LOW-TEMPERATURE HEAT-CAPACITY OF UREA

The heat capacity of urea was measured with an adiabatic calorimeter in the temperature range 15-310 K. The data were extrapolated to 0 K by a model function to derive some standard thermodynamic functions including the enthalpy increments DELTA0(T)H, the entropy increments DELTA0(T)S, and the Giauque function (= DELTA0(T)S - DELTA0(T)H/T). A simple model for the reproduction of the experimental heat capacities of urea, based on the Debye and Einstein functions, is described. The Debye characteristic temperature determined in this way was compared with those calculated from properties other than the heat capacity. Any positive evidence of a suggested phase transition in urea around 190 K was not observed in the present heat capacity measurements. Possible existence of a phase with a Gibbs energy lower than that realized in the present investigation is discussed briefly.