Abstract
New high pressure type of hydrogen sealed cells is designed and developed for realizing the triple point of equilibrium hydrogen, which is possible to replace sample gas without destruction. The triple point of equilibrium hydrogen has been realized using a calorimetric method and the new sealed cells, which include ferric hydroxide as a catalyst for the ortho-para equilibration. The triple point of equilibrium deuterium has also been realized using a sealed cell fabricated at Istituto di Metrologia G. Colonnetti, which include gadolinium oxide as a catalyst for the ortho-para equilibration. Anomalous increases on heat capacity curves of each substance, which markedly disturb melting curves of the triple point, are observed at temperatures just below the triple point. In the case of equilibrium hydrogen, a reduction of the amount of the catalyst suppresses the anomalous increases and allows one to obtain more reliable melting curves of the triple point.
