Measuring low vapor pressures employing the Knudsen effusion technique and a magnetic suspension balance.

A new Knudsen effusion apparatus employing a magnetic suspension balance (MSB) to measure low vapor pressures <1 Pa is presented. The intention of the work is developing a method for measuring vapor pressures that covers a wider range of pressure than established procedures do. A cooled condensation plate is used to collect a large fraction of effused molecules which have left a Knudsen cell. This fraction is calculated from the cosine law of effusion with the geometry of the system. The condensation plate is cooled indirectly with liquid nitrogen so that the molecules which impinge on the plate condense at its surface. The mass increase of the condensation plate is continuously measured with the MSB. The new system is tested with benzoic acid at 298-313 K and with anthracene at 343 and 353 K. Good results with a deviation lower than 5% are achieved with benzoic acid at 298 K. However, the deviation of our results from reference vapor pressures increases with increasing temperature. This behavior may result from not fully isothermal conditions in the Knudsen cell.