Article pubs.acs.org/jced
Phase equilibrium in the System KH2PO4 + KCl + H3PO4 at 298.15 K and 308.15 K Sheng-qiang Lin, Jian-hua Tang,* Jian Teng, and Debiao Liu Department of Chemical Engineering and Technology, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China ABSTRACT: The solubility of (KH2PO4 + KCl + H3PO4) at 298.15 K and 308.15 K were determined, using isothermal solution saturation. The equilibrium phase solid was verified by X-ray diffraction (XRD). On the basis of the experimental data, the phase diagrams were plotted. The crystalline regions of KH2PO4 and KCl were determined. The phase equilibrium in different phosphoric acid concentrations are compared and discussed. This comparison further illustrates that the phosphoric acid concentration can influence the equilibrium besides temperatures. All results can offer a fundamental basis for crystallization and separation processes.
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INTRODUCTION
Materials. Potassium dihydrogen phosphate with analytical grade (KH2PO4, ≥99.5 wt %) is purchased from Tianjin Bodi Chemical Holding Co. Ltd., China. Potassium chloride (KCl, ≥99.5 wt %) used in the work is purchased from Chengdu Kelong Chemical Reagent Co. Ltd., China. Phosphoric acid (H3PO4, ≥99.5 wt %) used in the work is purchased from Chengdu Kelong Chemical Reagent Co. Ltd., China. Doubly deionized water (electrical conductivity ≤1 × 10−4 S·m−1) is used in the work. The description of samples used in the experiment is shown in Table 1. Apparatus. A SHZ-88 type constant temperature water bath oscillator with a precision of 0.3 K is employed for phase equilibrium measurement. The oscillator was made in Jintan Medical Instrument Corporation, Jiangsu, China. The Philips X Pert Pro MPD X-ray diffraction (XRD) analyzer is employed for XRD characterizations. Experimental Methods. The method of isothermal solution saturation8−10 is employed to determine the solubility data. The equilibrium solid phase is tested by XRD to verify the crystalloid composition. In a pre-experiment, the liquid phase of the samples is analyzed at every 2 h and it is stated that the phase equilibrium is reached in 14 h. After the equilibrium was reached, the oscillation was stopped and the system was allowed to stand for 4 h to make sure that all the suspended crystals settled. Then, the liquid phases were transferred to a 250 mL volumetric flask which is a gas tightness device.
Potassium dihydrogen phosphate, KH2PO4, an important chemical material and compound fertilizer, provides both potassium and phosphorus. KH2PO4 does not contain chloride and in agriculture is used as fertilizer for a variety of soils and crops. Moreover, KH2PO4 is utilized for flavoring additives, the preparation of buffer solution, and microbiological culture media, and also used in the industry of medicine.1 Potassium chloride, as a basic raw material, is widely used in the manufacturing of various potassium salts or alkali. Potassium dihydrogen phosphate can be manufactured by a reaction of potassium chloride with phosphoric acid or phosphate.2,3 However, the potassium dihydrogen phosphate solution produced often contains KCl and H3PO4.4,5 The crystallization process is an important step in producing KH2PO4.6 The phase equilibrium