A. G. KEENAN
780
Vol. 61
cal./deg. for one mole of normal deuterium.25 By equating the experimental value of AH t o that obtained by integration of a theoretical Schottky heat capacity curve for the Stark-t pe splitting discussed above, a value of 6.5 cal.7mole is obtained for the energy difference in normal deuterium (an equivalent excitation temperature of 3°K.). As will he seen later, this can be a t best only an average figure, since the use of a Schottky curve is not actually justified, but it indicates that the experiinentnl A H i R in accord with the proposed mechanism. The experimental value of AS, 0.60 cal./deg./mole of n-Dz, is lower than the theoretical one by an amount which is outside experimental error. However, the data also indicate that the heat capacit,y is not vanishing even a t 0.3"K., so that the difference is very probably undetected entropy below this temperature. A comparison of the results for deuterium and those for hydrogen reveals that the anomaly appears a t approximately the same temperature and is similar in naturo. The energy of splitting may be expected to vary inversely with the moments of inertia of the two molecules, so that a much lower temperature would therefore be predicted for the anomaly. However, a compensation is produced by the wider lattice spacing of hydrogen (which results from its larger zero-point energy) , since the crystalline potential must also vary inversely with high powers of the intermolecular distance. This may be considered as additional evidence for the influence of the lattice. On the basis of these observations we believe that the anoinaly observed in this research corresponds to removal of the rotational degeneracy by tJhecrystalline field. I t was not possible to fit the data obtained in this
research to either a Schottky curve derived as assumed above, or to a curve derived from three equally spaced non-degenerate levels (a Zeemantype splitting). In the case of hydrogen even the limited early data did not yield constant values for the energy differencelZs as required for a pure Schottky anomaly, and the recent and more extensive results of Hill have shown conclusively that this type of anomaly is not obtained in hydrogen. It is obvious that the crysta.lline potential must depend to some extent on the population of the sublevels m = 0, Iil and on the distribution of molecules in these levels throughout the solid, so that the anomaly should bear characteristics of a cooperative phenomenon. This was substmtisted by a pronounced dilution effect in the results for 97% orthodeuterium where the anomalous contribution of the 3% para form was considerably suppressed (see Fig. 5 ) . Although no A-type transition was found in this research, the results of Hill indicate that this type of transitmionmay occur a t higher concentrations of the J = 1 statJe. Further information on this question will become available from heat capacity measurements on pure ortho-hydrogen and para-deuterium, which can now he pre~ a r e d and , ~ in~ which ~ ~ ~ these dilution effects due to the presence of the J = 0 states would be eliminated. An investigation of this nature is now in progress in this Laboratory. Acknowledgments.-Sincere thanks are due to Mr. Leo E. Davis and Mr. James Seiler for help in the measurements, to Mr. Lest.er E. Cox for aid in the design and construction of the apparatus, and t o Mr. Gwynne A. Wright for the liquefaction of the large quantities of liquid helium used in this resea.rch.
0 level will be the one with loweot energy: the reverse situation will yield an entropy lorn of l/sR(ln 3 - In 2 ) , or 0.27 e.u., which is not in agreement with the
(26) R. B. Scott and F. G . Brickwedde, J . Research NaB. Bur. Standards, 19, 237 (1837). (27) Y. L. Sandler, J . Phua. Chem.. 68, 58 (1954). (26) C. M . Cunningham, D. 8. Chapin and H . L. Johnston. Abstr. 120th Meeting A.C.S., September 1951, p. 19R.
(28) Thin value asnumen that the m
-
experimental result.
CRYOSCOPIC BEHAVIOR OF WATER, AND NITRIC ACID IN FUSED AMMONIUM NITRATE' BY A. G. KEENAN Deparlmenl of Chemistry, Illinois Institute of Technology, Chicago 16, Illinois R h e d January 34, I987
H,O and " 0 3 have been studied cryoscopicallg as solutes in ",NO3 melts. HIO alone, as well as mixtures of the two Rolutest show v-factors of unity based on stoichiometric mole fraction Concentrations determined l y analysis of the melts. A conmtent Ret of molecular speriea is postulated to accoiint for the observed v-factor and the results relatd to proposed mechanisms for the thermal decomposition of NHINOJ.
