Realization of the triple points of equilibrium hydrogen and equilibrium deuterium

The triple points of equilibrium hydrogen and equilibrium deuterium have been realized using a calorimetric method and sealed cells, which include hydrogen (or deuterium) and 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.