Phase Equilibrium in the Ternary System NaH2PO4 + Na2SO4 + H2O

Oct 8, 2015 - The phase equilibrium at different temperatures are compared and discussed. This comparison further illustrates that the temperature can...
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Phase Equilibrium in the Ternary System NaH2PO4 + Na2SO4 + H2O at 323.15 K and 343.15 K Yang Xiao, Wei Liu,* and Xu Han College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan, 610059, People’s Republic of China ABSTRACT: The ternary system of NaH2PO4 + Na2SO4 + H2O at 323.15 K and 343.15 K is studied by the method of isothermal solution saturation. The equilibrium solid phases are investigated by the Schreinemaker’s method of wet residues and verified by X-ray diffraction. The solubility of the system is determined. On the basis of the experimental data, the phase diagrams are plotted and the crystallization regions are determined. The phase equilibrium at different temperatures are compared and discussed. This comparison further illustrates that the temperature can influence the equilibrium of the ternary system. The research can offer meaningful data support for separation processes and further theoretical studies.

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INTRODUCTION Sodium dihydrogen phosphate, an important chemical product, is widely used in chemical industry, agriculture, medicine, food, and so on. Sodium dihydrogen phosphate is also an important raw material for other varieties of phosphate salts. Pure sodium dihydrogen phosphate can be used as food additives, and it is also used in medicine for the treatment of hypophosphatemia.1,2 Salt-water phase equilibrium and the phase diagram, an important predicting tool to use for describing the thermodynamic behavior of the crystallization and separation, play a very important guiding role for the relevant process conditions.3,4 Sodium dihydrogen phosphate solution often contains many impurity ions, such as SO42− ions,5 in the production process.6,7 And therefore, to optimize the process and prepare the pure sodium dihydrogen phosphate, it is necessary to study phase equilibrium of the ternary system (NaH2PO4 + Na2SO4 + H2O). According to the previous study at 298.15 K,8 it turns out that there are two invariant points (NaH2PO4·2H2O + Na2SO4; Na2SO4 + Na2SO4·10H2O), three univariant curves, and three crystallization regions corresponding to NaH2PO4· 2H2O, Na2SO4 ,and Na2SO4·10H2O. According to the previous research at 313.15 K,9 it is found that there are one invariant point (NaH2PO4·2H2O + Na2SO4), two univariant curves, and two crystallization regions corresponding to NaH2PO4·2H2O and Na2SO4. However, the data provided is far from enough to solve the practical separation above, so an extensive study at other temperatures needs to be done. This paper could help fill in the blanks of data in this study aspect, and new experimental data given in this study is useful for optimizing the crystallization and separation processes of NaH2PO4 in the industrial production. Additionally, the research can provide fundamental data support for industry and further forthcoming studies.

Sodium dihydrogen phosphate (Na2H2PO4, ≥ 0.995 mass fraction) and sodium sulfate (Na2SO4, ≥ 0.995 mass fraction) are purchased from Tianjin Bodi Chemical Holding Co. Ltd., China. A HZS-HA type constant temperature water bath oscillator with the standard uncertainty of 0.3 K is employed for phase equilibrium measurement. The oscillator is made in Donglian Electronic & Technology Development Co. Ltd., Beijing, China. The Philips X Pert Pro MPD X-ray diffraction (XRD) analyzer is employed for XRD characterizations. Experimental Method. The method of isothermal solution saturation10−12 is employed to determine the solubility of the ternary system. The famous Schreinemaker’s method of wet residues13−16 is employed to analyze the composition of the equilibrium solid phase indirectly, and the equilibrium solid phase is also tested by XRD to verify its composition. In this pre-experiment, the liquid phase of the samples is analyzed every 2 h. It demonstrates that the equilibrium is reached when the analytical results remain constant. It is shown that the phase equilibrium is reached in 10 h. According to a fixed ratio, and making sure that one of the components is in excess, the experimental components are added into a series of conical flasks (250 mL) gradually, and the sealed flask is placed into the constant temperature bath oscillator. The oscillator vibrates continuously at the two specific temperatures: 323.15 K and 343.15 K (the standard uncertainty of 0.3 K). After equilibrium, the oscillation is stopped and the system is allowed to stand for 3 h to make sure that all the suspended crystals settle. The liquid phase and wet residues are transferred to 250 mL volumetric flasks, respectively. Finally, these samples are quantitatively analyzed by chemical methods. More details of the experimental method and the procedure of the preparation, collection, and transfer of samples are depicted in the previous studies.8,9

METHODOLOGY Materials and Apparatus. Doubly deionized water (electrical conductivity ≤1·10−4 S·m−1) is used in the work.

