SolidâLiquid Phase Equilibrium for the Ternary System (Potassium

Article pubs.acs.org/jced

Solid−Liquid Phase Equilibrium for the Ternary System (Potassium Chloride + Potassium Dihydrogen Phosphate + Water) at (298.15 and 313.15) K Wei Shen, Yongsheng Ren,* Xiaorui Zhang, and Yuqin Shi Department of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, P. R. China ABSTRACT: Solid−liquid phase equilibrium and physicochemical properties (pH, nD, ρ) for the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K were investigated by the method of isothermal solution saturation and Schreinemaker’s wet residue. On the basis of the experimental data, corresponding phase diagrams and diagrams of physicochemical properties vs composition were plotted. In the phase diagrams, there are one eutonic point, two uninvariant curves, and two crystallization regions corresponding to KCl, KH2PO4. The phase diagrams of the ternary system show similar tendencies at different temperatures. At each temperature, the crystallization region of KH2PO4 is larger than that of KCl, and the crystallization region of KH2PO4 becomes larger as the temperature increases. Physico-chemical properties of the solid−liquid phase equilibrium solution vary regularly with the composition of KH2PO4 concentration. The experimental data have been satisfactorily compared with literature data. The measured data and phase equilibrium diagrams for the ternary system can provide the fundamental basis for the industrial production of KH2PO4 by the method of extraction with industrial KCl and industrial H3PO4 aqueous mixtures.

1. INTRODUCTION KH2PO4, as a widespread useful fine chemistry product is used in industrial and agricultural fields.1 KH2PO4 has aroused considerable interest because of its wide frequency conversion, high efficiency, and high damage threshold against high power laser.2 In addition, it is also a commercially valuable intermediate for producing the widest variety of derivatives, such as other potassium salts, penicillin, and sodium glutamate.3 Many processes for the production of KH2PO4 have been reported in the literature, such as neutralization technology, direct chemical conversion method, crystallization method, ion exchange method, and extraction technology.4−6 Among them, extraction technology was considered to be very promising because it has the advantages of low energy consumption, mild reaction conditions, and high product purity.7 The KH2PO4 produced with the method of extraction technology often contains many impurities, such as chloride ion.8 As we all know, the solubility of KCl is different from that of KH2PO4. Because there is a small quantity of KCl in the process of producing KH2PO4, it is necessary to separate KCl from the mixture solution. Thus, the crystallization process is a crucial step in producing KH2PO4 with the method of extraction technology. Special attention is paid to the crystallization process, especially the purity,9,10 temperature, and growth rate.11,12 To produce pure KH2PO4, knowledge of accurate solubility is needed for the design of the separation processes such as the crystallization process. SLE measurements and phase equilibrium diagrams of the system KCl + KH2PO4 + H2O at (298.15 and 313.15) K can provide the fundamental © XXXX American Chemical Society

basis for the production of KH2PO4 by the method of extraction with industrial KCl and industrial H3PO4 aqueous mixtures. Through previous publications, the solubility of the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K had been added to Vol. 31 of the IUPAC Solubility Data Series.13 In their published paper, Krasil’shtschikov et al. and Khaliieva13 studied the ternary system KCl−KH2PO4−H2O at (298.15 and 313.15) K by means of the isothermal method. In their paper, equilibrium was checked by repeated analysis, and standard analytical methods were used to determine the amount of chloride and dihydrogen phosphate ions in their experiments. Although the solubility of the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K had been reported, we did not know the details of these experiments from the literature. After searching the literature, we found that these data were reported before 50−80 years and no more recent data were found for the ternary system. As we all know, our experimental conditions and analytical methods have been gradually improved. Therefore, it is essential to supply more SLE data and compare it with literature values. In this study, we provided the completely phase equilibrium data and physicochemical properties (pH, nD, ρ) for the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K. At the same time, the objectives of this research are (1) to compare Received: February 5, 2015 Accepted: May 22, 2015

A

DOI: 10.1021/acs.jced.5b00113 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Purities and Suppliers of Chemicals mass fraction purity

CAS number

KCl

≥ 99.5 %

7447-40-7

KH2PO4

≥ 99.5 %

7778-77-0

chemical

a

purification method

source Tianjin Kermel Chemical reagent Co. Ltd., China Tianjin Kermel Chemical reagent Co. Ltd., China

analytical methoda

none

Volhard method

none

quinoline phosphomolybdate gravimetric method

Relative standard uncertainties ur are ur(KCl) = 0.05, ur(KH2PO4) = 0.01.

