NOTES
Oct., 1956
THE HEATS OF COMBUSTION OF DYSPROSIUM AND YTTERBIUM'
TABLEI THEHEATOF COMBUSTION OF DYSPROSIUM
BY ELMERJ. HUBER,JR., EARLL. HEADAND CHARLES E. HOLLEY, JR. Contribution from the University of California, Lo8 Alamoe Scientijic Laboratory, Los Alamos, New Mezico Received April 20, 1966
This paper is the sixth in a series reporting measnrements of the heats of formation of the rare earth oxides.2-6 The method, involving the determination of the heat evolved from the combustion of a weighed sample of the metal in a bomb calorimeter a t a known initial pressure of oxygen, has been described elsewhere.6 The same units and conventions are used here. Dysprosium and Ytterbium Metals.-The dysprosium and ytterbium metals were analyzed and found to have the following per cent. impurities C
Dy Yb
H
N
0
Ca
Li
1457
Y
0.0224 0.0194 0.007 0.036 0.01 0.005 ,036 .025 ,004 .003 .02 ,005 0.05
No other metallic impurities were detected. Each of the metals thus contained about 0.1% impurities. If it is assumed that the non-metallic impurities are combined with dysprosium and ytterbium as the carbide, hydride, nitride and oxide, the dysprosium is 97.92 mole per cent. metal (atomic weight Dy = 162.51) and the ytterbium 97.23 mole per cent. metal (atomic weight Yb = 173.04). An X-ray pattern of the dysprosium showed a two atom hexagonal close-packed unit cell and a calculated density of 8.534 g./cc. A pattern of the ytterbium revealed a face-centered cubic unit cell and a calculated density of 6.966 g./cc. A small amount of impurity was observed in the ytterbium which is believed to be the hydride. Combustion of Dysprosium and Ytterbium.Each of the metals was burned on sintered discs of the corresponding oxide in oxygen at 25 atmospheres pressure. The oxides were approximately 99.9% pure. Neither of the metals showed any increase in weight when exposed to O2 a t 25 atm. pressure for one hour. Combustion varied from 99.84 t o 100.OOyo.of completion. The average initial temperature for the dysprosium runs was 25.0" and for the ytterbium runs 25.1". The results are listed in Table I. The average values of 5718.9 i 11.6 and 5251.1 5.0 j./g. must be corrected for the impurities present. Correction for Impurities.-The calculated percentage composition of the dysprosium by weight is Dy metal 98.01; DyH2, 1.58; Dy20s, 0.28; DyN, 0.088; C, 0.0224; Ca, 0.01; Li, 0.005. The carbon is probably present as DyC, but the heat of formation of the latter is not known and is probably small. The heat of combustion of Dy metal cor-
*
(1) This work was performed under the auspices of the A.E.C. (2) E. J. Huber, Jr., and C. E . Holley, Jr., J . Am. Chem. Soc., 74, 5530 (1952). (3) E.J. Huber, Jr., and C. E. Holley, Jr., ibid., 75, 3594 (1953). (4) E. J. Huber, Jr., and C. E. Holley. Jr., ibid., 75, 5645 (1953). ( 5 ) E. J. Huber, Jr., and C . E. Holley, Jr., %bid.,77, 1444 (1955). (6) E.J. Huber, J r . , C. 0. Matthews and C. E. Holley, Jr., %bid..77, 6493 (1955).
Mass Dy burned,
Wt. Mg,
mg. 7.15 6.55 7.1 7.7 7.6
g.
2.11735 1.91355 2.0192 1.9711 2.07375
Wt. DyiOa. g.
45.1 50.0 43.3 38.9 45.1
Energy from FirJ./deg., total7 10031.6 10032.9 10031.2 10030.0 10031.6
AT, OK. 1,2240 1,1064 1.1685 1.1458 1,2059
I?@;,
1.
12.1 17.8 11.9 12.9 12.0 Av. Standard dev.
jfL:
5710.1 5707.2 5712.4 5727.6 5737.2 5718.9
Dev. from mean 9.3 12.3 6.6 9.9 18.1 10.8 5.8
THEHEATOF COMBUSTION OF YTTERBIUM Mass Yb burned, g.
