The measurement of Henry's constant for hydrogen in high subcritical and supercritical aqueous systems

Studies are reported on the Hg|HgO|ZrO₂(Y₂O₃)|NaOH(aq)|H₂(Pt) cell at temperatures ranging from 298.15 K (25°C) to 723.15 K (450°C) and at a pressure of 275 bar, to assess the viability of ceramic sensors for measuring Henry's constant for hydrogen in high subcritical (473.15 K < T < 647.30 K) and supercritical (T > 647.30 K) aqueous systems. Previous work has shown that this cell is thermodynamically well-behaved, in that the measured potential is in good agreement with that calculated from thermodynamic data for the cell components. Thus, the Hg|HgO|ZrO₂(Y₂O₃)|H⁺,H₂O ceramic membrane electrode, when combined with a platinum electrode, provides a convenient means of evaluating hydrogen activity and Henry's constant for hydrogen in high subcritical and supercritical aqueous systems. The apparent standard Gibbs energies of formation of H₂(aq) at temperatures within the range 473 and 723 K and at a pressure of 275 bar have been obtained. The calculated Henry's constants for dissolved molecular hydrogen in pure water were compared with the available literature data, and good agreement was found over the entire temperature range explored in this work.