Article pubs.acs.org/jced
Phase Equilibrium for the Ternary System NaH2PO4 + KH2PO4 + H2O at 288.15 and 318.15 K Hao Zhang,† Yang Xiao,*,† Zi-wei Zhang,† Jian-feng Zhou,‡ and Xue Yan‡ †
College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, People’s Republic of China Tarim Oilfield Company, PetroChina, Korla, XinJiang 841000, People’s Republic of China
‡
ABSTRACT: The phase equilibrium of KH2PO4 + NaH2PO4 + H2O at 288.15 and 318.15 K is investigated by isothermal solution saturation method and wet residues. The equilibrium solid phases are analyzed by the Schreinemaker’s method of wet residues and verified by X-ray diffraction (XRD). Based on the experimental data, the diagram of density versus composition and the phase diagram are plotted. The crystallization regions are determined. The density of the saturated solution is calculated, and the model precision is tested. The phase equilibrium at different temperatures are compared and discussed. All results can offer fundamental data support for separation processes and further theoretical studies.
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INTRODUCTION
METHODOLOGY Apparatus and Materials. Potassium dihydrogen phosphate with analytical grade (KH2PO4, ≥99.5 wt %) is purchased from Tianjin Bodi Chemical Holding Co. Ltd., China. Doubly deionized water (electrical conductivity ≤1 × 10−4 S·m−1) is used in the work. Sodium dihydrogen phosphate (NaH2PO4, ≥99.5 wt %) used in the work is purchased from Chengdu Kelong Chemical Reagent Co. Ltd., China. A sample description table is provided in Table 1.
KH2PO4, as an important compound fertilizer and chemical material, is widely used in the chemical, pharmaceutical, agricultural, and food industries.1 NaH2PO4 is an important raw material for the production of other varieties of phosphate salts.2,3 Salt-water system phase equilibrium and the phase diagram, a very significant predicting tool to employ for describing the thermodynamic behavior of the crystallization and separation, play a very important guiding role for the relevant process conditions.4−7 When potassium dihydrogen phosphate is prepared by double decomposition reaction of potassium chloride and sodium dihydrogen phosphate,8−11 the potassium dihydrogen phosphate solution prepared often contains some impurities, like sodium ion. The crystallization process is an important step in preparing KH2PO4. To optimize the process and prepare the pure potassium dihydrogen phosphate, it is necessary to study phase equilibrium of NaH2PO4 + KH2PO4 + H2O. Solubility data in the system KH2PO4 + NaH2PO4 + H2O can be found in previous studies.12,13 Authors of ref 12 reported the phase equilibrium data of KH2PO4 + NaH2PO4 + H2O at 303.15 K, while authors of ref 13 investigated a small amount of solubility data of this ternary system about 50 years ago. However, the data provided is insufficient to solve the practical separation above, and so an extensive study at other temperatures needs to be done. The phase equilibrium data of KH2PO4 + NaH2PO4 + H2O at 288.15 and 318.15 K have not been reported yet. This paper can help fill in the blank of data, and new experimental data given in this study is useful for design and optimization of the crystallization process of KH2PO4. Additionally, all results can provide fundamental data support for industry and further theoretical studies. © XXXX American Chemical Society
Table 1. Purities and Suppliers of Chemicals chemical
mass fraction purity
KH2PO4 NaH2PO4
≥99.5 wt % ≥99.5 wt %
source Tianjin Bodi Chemical Holding Co. Ltd., China Chengdu Kelong Chemical Reagent 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 and 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 saturation14,15 is employed to determine the solubility of the ternary system. The famous Schreinemaker’s method of moist residues16−18 is applied to analyze the equilibrium solid phase component in the experiments indirectly, and the solid phase is also tested by XRD to verify the crystalloid composition. Received: November 30, 2015 Accepted: May 23, 2016
A
DOI: 10.1021/acs.jced.5b01020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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In a pre-experiment, the liquid phase of the samples is analyzed at every 2 h. It demonstrates that the equilibrium is reached when the analytical results keep constant. It is shown that the phase equilibrium is reached in 10 h. According to a certain proportion and making sure that one of the salts is excessive, the experimental components are added into a series of conical flasks (250 mL) gradually, and the sealed flask is placed into the oscillator. The oscillator vibrates continuously at the two specific temperatures: 288.15 and 318.15 K (the standard uncertainty of 0.3 K). After equilibration, the oscillation is stopped, and the system is allowed to stand for 2 h to make sure that all the suspended crystals settle. The wet residues and liquid phase are transferred to a 250 mL volumetric flask, respectively. Meanwhile, some other liquid phases are used to measure density individually. Finally, these samples are quantitatively analyzed by chemical methods. More details of the experimental method and the procedure of the transfer, collection, and preparation of samples are described in our previous papers.3,12,15 Analysis. The H2PO4− concentration is analyzed by the quinoline phosphomolybdate gravimetric method,19 and the
Figure 1. Solubility for NaH2PO4 or KH2PO4 in pure water at 288.15 and 318.15 K. △, literary solubility of NaH2PO4 in water;21 ■, experimental solubility of NaH2PO4 in water; ▽, literary solubility of KH2PO4 in water;22 ●, experimental solubility of KH2PO4 in water.
