Phase Equilibria of Alkali Nitrates ( NaNO 3 , KNO 3 , RbNO 3 , CsNO 3 ) in Glycerol + H 2 O System at Various Temperatures

Aqueous solutions containing salts are of increasing importance in separation and purification processes in chemical engineering. For example, the addition of an organic solvent to the aqueous solution of a salt normally decreases the solubility of the salt, which is called “salting out effect”. Therefore, investigations of the physicochemical properties of inorganic salts in organic solvents + water, such as solubility data together with density, refractive index and so on, are of great scientific importance. For instance, much research has been done on the physicochemical properties of NaCl/KCl + alcohol + H2O systems. Gomis et al. investigated the ternary systems of alcohol (propan-1-ol, propan-2-ol, pentan-2-ol, pentan-3-ol, 2-methylbutan-2-ol, 2-methylbutan-1-ol, and pentan-1-ol) + MCl (M = Na, K) + water at 298.15 K.1–4 Galleguillos et al.5 reported the solubilities, densities, and refractive indices of systems NaCl/KCl + ethanol + H2O at different temperatures. Some researchers focused on the study of systems that include alkali nitrates. For example, reported were the solubility, refractive index, density, electrical conductivity and viscosity of NaNO3 + H2O + poly(ethylene glycol),6 KNO3 + H2O + propan-2-ol,7 and LiNO3 in organic solvent (methanol, ethanol, propan-1-ol, propan-2-ol and butan-1-ol).8


Introduction
Aqueous solutions containing salts are of increasing importance in separation and purification processes in chemical engineering.For example, the addition of an organic solvent to the aqueous solution of a salt normally decreases the solubility of the salt, which is called "salting out effect".Therefore, investigations of the physicochemical properties of inorganic salts in organic solvents + water, such as solubility data together with density, refractive index and so on, are of great scientific importance.For instance, much research has been done on the physicochemical properties of NaCl/KCl + alcohol + H 2 O systems.Gomis et al. investigated the ternary systems of alcohol (propan-1-ol, propan-2-ol, pentan--2-ol, pentan-3-ol, 2-methylbutan-2-ol, 2-methylbutan-1-ol, and pentan-1-ol) + MCl (M = Na, K) + water at 298.15 K. [1][2][3][4] Galleguillos et al. 5 reported the solubilities, densities, and refractive indices of systems NaCl/KCl + ethanol + H 2 O at different temperatures.Some researchers focused on the study of systems that include alkali nitrates.For example, reported were the solubility, refractive index, density, electrical conductivity and viscosity of NaNO 3 + H 2 O + poly(ethylene glycol), 6 KNO 3 + H 2 O + propan-2-ol, 7 and LiNO 3 in organic solvent (methanol, ethanol, propan-1-ol, propan-2--ol and butan-1-ol). 80][11][12] As a continuation of our work in this field, this paper presents the systematic study we carried out on the properties, including solubility, density and refractive index of the ternary systems of NaNO 3 /KNO 3 /RbNO 3 /CsNO 3 + glycerol [CH 3 CH(OH)CH 2 OH] + H 2 O at 35 and 45 °C.The data for the saturated systems were measured and correlated with an empirical equation.We also present a simple method for analysing the composition of the ternary systems by combining the refractive index and density.

Experimental Materials
The reagents used were glycerol (purity > 99.0 %, Shanghai), sodium nitrate and potassium nitrate (purity > 99.5 %, Shanghai), rubidium nitrate (purity > 99.5 %, Shanghai) and cesium nitrate (purity > 99.5 %, Sichuan).All of the chemicals used in this study were used without further purification.The above salts were preheated at 110 °C and dried to constant weight for 48 h.All the reagents were then stored over silica gel in desiccators.Double distilled water was used throughout this work.

