Aqueous nonelectrolyte solutions. Part XVII. Formula of hydrogen sulfide hydrate and its dissociation thermodynamic functions

Literature data for the saturation vapor pressure P(hl(1)g) of hydrogen sul fide hydrate with water, at 43 temperatures between quadruple points Q(hs(1 )l(1)g) at -0.4degreesC and Q(hl(1)l(2)g) at 29.484degreesC, are properly r epresented by a six-parameter equation togive a standard error (SE) of 0.13 % on a hydrate pressure measurement of unitweight. Equilibrium hydrogen sul fide and water fugacities and the gas and aqueous phase compositions are de rived using the Redlich-Kwong equation of state.Literature data for the sat uration vapor pressure P(hs(1)g) of hydrogen sulfide hydrate with ice, at 1 5 temperatures between -1.249 and -21.083degreesC, are properly represented by a two-parameter equation to give a SE of 0.26% on a single hydrate pres sure measurement. Quadruple point Q(hs(1)l(1)g) is evaluated at temperature -0.413degrees with SE 0.042degreesC and at pressure 94.7 with SE 0.26 kPa. Using the thermodynamic method, described for deuterium sulfide D-hydrate, the equilibrium fugacities of hydrogen sulfide are used to define 43 equil ibrium constants Kp(h--> l(1)g) forhydrate dissociation into water and hydr ogen sulfide gas. The temperature dependence of ln Kp(h--> l(1)g) is repres ented by a three-parameter thermodynamic equation which gives both values a nd standard errors (i) for Kp(h-->l(1)g), and for Delta H-ot(h--> l(1)g) an d Delta Cp-ot(h--> l(1)g), the standard enthalpy and heat capacity changes for hydrate dissociation and (ii) for n = r the approximate formula number of the hydrate H2S . nH(2)O at each experimental temperature. The formula H 2S . 6.119H(2)O with standard error 0.029H(2)O is found for hydrogen sulfid e hydrate with water at lower quadruple point Q(hs(1)l(1)g) -0.413degreesC: an approximateformula H2S . 5.869H(2)O with SE 0.026H(2)O is found at uppe r quadruple point Q(hl(1)l(2)g) 29.484degreesC. These estimates for the for mula of hydrogen sulfide hydrate at its quadruple points are not significan tly different from those found for the deuterium sulfide D-hydrate.