Abstract: A brown and transparent ionic liquid,[C₂mim][FeCl₄], was prepared by mixing anhydrous FeCl₃ with 1-methyl-3-ethylimidazolium chloride([C₂mim][Cl]), with molar ratio 1∶1 under stirring in a glove box filled with dry argon. The molar enthalpies of solution, Δ_sH_m, of [C₂mim][FeCl₄] in water with various molalities were determined by a solution-reaction isoperibol calorimeter at 298.15 K. In terms of Pitzer electrolyte solution theory, the standard molar solution enthalpy of [C₂mim][FeCl₄], Δ_sH⁰_m, and the sum of Pitzer parameters: (β^(0)L_Fe,Cl+β^(0)L_[C2min],Cl+Φ^L_Fe,[C2min]) and (β^(1)L_Fe,Cl+β^(1)L_[C2min],Cl) were obtained, respectively. Using the values of the solution enthalpies and crystal energy of the ionic liquid, the dissociation enthalpy of [FeCl₄]^- anion, ΔH_dis≈5659 kJ/mol, for the reaction: [FeCl₄]^-(g)→Fe³⁺(g)+4Cl^-(g), was estimated. It is shown that the enthalpy of solution of the[C₂mim][FeCl₄] is small because the large hydration enthalpies of Fe³⁺, 4Cl^- and [C₂mim]^- was compensated by the large dissociation enthalpy of [FeCl₄]^- anion, Δ_dH_m, in dissolution process of the IL.

Key words: Ionic liquid, Enthalpy of solution, Pitzer’s theory, Isoperibol calorimeter