Article pubs.acs.org/jced
Phase Equilibrium for the Ternary System KH2PO4 + NaH2PO4 + H2O at 303.15 K Si Shen, Jian-hua Tang,* Zhong-hua Liu, Li-quan Kang, and Dun Mao Department of Chemical Engineering and Technology, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China ABSTRACT: The phase equilibrium of the ternary system (KH2PO4+ NaH2PO4 + H2O) at 303.15 K was investigated by a combination of the isothermal solution saturation method and the moist residues method. The solid phase formed in the equilibrium state was verified by X-ray diffraction (XRD). A phase diagram and a densities versus composition diagram were constructed on the basis of the experimental data of the solubility and densities. It was discovered that there was one invariant point (KH2PO4 + NaH2PO4· 2H2O), two single saturated liquid curves, and two fields of crystallization corresponding to KH2PO4 and NaH2PO4·2H2O, with no double salt forming in this ternary system. Our research obtained new experimental data for this ternary system and gives guidance for potassium dihydrogen phosphate preparation and separation.
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INTRODUCTION
China. Our experiment employed redistilled water (electrical conductivity ≤ 1 × 10−4 S·m−1) to prepare the experimental solutions. Instrument. A stable temperature horizontal shaking bath (SHZ-88, Jintan Medical Instrument Corporation, Jiangsu, China) was employed to measure phase equilibrium measurement. The temperature range of this shaking bath was from 273.15 K to 373.15 K and can be restricted in ± 0.3 K. The Philips X PertPro MPD X-ray diffraction (XRD) analyzer was applied for solid-phase X-ray analysis. Experimental Methods. The method of isothermal solution saturation was applied to ascertain the solid−liquid equilibrium data in our study.5−8 At the temperature of 303.15 K (± 0.3 K), the solubility of NaH2PO4 goes down when pure KH2PO4 is added in the saturated solution of pure NaH2PO4, which is due to the same ion effect. The content of both KH2PO4 and NaH2PO4 reamins consistent with any further addition of KH2PO4. The solution in which both KH2PO4 and NaH2PO4 will not dissolve is taken as the co-saturation solution, thus the composition of this liquor is invariable at constant temperature and pressure. This point is called the invariant point in phase diagrams. Before the solution achieves the co-saturation point, only NaH2PO4 is saturated in the water, the KH2PO4 is dissolved entirely, while the crystals remain unsaturated during this process. The similar approach is used to obtain the after part of this phase diagram by putting the pure NaH2PO4 into saturated solution of KH2PO4. There is difficulty in separating crystals from mother liquor absolutely. So, Schreinemaker’s wet residue method is applied to confirm the chemical composition of the solid phase in this
Potassium dihydrogen phosphate (KDP) has many important properties that are required for a good synthetic fertilizer, one being a high density of vital elements in a form that is easily absorbed by plants, without elements that are undesired. At the same time, KDP is easily transported and applied,1 making it generally used in the field of agricultural. Moreover, KDP is also utilized for flavoring additives, the preparation of buffer solution, microbiological culture media, and is also used in the medical industry. Monosodium dihydrogen phosphate (NaH2PO4)is one of the most important products in the phosphate industry. It is widely applied in plating, boiled water treatment, and the tanning fields.2 It can be applied as flame retardants to avoid wood and paper burning.3 However, the product of KDP contains a small quantity of NaH2PO4 or there is a little KDP mixed in the product of NaH2PO4.4 A suitable concentration of the K+ and Na+ can be acquired at the crystallization process based on the phase equilibrium. But to the best of our knowledge, there is no complete phase equilibrium data of KH2PO4 + NaH2PO4 + H2O system at 303.15 K. The objective of our experiment is to measure the solubility and construct a phase diagram for the potassium dihydrogen phosphate (KH2PO4)−monosodium dihydrogen phosphate (NaH2PO4)−water (H2O) system at 303.15 K, Our work is supposed to help perfect the research about this system and provide theoretical guidance for inorganic salt production at the same time.
