Temperature and composition-dependent properties of the two-component system d- and l-camphor at ‘ordinary’ pressure

The properties of dextrorotatory and racemic camphor have been investigated as a function of temperature and composition. A thorough literature survey has been undertaken for temperatures and enthalpies of all transitions and for structural information of the phases existing under ordinary pressure. The transition temperatures and enthalpies have been measured with differential scanning calorimetry. Cell parameters of all phases have been determined as a function of temperature up to the melt and the specific volume of liquid camphor has been determined. A temperature–composition phase diagram is presented containing all condensed-phase transitions, including an analysis of the melting transition. The melting point of racemic camphor is at least 2 K lower than the melting point of d- (and l-) camphor at 451 ± 2 K, whereas the melting enthalpy is the same within error. Below the melting point, mixtures of solid d- and l-camphor are solid solutions. Because the structures of the racemic compound near the melting point and the solid solution are equivalent for time and volume averaged measurements, they cannot be distinguished. One could call this a case of ‘critical symmetry’.