Article pubs.acs.org/jced
Phase Equilibrium for the Ternary System NaH2PO4 + Na2SO4 + H2O in Aqueous Solution at 298.15 K Xi-Zhou Chen, Jian-Hua Tang,* Ya-Nan Cui, Mei Liu, and Dun Mao Department of Chemical Engineering and Technology, Sichuan University, Chengdu, Sichuan, 610065, People’s Republic of China ABSTRACT: The ternary system (NaH2PO4 + Na2SO4 + H2O) at 298.15 K was investigated by using an isothermal solution saturation method. The solubilities and densities of the solution were determined. According to the experimental results, the phase diagram and the diagram of densities versus composition were plotted. The phases of the equilibrium solids were analyzed by X-ray diffraction (XRD), and crystalline regions of both solid phases were determined. It was found that there were two invariant points (NaH2PO4·2H2O + Na2SO4; Na2SO4 + Na2SO4·10H2O), three univariant curves, and three crystallization regions corresponding to sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), sodium sulfate (Na2SO4), and sodium dihydrogen phosphate decahydrate (Na2SO4·10H2O; also known as Glauber’s salt) in this ternary system. The system belonged to a simple eutectic type, and neither double salts nor solid solutions were formed. This research provided missing information of the phase diagram in this ternary system, which in turn can provide fundamental data for industry and future study. These fundamental data could be useful for separation and crystallization processes in chemical industry.
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INTRODUCTION
METHODOLOGY Materials. The analytical reagent sodium phosphate monobasic dehydrate (NaH2PO4, 0.990 mass fraction) and sodium sulfate (Na2SO4, 0.990 mass fraction) used in this work were obtained from Tianjin Bodi Chemical Holding Co. Ltd., China. Doubly deionized water (electrical conductivity ≤ 1· 10−4 S·m−1) was used in this work to prepare the experimental solutions. Instruments. A constant temperature bath oscillator (SHZ88, Jintan Medical Instrument Corporation, Jiangsu, China) with the temperature range of 273.15 K to 373.15 K was used for phase equilibrium measurement. The temperature-controlling precision of this oscillator was ± 0.3 K. The Philips X Pert Pro MPD X-ray diffraction (XRD) analyzer was used for solidphase X-ray analysis. Experimental Methods. The solubility of this ternary system was determined through the isothermal solution saturation method in this study.3−5 At the temperature of 298.15 K (± 0.3 K), the solutions in the experiment were compounded by adding another component gradually on the basis of the binary subsystem salt saturation points. Thus chemical NaH2PO4 was added into the saturated solution of Na2SO4; the solubility of Na2SO4 decreased because of the Na+ ion effect. The solution of Na2SO4 would keep decreasing to a stopping point with further addition of NaH2PO4. The solution in which NaH2PO4 and Na2SO4 are both saturated is known as the cosaturation solution. The sign of the equilibrium is
Monosodium dihydrogen phosphate, which is composed with sodium and phosphate counterions, is also known as sodium phosphate monobasic dehydrate. As a chemical intermediate, monosodium phosphate plays an important role in chemical materials, which is widely used in the fields of tanning, boiled water treatment, plating,1 and so on. It is also used as a laxative and pH buffer2 when combined with other sodium phosphates. Moreover, monosodium phosphate is also used as fuel and detergent additives. In recent years, it has been developed rapidly and worldwide in food and fermentation industries. Sodium sulfate anhydrous, called thenardite, as a chemical is used in the manufacturing of sodium sulfide and sodium silicate. The mineral form is known as mirabilite and is applied to making paper, glass, printing and dyeing, synthetic fiber, leather, and so on. However, the completely phase equilibrium data of the NaH2PO4 + Na2SO4 + H2O system at 298.15 K have not been reported yet. No report has been found to describe if monosodium dihydrogen phosphate can react with sodium sulfate to form a double salt. In this paper, the phase equilibria of the ternary system at 298.15 K were studied on the basis of our former research. In this work, we determined phase equilibrium data of the monosodium phosphate (NaH2PO4) + sodium sulfate (Na2SO4) + water (H2O) system at 298.15 K, and we plotted the phase diagram and the diagram of densities versus composition which could help to fill in the blanks of data of this research aspect. Meanwhile the crystallized solid phases in the system have been investigated experimentally and theoretically. This can provide basic solubility data for phosphorus chemical industry development. © 2014 American Chemical Society
Received: October 23, 2013 Accepted: January 27, 2014 Published: February 5, 2014 481
dx.doi.org/10.1021/je400932e | J. Chem. Eng. Data 2014, 59, 481−484
Journal of Chemical & Engineering Data
Article
Table 1. Mass Fraction Solubility of the Ternary NaH2PO4 + Na2SO4 + H2O System at Temperature = 298.15 K and Pressure = 0.1 MPaa densities ρ/(g·cm−3)
composition of liquid phase, 100wb no.