of KH2PO4 + KCl + H3PO4 is very important in crystallization and separation processes. Concentration of phosphoric acid and temperatures will affect the phase equilibrium of the system. However, the phase equilibrium data of KH2PO4 + KCl + H3PO4 system at 298.15 K and 308.15 K are scarce in the literature.7 The partial solubility of KH2PO4 + KCl + H3PO4 and their mixture solution (at 288.15, 298.15, and 308.15 K) has been reported in ref 7, but the complete solubility (e.g., cosaturated point) and the ternary system phase diagram of KH2PO4 + KCl + H3PO4 (at 288.15, 298.15, and 308.15 K) has not been reported. In this work, we determined phase equilibrium data of the ternary system (at 298.15 and 308.15 K), and the phase diagram was plotted of the more detailed equilibrium data of the ternary system under the respective mass fraction of phosphoric acid (20%,40%). These data can help to crystallize and separate the mixed solution. © 2017 American Chemical Society
METHODOLOGY
Received: June 17, 2017 Accepted: November 13, 2017 Published: November 22, 2017 4169
DOI: 10.1021/acs.jced.7b00554 J. Chem. Eng. Data 2017, 62, 4169−4173
Journal of Chemical & Engineering Data
Article
Table 1. Sample Description Table chemical name
source
initial mole fraction purity
purification method
finalmole fraction purity
potassium dihydrogen phosphate potassium chloride
Bodi
0.993
recrystallization
0.996
Kelong
0.992
recrystallization
0.997
phosphoric acid
Kelong
0.993
recrystallization
0.995
doubly deionized water
self-made
electrical conductivity ≤1 × 10−4 S· m−1
none
analysis method atomic absorption spectrometry atomic absorption spectrometry atomic absorption spectrometry
Table 2. Mass Fraction Solubility of KH2PO4 and KCl, Respectively, In Aqueous Solution of Phosphoric Acid at 298.15 and 308.15 K and Pressure = 0.1 MPaa mass fraction of phosphoric acid KH2PO4 % KCl % a
298.15 308.15 298.15 308.15
K K K K
0%
10%
20%
30%
40%
50%
60%
70%
80%
19.85 22.96 26.45 27.84
22.31 26.10 23.18 25.03
25.03 28.47 20.08 22.54
27.89 30.58 17.51 18.71
30.49 32.71 15.18 16.44
32.51 35.32 12.77 13.91
35.11 37.25 11.78 12.39
39.19 40.89 11.63 12.37
39.77 41.06 11.45 12.18
Standard uncertainties u(T) = 0.3 K, u(p) = 5 kPa. ur(w(KCl)) = 0.01, ur(w(KH2PO4)) = 0.02, ur(w(H3PO4)) = 0.02.
More details of the experimental method and the procedure of the preparation, collection, and transfer of samples were described in our previous papers. Analysis. The P2O5 concentration was analyzed by the quinoline phosphomolybdate gravimetric method,11 and the average relative deviation of the determination is less than ±0.2%. The chloride is measured by Volhard method,12,13 and the average relative deviation of the determination is less than ±0.5%. The K2O concentration is determined by means of the sodium tetraphenylborate gravimetric method,14 and the average relative deviation of the determination is less than ±0.1%. Each experimental result is achieved from the average value of three parallel measurements. The equilibrium solid phase is verified by X-ray diffraction (XRD).
Table 3. Mass Fraction Solubility of the Ternary KH2PO4 + KCl + H3PO4 + H2O System at Mass Fraction of Phosphoric Acid (20%,40%), Temperature = 298.15 K and Pressure = 0.1 MPaa composition of liquid phase, 100w no.
