The glass transition temperatures of phosphoric acids

Abstract

THE vitrification of phosphoric acids is relatively easy, a glass often being formed simply on cooling from the liquid melt. Special procedures, however, are required to crystallise these materials. The glass transition temperature, T_g, is the temperature above which there is relatively rapid molecular motion; below T_g translational motion of molecules is inhibited. In standard texts¹ it is stated that the glass transition temperature of orthophosphoric acid, H₃PO₄, is −121 °C. The source of the determination is not always cited but it seems to be that of Kobeko et al.² who determined T_g from measurements of the temperature dependence of the electrical conductivity of phosphoric acid. Although published almost 40 yr ago there does not seem to have been a more recent determination. We present here evidence from broad-line nuclear magnetic resonance (NMR) studies that the estimate of −121 °C for the glass transition temperature of orthophosphoric acid is wrong. A discussion of the nterpretation of the electrical conductivity of phosphoric acid would be in appropriate beyond observing that several different ionic or molecular processes may be responsible for the transfer of electrical charges. Thus, an unambiguous determination of a glass transition from such data may be far from straightforward.