2.02835 2.0623 2.1622 2.3298 2.4132
Wt. Wt. Mg, YbaOs, mg. g. 7.4 65.6 7.2 33.2 8.0 36.1 7 . 2 60.0 7.7 34.8
J./deg., total7 10036.3 10028.2 10028.9 10034.9 10028.6
Energy from FirDev. Yb, from jJg. mean 12.6 5260.0 8.8 12.3 5247.1 4.1 21.5 5250.9 0.2 17.1 5245.7 5.4 15.0 5251.9 0.7 Av. 6251.1 3.8 Standard dev. 2.5 AT, OK. 1,0825 1,0980 1.1539 1.2373 1.2842
137, 1.
recteds for impurities is 5726.4 j./g. The correctjion due to impurities amounts to 0.13% of the uncorrected value.s The percentage composition of the ytt,erbium is Yb metal 97.64; YbH2,2.17; Yb203,0.0246; YbN, 0.053; C, 0.036; Ca, 0.02; Li, 0.005; Y, 0.05. The corrected heat of combustion for ytterbium gives a value of 5,23L.O j./g. The correction for impurities amounts to 0,38770 of the uncorrected value. Calculation of the Uncertainty.-The uncertainty to be attached to the corrected values includes the uncertainty in the energy equivalent which is 0.04Oj,, the uncertainty in the calorimetric measurements which is 11.6 j./g, or 0.20% for the dysprosium and is 5.0 j,/g. or 0.095% for the ytterbium, and the uncertainty introduced in the correction for the impurities.6 The combined uncertainit~yis 11.9 j./g. for the dysprosium and 6.5 j./g. for the ytterbium. The values for the heats of combustion give for the reaction in the bomb a value of AE25.~0 = - 1861.2 f 3.9 kj./mole for dysprosium and AEzs.00 = - 1810.3 f 2.2 kj./mole for the ytterbium. Composition of the Dysprosium and Ytterbium Oxides.-Each oxide formed was tan in color. An X-ray pattern of the dysprosium oxide showed it to be composed of roughly equal proportions of B and C-types of the sesquioxide. Unfortunately the energy of transition between these two forms is not known. It is probably small, however. The X-ray pattern of the ytterbium oxide showed lines (7) The specific heats of DyzOs and YbzOs are estimated to be 0.20 and 0.25 j . / g J 0 , respectively. The specific heat of Pt is taken The as 0.136 j./g./" and the specific heat of Oa a8 0.651 i./g./'. amount of MgO formed is so small that its contribution to the energy equivalent of the calorimeter may be neglected. (8) The heats of formation of DyHz and YbHi are estimated a t -45 kcal./mole from the known values for NdHz and PrHa. See R. N. R. Mulford and C. E. Holley, Jr., THIS JOURNAL, 59, 1222 (1955). The heats of formation of D y N and YbN are estimated at -75 kcal./mole from the published values of LaN and CeN. See Selected Values of Chemical Thermodynamic Properties, N.B.S.Circular 500, 1952, pp. 350, 354. The heats of combustion of graphite (to Cod. calcium (to CaO), lithium (to LiaOz), and yttrium ( t o YaOa), are taken as 33,000,16,000,45,800 and 10,300 j./g., respectively. The heats of formation of HiO(g) and NO2 are taken a8 -58 and f 8 kcal./mole.
NOTES
1458
only of the C-type sesquioxide. Analysis by the method of Barthauer and Pierceg showed no oxygen above that necessary for the sesquioxide. Heats of Formation of Dy203 and YbzOa.Using methods of calculation reported elsewhere6 the heat of formation of Dyz03, AHz6“ = - 1865.4 i 3.9 kj./mole. In defined calories this is -445.84 i 0.93 kcal./mole. For the heat of formation of 2.2 kj./mole or Ybz03, AHz60 = - 1814.5 - 433.68 =k 0.53 kcal./mole. No literature values are available for comparison. It should probably be remarked, however, that this value for the heat of formation of Dy203 is larger in magnitude by about 10 kcal. than would be expected on the basis of the value for Yb203 (C-form) and the reported values for Gdz03 (B-form) (- 433.94 i 0.86 kcal./mole)5 and Smz03 (B-form) ( - 433.89 i 0.48 kcal./mole) .6 The measurements were made at intervals over a two-month period along with measurements on other materials, two of the five measurements on dysprosium being made before the ytterbium measurements and the other three afterwards so that it seems unlikely that there is a calorimetric error of the necessary magnitude involved. Also, since Smz03 and Gdz03 form B-type oxides and Yb2O3forms the C-type oxide, the fact that Dyz03 forms a mixture of B and C-types can hardly be the explanation. Acknowledgments.-The authors wish to acknowledge the valuable assistance of D. Pavone, F. H. Ellinger, 0. R. Simi and E. Van Kooten in the analytical work. They also appreciate the courtesy of Dr. F. H. Spedding of the Ames Laboratory, A.E.C., through whom the metals and oxides were obtained.
