An Experimental Investigation of the Effect of Pressure on Phase Equilibria of Sodium Tungstate and of Related Thermodynamic Properties

Abstract

In order to clarify some uncertainties concerning the physical properties of sodium tungstate the effect of pressure on phase equilibria to 1000 bars, the heats of inversion and of melting, and the specific volume (density) at 30° were determined. Sodium tungstate is trimorphous at atmospheric pressure, form I being stable from 695.5° (melting point) to 588.8°, form II stable from 588.8° to just below 587.6°, and form III stable below this temperature. The I⇌II inversion is estimated to take place at 588.8° with a heat change of 14 joules/gram but no volume change (independent of pressure). Because of the large amount of undercooling in the I→III inversion, the II⇌I transition could be obtained metastably to 613 bars. The III⇌I inversion takes place promptly and reproducibly on heating but not so on cooling, the temperature change being dependent in large measure on the past history of the charge. Heating curve values lead to the expression t = 588.8+0.025(p — 40) with a heat change of 105 joules/gram and volume change of 0.035 cm3/gram. The triple point lies at t = 588.8° and p = 40 to 80 bars. The II⇌III inversion is very similar in behavior to the I⇌III inversion; here t = 587.6+0.029 p on the heating curve. In order to represent specific volume as a function of temperature from 20° to 1300° the data of Jaeger and of Austin were also utilized. The published density of sodium tungstate was found to be in error and therefore a redetermination was made. The value obtained by us, 5.13±0.01, is 20 percent higher than that given by F. W. Clarke (J. L. Davis). The low-temperature modification expands from 0.1950 cm3/gram at 30° to 0.2032 cm3/gram at 587.6°. The salt inverts at this temperature, attaining the volume 0.2382 cm3/gram, and finally, it again increases in volume at the melting point, 695.5°, to 0.2565 cm3/gram.