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METHODOLOGY Materials. Potassium dihydrogen phosphate (KH2PO4, 0.995 mass fraction) and analytical reagent monosodium dihydrogen phosphate (NaH2PO4, 0.990 mass fraction) were purchased from Tianjin Bodi Chemical Holding Co. Ltd., © XXXX American Chemical Society
Received: November 5, 2014 Accepted: February 6, 2015
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DOI: 10.1021/je5009872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 1. Mass Fraction Solubility of the Ternary KH2PO4+ NaH2PO4+ H2O System at Temperature T = 303.15 K and Pressure p = 0.1 MPaa composition of liquid phase, 100wb no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
KDP 0 3.18 4.91 6.88 8.24 8.87 9.11 9.41 9.8 10.63 11.01 12.49 13.42 14.15 15.25 16.12 18.66 20.21 21.47
composition of wet residue phase, 100wb
densities ρ/(g·cm−3)
MSP
KDP
MSP
equibrium solid phase
53.15 51.48 50.74 50.13 49.22 48.93 45.33 42.45 40.8 38.01 34.75 30.82 27.73 25.53 20.84 16.9 8.5 4.69 0
d
ND 67.97 66.8 64.54 63.02 49.52 25.68 23.66 21.87 19.89 17.34 15.32 13.36 11.29 9.05 6.52 3.49 2.04 ND
MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d+KDP KDP KDP KDP KDP KDP KDP KDP KDP KDP KDP KDP KDP KDP
ND 1.29 2.06 3.44 4.54 27.31 48.73 50.35 52.78 54.39 55.73 57.12 59.22 62.29 65.03 68.48 70.54 73.31 ND
c
exp
calcd
REe
1.4920 1.5026 1.5266 1.5377 1.5398 1.5415 1.4980 1.4731 1.4561 1.4352 1.4064 1.3805 1.3588 1.3412 1.3046 1.2715 1.2199 1.1977 1.1659
1.4920 1.5078 1.5185 1.5335 1.5383 1.5420 1.5030 1.4737 1.4595 1.4376 1.4063 1.3798 1.3570 1.3417 1.3051 1.2747 1.2184 1.1971 1.1659
0 0.0052 −0.0081 −0.0042 −0.0015 0.0005 0.0050 0.0006 0.0034 0.0024 −0.0001 −0.0007 −0.0018 0.0005 0.0005 0.0032 −0.0015 −0.0006 0
Standard uncertainties u are u(T) = 0.3 K, u(p) = 0.05 MPa, u(MSP.d) = 0.01 (mass fraction), u(KDP) = 0.01 (mass fraction), u(ρ) = 0.01 g·mL−1. w, mass fraction. cMSP, NaH2PO4; MSP.d, NaH2PO4·2H2O; KDP, KH2PO4. dND, not determined. eRE, relative error = (calcd value − exp. value)/calcd value.
a b
put the unweighed solid phase into the drying closet; the obtained dry samples were used for X-ray diffraction (XRD) in the next procedure. Analysis. The concentration of P2O5 was determined by the quinoline phosphomolybdate gravimetric method,9 and the mean error of the determined concentration was under 0.01. The method of gravimetric sodium tetraphenylborate was applied to determine the concentration of K2O,10 and the mean relative deviation of the determined concentration was under 0.003. The concentration of sodium dihydrogen phosphate was determined by the subtraction method. We calculated the density of solution by weighing 1 mL of the equilibrium solution, and the absolute error of the measure value for density was expected to be within 0.01 g·mL−1. The solid of equilibrium phase was verified by XRD.
experiment. The basic foundation of this method consists of linear rules, because the wet solid phase is composed of the pure solid phase and liquid phase; therefore, on the phase diagram, the point that reflects the component of the wet solid phase should be on the straight line of the saturated liquid and the pure solid in the equilibrium. Experimental Procedures. A known mass of monosodium dihydrogen phosphate and KDP were dissolved in accurate redistilled water and loaded in a conical 250 mL-flask. The flask was sealed and moved to a temperature-stable horizontal shaking bath. The oscillator vibrated continuously with a temperature controlled at around 303.15 K (inconclusive, ± 0.3 K), which was monitored by a mercury thermometer. We analyzed the liquid phase of the samples utilizing a chemical analysis approach at every 2 h in this experiment. It demonstrated that the dissolution reached the equilibrium states when the analytical results were kept constant. It turned out that the dissolution equilibrium point is 6 h. When the concentrations of P2O5 in the solution remained stable for another 2 h, the oscillator stopped vibrating, and the temperature was set at around 303.15 K to allow the system rest for 2 h to achieve equilibrium. After attaining an equilibrium state, the saturated solution was transferred into a measuring flask of 250 mL using a 1 mL pipet at 303.15 K, and the wet solid phase was transferred with a spoon into a small beaker (100 mL). The weight of the volumetric flask and the beaker were measured and recorded before and after the saturated solution or the wet residues were added, respectively. The next step was to dissolve the wet residues in redistilled water, and transfer the liquid to a measuring flask (250 mL). To make sure that all wet residues were transferred into the measuring flask, the small beaker was gargled 4 to 5 times. The solutions in the volumetric flasks were brought to volume with deionized water. Under constant temperature and pressure, we
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RESULTS AND DISCUSSION
