DETERMINATION OF TEMPERATURE-DEPENDENT PA RTIAL PRESSURES IN CLOSED SYSTEMS - A NEW METHOD - THE HEAT OF FORMATION FOR PTI2(S)

A new method to determine temperature dependent partial pressures of g aseous species in equilibria with condensed phases in closed systems ( silica ampoules) at temperatures up to 1000 degrees C and pressures p( i) 0.01 <p(i) < 10 bar is presented. It is based on the determination of the change of mass in the gasphase caused by solid-gas transition a t higher temperatures of substances which are deposited at one end of the ampoule. The results of the measurements give informations about r eaction mechanisms, enthalpies and entropies. The reliability of the m ethod is demonstrated at the example of the system Pt/I-2. The heat of formation and the entropy of PtI2(s) (Delta(B)H degrees(PtI2(s),298) =-51.4kJ mol-l, S degrees(PtI2(s), 298) = 119.3 J . K-1. mol(-1)) are computed from experimental results. The heat of thermal decomposition of PtI2(s) was reconsidered by Knudsen Mass Spectrometry.