A MICROTHERMOGRAVIMETRIC SYSTEM FOR THE MEASUREMENT OF VAPOR-PRESSURE BY A TRANSPIRATION METHOD

A novel automatic recording microthermogravimetric system for the measurement of vapour pressures at high temperatures by transpiration method was designed and fabricated. The performance of the system was checked by measuring the vapour pressure of anhydrous cadmium chloride in flowing dry argon in the temperature range 713-833 K. The vapour pressure of cadmium chloride in this range could be expressed as ln(P/Pa) = -(20 109.10 +/- 262.32)/T + (29.46 +/- 0.344) The enthalpy of sublimation of CdCl2 derived from this equation at the mean temperature of investigation was found to be 167.2 +/- 2.2 kJ mol-1 and compared very well with the value of 166.5 +/- 4.7 kJ mol-1 reported by Skudlarski et al. (J. Chem. Thermodyn., 19 (1987) 857) from their recent Knudsen effusion mass spectrometric measurements. The present system can be used for measurement of vapour pressures up to 1425 K and is the first of its kind in which the mass loss due to vaporization is monitored continuously during the conventional transpiration experiment. The system described in this paper effects considerable saving of time in the experiment and minimizes the errors associated with mass measurements in the conventional transpiration method currently in use.