TRIPLE POINT OF PURE NORMAL-DEUTERIUM

The triple point temperature of pure normal-deuterium was determined on IPTS-68 using a calorimetric technique in an adiabatic cryostat and samples of about 0.2 mol of deuterium, 99.86% pure, permanently sealed in small metal cells. The temperature value was found to be 18.729 ± 0.002 K. Two thirds of the quoted uncertainty was due to the differences in IPTS-68 calibrations from NBS, NPL and PRM); differences in a range of 4.2 mK were observed using five calibrated thermometers. Natural para-ortho conversion rate was observed to be 7 ± 2h-1 at 18.7 K; the rate of conversion back to normal composition for the gas at 20 MPa and ambient temperature was 3 ± 1h1. The enthalpy of melting was 199 ± 1 J mol-1. © 1979.