Table 2. Mass Fraction Solubility of the Ternary KH2PO4 + NaH2PO4 + H2O System at Temperature = 288.15 and 318.15 K and Pressure = 0.1 MPaa composition of liquid phase, 100w
composition of wet residue phase, 100w
no.
100w1b
100w2
100w1
100w2
1, E 2 3 4 5, F 6, F 7 8 9 10 11 12 13 14 15 16, N
0.00 3.11 5.59 8.19 8.67 8.67 8.77 9.26 9.90 10.33 11.05 11.95 13.06 13.97 15.23 16.86
43.60 42.78 41.87 41.03 40.68 40.68 38.67 35.07 31.89 29.29 25.63 20.85 16.34 11.91 6.26 0.00
NDd 1.50 2.51 3.76 13.92 43.26 59.56 60.00 60.19 61.88 61.32 61.63 63.26 64.08 63.45 ND
ND 60.74 61.08 60.40 57.79 35.30 17.02 15.42 14.15 12.47 11.12 9.10 6.91 4.97 2.70 ND
1, H 2 3 4 5 6, T 7, T 8 9 10 11 12 13 14 15 16 17
0 2.42 4.61 6.62 9.13 10.00 10.00 10.70 11.36 12.45 13.52 14.89 16.40 17.63 19.29 20.75 22.06
59.92 58.89 57.99 57.25 56.36 56.13 56.13 53.50 50.53 46.84 42.86 38.16 33.85 28.98 24.19 20.20 15.68
ND 1.23 2.38 3.07 4.30 11.35 40.83 61.85 63.50 64.56 63.69 64.56 65.67 66.38 64.65 65.57 65.92
ND 73.38 72.54 73.52 72.90 71.33 43.92 22.89 20.92 18.98 17.97 15.92 13.90 11.88 10.64 8.79 6.85
densities ρ/(g·cm−3) equilibrium solid phase 288.15 K NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O NaH2PO4·2H2O + KH2PO4 NaH2PO4·2H2O + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 318.15 K NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O NaH2PO4·H2O + KH2PO4 NaH2PO4·H2O + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 B
exp. value
calcd. value
relative errorc
1.3884 1.4090 1.4275 1.4403 1.4424 1.4428 1.4228 1.3925 1.3650 1.3397 1.3118 1.2731 1.2421 1.2100 1.1685 1.1264
1.3884 1.4107 1.4260 1.4434 1.4445 1.4445 1.4238 1.3905 1.3637 1.3413 1.3114 1.2731 1.2403 1.2073 1.1673 1.1264
0.0000 0.0012 −0.0011 0.0022 0.0015 0.0012 0.0007 −0.0015 −0.0010 0.0012 −0.0003 0.0000 −0.0015 −0.0023 −0.0010 0.0000
1.5733 1.5857 1.6046 1.6142 1.6384 1.6439 1.6407 1.6168 1.5935 1.5555 1.5238 1.4840 1.4489 1.4099 1.3709 1.3471 1.3108
1.5733 1.5886 1.6030 1.6174 1.6360 1.6435 1.6435 1.6187 1.5896 1.5573 1.5220 1.4824 1.4497 1.4088 1.3741 1.3467 1.3130
0.0000 0.0018 −0.0010 0.0020 −0.0015 −0.0002 0.0017 0.0012 −0.0025 0.0012 −0.0012 −0.0011 0.0006 −0.0008 0.0023 −0.0003 0.0017
DOI: 10.1021/acs.jced.5b01020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 2. continued composition of liquid phase, 100w b
no.
