LOW-TEMPERATURE THERMAL PROPERTIES Experimental Low-temperature heat capacity measurements were determined by means of a precision adiabatic calorimeter. Detailed descriptions of the apparatus and operating procedure were given elsewhere (16). The experimental determinations were made from 11 to 311 K in three ranges as follows: liquid nitrogen range, 80 to 311 K; solid nitrogen range, 47 to 80 K; and liquid helium range, 11 to 50 K. A nominal bulk volume of 90 ml of sample was used for the measurements. The sample weighed 64.090 g in vacuum. Temperatures reported are based on the International Practical Temperature Scale of 1968 (IPTS-68) (4). Energy units given in this paper refer to the thermochemical calorie (1 cal = 4.1840 joules). Atomic weights are taken from the 1973 Table of Atomic Weights (8). Temperatures below 15 K were determined with a National Bureau of Standards (NBS) calibrated germanium resistance thermometer. A high-precision platinum resistance thermometer was used from 15 to 311 K. Accurate potentiometric measurements in conjunction with these thermometers permitted a temperature resolution of ±0.001 K or better. The sample was loaded into a gold-plated copper calorimeter (sample container) in an argon-filled dry box. The argon was subsequently pumped out of the filled calorimeter and replaced with 0.0024 mole of helium gas to improve thermal conductivity within the sample mass. Results Experimental heat capacity data are listed in table 2. A plot of these data as a function of temperature is given in figure 1. Polynominal functions were fitted to the experimental data with a digital computer. The functions were used to calculate heat capacities at regular temperature intervals. Calculated heat capacities as well as the related functions S° and H°-Ho are given in table 3. Values of S° and H°-H Values of S and H°-H at 10 K were determined by extrapolation to be 0.12 cal/deg-mole and 0.89 cal/mole, respectively. ENTHALPY OF FORMATION AT 298 K Experimental The The enthalpy of formation was determined by solution calorimetry. calorimeter employed and the method of operation were described by Ko (13). A few changes were made as follows: 1. The Hallikainen Thermotrol temperature controller was replaced by a Precision temperature controller (Bayley Instrument Co., Danville, Calif.). With this controller the temperature of the water bath was maintained to ±0.002° C. 2. The stirring mechanism for the calorimeter formerly used a rubber O-ring. This was replaced by a geared-type pulley system employing a no-slip drive belt to give a more uniform stirring speed. 3. The FORTRAN computer program for calculating results from calorimetric experiments was replaced by a new program developed for a Wang 600 calculator system (Wang Laboratories, Inc.). The calorimeter was calibrated by measuring the heats of solution of THAM [Tris (hydroxymethyl) aminomethane] in 0.10-N HC1 and of ZnO in The results, -7.100±0.005 and -15.811±0.007 kcal/ mole, respectively, were in good agreement with accepted values. 4.360 molal HC1 at 298 K. The solution media used in this study were 2.131.1 g of HC1.12.731H2O and 2,592.9 g of H2SOH PO 0.01Na Cr20, 15H20. The temperature of operation was 298.15 K. Weights were corrected to vacuum. The assignment of precision uncertainties was based on the following criteria: (1) When several individual heat values were measured for a reaction, the precision uncertainty was taken as twice the standard deviation of the mean; (2) when the heats of two or more reactions were combined, the uncertainty of the resulting reaction was taken as the square root of the sum of the squares of the individual uncertainties. Aluminum Sulfate Hydrate No relevant thermodynamic data were available for the reference compound, Al2(SO4)3 12H2O(s). Consequently, it was necessary to first determine the enthalpy of formation of this compound. The solution medium used was 2,131.1 g and of HC1.12.731H 0. The details of the reaction scheme for determining the enthalpy of formation of A1, (SO) 12H0 are given in table 4. The symbols 5, 1, 8, and sol denote substances that are crystalline, liquid, gaseous, in solution. The reactions are written in an abbreviated form sufficient to show that stoichiometry was maintained. Also listed in table 4 are the average measured heat values and uncertainties for the individual reactions. A quantity of 0.005 g-mole of Al2(SO4)3·12H20 was used as the weight basis for this scheme, and other substances conformed stoichiometrically with this quantity. Reference to specific manufacturers and brands of equipment is made for identification only and does not imply endorsement by the Bureau of Mines. Reactions 1-2 were measured consecutively in 2,131.1 g of solvent. Reactions 3-4 were measured consecutively in a fresh portion of 2,131.1 g of solvent. Reactions 1 and 3 were both measured previously in this laboratory. Coughlin (5) reported a heat of solution value of -127.050±0.120 kcal/mole for reaction 1. Ko (13) determined the heat for reaction 3 to be -0.0761±0.0001 kcal/mole. These values are adopted here in the reaction scheme. The experimental heats of solution for reactions 2 and 4 are given in table 5 along with their mean values and uncertainties. Reactions 1-4 and their heats can be combined to give the metric reaction 5 and a heat value of -254.90±0.25 kcal for AH Combination of AH with AH, the enthalpy of formation of H2SO K, gives the formation reaction 7 for Al2 (SQ)3 12H2 0. H2 (g) + S(rh) + 202 (g) + 6H2O(1) H2SO 6H20(1) = ΔΗ = -208.944±0.100 kcal/mole. The enthalpy change for reaction 6 was from Wagman (17). 2A1(s) + 3S (rh) + 602 (g) + 12H2O(1) = A12 (SO4)3 12H0(s) The details of the reaction scheme for A1203 ·2.0S02 ·5.3H20 are given in table 6. The solution medium was 2,592.9 g of tion of sulfur of +4 to sulfate. The SOH PO 0.01Na2Cr20, 15H20. The presence of Na2Cr2O, insured the oxidatitrating the excess Na2 Cr2O, with ferrous ammonium sulfate solution. A test was made for complete oxidation by amount of Na2 Cr2O, actually consumed in oxidation of S02 to SO2 confirmed the theoretical calculation according to stoichiometry. table 6 are the average measured heat values and uncertainties for the Also listed in individual reactions. A quantity of 0.005 g-mole of A120, 2.OSO2 5.3H2O was used as the weight basis for this scheme. stoichiometrically with this quantity. Other substances conformed TABLE 6. Reaction scheme for A120.2.0S02 5.3H20 at 298 K (7) Uncertainty, kcal -516.90 2.42 8.75 0.11 -122.92 .11 -484.06 .29 + 9S02 (sol) + 54H2O(sol).. (12) 9[H2SO •6H0] (1) = 18H+ (sol) ΔΗ13 1/3 (ΔΗ, + ΔΗ, = 1 (13) 2[H2SO 100H20] (1) + A12 (SQ)з · 12H2O(s) 187.720(1) + A1203·2.0502·5.3H20(s) = +3[H2SO •6H2O](1).. Reactions 8-9 were measured consecutively in 2,592.9 g of solvent. Reactions 10-12 were measured consecutively in a fresh portion of 2,592.9 g of solvent. The experimental heats of solution for these reactions are given in table 7 along with their mean values and uncertainties. Reactions 8-12 and |