The apparent and partial molal volume of aqueous sodium chloride solutions at various temperatures

The apparent, φ_V, and partial, V̄₂, molal volumes of dilute (0.01 to 1.0 m) aqueous NaCl solutions have been determined as a function of temperature (0 to 55°) from precision density measurements. The Debye-Hückel theoretical limiting slope for the φ_V and V̄₂ as a function of the square root of molar concentration is approached at all temperatures studied. The deviations of φ_V and V̄₂ are strongly dependent on temperature. These results indicate that ion-ion interactions are strongly related to the effect of temperature on the structure of the hydrated ions or the structure of water between the interacting ions. Ion pairing (cation-anion) is examined as a possible cause for the observed deviations and a general method of calculating association constants from V̄₂ data is presented. Stoichiometric association constants, K_A* = 0.35, 0.18 and 0.11, respectively, at 0, 25, and 55°, were found to represent the V̄₂ data over the concentration range from 0.25 to 1.0c.