Thermodynamics of ionization of water at temperatures 278.15⩽⩽⩽T / K⩽⩽⩽ 393.15 and at the pressurep = 0.35 MPa: apparent molar volumes and apparent molar heat capacities of aqueous solutions of potassium and sodium nitrates and nitric acid

Abstract

Listen

Translate article

The apparent molar volumes Vφand apparent molar heat capacitiesCp, φ of aqueous solutions of potassium and sodium nitrate and of nitric acid have been determined over the temperature range 278.15 ⩽T/K⩽ 393.15 and molality range 0.015 ⩽m/ (mol · kg−1) ⩽ 0.5 at the pressure p= 0.35 MPa. Densities of these solutions were measured using a vibrating-tube densimeter (DMA 512, Anton Paar, Austria). The density results were used to calculateVφ for each solution. The massic heat capacities of these same solutions were measured using a twin fixed-cell, differential-output, temperature-scanning calorimeter (NanoDSC model 6100, Calorimetry Sciences Corporation, Spanish Fork, UT, U.S.A.) and these measurements were used to calculate Cp, φfor each solute. The Vφand Cp, φresults were fit to semi-empirical equations by regression. These equations were used to calculate the change in volume ( m, T,ΔrVm ) and the change in heat capacity ( m,T , ΔrCp, m) for ionization of water in KNO3 (aq) and NaNO 3(aq). The ( m, T,ΔrCp, m ) results were integrated to obtain the change in enthalpy ( m, T, ΔrHm), the molality quotient ( m, T, pQw), and change in entropy ( m,T , ΔrSm) for the ionization reaction of water.