NaH2PO4
Na2SO4
equilibrium solid phase
1 2 3 4 5 6 7, E 8 9 10 11 12 13, F 14 15 16 17 18 19 20 21 22
49.12 47.54 45.8 44.8 43.77 41.8 39.51 38.28 35.52 32.4 30.54 28.59 26.4 24.15 20.7 18.66 16.58 14.08 9.03 4.23 2.7 0
0 1.56 3.62 4.05 5.4 7.94 9.97 10.55 11.57 13.68 14.5 15.52 16.67 17.37 17.25 17.03 16.64 17.17 20.07 22.95 24.07 25.23
MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d + S S S S S S S + S10 S10 S10 S10 S10 S10 S10 S10 S10 S10
c
exp. value
calcd value
relative errord
1.3837 1.3851 1.3912 1.3933 1.3989 1.4112 1.4243 1.4161 1.4035 1.3963 1.3856 1.3791 1.3704 1.3522 1.3207 1.3019 1.2811 1.2622 1.2513 1.2393 1.2352 1.2345
1.3837 1.3874 1.3955 1.3913 1.3977 1.4094 1.4121 1.4074 1.3937 1.3887 1.3821 1.3760 1.3693 1.3570 1.3247 1.3044 1.2822 1.2666 1.2551 1.2456 1.2447 1.2345
0.0000 0.0017 0.0031 −0.0014 −0.0009 −0.0013 −0.0087 −0.0062 −0.0070 −0.0055 −0.0026 −0.0022 −0.0008 0.0035 0.0030 0.0019 0.0008 0.0035 0.0030 0.0050 0.0076 0.0000
Standard uncertainties u are u(T) = 0.3 K, u(p) = 0.05 MPa, u(MSP.d) = 0.01 (mass fraction), u(U) = 0.01 (mass fraction), u(ρ) = 0.01 g·mL−1. w, mass fraction. cMSP, NaH2PO4; MSP.d, NaH2PO4·2H2O; S, Na2SO4; S10, Na2SO4·10H2O. dRelative error = (calcd value − exp. value)/calcd value.
a b
The crystals were treated by drying oven under constant pressure and temperature; thus, dry samples were obtained for X-ray diffraction (XRD). Analysis. The P2O5 concentration was measured through the quinoline phosphomolybdate gravimetric method.6 And the average deviation of the determination was less than 0.01. The concentration of SO42− in the liquid phase was analyzed by gravimetric method of barium chloride.7,8 The mean relative deviation of the determination was less than 0.01. The density was calculated by weighing 1 mL of saturated solution, and the absolute uncertainties in the density measurements were estimated to be within 0.01 g·mL−1. The water content was determined by subtraction.9 The equilibrium phase solid was determined by X-ray diffraction.
determined on the unchanged concentration of the solution which the composition of this solution would not change under constant temperature and pressure. In phase equilibria, this point is called the invariant point. Since the method was adding NaH2PO4 into a saturated solution of Na2SO4 before the solution reached the invariant point, NaH2PO4 was dissolved in water completely, while the crystals remained unsaturated in the process. Then the method is reverse which was adding pure Na2SO4 into saturated solution of NaH2PO4 before the solution reached the invariant point. Because the traditional Schreinemaker’s method would yield relatively great errors in many cases, the filtration way was used to separate crystals and mother liquor after the equilibrium. The solid phases were analyzed by X-ray diffraction to ascertain the crystalloid form. Experimental Procedures. A known mass of sodium dihydrogen phosphate (NaH2PO4), sodium sulfate (Na2SO4), and doubly deionized water which had been boiled once were loaded into a Erlenmeyer flask (250 mL). The flask was sealed and put in the bath oscillator at constant temperature. The oscillator vibrated continuously with temperature controlled at around 298.15 K (uncertainty, ± 0.3 K), and monitored using a mercury thermometer. In this process it took 8 h of vibration under constant temperature (298.15 K), when the concentrations of both NaH2PO4 and Na2SO4 in the solution remained unchanged for another 2 h, the oscillator was stopped, after the system reached equilibrium, the filtration way was used to separate crystals and saturated solution from the flask. The saturated solution was removed into a 100 mL beaker using a 1 mL (uncertainty, ± 0.01 mL) pipet at 298.15 K. The beaker was weighed before and after the solution was added. The crystals were removed into a small beaker with a scoop.
