[en] The first precise measurements of specific heat changes have been performed for a C₃N₂H₅ClO₄ crystal using an ac calorimeter. Phase transitions at about 373 and 219 K were revealed. The continuous second-order phase transition at 373 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of the same order of magnitude as those obtained for hard ferroelectric materials. Thermal parameters (such as the excess enthalpy, ΔH, and the excess entropy, ΔS) of the continuous transition were estimated and discussed. During the first cooling, a distinct (∼2 K) supercooling effect was observed at the discontinuous transition at about 219 K