Apparatus and procedure
The phase equilibrium study was carried out by mixing known masses of glycerol and water with excess salt.A detailed description of the equipment used in this paper has been presented in former work. 9All the samples were prepared by mass using an analytical balance with precision of Phase Equilibria of Alkali Nitrates (NaNO 3 , Jiang, a and M.-C.Hu a* ± 1 × 10 −4 g (Mettler Toledo, AL204).The samples were fixed on the carrier plate and stirred for 24 h.After standing for another 48 h ensuring that equilibrium was established, samples of the solution were taken for phase composition analysis.
Refractive indices and densities of the solutions were determined using RXA 170 refractometer and DMA 4500 vibrating tube densimeter (Anton Paar) with precision of ± 4 × 10 −5 and 1 × 10 −5 g cm −3 , respectively.Both instruments were calibrated prior to initiation of each series of measurements, using air and deionized distilled water as reference substances.
For ternary systems containing organic compound (1) + salt (2) + water (3), the refractive index and density of the solution depends both on the content of the organic component and the salt. 13,14For dilute aqueous solutions containing glycerol and a salt, the relation between the refractive index, n D , or density, ρ, and the mass fractions of glycerol w 1 , and a salt, w 2 is given by: where ρ is the density, n D is the refractive index, w 1 is the mass fraction of the alkali metal nitrates in the solution, w 2 is the mass fraction of the glycerol in the solution.a 1 , b 1 , c 1 , a 2 , b 2 and c 2 are parameters of the equation.The samples for calibration were prepared containing the salt with mass fraction of 0, 3 %, 6 % and glycerol with known and varying content.Then the density and refractive index of the samples were determined at 25 °C and plotted against the mass fraction of glycerol as shown in Fig. 1.The values of the six coefficients for the studied systems were obtained by the calibration plots listed in Table 1.However, it should be noted that this equation is only valid for dilute solutions of glycerol and salt (w 1 < 0.06, w 2 < 0.45).Therefore, it was necessary to dilute the samples before measurement of refractive index and density.Then the combined equations 1 and 2 can be solved, and w 1 and w 2 in the ternary systems can be determined.

Results and discussion
The solubility, density and refractive index data of the ternary saturated solution NaNO 3 / KNO 3 /RbNO 3 /CsNO 3 (w 1 ) + glycerol (w 2 ) + H 2 O (w 3 ) at 35 °C and 45 °C are listed in Table 2. Table 3   Fig. 2 depicts the variation trend of solubility, density and refractive index versus the mass fraction of glycerol for the systems RbNO 3 /CsNO 3 (w 1 ) + glycerol (w 2 ) + H 2 O (w 3 ) at 25 and 35 °C.As shown in Fig. 2a for the systems at fixed temperature, with increasing mass fraction of glycerol, the solubility of the salt in the mixed solvent decreased.Furthermore, the higher the temperature, the higher is the solubility.However, the variation tendency of density for the systems is slightly different from that of solubility as presented in Fig. 2b.For RbNO 3 + glycerol + H 2 O systems, the density tendency is similar to that of solubility, which suggests that the density is mainly affected by the solubility.But for CsNO 3 + glycerol + H 2 O, in the entire concentration range of glycerol, the density changed slightly.This phenomenon may be caused by both the density and the content of glycerol.The higher the glycerol content in the mixed solvent, the lower is the solubility of CsNO 3 .But at the same time, because of the higher density of glycerol itself, the higher the content of glycerol in the mixed solvent, the higher is the density of solution.The effect of the glycerol content was more obvious in the systems KNO 3 + glycerol + H 2 O.That is, the density first decreases and then increases with increasing of the glycerol content.The refractive indices of the systems are given in Fig. 2c.First, the refractive index of the solution rises with the increase of the glycerol content, showing the contrary variation trend to solubility and density.This is, however, completely different from the systems of NaCl/KCl + ethanol + H 2 O 5 and KNO 3 + propan-2-ol + H 2 O. 7 Moreover, the refractive indices of the systems almost overlapped, except for RbNO 3 + glycerol + H 2 O at low concentrations of glycerol.This can be explained by the two contrary factors that affect the refractive index of the systems.It is known that the refractive index increases with the increase in salt concentration and decreases with the rise in temperature.Thus, the increase by higher solubility induced by higher temperature and the decrease in temperature itself balanced themselves out.

T a b l e 1 -Density and refractive index parameters of equations 1 and 2 T a b l i c a 1 -Parametri jednadžbi 1 i 2 za gustoću i indeks loma
For the studied systems, the empirical equation (3) 5 can be used for the fitting of solubility, density and refractive data.
The resulting parameter values and relative standard deviation δ are listed in Table 4.
wherein Y represents the solubility, density, or refractive index in the studied systems, A i denote fitting parameters and w 2 represents the mass fraction of glycerol.The standard deviations ranged from 0.0002 to 0.0032.It can be concluded that the equation can be satisfactorily used to correlate the experimental data.

Conclusions
In this paper, the solubility, density and refractive index of the ternary system NaNO 3 /KNO 3 /RbNO 3 /CsNO