Phase Equilibrium of NaNO3/KNO3 in Glycerol + H2O Mixed Solvent System at 288.15 K and 298.15 K

“Salting out effect” means that the addition of an organic solvent to an aqueous solution of a salt will reduce the solubility of salts and thus induce its precipitation.1 Therefore, the 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. Various systems of salt and mixed solvent systems have been investigated to assess the potential applicability of such method as a technique for separation of salts. Many of the reports focus on systems composed of alkaline metal chlorides, alcohols, and water. For example, the properties of 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 were systematically studied by Gomis et al.2–5 Moreover, physicochemical properties 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 were reported.


Introduction
"Salting out effect" means that the addition of an organic solvent to an aqueous solution of a salt will reduce the solubility of salts and thus induce its precipitation. 1 Therefore, the 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.Various systems of salt and mixed solvent systems have been investigated to assess the potential applicability of such method as a technique for separation of salts.Many of the reports focus on systems composed of alkaline metal chlorides, alcohols, and water.For example, the properties of 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 were systematically studied by Gomis et al. [2][3][4][5] Moreover, physicochemical properties 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) 8 were reported.
0][11] In this work, the equilibrium solubility of NaNO 3 and KNO 3 in the mixed solvent HOCH 2 CH(OH) CH 2 OH + H 2 O were determined by the density-refractive index method.The density and refractive index of the saturated and unsaturated solutions were also measured.

Experimental Section
Materials.All the chemicals (purity > 99.5 %), including sodium nitrate, potassium nitrate, and glycerol were purchased from Sinopharm Chemical Reagent Co. Ltd., and used without further purification.The salt was dried to constant weight for 48 h at 110 °C and stored in desiccators prior to use.Double-distilled water was used throughout this work.
Apparatus and Procedure.All samples were prepared by mass with electronic balance (AL204, Mettler Toledo, accuracy of ±1 • 10 −4 g).0][11] The mixed solvent (glycerol and water) was firstly placed in a 10 ml syringe (the mass fraction of glycerol in the mixed solvent varied from 0.0 to 1.0).Excessive amounts of NaNO 3 and KNO 3 were then added into the mixed solvent.Next, the samples were stirred for 48 h and settled for a further 24 h to ensure that equilibrium was established.The temperature was controlled within an accuracy of ±0.1 K.After equilibrium was achieved, the solutions were withdrawn and analysed.The unsaturated solutions were prepared by adding salt to the mixtures of glycerol -water at a certain ratio.
Refractive index and density of both saturated and unsaturated solutions were determined using RXA 170 refractometer (Anton Paar) and DMA 4500 vibrating tube densimeter (Anton Paar) with a precision of 4 • 10 −5 and ±1 • 10 −5 g cm -3 , respectively.Both the instruments were calibrated prior to initiation of each series of measurements, using air and double-distilled water as reference substances.
The content of the salts and glycerol was analysed using density-refractive index method, 9 because refractive index and density both depend on quantity of dissolved salts and concentration of glycerol in ternary systems NaNO 3 /KNO 3 (1) + glycerol (2) + H 2 O (3), based on the following equations: where ρ is the density, n D is the refractive index, w 1 and w 2 are the mass fraction of the salts and glycerol in the mixed solution, respectively.The samples for calibration were prepared containing the salt with mass fraction of 0 %, 3 %, 6 %, and glycerol with known varying content.The density and refractive index of the samples were then determined at 298.15 K and plotted against the mass fraction of glycerol.
The values of parameters of equations 1 and 2 were obtained by the calibration curves 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.The combined equations 1 and 2 can then be solved, and w 1 and w 2 in the ternary systems can be determined.