100w1
18 19 20, K
23.12 24.61 26.85
composition of wet residue phase, 100w
100w2
100w1
100w2
10.69 5.64 0.00
64.29 63.04 ND
5.00 2.81 ND
densities ρ/(g·cm−3) equilibrium solid phase 318.15 K KH2PO4 KH2PO4 KH2PO4
exp. value
calcd. value
relative errorc
1.2713 1.2401 1.2045
1.2731 1.2378 1.2045
0.0014 −0.0019 0.0000
a Standard uncertainties u(T) = 0.3 K, ur(p) = 0.05, ur(w1) = 0.01, ur(w2) = 0.01, and ur(ρ) = 0.001. bw1, mass fraction of KH2PO4; w2, mass fraction of NaH2PO4. cRelative error = (calcd. value − exp.value)/calcd.value. dND, not determined. E, F, N, H, T, and K have the same meaning as described in Figures 2 and 3.
relative standard uncertainty is 0.01. The K2O concentration is determined by precipitation with sodium tetraphenylborate,20 and the relative standard uncertainty is 0.01. The density is measured by a specific weighing bottle method, and the relative standard uncertainty is 0.001. Each experimental result is achieved from the average value of three parallel measurements. The equilibrium solid phase is analyzed by XRD characterizations.
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RESULTS AND DISCUSSION In Figure 1, the experimental data are compared with literature data,21,22 and it is discovered that the experimental results fit well with the literature results, which demonstrates that the experimental devices and methods are feasible. The phase equilibrium data is given in Table 2, and the ternary phase diagram is plotted in Figure 2. The diagram at 318.15 K is
Figure 3. Equilibrium phase diagram of the ternary system KH2PO4 + NaH2PO4 + H2O at 318.15 K. ■, equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of KH2PO4; B, pure solid of NaH2PO4; W, water; D, pure solid of NaH2PO4·H2O; H, solubility of NaH2PO4 in water; K, solubility of KH2PO4 in water; T, cosaturated point of KH2PO4 and NaH2PO4·H2O.
Figure 2. Equilibrium phase diagram of the ternary system KH2PO4 + NaH2PO4 + H2O at 288.15 K. ■, equilibrium liquid phase composition; ●, moist solid phase composition; A, pure solid of KH2PO4; B, pure solid of NaH2PO4; W, water; C, pure solid of NaH2PO4·2H2O; E, solubility of NaH2PO4 in water; N, solubility of KH2PO4 in water; F, cosaturated point of KH2PO4 and NaH2PO4·2H2O.
similar to Figure 2 and is given in Figure 3. The diagram of density versus composition is given in Figure 4. In Figures 2, 3, 4, and 5, A, B, C, D, and W denote solid KH2PO4, NaH2PO4, NaH2PO4·2H2O, NaH2PO4·H2O, and H2O, respectively. Point F, an invariant point, reflects the cosaturated solution of KH2PO4 and NaH2PO4·2H2O at 288.15 K. N represents the solubility of KH2PO4, and E represents the solubility of NaH2PO4 in water at 288.15 K. The saturated liquid line EFN consists of two branches. Branch EF corresponds to the saturated NaH2PO4 solution and visualizes changes of the
Figure 4. Density versus composition. □, 318.15 K; △, 303.15 K;12 ■, 288.15 K. H, T, K, E, N, and F have the same meaning as described in Figures 2 and 3.
