Aqueous nonelectrolyte solutions. Part XVI. Formula of deuterium sulfide D-hydrate and its dissociation thermodynamic functions

A method has been devised to approximate both the hydrate formula number n and the standard thermodynamic functions for hydrate dissociation from the temperature change of the hydrate former fugacity along a univariant three- phase (h1(1)g) equilibrium line. Thermodynamic equations are derived, their validity discussed, and an iterative method for their solution is describe d. The univariant (h1(1)g) equilibrium dissociation of deuterium sulfide D- hydrate (D2S . nD(2)O phase h) into gaseous deuterium sulfide (g) and liqui d deuterium oxide (1(1)) has been treated to give approximate formulae and dissociation constants at 58 temperatures from 2.798 to 30.666 degrees C. D issociation equilibrium constants Kp(h-->1(1)g) have been represented as a function of temperature by a four-parameter equation which yields both valu es and standard errors (i) for Delta H-ot(hr-->1(1)g) and Delta Cp-ot(h-->1 (1)g) the standard enthalpy and heat capacity changes for D-hydrate dissoci ation and (ii) for n = r the approximate formula number of the D-hydrate at each experimental temperature. The formula D2S . 6.115D(2)O with standard error 0.018D(2)O is found for deuterium sulfide D-hydrate at lower quadrupl e point Q(hs(1)1(1)g) 3.392 degrees C; an approximate formula D2S . 5.840D( 2)O with SE 0.019D(2)O is found at upper quadruple point Q(hs(1)l(2)g) 30.7 70 degrees C.