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RESULTS AND DISCUSSION The solubility of KH2PO4 and KCl in different concentrations of phosphoric acid solution is shown in Table 2. The phase equilibrium experimental data are shown in Table 3 and Table 4. The ion concentration values are measured in mass fraction in this equilibrium system. According to the data listed in Table 2, the solubility diagrams of KH2PO4 and KCl at 298.15 K and 308.15 K are shown in Figure 1 and Figure 2. On the basis of the experimental data shown in Table 3 and Table 4, the phase diagrams are shown in Figure 3 to Figure 6. As indicated in Figure 1 and Figure 2, with an increase in the concentration of phosphoric acid, the solubility of KH2PO4 gradually increased, and then the rising trend slowed down. However, the solubility of KCl gradually decreased, and then the downward trend slowed down. Figure 1 and Figure 2 show that the concentration of phosphoric acid in the solution will promote the dissolution of KH2PO4 and inhibit the dissolution of KCl. Figures 3 to 6 represent phase diagrams at different temperatures and concentrations of phosphoric acid, and the values represented by the points, lines, and faces in the figure are similar. For example, in Figures 3, point E, an invariant point at 298.15 K, reflects the cosaturated solution of KH2PO4 and KCl. Points C and D show the solubility of different single salts in 20% solution of phosphoric acid. C represents the solubility of KCl in phosphoric acid at 298.15 K. D represents
100w1b
1,C 2 3 4 5,E 6 7 8 9 10,D
0 3.59 7.45 11.59 11.94 14.25 16.16 18.60 21.77 25.03
1,F 2 3 4 5 6 7,H 8 9 10 11,G
0 3.67 7.35 11.06 14.31 17.87 21.28 23.42 24.86 26.77 30.49
100w2
equibrium solid phase
H3PO4 % = 20% 20.08 18.65 17.71 16.25 16.12 12.94 9.71 6.43 3.45 0 H3PO4 % = 40% 15.18 14.25 13.26 12.29 11.06 10.32 9.09 6.91 4.94 3.09 0
KCl KCl KCl KCl KCl + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KCl KCl KCl KCl KCl KCl KCl + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4
a Standard uncertainties u(T) = 0.3 K, u(p) = 5 kPa, ur(w2) = 0.01, ur(w1) = 0.02. bw1, mass fraction of KH2PO4; w2, mass fraction of KCl, ur(w(H3PO4)) = 0.02. C, E, D, F, H, and G have the same meaning as described in Figures 3 and 4.
the solubility of KH2PO4 in phosphoric acid at 298.15 K. The saturated liquid line CED consists of two sections. The curve between points D and E indicates that KCl has been saturated in phosphoric acid, while KH2PO4 has been precipitated by salting out. Additionally, the curve between points C and E presents the saturation process of KH2PO4, while KCl has been 4170
DOI: 10.1021/acs.jced.7b00554 J. Chem. Eng. Data 2017, 62, 4169−4173
Journal of Chemical & Engineering Data
Article
Table 4. Mass Fraction Solubility of the Ternary KH2PO4 + KCl + H3PO4 + H2O System at mass fraction of phosphoric acid (20%,40%), Temperature = 308.15 K and Pressure = 0.1 MPaa composition of liquid phase, 100w no.
100w1b
1,I 2 3 4 5 6,K 7 8 9 10 11,J
0 4.67 7.30 9.68 12.17 13.29 15.45 17.71 22.17 25.20 28.47
1,L 2 3 4 5 6 7 8,N 9 10 11 12 13,M
0 3.42 7.38 11.80 15.86 19.12 20.73 23.43 24.09 25.80 28.23 31.09 32.71
100w2 H3PO4 % = 20% 22.54 20.85 19.47 18.65 17.24 16.79 13.71 10.82 5.21 2.76 0 H3PO4 % = 40% 16.44 15.71 14.95 13.50 12.01 11.06 10.32 9.61 8.36 6.87 4.47 2.26 0
equibrium solid phase KCl KCl KCl KCl KCl KCl + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4
Figure 2. Solubility of KCl in phosphoric acid solution.
KCl KCl KCl KCl KCl KCl KCl KCl + KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4
Figure 3. Equilibrium phase diagram of KH2PO4 + KCl + H3PO4 in 20% phosphoric acid solution at 298.15 K.
a
Standard uncertainties u(T) = 0.3 K, u(p) = 5 kPa, ur(w2) = 0.01, ur(w1) = 0.02. bw1, mass fraction of KH2PO4; w2, mass fraction of KCl, ur(w(H3PO4)) = 0.02. I, K, J, L, N, and M have the same meaning as described in Figures 5 and 6.
Figure 4. Equilibrium phase diagram of KH2PO4 + KCl + H3PO4 in 40% phosphoric acid solution at 298.15 K. Figure 1. Solubility of KH2PO4 in phosphoric acid solution.
