Apparent molar heat capacity and relative enthalpy of aqueous NaOH between 323 and 523K

The apparent molar heat capacity, C _p,, of aqueous NaOH has been measured at temperatures between 50 and 250°C and molalities from 0.05 to 1.5 mol-kg^–1. Enthalpies of dilution L were also determined at 99°C and apparent molar relative enthalpies L were calculated up to 1.9 m. Measurements were performed by means of a flow calorimetric apparatus constructed in our laboratory and standardized for C _p, and L with aqueous Na₂SO₄ and with the formation of water from its ions, respectively. Characteristics and performance of this calorimeter are described in detail. Pitzer's semiempirical equations are used for the representation of the results and a general fitting of C _p, data is reported using also recent literature values measured between 4 and 55°C. The fitted parameters are finally utilized, through an integration procedure, to derive a general equation to calculate L at any temperature between 4 and 250°C.