## Abstract

The densities of a selection of binary mixtures of fused alkali nitrates with alkaline earth nitrates have been measured by means of a manometric densitometer. Within the range of liquid stability all results are expressible as a linear function of temperature. For any pair of salts the isotherms of equivalent volume as a function of equivalent composition have been found to be straight lines. Although decomposition did not permit observations over the whole temperature range, any excess volumes of mixing are sufficiently small to justify extrapolation to determine the theoretical equivalent volumes of the alkaline earth nitrates. The partial equivalent volumes $\ovlerline{V}_m$ (the volume of melt containing one gram-ion of nitrate ion) of the constituents are thus effectively independent of composition at a given temperature and are given by: $\mathrm{NaNO}_3 \ovlerline{V}_m &= 44\cdot18 + 1\cdot 76 x 10^{-2} (t-300) \\ \mathrm{KNO}_3 \ovlerline{V}_m &= 53\cdot 25 + 2\cdot 29 x 10^{-2} (t-300) \\ \mathrm{RbNO}_3 \ovlerline{V}_m &= 58\cdot 37 + 2\cdot62 x 10^{-2} (t-300) \\ \mathrm{CsNO}_3 \ovlerline{V}_m &= 66\cdot 25 + 3\cdot 06 x 10^{-2} (t-300) \\ \frac{1}{2}\mathrm{Mg} (\mathrm{NO}_3)_2 \ovlerline{V}_m = 38\cdot 80 + 1\cdot 12 x 10^{-2} (t-300) \\ \frac{1}{2}\mathrm{Ca(NO}_3)_2 \ovlerline{V}_m = 38\cdot 70 + 1\cdot 01 x 10^{-2} (t-300) \\ \frac{1}{2}\mathrm{Sr(NO}_3)_2 \ovlerline{V}_m = 39\cdot 55 + 1\cdot 02 x 10^{-2} (t-300) \\ \frac{1}{2} \mathrm{Ba(NO}_3)_2 \ovlerline{V}_m = 41\cdot 75 + 1\cdot 03 x 10^{-2} (t-300)$ where t is the temperature in degrees Celsius. V$_m$ is correct to $\pm$0.1% and the expansion coefficient to 1.3%. An interpretation of the effectively ideal behaviour of these systems is discussed on the basis of simple structural models for the packing of nitrate ions.