Received: June 25, 2015 Accepted: October 2, 2015

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© XXXX American Chemical Society

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DOI: 10.1021/acs.jced.5b00531 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Analysis. The sulfate ion concentration is analyzed by gravimetric method-precipitation with barium chloride,17−19 and the relative standard uncertainty of the determination is 0.01 by this method. The concentration of H2PO4− is measured by the quinoline phosphomolybdate gravimetric method,20 and the relative standard uncertainty of the determination is 0.01 by this method. Each experimental result is achieved from the average value of three parallel measurements. The equilibrium solid phase is verified by XRD characterizations.

Table 1. Mass Fraction Solubility of the Ternary NaH2PO4 + Na2SO4 + H2O System at Temperature = (323.15 and 343.15) K and Pressure = 0.1 MPaa composition of liquid phase, 100 w no.

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RESULTS AND DISCUSSION In Figure 1, the experimental data are compared with literature data21−23 and it is found that the experimental data are in good

Figure 1. Solubility for NaH2PO4 or Na2SO4 in pure water at (323.15 and 343.15) K: ▽, literary solubility of NaH2PO4 in water;21 ●, experimental solubility of NaH2PO4 in water; △, literary solubility of Na2SO4 in water;21,22 ■, experimental solubility of Na2SO4 in water.

agreement with the literature values, which demonstrates that experimental methods and devices are feasible in this study. The phase equilibrium experimental data is shown in Table 1. According to the data listed in Table 1, the ternary phase diagram is plotted in Figure 2. The phase diagram at 343.15 K is similar to that in Figure 2, and is given in Figure 3. In Figures 2, 3 and 4, A, B, C, and W represent solid Na2SO4, solid NaH2PO4, NaH2PO4·H2O, and H2O, respectively. Point T, an invariant point at 323.15 K, reflects the cosaturated solution of Na2SO4 and NaH2PO4·H2O. R represents the solubility of Na2SO4 in water at 323.15 K. S represents the solubility of NaH2PO4 in water at 323.15 K. The saturated liquid line RTS consists of two branches. Branch RT corresponds to the saturated Na2SO4 solution and visualizes changes of the Na2SO4 concentration with the concentration of NaH2PO4 increasing in the equilibrium solution. Branch TS corresponds to the saturated NaH2PO4 solution and indicates changes of the NaH2PO4 concentration with the concentration of Na2SO4 increasing in the equilibrium solution. As indicated in Figures 2 and 3, along the curve RT, we connect the composition points of the wet residue phase with the liquid phase and then extend, the intersection of these straight lines is approximately the solid phase component for Na2SO4. The same method is utilized to determine the equilibrium solid phase component of TS, and the intersection

100 w1b

100 w2

1,S 2 3 4,T 5,T 6 7 8 9 10 11 12 13 14 15 16 17,R

61.43 60.58 59.71 58.97 58.97 53.99 51.08 46.64 41.78 37.66 32.96 26.99 20.48 15.48 11.34 4.73 0.00

0.00 1.23 2.24 3.05 3.05 4.86 6.08 7.87 9.84 11.37 13.39 16.63 20.25 22.68 25.22 29.07 31.78

1,F 2 3 4,D 5,D 6 7 8 9 10 11 12 13 14 15 16 17,E

66.20 65.46 64.63 63.97 63.97 57.04 49.83 42.54 37.28 31.94 26.78 21.46 16.48 10.31 5.65 3.08 0.00

0.00 0.88 2.06 3.00 3.00 4.97 7.20 10.12 12.38 14.75 17.25 19.71 22.29 25.41 27.66 29.21 30.56

composition of wet residue phase, 100 w 100 w1

100 w2

323.15 K NDc ND 69.12 0.93 68.41 1.64 71.52 11.31 46.29 39.07 15.50 72.73 14.39 73.50 12.22 75.90 11.33 75.63 10.66 74.81 9.55 74.86 8.58 73.49 6.75 73.83 5.46 72.62 4.22 72.35 1.87 72.19 ND ND 343.15 K ND ND 80.71 0.54 82.06 1.08 85.07 6.23 44.82 48.53 13.86 77.05 12.66 76.56 11.46 75.79 10.91 74.49 9.11 75.85 8.53 73.81 6.62 75.33 5.73 73.22 3.86 72.40 2.30 72.70 1.31 73.72 ND ND

equilibrium solid phase NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O + Na2SO4 NaH2PO4·H2O + Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 NaH2PO4 NaH2PO4 NaH2PO4 NaH2PO4 + Na2SO4 NaH2PO4 + Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4

a Standard uncertainties u(T) = 0.3 K, ur(p) = 0.05, ur(w1) = 0.01, ur(w2) = 0.01. bw1, mass fraction of NaH2PO4; w2, mass fraction of Na2SO4. cND, not determined. S, T, R, E, D, and F have the same meaning as described in Figures 2 and 3.