NaH2PO4 concentration with increasing the KH2PO4 concentration. Branch FN corresponds to the saturated KH2PO4 solution and indicates changes of the KH2PO4 concentration C
DOI: 10.1021/acs.jced.5b01020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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denotes crystalline region of NaH2PO4·2H2O. Zone AFC denotes the mixed crystalline region of KH2PO4 + NaH2PO4·2H2O. Figure 4 indicates the relationship between the mass fraction of NaH2PO4 in the solution and the density at 288.15 K, 303.15 K,12 and 318.15 K. With an increase of the concentration of NaH2PO4, the density has the tendency to increase, and then the density declines slightly afterward. At the cosaturated point F(T), the density reaches a maximum value. The density of the liquid phase is calculated, according to the following density empirical formula developed in the previous study.23
ln
ds = d0
∑ Ai ·wi
where d0 = 0.999099 g·cm−3 and d0 = 0.990208 g·cm−3, the density of the pure water at 288.15 and 318.15 K;24 Ai is the constant of each possible component i in the system. wi is the salt of i in the solution in mass fraction. Constants Ai of NaH2PO4 and KH2PO4 for calculation are 0.007547 (0.007727) and 0.007113 (0.007299) at 288.15 (318.15) K, respectively. The experimental result is contrasted with the calculated result to determine the relative error. The related data are in Table 1. Test of the model precision uses the average relative error criterion, and the average relative error of the empirical equation is 0.11%, which demonstrates that the calculation method is feasible. A comparison between the phase equilibrium for NaH2PO4 + KH2PO4 + H2O at 288.15 K, 303.15 K,12 and 318.15 K is presented in Figure 5. The diagram further reveals that the temperature can affect the phase equilibrium. With the temperature increasing from 288.15 to 318.15 K, it is discovered that (1) the crystalline region of KH2PO4 is much larger than crystallization region of NaH2PO4·2H2O (NaH2PO4·H2O) at both temperatures, and the unsaturated area expands apparently. (2) The invariant point shifts upward from F to T, which illustrates that the salting out effect of NaH2PO4 to KH2PO4 increases more significantly. (3) With the temperature rising, the crystallization region of NaH2PO4·2H2O at 288.15 and 303.15 K12 transforms into NaH2PO4·H2O at 318.15 K. The differences in the KH2PO4 and NaH2PO4 solubility between the invariant points and aqueous solutions are shown in Figure 7. Differences between the KH2PO4 solubility at
Figure 5. Solubility isotherms of the ternary system NaH2PO4 + KH2PO4 + H2O at 288.15, 303.15, and 318.15 K. ○, 288.15 K; △, 303.15 K;12 ●, 318.15 K; A, B, C, D, E, F, N, H, T, K, and W have the same meaning as described in Figures 2 and 3.
with increasing the NaH2PO4 concentration in the equilibrium solution. As indicated in Figures 2 and 3, along the curve EF, we connect the composition points of wet residue phase with liquid phase and then extend, the intersection of these straight lines is approximately the equilibrium solid phase for NaH2PO4·2H2O. The same method is utilized to analyze the equilibrium solid phase component of FN, and the intersection is KH2PO4. Similarly, the equilibrium solid phases of A and D at 318.15 K are KH2PO4 and NaH2PO4·H2O, respectively. As indicated in Figure 6, the equilibrium solid phase of T is
Figure 6. X-ray diffraction pattern of the invariant point T.