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RESULTS AND DISCUSSION The phase equilibrium experimental results of solubility and density measured for the ternary NaH2PO4 + Na2SO4 + H2O system at 298.15 K are shown in Table 1. The ion concentration values in this equilibrium system were measured in mass fraction. ρ is the density, and it expressed the equilibrated solution in grams per cubic centimeter, that is, g· cm−3. According to experimental data listed in Table 1, the phase diagram of the ternary system was shown in Figure 1. As shown in Figure 1, A, B, C, and W represent the pure solid of NaH2PO4·2H2O, pure solid of Na2SO4·10H2O, pure solid of Na2SO4, and H2O, respectively. There are two invariant points, three univariant curves, and three regions of single salt crystallization in the phase diagram. E is an invariant point at 298.15 K, which could reflect the cosaturated solution of NaH2PO4·2H2O and Na2SO4. F is an invariant point at 298.15 K, which could reflect the cosaturated solution of Na2SO4 and 482
dx.doi.org/10.1021/je400932e | J. Chem. Eng. Data 2014, 59, 481−484
Journal of Chemical & Engineering Data
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It can be seen from Figure 1 that the area of WDEFG is the unsaturated region at 298.15 K; ADE represents the crystalline region of NaH 2PO 4·2H2 O, while CEF stands for the crystallization region of Na2SO4 and BFG stands for the crystallization region of Na2SO4·10H2O. It seems that the crystallization region of Na2SO4 (CEF) is the largest area, and the crystallization region of Na2SO4·10H2O (BFG) is a little larger than the crystallization region of NaH2PO4·2H2O (ADE); thus the crystallization region of NaH2PO4·2H2O (ADE) is the smallest area. The area AEC is the mixed crystallization region of NaH2PO4·2H2O + Na2SO4, and the area BFC is the mixed crystallization region of Na2SO4 + Na2SO4·10H2O. Points E and F are the invariant point for the system NaH2PO4 + Na2SO4 + H2O at 298.15 K. Figure 3 shows the relationship between the density and the mass concentration of NaH2PO4 in the solution. It can be Figure 1. Equilibrium phase diagram of the ternary system NaH2PO4 + Na2SO4 + H2O at 298.15 K. A, pure solid of NaH2PO4·2H2O; B, pure solid of Na2SO4·10H2O; C, pure solid of Na2SO4; W, water; D, solubility of NaH2PO4 in water; G, solubility of Na2SO4 in water; E, invariant point of NaH2PO4·2H2O + Na2SO4; F, invariant point of Na2SO4 + Na2SO4·10H2O.
Na2SO4·10H2O. The curve between points D and E indicates that Na2SO4 has been saturated in the water, while NaH2PO4· 2H2O has been precipitated. The curve between points E and F indicates NaH2PO4 has been saturated in the water, while Na2SO4 has been precipitated. Additionally, the curve between points F and G indicates NaH2PO4 and Na2SO4 has been saturated in the water, while Na2SO4·10H2O has been precipitated. As shown in Table 1 and Figure 1, the solid phases were analyzed by XRD to ascertain the crystalloid form. Before the XRD test, every wet residue sample was dried under 298.15 K and ground by using agate mortar, with the help of chemical analysis and X-ray diffraction analysis, the solid phases of A were determined to be sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), it turned out that this system was a simple eutectic type. This system is a type of simple common saturation and without complex salt and solid solution at the investigated temperature. As shown in Figure 2, the phase of the invariant point E equilibrium solid was detected by XRD analysis and determined to be coexistence of the salts NaH2PO4·2H2O and Na2SO4.
Figure 3. Density vs composition.
found that the solution density of this ternary system changed regularly with the content change of NaH2PO4. According to Table 1 and Figure 3, with the concentration of NaH2PO4 increasing in the solution, the density tends to increase. It seems that the densities all reach a maximum value at the invariant point E. Then the curve decreases slightly afterward. It turns out that the solution density of the ternary system changed regularly with the content change of monosodium phosphate, and all reach a maximum value at the invariant point E, and the compositions of NaH2PO4 and Na2SO4 in the liquid phase with mass fraction (100w) are 39.51 and 9.97, respectively. Based on the following empirical equations of density in electrolyte solutions developed in the previous study,10 the density of the solution was calculated. The experimental data were compared with the calculated data to determine the relative error. All of the data mentioned above are listed in Table 1.
ln
da = d0
∑ Ai ·wi
where d0 = 0.997044 g·cm−3, the density of water at 298.15 K; Ai is the constant of each possible component i in the system at 298.15 K. wi is the salt of i in the solution in mass fraction. Constants Ai of NaH2PO4 and Na2SO4 for calculation are 0.006672 and 0.008467, respectively. The calculated results and experimental values are presented in Table 1 for comparison.
Figure 2. X-ray diffraction pattern of the invariant point E (NaH2PO4· 2H2O + Na2SO4). 483
dx.doi.org/10.1021/je400932e | J. Chem. Eng. Data 2014, 59, 481−484
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CONCLUSIONS The equilibrium of the ternary system NaH2PO4 + Na2SO4 + H2O at 298.15 K was studied with the isothermal solution saturation method. The data of solubility and density of this system were determined in this process. According to the experimental data, the phase diagram and density diagrams of this system were plotted, the solid phases which are balanced with saturated solutions were analyzed by XRD, and crystalline regions of both solid phases were determined. There were two invariant points in this phase diagram, and there were no double salt and solid solution formed in this ternary system. All results obtained in this experiment can provide fundamental data support in this ternary system; these fundamental data could be useful for separation and crystallization in future studies.
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AUTHOR INFORMATION
Corresponding Author
*E-mail (J.T.):
[email protected]. Notes
The authors declare no competing financial interest. E-mail (X.C.):
[email protected].
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REFERENCES
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