Some observations concerning the loss of water from hydrated cerous magnesium nitrate

Abstract

The fact that the paramagnetic susceptibility of cerous magnesium nitrate hydrate (CMN) obeys the Curie law down to about 0.006 K has led to its wide use as a low temperature magnetic thermometer. Under unfavourable experimental conditions, however, the salt tends to lose its water of crystallization, and any consequent alteration of its magnetic properties will cause uncertainties in the magnetic temperature scale. The temperature dependence of the water vapour pressure of CMN between 25° and 85°C can be described by the relati log 10 P = 10.05 − 3.06 × 10 3/T where P is in torr and Tin kelvin. Thus at room temperature ( T = 25° C, P = 0.60 torr) dehydration will occur if the atmospheric water vapour pressure falls below 0.6 torr, a value corresponding to a relative humidity of 2.5%. The present study also shows that under dynamic vacuum at normal temperatures CMN loses up to two-thirds of its combined water. In the case of a loose power sample at a temperature of 25°C this loss is found to be essentially complete in about 250 hours, while, as a rough rule, the loss rate doubles for each temperature rise of 5°C. The rate-determining step is diffusion in the bulk of the material and the activation energy for H 2O diffusion in CMN is found to be 117 kJ per mole CMN.