Thermodynamic Properties and Phase Equilibria in the H₂O–Ca(NO₃)₂ System

Abstract

A comprehensive data set of thermodynamic and calorimetric properties of calcium nitrate aqueous solutions, including the heats of dilution, heat capacity, and phase equilibria in a wide temperature and concentration range is thoroughly collected and critically reviewed. A temperature-dependent Pitzer− Simonson−Clegg model is proposed to describe reliably the properties of liquid phase as well as solid−liquid (SLE) and vapor− liquid (VLE) equilibria, including metastable ones, over T = (243.15 to 425.75) K and up to x(Ca(NO 3 ) 2 ) = 0.3. Improved accuracy is obtained as compared to that of earlier models along with incorporating many diverse experimental data types simultaneously.

Thermodynamic Properties and Phase Equilibria in the H2O–HNO3–Ca(NO3)2–UO2(NO3)2 System

“…To reproduce the available thermodynamic properties of the H 2 O–HNO 3 –Ca­(NO 3 ) 2 system we determined optimal values of the W Ca 2+ /H + /NO 3 – , U Ca 2+ /H + /NO 3 – , and Q H 2 O/Ca 2+ /H + /NO 3 – parameters. In addition, we had to slightly reoptimize the solubility products for two hydrates (Ca­(NO 3 ) 2 ·3H 2 O and Ca­(NO 3 ) 2 ·2H 2 O) previously proposed by Novikov et al The following considerations show that the reoptimization was appropriate.…”

“…According to the literature, even at T = 273.15 K there are stability regions for Ca­(NO 3 ) 2 ·3H 2 O and Ca­(NO 3 ) 2 ·2H 2 O in the ternary system H 2 O–HNO 3 –Ca­(NO 3 ) 2 , while in the binary subsystem H 2 O–Ca­(NO 3 ) 2 they are stable in a very narrow temperature range near T = 323.15 K. This must have led to the situation that the expressions for ln K (Ca­(NO 3 ) 2 ·3H 2 O) and ln K (Ca­(NO 3 ) 2 ·2H 2 O), which have been evaluated by Novikov et al upon the sole “binary” data, are surely reliable to predict SLE near 323.15 K but do not keep their accuracy when extrapolated deeply with respect to temperature. During the current research it was shown that there are no values of the ternary PSC parameters being able to provide an accurate representation of SLE-data in the H 2 O–HNO 3 –Ca­(NO 3 ) 2 system at T = 298.15 K or lower temperatures when using the expressions for ln K (Ca­(NO 3 ) 2 ·3H 2 O) and ln K (Ca­(NO 3 ) 2 ·2H 2 O) proposed by Novikov et al .…”

“…Solubility products of Ca­(NO 3 ) 2 ·3H 2 O (brown) and Ca­(NO 3 ) 2 ·2H 2 O (dark blue) as functions of temperature. Solid lines represent the values obtained in the current study, dotted lines represent the expressions evaluated by Novikov et al Dashed lines are borders of the stability regions of Ca­(NO 3 ) 2 ·3H 2 O (brown) and Ca­(NO 3 ) 2 ·2H 2 O (dark blue) in the binary H 2 O–Ca­(NO 3 ) 2 subsystem (the upper bound being equal to 323 K for both salts).…”

“…In a sense of the aforementioned criterion, this fact means that Carslaw’s model is appropriate for the purposes of the current research. Similar considerations with regard to Novikov’s model of the H 2 O–Ca­(NO 3 ) 2 system prove it to be a model of choice as well.…”

“…The aforementioned facts combined make Carslaw’s model the most attractive basis to build up a thermodynamic model of the H 2 O–HNO 3 –Ca­(NO 3 ) 2 –UO 2 (NO 3 ) 2 system. The presence of a comprehensive thermodynamic description of the H 2 O–Ca­(NO 3 ) 2 subsystem, also based on PSC equations, makes adoption of Carslaw’s model even more favorable. Therefore, the objective of this study was to develop a thermodynamic model of the H 2 O–HNO 3 –Ca­(NO 3 ) 2 –UO 2 (NO 3 ) 2 system using PSC equations to represent the properties of a liquid phase.…”