*
Vol. 60
in a slow stream of dry or nitrogene to displace moisture laden air and minimize the possibility of contamination by hydrolysis, Potassium chloride, precipitated from strong hydrochloric acid, fused in this way usually contains no more than 0.001% of free alkali.6b For very hygroscopic material, as lithium chloride, a more rapid stream of the inert gas, and electrolysis of the molten salt a t low current density t o remove the hydroxides have been recommended5 as additional pretreatments before use in high temperature electrochemical investigations. The gain or loss in weight of the alkali chlorides as a function of temperature and time, using modern high vacuum techniques and a dry inert gas to purge the samples, would provide a ready reference for the drying of these salts for electrochemical studies. The present communication describes the results of a series of weight loss determinations designed to meet this need and to supplement the existing information concerning the dryness and purity of molten
In the course of investigations on the conductance of alkali metal titanium fluorides as solutes in potassium chloride-lithium chloride eutectic melts at 350-500°, a need was felt for a criterion of purity in the preparation of dry alkali chlorides. The importance of potassium chloride as conductance reference standard in both aqueous2J and non-aqueous (fused salt) measurements is well recognized. While Jones and Bradshaw3 recommended fusion of the potassium chloride to remove last traces of water, modern practice is t o effect this
Experimental An all glass vacuum-transfer system constructed for work under controlled atmospheres was used in the present investigation. The lithium, sodium and potassium chlorides, Fisher rea ent grade chemicals, were powdered to pass a 30 mesh A.S.%.M. sieve and used‘without further purification. The vials used to hold the samples (about 20 g. salt) for the measurements were a simple test-tube design made from 25 mm. Pyrex or Vycor tubing and connected to the vacuum manifold through a 14/35 T male joint. An indentation in the bottom was made to receive a calibrated chromelalumel thermocouple for temperature measurements. A small General Electric Co. combustion furnace was used to heat these vials. Argon or nitrogen, carefully purified and dried, was used to provide the inert atmosphere. All weighings were corrected for air buoyancy. The general procedure for measurement of the weight losstemperature dependence was as follows. The salt was placed in a weighed dry vial to obtain the weight before drying. The sample was :hen evacuated to 1 0 4 mm. at room temperature or a t 100 . The criterion, that the leak rate on the vacuum manifold system with the sam le in be the same as that with the sample out, was taken as t i e indication that the sample was “dry” a t that temperature. The use of a dry argon or nitrogen “flush” a t the temperature of drying to hasten the removal of water vapor was also introduced. The rate of drying would normally be controlled by the rate of diffusion of the water vapor through the salt mass. It was found with lP, g. of KCl, for example, that with three dry gas “flushes, the sample would pump down to mm. in 30 minutes or less, whereas without this practice a period of several hours was required to achieve dryness. To remove a sample for weighing, the manifold and vial were filled with the dry pre-purified inert gas at a pressure slightly greater than atmospheric. The vial was removed and the stopcock grease adhering to the joint. wiped off with a cloth damp with carbon tetrachloride under this flow of gas. The tube was then uickly capped for weighing. The reproducibility of wei&ings after such treatment was k0.2 mg. Extended evacuation a t 10-8 mm. did not produce any further decrease in weight. The weight loss having been achieved a t the temperature in question, this procedure was repeated with the same sample a t the next higher temperature, until in increments of approximately loo”, the upper limit of temperature was reached.
(1) This work was supported b y the U. 8. Office of Naval Research under Contract Nonr-591(06). (2) F. Kohlrauach, L. Holborn and H. Diesselhorst, Wded. Ann., 64, 417 (1898). (3) G . Jones and B. C. Bradshaw, J . A m . Chem. Soc., E E , 1780 (1933). (6) E. R . Van Artsdalen and I. S. Yaffe, THISJOURNAL, EO, 118 (1955).
(5) 6. Senderoff and A. Brenner, J. Electrochem. Soc., 106, 16 (1954). (6) (a) G. D. Pinching and R. G. Bates, J . Research Natl. Bur. Standards, 81, 311 (1946). (b) Heating to fusion ensures removal of occluded hydrochloric acid from the salt precipitated in a strong solution of the acid. (7) C. Duval, “Inorganic Therniogravimetric Analysis,” Elsevier Publ. Co., Houston, Texas, 1053.
(9) G. L. Barthauer and D. W. Pierce, Ind. Eng. Chem., 18, 479 ( 1 946).
THE PREPARATION OF DRY ALKALI CHLORIDES FOR SOLUTES AND SOLVENTS I N CONDUCTANCE STUDIES BY HENRYJ. GARDNER,CHARLES T. BROWNAND GEORQE
J. JANZ
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N . Y . Received April 67. 1966
