JOURNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry Measurement of the Relative Enthalpy of Pure a-Al2O3 (NBS David A. Ditmars and Thomas B. Douglas Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (June 11, 1971) The relative enthalpy of NBS Standard Reference Material No. 720 (99.98 percent pure, singlecrystal a-Al2O3. a calorimetric heat-capacity standard) was measured over the range 273 to 1173 K by the drop method using a highly precise Bunsen ice calorimeter. Enthalpy data over the same temperature interval were obtained also on the Calorimetry Conference Sample of this substance. These results are believed to be more accurate than similar NBS results on the latter sample published in 1956, and show no significant discontinuity with the NBS data on the same substance that covered the ranges 13 to 380 K and 1173 to 2257 K. The average deviation from the mean for all enthalpy measurements on the SRM 720 sample was 0.017 percent, and the smooth enthalpy values derived from the data were estimated to be accurate to 0.1 percent. The precautions observed in order to minimize measuring errors are described in detail. The data are compared with many sets of the most reliable published data available and new recommended values for the thermodynamic functions of a-Al2O3 are presented for the interval 0 to 1200 K. Key words: Alumina; aluminium oxide; corundum; drop calorimetry; enthalpy; heat capacity standard; 1. Introduction Calorimetric standard substances are necessary in order to facilitate meaningful comparison of different calorimeters or the same calorimeter at different times. One such substance, a-Al2O3 (“corundum"), was recommended as a calorimetric heat capacity standard in 1948 by a committee of the Fourth Conference on Low Temperature Calorimetry. It was felt that its ready availability in highly pure form as synthetic sapphire, together with its chemical and mechanical stability. would make it suitable for use from the cryogenic temperature range to near its melting point (2327 K ± 6 K [1]1). The National Bureau of Standards ("NBS"), as part of its overall responsibility for the maintenance of standards in several areas of science and industry, subsequently undertook the measurement of the heat capacity and enthalpy of a special sample of pure a-Al2O3 from approximately 0 to 1200 K [2]. Although the NBS distributed for the Calorimetry Conference, specimens from this sample of a-Al2O3 α 'Figures in brackets indicate the literature references at the end of this paper. *All inquiries concerning the availability of this material and details concerning purchases should be directed to the Office of Standard Reference Materials, Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234. It is currently supplied, together with a certification of values of its enthalpy and heat capacity in the temperature range 273.15 to 2250 K. at a cost of $56. per unit of 15. grams. (hereafter called the "Calorimetry Conference Sample") to qualified laboratories, this material was never considered a formal part of the NBS Standard Samples Series. As the NBS stock of the Calorimetry Conference. Sample neared depletion, the question of adopting a more suitable physical form for the synthetic sapphire standard arose. The Calorimetry Conference Sample was in the form of crushed crystals ranging in size from 0.5 to 2. mm. Past experience had shown that the sharp edges and diversity of size of the particles of the old sample often led to considerable difficulty in filling and emptying some sample containers of calorimetrically desirable design. It was felt that cylindrical segments was one form which was compatible with the technology of producing the synthetic sapphire and which promised to avoid the handling difficulty referred to above. The new synthetic-sapphire heat-capacity standard is being incorporated into the NBS Standard Reference Materials Program and will hereafter be referred to as "SRM 720."2 This report describes measurements by the "drop" method with a Bunsen ice calorimeter, of the relative enthalpy of SRM 720 from 273 to 1173 K, together with a remeasurement of the Calorimetry Conference Sample. Owing to the improved precision and accuracy |