Thermodynamics of anhydrous potassium ferrocyanide and of potassium ferrocyanide trihydrate. Ferroelectric transition in the trihydrate

Abstract

The heat capacity of potassium ferrocyanide trihydrate (KFCT) and the anhydrous salt has been measured from 10 K to 300 K. The contribution made to the heat capacity of KFCT by the ferroelectric → paraelectric transition has been assessed (a) by using the heat capacity of the anhydrous salt as a reference, (b) by subtracting from the total heat capacity of KFCT the contribution from the internal vibrations of the ferrocyanide ion. The values so obtained for the entropy of transition are (a) 7.8 J K^–1 mol^–1, (b) 9.7 J K^–1 mol^–1. Measurements have also been made, at 25°C, of the heat of solution of the two salts and of the water vapour dissociation pressure of the KFCT + anhydrous salt system. The results give a value of 593.7 J K^–1 mol^–1 for the molar entropy of KFCT derived from the equilibrium study, in almost exact agreement with the calorimetric entropy of 593.8 J K^–1 mol^–1, showing that the water molecules in KFCT in the ferroelectric phase are uniquely ordered at 0 K.

The standard entropy of the ferrocyanide ion in aqueous solution at 25°C, S°(Fe(CN)^4–₆, 298.15 K), is estimated to be 76.6 J K^–1 mol^–1.