A QUARTZ-CRYSTAL MICROBALANCE TO DETERMINE ENTHALPIES OF SUBLIMATION AT INTERMEDIATE TEMPERATURES BY THE KNUDSEN EFFUSION METHOD

The principle of operation, design, construction and testing of a quar tz crystal microbalance for indirect determination of the enthalpy of sublimation is described: The instrument allows fast and reliable meas urement of the mass flow from a Knudsen effusion cell at different tem peratures. The enthalpy of sublimation can be derived using the Knudse n equation for the vapour pressure combined with the Clapeyron equatio n. The test substance used in this study was thiourea and experiments were made in the range 378.45-395.89 degrees K. The derived value of D elta(sub)H(m) (387.17 K) = (103.95 +/- 0.32) kJ mol(-1) is in good agr eement with the reported calorimetric value Delta H-sub (389.17 K) = ( 103.54 +/- 0.43) kJ mol(-1).