Molar Volumes and Heat Capacities of Aqueous Solutions of Short-Chain Aliphatic Sodium Carboxylates at 25 °C

Abstract

Densities and isobaric heat capacities of the aqueous solutions of the sodium salts of the four smallest straight-chain aliphatic carboxylates: sodium methanoate (formate), ethanoate (acetate) n-propanoate, and n-butanoate have been measured at 25 degrees C and 0.1 MPa by vibrating-tube densimetry and Picker flow calorimetry, respectively. All systems were investigated at concentrations ranging from approximately 0.02 mol.kg(-1) near saturation, which extends the existing database to higher concentrations for all of these salts. Apparent molar isobaric heat capacities and volumes derived from the data were fitted with an extended Redlich-Meyer equation to determine the standard state partial molar quantities. At concentrations where comparisons were possible, the present results generally agreed well with previous determinations. An exception was the heat capacities of sodium formate solutions, where the present results at low solute concentrations appear to be influenced by an unidentified thermal effect. The apparent molar quantities, especially the heat capacities, provide indirect evidence of aggregation by the longer-chain aliphatic carboxylates at high concentrations.