It has been shown in a previous pnblication2 that ammonium nitrate can be employed as a useful cryoscopic solvent despite its well-known thermal decomposition a t higher temperatures. In the present work HzO and HNO, were chosen as solutes ( 1 ) This research was supported by the United Statra A i r Forre through the Air Force Office of Scientific ReseRrrh of the Air Rraearch and Development Command under Contract No. A F 18(fi00)-1148. Reproduction in whole or in part is permitted for any purpose of the United States Government. (2) A. 0.Keenan, THISJOURNAL, 60, 1356 (1956).
for cryoscopic study because of their interest in connection with proposed mechanisms for the thermal decomposition of NHdNOa. Only HzO is mentioned in the prior l i t e r a t ~ r e . ~The method used there was an approximate one, with an expected accuracy of + lo, and the lowest concentration studied was somewhat above the highest concentration in tthe present data. (3) I . L. Millican, A. F. Joseph and T. M. Lowry, J . Chem. Soc.. i l l , 859 (1922).
CRYOSCOPIC BEHAVIOR OF HzO A N D "03
June, 1957
The frcezirig point apparatus and the manipulations involving the ammonium nitrate solvent have already been dcscrihed.2 Distilled water and approximately 90% nitric ucid were used as solutes. The nitric acid was free of NO, according to the criteria given by Robertson, et al.4 I n one run 9GYo acid was used and this result agreed with the others. Appropriat>eamounts of water or nitJric acid, as judged by experience, were added to the and allowed to equilibrate in a closed flask at room temperature for about one hour. A freezing point determination was then made in the usual manner.2 When the cooling curves on the recorder chart indicated that crystallization of solid phase had be un (but before any appreciable amount had separated out) t8e melt was poured into two tared flasks. After weighing, water was added to one flask and duplicate aliquots titrated with 0.05 N NaOH using a Sargent-R.lalmstadt auttomatic differential t,itrator to control the titrant. The base was standardized with arid containing the same concentration of NHdNOs as the solution to be analyxcd. I n order to determine the water content, of the melt, methanol, which had been distilled from magnesium turnings through a 60-cm. vaouumjacketed, packed column, was used as solvent for the material in the sccond flask. Aliquots of this solution were t,itrated with ICarl Fiwher reagent using standard procedures.6 Again, blank determinations were used to correct for water contained in the solvent and picked u p during the transfer operations. The melts were colorless at all stage#, indicating the ahsence in the melt of NO? from any decomcatalyzes the decomposition of the HN03 added. "03 position of NH4N03,but the. NzO produced is insoluble in the melt.6 Any water retained in the melt, from t h ~ s source or from decomposition of "03, is determined as part of the total water content by the analysis following the freezing point determination. T o study the roducts of decomposition of the ammonium nitrate it,self, wfich are retained in the melt, the pure "4NO3 was hent,ed in the freezing point cell to temperatures in the range 235-250' for a,bout one hour and the above prorcdures then followed.
Results and Discussion The result,s are given in Table I and in Fig. 1 . l'hr line in Fig. 1 is taken froin the data of the previous paper2 arid represents the freezing-point depression for a solute showing ideal cryoscopic behavior with a v-factor' of l. While the scatter of the data is somewhat greater than in the previoris work, due largely to less reproducibility in t8heanalytical procedures, it is, nevertheless, quite definite that all of the systems follow the ideal depression line very closely. The data for HzO ngree with the prior literaturea within the expected tolerance. The ideal behavior of water as a solute io easily understood. An ammonium nitrate melt consiels of the ionic species NHd+ and NO3- exclusively, rontributions from the possible reaction
+ NO3-
NH, f FINO3
FUSED"4NOa
78 '
0
Experimental
NH,+
IN
(1)
I)citig iiogligihle i n the liqiiid phnsc, a t least8 for t,rmpcrntures near t8hefreezing point), a s shown by t8hrprrvioiis cryoscopic stjiidy.2 I n Filch a system thrre is 110 apparent react,ion which t8he water could r i i t d e r g o . On t,hn other hand, the low vapor pressure of water in the me1t indicates association (4) G. D. Roherteon, D . A f . Mason and W. H. Corooran, THIB JOURNAL, 59, 683 (1955). (5) J. Mitchell Rnd D. X4. Smith, "Aquametry." Interscience Publishers, Inc., New York. N. Y.,1948. (6) H. A. Bent, Doc'toral Thcsis, University of California, 1R52. (7) R . .I Gillespie. E. D. Hllnhes and C. I
