Solubility Equilibria and Crystallographic Characterization of the <scp>l</scp>-Threonine/<scp>l</scp>-<i>allo</i>-Threonine System, Part 1: Solubility Equilibria in the Threonine Diastereomeric System

Abstract

Solubility equilibria of threonine, allo -threonine, and their mixtures in water were measured in a temperature range between 10 and 80 °C. In comparison with allo -threonine, the threonine species exhibit higher solubility in water. The solubility phase diagram of the ternary system l - allo -threonine/ l -threonine/water shows maximum solubilities at l -threonine contents in the range of 56–66%, while the tie lines clearly indicate the presence of solid solutions. X-ray powder diffraction (XRPD) studies of various mixtures prove complete miscibility of l - allo -threonine and l -threonine molecules in the solid state, based on characteristic features of measured diffraction patterns and calculated lattice parameters. Applying the solubility data determined, fractional crystallization is suggested as an effective purification method of mixtures below or above the alyotropic composition.