THE VAPOR-PRESSURE OF SODIUM BETWEEN 660-DEGREES-C AND 962-DEGREES-C

Marcarino and Bassani have proposed the sodium liquid-vapour equilibri um as a means to approximate the ITS-90 between 660 degrees C and 962 degrees C with an accuracy of about 10 mK. We have also measured the s odium vapour pressure over this temperature range using a pressure-con trolled heat pipe. Our results differ from those of Marcarino and Bass ani by approximately 34 mK at 660 degrees C and 21 mK at 962 degrees C with a linear temperature dependence of the difference between these values. This relationship may be explained very satisfactorily by a di fference in the impurity content of the two samples. We show that an a dditional 200 x 10(-6) mass fraction of potassium in our sample as com pared with theirs is sufficient to explain the discrepancy. As sodium of sufficient purity to ensure repeatability of its use as a temperatu re standard at the millikelvin level is not commercially available, it appears necessary to characterize the pressure/temperature relationsh ip for each sample prior to relying on a measurement of the pressure t o determine the temperature of the heat pipe. The alternative is to re ly on the heat pipe as an intercomparison device with the temperature given by a calibrated thermometer.