Vol. 61 illustrated in Figs. 11, 2 and 3. Tt, is not due to ,change of temperature since the sample has had time to reach the bath temperature before pressure measurements are started. 3t is not due to decomposition either since ,thesame equilibrium value (within uncertainties due to impurities) can be reached from either side and since refluxing between measurements removes any decomposition products. This behavior is also found for other physical properties of aluminum isopropoxide and will be discussed in a later paper. It is also in agreemen% with some remarks made by Mehrotra2 about changes of degree of association with aging. The possibility of time dependent behavior was examined in the other alkoxides. It was found to a slight extent in the propoxide and the n-butoxide only. Table I1 shows the experimental data and compares it with the vapor pressure calculated from the constants in Table I. The deviation of the observed points from the smooth log p us. 1/T plots are greater than the experimental error of temperature and pressure measurement and are probably due to the presence of impurities. Measurements at the higher temperatures are uncertain because of rapid decomposition. The discrepancy between data reported here and the data of Mehrotra is also greater than can be accounted for by temperature and pressure errors. The largest difference occurs with the isopropoxide and the n-butoxide. This is easy to understand in the case of the isopropoxide since failure to take into account its time dependent behavior could cause a large error in vapor pressure. ELECTRICAL CONDUCTANCE IN MOLTEN SALT MIXTURES BYBENSON R. SUNDHEIM Department of Chemistry, New York Uniuerdy, Washington Square, Naw York 8, N . Y . Received Au~usl90,IS66
Some precise measurements of the equivalent electrical conductance of solutions of molten salts have been published recently.’ Two of the systems studied, KCl/LiCl and KCl,”aCl, show pronounced minima in the graph of equivalent conductance versus mole fraction of KC1 while the third, KCl/KI, shows a negative deviation from simple additivity of conductances. It is remarkable that the system with the greatest deviation from additivity of conductance, KCl/LiCl, displays zero volume change upon mixing, Le., the equivalent volume is a perfectly linear function of composition. The other two systems, where compound formation or solid solution is known to occur in the solid state, show very slight volume changes upon mixing. Certain interesting features of the conductivity of these solutions may be seen when the data are presented in a different way. The conductance of a solution of two salts is an extensive property which ma.y be represented (1) E. R. Van Artsdalen and I. S. Yaffe, THISJOURNAL, 69, 118 (1955).
formally ‘in terms of the “partial equivalent Icon\ductances” of the components.
+
A = n&
n& ziii 4-z& (bA/bni)T,-k hi
Aeq
,where hes denotes the equivalent conductance, xi the mole fraction and Iii the partial equivalent conductance of the ith component. The physical significance of Li may be seen as follows: a pair d parallel plates is spaced one em. apart and contahs a very large amount of the appropriate mixture. One equivalent of a pure salt is added to the mixture between the plates. The increase in conductance is the partial equivalent conductance of the salt added at the composition of the mixture. These partial equivalent conductances are the quantities which are most meaningfully compared from one solution to another. Their calculation is facilitated by a construction similar to that given by Lewis and Randall2for volumes. The partial equivalent conductance of one salt a t vanishingly small concentration in a second salt is particularly interesting. It is a direct measure of the mobility of the added salt in the foreign salt. These limiting conductances have been computed from the data of Van Artsdalen and Yaffe and are presented in Table I. The important features brought to light are the following. 1. I n all cases the limiting equivalent conductance of one salt in another salt is less than in the pure salt. Thus the partial equivalent conductance of LiCl is less in KC1 than in LiCl and conversely the partial equivalent conductance of KC1 is less in LiCl than in KC1. TABLE I PARTIAL EQUIVALENT CONDUCTANCES AT 800”, MHOCM.-’
LiCl KC1 KI KC1
-
0)
i ( x = 1)
108.5 84.0 75.5 - 175 78.8 77.3
134.7 110.5 197.5 110.5 108.0 110.5
i(x
NaCl KC1
2. The decrease in the equivalent conductance when a salt is dissolved in a second salt from its value in the pure salt, [&(x = 1) - Xi(X = 0)], is larger the greater the difference in relative size of the ions of the different salts. Thus the depression for LiCl in KC1 is greater than that for KC1 in NaC1. Similarly the depression for KCl in LiCl is greater than for KC1 in NaCl. The same effect appears to obtain for anions. 3. This effect is so pronounced in the case of KC1 in LiCl that the addition of KCl to a large amount of LiCl actually decreases the total conductance. At the same time the partial molal volumes in this systems are ideal, so that no profound change in the quasi-lattice can have occurred. 4. It is possible to calculate the “heats of activation” for the separate partial equivalent conductances. The most interesting and probably the (2) G. N. Lewis and M. Randall, “Thermodynamica,” McGraw-Hill Book Co., New York, N. Y., 1963.