KH2PO4 and KCl standard data, and the equilibrium solid phase of the invariant point E is verified to be the coexistence of KH2PO4 and KCl. Consequently, the system appertains to a simple eutectic type and does not form complex salt and solid solutions at the investigated temperatures and concentration of phosphoric acid. A comparison between the phase equilibrium for KH2PO4 + KCl + H3PO4 at 298.15 and 308.15 K in different phosphoric
precipitated by salting out. Figures 4 to 6 are similar to Figure 3. With the help of XRD, we analyzed the cosaturation points (E, H, K, N) in Figures 3 to 6 and found that the equilibrium solid phases at the four cosaturated points were all mixtures of KH2PO4 and KCl. The XRD pattern at point E is taken as an example; in Figure 7, all the main peaks are consistent with the 4171
DOI: 10.1021/acs.jced.7b00554 J. Chem. Eng. Data 2017, 62, 4169−4173
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Figure 8. Equilibrium phase diagram of KH2PO4 + KCl + H3PO4 in 20% and 40% phosphoric acid solution at 298.15 and 308.15 K.
Figure 5. Equilibrium phase diagram of KH2PO4 + KCl + H3PO4 in 20% phosphoric acid solution at 308.15 K.
the equilibrium besides temperatures. The temperature remains the same, when increasing the concentration of phosphoric acid from 20% to 40%; it is discovered that the cosaturation point from E moves to H at 298.15 K, and that from K moves to N at 308.15 K. Obviously, the crystalline region of KH2PO4 becomes smaller, while the crystalline region of KCl becomes larger. In addition, when the concentration of phosphoric acid is kept constant, the temperature rises from 298.15 K to 308.15 K, and the crystalline regions of KH2PO4 have decreased a little and the crystalline regions of KCl have a increased a little when the concentration of phosphoric acid is 20% and 40%, respectively. The experimental results show that the temperatures and concentration of phosphoric acid can affect the phase equilibrium of system KH2PO4 + KCl + H3PO4. In order to obtain pure KH2PO4, the crystallization process should maintain the low phosphoric acid concentration and low temperature conditions.
Figure 6. Equilibrium phase diagram of KH2PO4 + KCl + H3PO4 in 40% phosphoric acid solution at 308.15 K.
acid concentrations is shown in Figure 8. This diagram further illustrates that phosphoric acid concentration can also influence
Figure 7. X-ray diffraction pattern of the invariant point E. 4172
DOI: 10.1021/acs.jced.7b00554 J. Chem. Eng. Data 2017, 62, 4169−4173
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(12) Sheen, H. T.; Kahler, H. L. Effect of ions on Mohr method for chloride determination. Ind. Eng. Chem., Anal. Ed. 1938, 10 (11), 628− 629. (13) Zhu, T.; Li, C. M. Determination of the content of chloride ion in fertilizer material by Volhard method. Chem. Fert. Ind. 2002, 1, 23− 26. (14) SN/T 0736.7-1999. Chemical Analysis of Fertilizers for Import and Export-Determination of Potassium Content; Dongying City Agricultural Bureau: China, 1999.
CONCLUSIONS The phase equilibrium of KCl + KH2PO4 + H3PO4 at 298.15 K and 308.15 K were investigated. The solubility data were obtained. On the basis of the solubility data measured, the phase diagrams were plotted, the solid phase which was in equilibrium with the saturated solution was detected, and crystalline regions of both solid phases were determined. When the phosphoric acid concentration is increased from 20% to 40% at 298.15 K and 308.15 K, the crystalline region of KH2PO4 becomes smaller, while the crystalline region of KCl becomes larger. When the temperature is increased from 298.15 to 308.15 K when the concentration of phosphoric acid is 20% and 40%, the crystalline regions of KH2PO4 decrease a little and the crystalline regions of KCl have a small increase. All results can offer fundamental data support for further theoretical studies.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jian-hua Tang: 0000-0003-0018-4084 Funding
The work described in this paper was fully supported by a grant from the National Natural Science Foundation of China (No. 21476151). Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.jced.7b00554 J. Chem. Eng. Data 2017, 62, 4169−4173
