The Heat Capacity and the Entropy of Transition of Würster’s Blue Perchlorate

The heat capacity of Würster’s Blue perchlorate (N,N,N′,N′-tetramethyl-p-phenylene-diamine perchlorate) has been measured between 66 and 200°K. A phase transition occurs at 189.9°K with its tail extending down to about 160°K. The transition is probably of the first order. The heat and the entropy of the transition are 408.6±4.5 cal./mol. and 2.19±0.02 cal./deg.mol. respectively. The magnetic susceptibility of the specimen at room temperature indicates that the paramagnetic spin density is 100±3%. The experimental results have been compared with current theories, and it has been inferred that some unknown structural changes are involved in the phase transition.