Corresponding states and the glass transition for alkali metal nitrates

In order to establish a set of corresponding temperatures for the much-studied molten alkali metal nitrates, estimates of the "ideal" glass transition temperatures for NaNO₃ and KNO₃ are made from available thermodynamic data and shown consistent with "experimental" glass transition temperatures obtained by extrapolation from binary solution (hydrate melt) data. T_g values are obtained in the same manner for the other alkali metal nitrates, Li, Rb, and CsNO₃, the spread of values among the five salts found being only 18°, thereby largely justifying isothermal comparisons of their properties despite the 160° spread in melting points. The order-disorder transition in sodium nitrate is examined and the relation of this class of transition to the glass transition phenomenon is considered. Equations from a zeroth-order, order-disorder theory for the glass transition are used to give a qualitative account of the change in heat capacity at the glass transition for sodium nitrate.