117
NOTES
Jan., 1957
each firing temperature. Foex also observed that no sintering of particles seems to take place below 1000" and from this he surmised that growth occurred only among closely joined crystallites. In this note the effects of oxalate precipitation CONDUCTANCII AT temperature, calcination temperature and time upon the particle size, surface area and crystallite AG(z = 1) size of thorium oxide are presented.
only significant ones are those for zero concentration. These were computed for the three systems mentioned and are presented in Table 11. TABLE I1 APPARENT ACTIVATION ENERQIES FOR SOO', KCAL. AE(z = 0)
NaCl KC1 LiCl KC1 KI KCl
4.95
2.8
5.75
3.6
2.2 5.8
2.0
6.6
3.5
3.5
3.6
3.6
The uncertainties estimated for the entries in both tables are rather large since there are insufficient data at low mole fractions to establish reliable slopes. In order to further test these conclusions the data of Bloom and Heyman3 and Bloom, et u Z . , ~ were converted into this form. These data are not as well suited for these calculations. Nevertheless, insofar as comparisons are possible, the conclusions listed above are valid for the 13 pairs of salts studied by these workers.6 It is a deasure t o acknow1edg.e assistance from the Atomic Energy Commission Icontract AT 30-1 1938). (3) H.Bloom and E. Heyman. Proc. Roy. SOC.(London), 8188,302 (1947). (4) H. Bloom. I. W. Kraggs, J. J. Molloy and D. Welch, Trane. Faraday SOC.,49 (1953). (5) PbCls/CdCl* may be an exception.
CHARACTERISTIC PROPERTIES OF THORIUM OXIDE PARTICLES BY V. D. ALLRED,S. R. BUXTONAND J. P. MCBRIDE Oak Ridge National Laboratory, Oak Ridge, Tenneaaee Received Seplember 4, 3068
Thoria is of interest as a fertile fuel for the nuclear breeder reactor, as a starting material for thorium metal production and as a very high temperature ceramic. The pure oxide has a very high melting point (ca. 3200°), no reported allotropic transformation, and a high degree of chemical stability. The most widely reported method for the preparation of high purity thorium oxide is the thermal decomposition of thorium oxalate. The mechanism by which thermal decomposition takes place has been quite widely However, little work has been reported on the effect of the preparation variables on fundamental characteristics of the oxide product. Foex4 investigated the rate of change in density as a function of firing temperature for thorium oxide prepared by the thermal decomposition of a hydrous oxide. This work is of interest since the denaity change was characteristically associated with crystallite growth for (1) C. L. Duval, "Inorganic ThermogravirnetricAnalysis," Elsevier Press, New York, N. Y., 1953, p. 496. (2) R. Beckett and M. E. Winfield, A u B ~J.. Sci. Rss.. 4, 644 (1951). (8) R. W. M. D'Eye and P. G. 8ellman, J . Inorg. Nuclear Chsm., 143 (1965). (4) Marc Foex, Bull. eoc. chim. (France), I161 65, 231 (1949).
Experimental
a. Oxide Preparation.-Thorium oxalate precipitates were repared a t 10, 40, 70 and 100' by dropwise addition of 1$high purity oxalic acid to avigorouslystjrred 1 M high
purity thorium nitrate solution. The recipltates were digested at temperature for one hour, Rltered, washed and finally dried a t 110' for 24 hours. An oxide was prepared from each of the four materials by calcining the dried oxalate in platinum crucibles a t 375' for four hours and then at 400" for an additional 16 hours. The resulting oxides were used as stock material!. Tengram samples of each of the oxides were placed in small platinum crucibles and k e d in a reheated furnace a t 500, 650.750 or 900' for a period of 24 {ours. For calcination time-temperature studies several onegram samples of each oxide were put in small platinum crucibles and brought rapidly to temperature in a preheated furnace. At fixed time intervals samples were, removed and quenched by partially immersing the crucibles in water. 6 . Oxide Characterization.-Chemical analysis of the oxide products showed them to be extremely pure containing less than 100 .m. each of Ca and PO4' and less than 50 p.m. each oF&!04-, C1-, F-, Fe, Cr, Ni, Pb, B ! , Me, Na, Li, Si and total rare earths. Thermogravimetric gas adsorption data showed both Water and carbon dioxide to be strongly adsorbed on the thoria surface and they were present in the oxide samples in amounts proportional to the specific surface areas. The oxides from the 400' calcinations contained as much as 1%carbonate and 0.4% water while those from the 900' calcination contained less than 0.2% of either. Firing the oxide in excess of 1000" was necessary to com letely remove the water and carbon dioxide from the surzce. The shape, size and size distribution of the oxide particles were determined from electron micrographs and sedimentation particle size analysis. The method for sedimentation particle size analysis was developed at ORNL.6 The oxide was activated by neutron irradiation, dispersed a t
