.00005
99.51 99.51 99.66 99.66 99.49 99.49 99,49
At, "C.
0.601 0.600 0.602 0.600 0,603 0.602 0.603 Mean:
Heat of soln.,a kcal./mole
17.03 17.00 17.04 16.98 17.09 17.06 _____17.09 17.04 i 0 . 0 3
a Energy equivalent of the calorimeter system, 197.1 f 0.4 cal./deg.
ion in 6 ili HNOs was found to be -AH298 = 242.2 i 1.0 kcal./mole. I t is evident that this value is only a formal one, as part of the uranyl ions is present in the form of uranyl nitrate complexes. For this reason, the value given may strictly only be used for the same UOZ2+and HNO, concentrations. The heats of solution of UO?(nTOB)2.6H20 and of UO2(N03)2*3&0 in 6 N HSO, have been determined (Table 11)also. From the results obtained, the heat of
Table I1 : Heats of Formation of Uranyl Nitrate
Substance
Heat of solution in 6 N HNOa ( - AHnss). koal./ mole
UOP + ( a s ) UOz( NOaMaq)
Heat of formation ( - AHnua), kcal. /mole Present investigation Literature
LTO~(NO~)2.6HzO
-9.53
242.2 ... 762.3
UOa(KO,),~3HzO
-2.75
550.5
250" 342.8" 764.3* 759 i 3" 552. 3b 547"
W. M. Latimer, "Oxidation Potentials,'] Prena See ref. 3. tice-Hall, Inc., New York, N. Y., 1952.
transition of the equilibrium
(5) J. Coops, A. N. Balk, and M. W. Tolk, Rec. trav. chim., 75, 75 (1956). (6) E, H. P. Cordfunke, J . Inorg. Nucl. Chem., 23, 285 (1961). (7) E. H. P. Cordfunke and P. C. Debets, ibid., 26, 1671 (1964). (8) E. H. P. Cordfunke, ibid., 24, 303 (1962). (9) V. M. Vdovenko and A. P. Sokolov. Radiokhimiya, 1, 117 (1959). (10) B. J. Fontana, T.I.D. report 5290 (1947). (11) E H. P. Cordfunke and P. Aling, T r a n s . Faraday SOC.,in press.
JIEATS OF
FORMATION OF
HEXAVALENT URANIUM
3355
COMPOUND6
was calculated as -AH2y8 = -38.3 kcal./mole. This value is in good agreement with the values obtained indcpcndent ly froni vapor pressure nieasurenients by Vdovcnko, et ( - AHzy8= -38.7 kcal.!niolc), arid in this laboratoryl2(= -38.53 kcal./niolc). Using the hclat of forniation of the aqueous uranyl nitrate ~ o l u t i o nthe , ~ heats of formation of the hydrates of uranyl nitratc can bc calculated. Table I1 gives a survey of thc results. A cornparison with the literaturc shows a rcasonablc agreenicnt. The System I;'Os-H20--NfZs, At least three deteriiiinations of the heat of solution in 6.0 N nitric acid were niade on each conipound in the ternary system Uo3H20-NH3 investigated. In thc case of uranium trioxide a small correction was necessary for the impurities, mainly present as moisture and residual nitrogen. The results are listed in Table 111, together with the heats of forniation derived froni the heat of solution data with the heal of formation of the uranyl ion. In the case of the aninionium uranates, e.g., CO3.0..5l..iHzO (ADU-II18), dissolving according to the equation
c
tl QI
z
125
f-
I L
1
I
s
+
UOs43.5NH3.1.5HzO(s) 2.5H+(aq) +
+
U022+(aq) 0.5NH4+(aq)
450.