Table 2. Solubility (S), Density (ρ), and Refractive Index (nD) in Pure Water at T = (298.15 and 313.15) K and P = 88.35 kPa this worka

S/wt % T/K 298.15

313.15

a

−3

lit

this work

lit

RD

ρ/(g·cm )

nD

ρ/(g·cm−3)

nD

KCl

26.42

1.3678

1.17326

1.370425

20.49

1.1531

1.3561

1.108021 1.14713

1.355013

KCl

28.40

1.1956

1.3683

KH2PO4

25.04

0.0021 0.0086 −0.0084 0.0443 −0.0286 −0.0281 −0.0094 0.0114 −0.0139 0.0777 0.0024 −0.0016

1.1800

KH2PO4

26.47622 26.6523 26.224 21.441 19.9213 19.9313 20.3013 28.72823 28.0113 27.1513 25.1026 25.0026

1.1839

1.3612

salt

a

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.02 kPa, ur(KCl) = 0.05, ur(KH2PO4) = 0.01, ur(nD) = 0.0002, ur(pH) = 0.02, ur(ρ) = 0.005;

sealed and placed in the constant temperature bath oscillator. The oscillator vibrated continuously with temperature controlled at around (298.15 or 313.15) K, and the actual temperature of the complex was monitored by a mercury thermometer (uncertainty = ± 0.05 K). To determine the equilibrium time, several samples of the system at 298.15 K were analyzed for Cl− and H2PO4− after shaking 2, 3, and 4 h, separately. When the components of the solution did not change, equilibrium was established. Experimental results showed that it was sufficient to take 3 h to reach equilibrium. After 3 h, the bottles with solid−liquid were removed into a constant-temperature water bath for an additional 1.5 h to allow remaining solids to settle. When the system reached equilibrium, the saturated solution was removed into a 100 mL beaker at (298.15 or 313.15) K, and it was weighed accurately as the total mass of the liquid phase. A certain mass of liquid phase was taken into a volumetric flask with a dropper and diluted to the mark with deionized water for the analytical method. The remainder of liquid phase continued to rest in the constant-temperature water bath. Then it was used to measure the relative physicochemical properties (pH, nD, ρ) of the liquid phase according to the analytical methods. When a ternary solid−liquid system arrives at equilibrium, at least one solid and one liquid phase exists. The composition of the solid phase is usually not determined directly, because it is quite difficult to separate the crystals from the mother liquor completely. Thus, the wet residue was identified via the method of Schreinemaker’s wet residue.15−17 The total weight of the wet residues was also measured accurately. An appropriate quantity of the wet residues was transferred to a beaker by a scoop. Afterward the residues were dissolved with deionized water, transferred to a 100 mL volumetric flask, and diluted to the mark with deionized water. The small beaker was rinsed 4 to 7 times to ensure that all the wet residues were transferred into flask. Finally, the composition of equilibrium liquid phase

with literature data to give a reliable result; (2) to construct and analyze the phase diagrams of the ternary system KCl− KH2PO4−H2O at (298.15 and 313.15) K; (3) to supply the diagrams of physicochemical properties vs composition at the two different temperatures.

2. EXPERIMENTS 2.1. Reagents and Instruments. All reagents used were analytically pure KCl and KH2PO4 provided from Tianjin Kermel Chemical reagent Co. Ltd., China, with a mass fraction of 99.5%. All reagents were employed with no further purification. The source and CAS numbers were listed in Table 1. Doubly deionized water (electrical conductivity less than 10−4 S·m−1) was used for the solid−liquid phase equilibrium experiments and chemical analysis. An XMTD-4000-type constant-temperature water bath (Zhengzhou Ying Yuhua Instrument Co. Ltd., China) was used for the phase equilibrium measurement. A constant temperature bath oscillator (SHZ-C, Shanghai Langgan Laboratory Equipment Co. Ltd., China) was used to achieve the solid−liquid equilibrium measurements. The temperature of this oscillator could be controlled to ± 0.05 K. 2.2. Experimental Method. The method of isothermal solution saturation was employed in this study.14 The saturated solutions were acquired by adding an excess of KH2PO4 and KCl to water until KH2PO4 and KCl cannot be dissolved any more. In this similar way, the solid−liquid phase equilibrium data were obtained in KCl and KH2PO4 aqueous mixtures. Although KH2PO4 and KCl exist in the solution, only KCl or KH2PO4 is saturated in the water before the solution achieves cosaturation. Therefore, we get a series of points that represent the solubility of KCl and KH2PO4 in saturated solutions. For the measurement of phase equilibrium data, a known mass of KH2PO4 and KCl was added into a conical flask (250 mL) with doubly deionized water. Then, the conical flask was B



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and the wet residues were determined by the methods of chemical analysis. 2.3. Analytical Methods. The H2PO4− concentration was measured by the quinoline phosphomolybdate gravimetric method18 with a relative standard uncertainty of 0.01. The Cl− concentration was determined by Volhard method19,20 with a relative standard uncertainty of 0.05. A PB-10 pH meter supplied by the Sartorius Scientific Instruments (Beijing Co. Ltd.) was used to measure the pH of the equilibrium aqueous solutions with a relative standard uncertainty of 0.02. Before it was used, the pH meter was calibrated with standard buffer solutions of both mixing potassium dihydrogen phosphate (pH = 6.86) and potassium hydrogen phthalate (pH = 4.01). An Abbe refraction (WAY-2S) was used for measuring the refractive index (nD) with a relative standard uncertainty of 0.0002.21 The density (ρ) was measured with a specific weighing bottle method with a relative standard uncertainty of 0.005. All the measurements were maintained in a constant temperature bath monitored by a mercury thermometer (uncertainty = ± 0.05 K).

