Measurement of the Relative Enthalpy of Pure α-AI₂O₃ (NBS Heat Capacity and Enthalpy Standard Reference Material No....

The relative enthalpy of NBS Standard Reference Material No. 720 (99.98 percent pure, single-crystal α-Al₂O₃, a calorimetrie 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 SKM 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 α-Al₂O₃ are presented for the interval 0 to 1200 K.