analyzed by XRD. All the main peaks are consistent with the KH2PO4 and NaH2PO4·H2O standard data, and the equilibrium solid phase of the invariant point T is verified to be coexistence of KH2PO4 and NaH2PO4·H2O. The solid phases of A, C and D are certified to be KH2PO4, NaH2PO4·2H2O and NaH2PO4·H2O, respectively. Consequently, the ternary system appertains to a simple eutectic type, and does not form complex salt and solid solution at the investigated temperature. Figure 2 shows that WEFN denotes unsaturated area at 288.15 K. AFN denotes crystallization region of KH2PO4, while CFE
Figure 7. Comparison of the NaH2PO4 and KH2PO4 solubility in aqueous solutions and at cosaturated points F and T. D
DOI: 10.1021/acs.jced.5b01020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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(10) Chen, J. F.; Tan, G. X. The Production and Application of Phosphate; Chengdu Science and Technology Press: Chengdu, China, 1989. (11) Xie, Y. H.; Liu, J. F.; Yuan, J. S.; Zhu, H. J. The Methods of Producing Potassium Dihydrogen Phosphate. J. Salt Chem. Ind. 2005, 34, 9−11. (12) Shen, S.; Tang, J. H.; Liu, Z. H.; Kang, L. Q.; Mao, D. Phase Equilibrium for the Ternary System KH2PO4 + NaH2PO4 + H2O at 303.15 K. J. Chem. Eng. Data 2015, 60, 1072−1075. (13) Brunisholz, G.; Brunisholz, G.; Bodmer, M. The System H+− Na+−K+−Cl−−PO43‑−H2O. I. General Observations and the Ternary Systems NaCl−KCl−H2O, KCl−KH2PO4−H2O, NaCl−NaH2PO4− H2O, and NaH2PO4−KH2PO4−H2O. Helv. Chim. Acta 1963, 46, 2566−2574. (14) Shu, Y. G.; Lu, B. L.; Wang, X. R. Phase Diagram Analyses of Inorganic Chemical Production: Basic Theory; Chemical Industry Press: Beijing, 1985 (in Chinese). (15) Niu, Z. D.; Cheng, F. Q. The Phase Diagram of Salt-Water System and its Application; Tianjin University Press: Tianjin, China, 2002. (16) Schott, H. A Mathematical Extrapolation for the Method of Wet Residues. J. Chem. Eng. Data 1961, 6, 324. (17) Schreinemakers, F. A. H. Graphical deductions from the solution isotherms of a double salt and its components. Z. Phys. Chem. 1893, 11, 109−765. (18) Deng, T. L.; Zhou, H.; Chen, X. The Phase Diagram of SaltWater System and its Application; Chemical Industry Press: Beijing, 2013. (19) ESO/TC 47.ISO 3706-1976. Phosphoric Acid for Industrial Use (Including Foodstuffs)-Determination of Total Phosphorus (V) Oxide Content-Quinoline Phosphomolybdate Gravimetric Method; ISO Information Handling Services: Switzerland, 1976. (20) SN/T 0736.7-1999. Chemical Analysis of Fertilizers for Import and Export-Determination of Potassium Content; Dongying City Agricultural Bureau: China, 1999. (21) Eysseltová, J. Sodium dihydrogenphosphate. Alkali Metal Orthophosphates 1988, 31, 43−45. (22) Eysseltová, J. Potassium dihydrogenphosphate. Alkali Metal Orthophosphates 1988, 31, 213−219. (23) Lin, L. J.; Fang, C. H.; Fang, Y.; Qin, X. F. A new model for prediction density of electrolyte solutions. J. Salt Lake Res. 2006, 14 (2), 56−61. (24) Yang, W.; He, R. X. Physical and Chemical Testing II; Science Press: Beijing, China, 2014.
invariant points and in aqueous solution are 8.19% and 16.85% at 288.15 and 318.15 K, respectively, which illustrates that NaH2PO4 has a strong salting-out effect on KH2PO4 and the effect is stronger at higher temperatures. Differences between the NaH2PO4 solubility at invariant points and in aqueous solution are 2.92% and 3.79% at 288.15 and 318.15 K, respectively, which confirms KH2PO4 has a weak salting-out effect on NaH2PO4.
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CONCLUSIONS The phase equilibrium of KH2PO4 + NaH2PO4 + H2O at 288.15 and 318.15 K is investigated. The solubility and density are obtained. Based on the solubility data, the phase diagram is plotted; the solid phase that is in equilibrium with the solution is analyzed, and the crystallization areas are determined. The crystalline region of KH2PO4 is much larger than that of NaH2PO4·H2O (NaH2PO4·2H2O) at the investigation temperature. There are in all two crystallization regions, one invariant point, and two univariant curves in the phase diagrams. The empirical equation has a high precision to calculate the density of the saturated solution. With the temperature increasing from 288.15 to 318.15 K, the crystallization region of NaH2PO4·2H2O transforms into NaH2PO4·H2O. NaH2PO4 has a strong saltingout effect on KH2PO4, and the salting-out effect is stronger at higher temperatures. All results can offer fundamental data support for the separation processes of KH2PO4 in the industrial production and further theoretical researches.
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AUTHOR INFORMATION
Corresponding Author
*E-mail (Y.X.):
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.jced.5b01020 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