Table1 shows the phase equilibrium results of solubility and density for the ternary system of KH2PO4 + NaH2PO4 + H2O at the temperature of 303.15 K. The concentration of ions was measured by mass fraction in this equilibrium system, and ρ is the density for the balanced solution, the unit of which is g· cm−3. On the basis of the data which is listed in Table1, Figure1 displays the phase diagram of this ternary system and Figure2 demonstrates the plot of the correlation between density and composition. The XRD of the invariant point S is given in Figure 3. Figure1 indicates that W, A, and B represent H2O, solid KH2PO4, and solid NaH2PO4, respectively. Besides, R and T denote the different single salt’s solubility. R indicates the solubility of NaH2PO4, which is 53.15 in mass fraction (100w). T signifies the largest mass fraction (100w) of KH2PO4 that dissolved in purified water, the value is 21.47. The cosaturated solution of NaH2PO4 and KH2PO4 could be reflected by S, B
DOI: 10.1021/je5009872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
turned out that the intersection represents KH2PO4. With the characterization of XRD analysis, the solid phase components of A and B were demonstrated to be KH2PO4 and NaH2PO4· 2H2O, respectively. The solid of equilibrium phase for the invariant point S was verified by analysis of XRD and estimated to be coexisting with the salts of NaH2PO4·2H2O and KH2PO4, which could be seen in Figure 3. Every sample of wet solid phase was dried at 295.15 K during the procedure of the XRD test, and it turned out that this system was a simple eutectic type. Results in Figure1 indicated that the region of WRST denotes an unsaturated region at 303.15 K and RBS denotes the crystalline region of NaH2PO4·2H2O. In addition, ATS is the crystallization region of KH2PO4 and ASB is the blended crystallization region of KH2PO4 + NaH2PO4·2H2O. The result shows that the crystallization area of pure KH2PO4 is much smaller than the area of pure NaH2PO4·2H2O. The density of solution versus the mass concentration of NaH2PO4 was plotted in Figure2. It can be discovered that the density shows an upward tendency with the increase percentage of NaH2PO4 in the solution. However, the curve reaches its maximum at point S which presents cosaturation and decreases slightly afterward. The density of the solution was calculated according to the following empirical equation of density, which was developed in the previous study.11 The relative error was determined by comparison with the experimental results and calculated data, and Table 1 listed all of the data mentioned above.
Figure 1. Phase diagram for the ternary KH2PO4 + NaH2PO4 + H2O system at 303.15 K: ■, wet residue phase composition at 303.15 K; A, pure solid of KH2PO4; B, pure solid of NaH2PO4·2H2O; W, water; R, solubility of NaH2PO4 in water; S, cosaturated point of KH2PO4+ NaH2PO4; T, solubility of KH2PO4 in water.
ln
d = d0
∑ Ai . wi
where d0 = 0.995645 g·cm−3, representing the water density at 303.15 K; Ai is the constant of each possible component i of the system at 303.15 K. Wi is the mass fraction of salt i in the solution. The calculated constants of Ai for NaH2PO4 and KH2PO4 are 0.007607 and 0.007354, respectively.
Figure 2. Density vs composition.
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CONCLUSIONS The phase equilibrium for the system of KH2PO4 + NaH2PO4 + H2O at 303.15 K was investigated by the isothermal solution saturation method. On the basis of the solubility and density data obtained in the experiment, the phase diagram of this ternary system and density diagram were drawn. The solidphase was detected by the method of Schreinemaker’s wet residue and verified by XRD. The phase diagram includes one invariant point, two single saturated liquid curves, and two crystallization fields at constant temperature. There was no double salt formed in this ternary system. As it is the first investigation on the ternary system at 303.15 K, the data record obtained in our experiment may serve as the basic reference. The results on the solubility, phase equilibrium diagrams, and density of this ternary system may provide fundamental data support for the preparation and separation of KDP in the monosodium dihydrogen phosphate. The comparison of the crystallization region between pure KH2PO4 and pure NaH2PO4·2H2O (the former is much smaller than the latter) shows that crystallization evaporation can be very effective in separating KDP from the liquor.
Figure 3. X-ray diffraction pattern of the invariant point S (KH2PO4 + NaH2PO4·2H2O).
which is an invariant point. The line between points R and S indicates that KH2PO4 has been saturated in the water, while NaH2PO4·2H2O has been precipitated. Additionally, the curve between points S and T present the saturation process of NaH2PO4 as well as the precipitation of KH2PO4. Figure1 shows that the solubility curve RS links the component points of the liquid phase and wet residue phase and then lengthens to associate with the Y axis, and the point of intersection is approximately the solid-phase constituent for the NaH2PO4·2H2O. The balance solid-phase constituent of curve ST was determined by applying the same approach, and it
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. C
DOI: 10.1021/je5009872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/je5009872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