Results and Discussion
The solubility, density, and refractive index of the saturated solution for NaNO 3 / KNO 3 + glycerol + H 2 O at 288.15 K and 298.15K are listed in Table 2.Moreover, the comparison of the solubility, refractive index, and density of sodium nitrate and potassium nitrate in pure water at 298.15 K with literatures [12][13][14][15] are given in Table 3, which shows good agreement.
T a b l e 1 -Parameters of equations 1 and 2 The effect of glycerol and temperature on the solubility, density and refractive index are depicted in Fig. 1.It can be observed that the solubility of the two salts decreased with the addition of glycerol.With higher temperature solubility is greater.However, the NaNO 3 system is not very sensitive to the change of temperature.The changing trend of the density of the solution varies for different salts.For NaNO 3 + glycerol + H 2 O systems, the trend is similar to that of solubility.However, for KNO 3 + glycerol + H 2 O systems, the density of the solution increased with the addition of glycerol in the mixed solvent.Moreover, the density curves at the two temperatures almost overlapped in the glycerol-rich area.However, a decreasing trend of the density curve is observed for KNO 3 + propan-2-ol + H 2 O 15 systems when increasing the content of propan-2-ol.These phenomena can be attributed to three factors.Firstly, the density increased when the solubility of salts increased.Secondly, the density increased with increasing the content of glycerol, because the density of glycerol is greater than that of water.Thirdly, with higher temperature density will be lower.For the refractive index of the solutions, a monotonically increasing trend is observed with increasing the content of glycerol.Moreover, an interaction can be observed for KNO the fact that the refractive index of the saturated solution is mainly influenced by two factors: concentration of the salt and temperature.The refractive index increased with the increase in the mass fraction of the salt, and decreased with the enhancement of temperature.This phenomenon is different from that of KNO 3 + propan-2-ol + H 2 O system. 15or KNO 3 + propan-2-ol + H 2 O system, the refractive index first decreases (until w propan-2-ol ≈ 0.1 at 298.15 K and w propan-2-ol ≈ 0.2 at 313.15 K) and then increases with the increase in propan-2-ol content.An interaction was also observed for the refractive index of KNO 3 + propan-2-ol + H 2 O system at different temperatures.This is mainly due to the fact that the structure and properties of the organic solvent glycerol is very different from that of propan-2-ol.
For comparison, the solubility, refractive index, and density of the systems NaCl + glycerol + H 2 O is plotted in Fig. 1. 10 It can be seen that the changing trends of solubility and refractive index of all the systems are similar.That is, the main effect on solubility and refractive index is the content of glycerol.
However, for the changing of density, different phenomenon can be observed from Fig. 1b.For NaCl + glycerol + H 2 O system, the density of the solution increased with increasing the concentration of glycerol.However, the density of NaNO 3 + glycerol + H 2 O system at the two temperatures are changed with a contrary tendency with increasing the glycerol content.This suggests that different salts play a dominant role in changing of density.These phenomena may further explain that the solubility, temperature, and the organic solvent all affect the properties of ternary systems.
Experimental data for the mass fraction of the salt, density, and refractive index of the saturated systems are correlated using the following equation: 16 where w 2 is the mass fraction of glycerol in the solution.Y represents the mass fraction of salts in the solution (w 1 ), numerical value of density (ρ ⁄ g cm −3 ), or refractive index (n D ) of the solution.The obtained parameters A, B, C, D together with the relative standard deviation (δ) of the systems are listed in Table 4. From the obtained standard deviations, we conclude that equation ( 3) can be satisfactorily used to correlate solubility, density and refractive index data.
To complete this study, unsaturated systems NaNO .Both the density and refractive index increased with the increase of the glycerol-to-water ratio and with the increase of the salts content.However, the higher the temperature, the lower are the density and refractive index.3    The seven-parameter empirical equation was used to fit the unsaturated density and refractive index data: 16 where Y represents numerical value of density (ρ ⁄ g cm −3 ) or the refractive index (n D ) of the ternary system, where w 1 , w 2 , and w 3 are the mass fractions of salts, glycerol, and water in the solution, respectively.The coefficients of Eq. (4) (A i , i = 0 to 6) along with the corresponding standard deviations for the investigated systems are given in Table 7.Based on the obtained standard deviations, it can be seen Eq. ( 4) is satisfactory for the systems in this work.
T a b l e 6 -(continued) Density (ρ) and refractive index (n D ) for the unsaturated system of KNO

Conclusion
The equilibrium solubility, density, and refractive index of the ternary systems of NaNO 3 /KNO 3 + glycerol + H 2 O were determined at temperatures 288.15 and 298.15K in this work.For the saturated systems, the phase is a single solidliquid equilibrium with the solid being NaNO 3 or KNO 3 .The solubility for the saturated solutions was found to decrease with the increase in glycerol concentration, and increased with the increase in temperature.That is, the salting out effect is observed when adding organic solvent glycerol into water.The refractive index of all the systems increased with the addition of glycerol in the mixed solvent, because of the greater refractive index of glycerol itself.However, the temperature effect on the refractive index is complicated, because the temperature affects both the concentration of salts in the solution and the refractive index of the solution.The changing trend of density curves is completely different from that of solubility and refractive index.These phenomena imply that density of the solution is controlled by the solubility of different salts and temperature.The investigation of the unsaturated systems show that both density and refractive index increased with the increase in the glycerol-to-water ratio, and with the increase in salts content.
ACKNOWLEDGMENT This work was supported by Guangxi Natural Science Foundation (GXNSF74114).4) and the standard deviation (δ) of the correlation for the unsaturated systems NaNO 3 /KNO 3 + glycerol + H 2 O T a b l i c a 7 -Parametri A i (i = 0 -6) jednadžbe (4)