is NaH2PO4·H2O. Similarly, the equilibrium solid phases of A and B at 343.15 K are Na2SO4 and NaH2PO4, respectively. As shown in Figure 5, the equilibrium solid phase of the invariant point T is analyzed by XRD and verified to be the coexistence of Na2SO4 and NaH2PO4·H2O. With the help of XRD, the solid phases of A, B, and C are certified to be Na2SO4, NaH2PO4, and NaH2PO4·H2O, respectively. Consequently, the system belongs to a simple eutectic type and neither double salt nor solid solution is formed at the investigation temperature. Figure 2 shows that WRTS represents an unsaturated region at 323.15 K. ART represents the crystallization region of Na2SO4, while TCS denotes crystallization region of NaH2PO4· H2O. Zone ATC represents the mixed crystallization region of Na2SO4 + NaH2PO4·H2O. B

DOI: 10.1021/acs.jced.5b00531 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 2. Equilibrium phase diagram of the ternary system NaH2PO4 + Na2SO4 + H2O at 323.15 K: ■, equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of Na2SO4; B, pure solid of NaH2PO4; C, pure solid of NaH2PO4·H2O; W, water; R, solubility of Na2SO4 in water; S, solubility of NaH2PO4 in water; T, cosaturated point of Na2SO4 and NaH2PO4·H2O.

Figure 4. Solubility isotherms of the ternary system NaH2PO4 + Na2SO4 + H2O at 323.15 K and 343.15 K: ○, 343.15 K; ●, 323.15 K; A, B, C, D, T, R, E, S, F and W have the same meaning as described in Figures 2 and 3.

Figure 5. X-ray diffraction pattern of the invariant point T.

This diagram further illustrates that the temperature can influence the equilibrium of the ternary system. Increasing the temperature from 323.15 K to 343.15 K shows that (1) the crystallization region of Na2SO4 is much larger than the crystallization region of NaH2PO4·H2O (NaH2PO4) at both temperatures. (2) The solubility of Na2SO4 decreases sharply with an increase in the concentration of NaH2PO4. That is, NaH2PO4 has a strong salting-out effect on Na2SO4. The invariant point moves right from point T to D, which illustrates that the salting out effect of NaH2PO4 to Na2SO4 does not change significantly. (3) The curve RT is close to the curve ED, while curve TS is far from curve DF; because the solubility of NaH2PO4 increases, while the solubility of Na2SO4 decreases

Figure 3. Equilibrium phase diagram of the ternary system NaH2PO4 + Na2SO4 + H2O at 343.15 K: ■, equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of Na2SO4; B, pure solid of NaH2PO4; W, water; E, solubility of Na2SO4 in water; F, solubility of NaH2PO4 in water; D, cosaturated point of Na2SO4 and NaH2PO4.

A comparison between the phase equilibrium for NaH2PO4 + Na2SO4 + H2O at 323.15 K and 343.15 K is shown in Figure 4. C

DOI: 10.1021/acs.jced.5b00531 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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slightly. (4) With rising the temperature, the crystallization region of NaH2PO4·H2O transforms into NaH2PO4. The differences in the Na2SO4 and NaH2PO4 solubility between the pure aqueous solutions and invariant points are presented in Figure 6. Differences between the Na2SO4

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Figure 6. Comparison of the NaH2PO4 and Na2SO4 solubility in aqueous solutions and at cosaturated points T and D.

solubility in the aqueous solutions and at invariant points are (28.73 % and 27.56 %) at (323.15 and 343.15) K, respectively, which illustrates that NaH2PO4 has a strong salting-out effect on Na2SO4, and the effect has no significant change at higher temperatures. Differences between the NaH2PO4 solubility in the aqueous solutions and at invariant points are (2.46 % and 2.23 %) at (323.15 and 343.15) K, respectively, which confirms Na2SO4 has a weak salting-out effect on NaH2PO4.

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CONCLUSIONS The phase equilibrium of the ternary system NaH2PO4 + Na2SO4 + H2O at (323.15 and 343.15) K is investigated. The data of solubility are obtained. According to the solubility data, the phase diagrams are plotted, the solid phase which is in equilibrium with the saturated solution is analyzed, and crystallization regions of both solid phases are determined. In the phase diagrams, there are in all two crystallization regions, one invariant point, and two univariant curves. The system belongs to a simple eutectic type and the crystallization region of Na2SO4 is much larger than the crystallization region of NaH2PO4·H2O (NaH2PO4) at the investigation temperature. Increasing the temperature from 323.15 K to 343.15 K transforms the crystallization region of NaH2PO4·H2O into that of NaH2PO4. NaH2PO4 has a strong salting-out effect on Na2SO4, and the salting-out effect does not change significantly at higher temperature. All results can offer fundamental data support for the separation processes of NaH2PO4 in industrial production and further theoretical researches.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jced.5b00531 J. Chem. Eng. Data XXXX, XXX, XXX−XXX