Abstract
The principle of operation, design, construction and testing of a quartz crystal microbalance for indirect determination of the enthalpy of sublimation is described. The instrument allows fast and reliable measurement of the mass flow from a Knudsen effusion cell at different temperatures. The enthalpy of sublimation can be derived using the Knudsen equation for the vapour pressure combined with the Clapeyron equation. The test substance used in this study was thiourea and experiments were made in the range 378.45-395.89 K. The derived value of Delta subHm(387.17 K)=(103.95+or-0.32) kJ mol-1 is in good agreement with the reported calorimetric value Delta Hsub(389.17 K)=(103.54+or-0.43) kJ mol-1.
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