+ 2.5H20(1)
the heat of formation of the S H 4 + ion is necessary for the calculation of the heats of forniation of the uranatcs of ammoniuni. Therefore, the heat of solution of solid T\FHsNo3(IV) was determined in a solution of uranyl nitrate in 6 N Hx03, so that the same final concentraTable 111: Heats of Soiution in 6 A' HSO3 at 25' and Heats of Formation Heat of solution, Substance
400
Heat of formation,
- AHnas,
- AHHE,
kenl./niole
kcal./mole
291.8 292.6 293.5 290" 291.8 289.6 292.0
357,4 367.4 366.8 437.3 423.0 415.6 406.6 Estimated value for 6-UO3, based on the heat of solution of a mixture of 6-1:03 and a-lTo3 (see 1Iisc:ussion).
t I
0
\
\ \ \ \ \\
.
.
I
I
I
.
.
.
#
1.0
0.5
N4IIu0; Figure 2.
.
rat io
Heats of formation of uranates of ammonium.
tions mere obtained, as is the case for dissolving the uranate in nitric acid. From the heat of solution thus obtained (-AH298 = -4.17 f 0.02 kcal./mole), the heat of formation of the NH4+ ion was calculated as -AH288 = 32.60 kcal.,/rnoIe, using the heat of formation of solid NH4N03(IV) recommended by the National Bureau of standard^'^ ( - AHzY~ = 87.27 kcal./ mole) and the heat of formation of the S O 3 - ion (- AHzy8 = 50.5 kcal.jmole), derived froni the heat of for(12) Vnpublished results. (13) National Bureau of Standards Circular 500, U. 5. Goverernent Printing Office, Washington, D. C.. 1952.
Volume 68,)\'umber 11
.Vovember, 1964
3356
mation of UOz(T\’03)2(aq). The heats of formation of the uranates of ammonium found in the ternary system U03-HzO-NH38 are listed in Table 111.
Discussion The heats of solution obtained in the present investigation cannot be compared directly with those of other authors because of the marked effect of the conditions in the solution. Measurements on the hydrates of uranyl nitrate, for instance, have only been carried out in water. The heats of formation derived from these measurenients and those from ours (Table 11) are in good agreement. This is also the case for the hydrates of U 0 3 for which only old data were available3 until now. The heats of solution of UO, preparations depend to a marked extent on the mode of preparatioii and on the starting material. “Active” preparations might have a heat of solution up to 1.0 kcal./mole higher than that of the inactive preparations. The differences in energy content may be due to incomplete ordering of the crystal lattice, as is probably the case for E - U O ~ or , ~ to differences in particle size. The influence of the degree of crystallinity on the heat of solution is shown for p-uo~in Fig, 1, in which the crystallinity is expressed as the crystallite size (A.) calculated from the X-ray line broadening. Further, the measurements of the heats of solution of uranium trioxide are very sensitive to hydration of the samples because of the large differences in the values for the hydrates and the oxides. In order to prevent hydra-
T h e Journal of Physical Chemistry
E. H. P. CORDFUNKE
tion during the measurements (preperiod), the tube containing the sample was provided with a small vessel filled with Pz05. The heats of solution of uo3 listed in Table I11 are reasonably consistent with those by Hoekstra and Siege1,l who have carried out their measurements in 300 ml. of 5 N HX03. Comparison with their data is possible if 0.1 kcal./mole is added to our data, as was found by us experimentally. The only serious disagreement is the value for S-CO,. It was found previously’ that this oxide is difficult to obtain in a pure form. In fact, 6-uo3 is always contaminated with either aniorphous uO3 or with cr-UO3. From samples of 6-U03, containing approximately 30 wt. 7, of aUOa, a heat of formation of 6-uo3 was estimated (290 kcal./mole) from the heat of solution of the mixture (19.9 kcal./mole). Figure 2 shows the heats of formation of the conipounds in the system U03-HZO-NH3 as a function of the XH3/U03 ratio. From this, the heat of formation of the compound (NH4)zUzOY (ammonium diuranate or ADU) could be found by extrapolation. -AH298 = 393
f
1 kcal./mole
It was reported p r e v i ~ u s l ythat ~ ~ ‘ this ~ compound could not be prepared. Acknowledgment. The author wishes to thank Miss B. J. M. de Boer for experimental assistance. (14) P. C. Debets and B. 0. Loopstra, J . I n o r g . Aruc1. Chem., 25, 946 (1963).
