The vapour pressure of sodium between 660℃ and 962℃

摘要

Marcarino and Bassani have proposed the sodium liquid-vapour equilibrium as a means to approximate the ITS-90 between 660℃ and 962℃ with an accuracy of about 10 mK. We have also measured the sodium vapour pressure over this temperature range using a pressure-controlled heat pipe. Our results differ from those of Marcarino and Bassani by approximately 34 mK at 660℃ and 21 mK at 962℃ with a linear temperature dependence of the difference between these values. This relationship may be explained very satisfactorily by a difference in the impurity content of the two samples. We show that an additional 200×10~(-6) mass fraction of potassium in our sample as compared with theirs is sufficient to explain the discrepancy. As sodium of sufficient purity to ensure repeatability of its use as a temperature standard at the millikelvin level is not commercially available, it appears necessary to characterize the pressure/temperature relationship for each sample prior to relying on a measurement of the pressure to determine the temperature of the heat pipe. The alternative is to rely on the heat pipe as an intercomparison device with the temperature given by a calibrated thermometer.