Enthalpies of formation and heat capacity functions for maricite,NaFePO4(cr), and sodium iron(III) hydroxy phosphate,Na3Fe(PO4)2· (Na4/3H2/3O)(cr)

Abstract Maricite, NaFe(II)PO4, and sodium iron(III) hydroxy phosphate (“SIHP), Na3Fe(III)(PO4)2· (Na4 / 3H2 / 3O), are recently discovered compounds that play an important role in phosphate-based water treatments for high-pressure steam generators in the electric power industry. Heat capacities of maricite and SIHP have been determined as a function of temperature by differential scanning calorimetry, and enthalpies of solution have been measured at T =  298.15 K in an isothermal calorimeter. These results lead to the following expressions for the standard molar heat capacity and enthalpy of formation of maricite and SIHP:Cp,mo(NaFePO4, cr) / (J·K − 1·mol − 1)  =  107.0  +  8.991 · 10 − 2· (T / K)  −  9.371 · 105· (T / K) − 2, (295 K ≤ T ≤  470 K);Cp,mo{ Na3Fe(PO4)2· (Na4 / 3H2 / 3O), cr } / (J·K − 1·mol − 1)  =  171.0  + 0.3272 · (T / K)  +  1.538 · 106· (T / K) − 2, (320 K ≤ T ≤  700 K);ΔfHmo(NaFePO4, cr, 298.15K) =  − (1571.8  ±  3.0)kJ · mol − 1; and ΔfHmo{Na3Fe(PO4)2·(Na4 / 3H2 / 3O), cr, 298.15K} =  − (3830.8  ±  7.0)kJ · mol − 1.