Figure 2. SLE temperature for KH2PO4 + H2O: ●,△, ref 25; ▽,▲, ref 13; ▼, ref 1; ■,□, exp.

3. RESULTS AND DISCUSSION Many researchers have reported the solubility of KH2PO4 and KCl. To compare with literature data, experimental solubility data and literature data at different temperatures are shown in Table 2 and Figures 1 and 2. xe and xc represent literature value

The ternary phase diagrams were demonstrated in Figure 3 and Figure 4. Points A, U, and W represent the pure KCl, KH2PO4, and water. In Figure 3, points E1 and F1 are the solubility of the single salts of KCl and KH2PO4 in water at 298.15 K, and the solubilities of KCl and KH2PO4 are analyzed as 26.42 wt % and 20.49 wt %, respectively. With further addition of KH2PO4, the solubility of KCl will remain unchanged until KH2PO4 cannot be dissolved any more. The mixed solution is known as the cosaturation solution. Point C1 is a eutonic point at 298.15 K, which shows the two pure solids KCl and KH2PO4 saturated with equilibrium solution. The composition of the corresponding equilibrated solution at eutonic point C1 is 100w(KCl) = 24.37, 100w(KH2PO4) = 3.66. The solubility curve C1F1 links the component points of the liquid phase and wet residue phase, and then if they are extended the point of joint of these tie-lines approximates the pure solid-phase component for KH2PO4. Thus, curve C1F1 indicates that KH2PO4 has been saturated in the water. Similarly, along the solubility curve E1C1, if the component points of the liquid phase and moist solid phase are linked and then extended, the point of intersection of these tie-lines is approximately the solid-phase component for KCl. Therefore, curve E1C1 presents the saturation process of KCl. The phase diagram is divided into four regions in Figure 3: KCl crystallization region (I), KH2PO4 crystallization region (III), the mixed crystallization region of KCl+KH2PO4 (II), and the unsaturated region (IV). As shown in Figure 4, the phase diagram of this ternary system at 313.15 K consists of four regions, one eutonic point C2, and two uninvariant curves (E2C2, C2F2). Points E2 and F2 represent the saturation points of binary systems KCl−H2O and KH2PO4−H2O at 313.15 K, where the mass fractions of salts are 28.40 wt % and 25.04 wt %. It is determined that KH2PO4 and KCl are in equilibrium with liquid phase at the eutonic point C2. The composition of the corresponding equilibrated solution at point C2 is 100w(KCl) = 26.35, 100w(KH2PO4) = 4.40. The two uninvariant curves correspond to the saturation process of KCl (E2C2) and KH2PO4 (C2F2) at 313.15 K, respectively. There are four regions divided by two solubility curves. In Figure 4, the four crystallization zones are divided as follows: crystallization region of KCl (I), crystallization region of KH2PO4 (III), the mixed crystallization

Figure 1. SLE temperature for KCl + H2O: ▼, ref 13; △,▲, ref 22; ▽, ref 23; ●, ref 27; ■, □, exp.

and experimental value, respectively. The RD of the solubility of KH2PO4 and KCl at (298.15 and 313.15) K were calculated. RD values are shown in Table 2. As we can see from Table 2, the maximum RD of the solubility of KCl is 0.0114, and the maximum RD of the solubility of KH2PO4 is 0.0777. It can be seen that experimental data are in good agreement with literature data, which indicated that experimental methods are reliable in this experiment. 3.1. Experimental Results of Solid−Liquid Phase Equilibrium. The experimental results of the solubility and physicochemical properties (pH, nD, ρ) of the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K are shown in Table 3 and Table 4. Solution compositions were expressed in terms of mass fractions. The ternary phase diagrams were plotted, as given in Figure 3 and Figure 4, respectively. C



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Table 3. Solid−Liquid Equilibrium and Physico−Chemical Properties of the Ternary System KH2PO4 (1) + KCl (2) + H2O at T = 298.15 K and P = 88.35 kPaa composition of liquid phase no. 1,E1 2 3,C1 4,C1 5 6 7 8 9 10 11 12,F1

100w1b 0.00 1.47 3.60 3.66 5.27 6.38 8.36 9.80 12.23 15.13 16.40 20.49

100w2

b

26.42 24.79 23.49 24.37 18.74 15.81 11.02 8.96 5.33 4.08 2.03 0.00

composition of wet residue 100w1 c

ND 0.62 48.40 52.64 71.87 50.31 69.15 69.70 64.96 75.47 74.78 ND

physico-chemical properties of liquid phase

100w2

nD

pH

ρ/(g·cm−3)

equibrium solid phase

ND 64.18 25.51 17.89 6.94 7.38 0.33 2.75 1.01 0.30 0.44 ND

1.3678 1.3681 1.3696 1.3700 1.3639 1.3601 1.3594 1.3578 1.3560 1.3553 1.3557 1.3561

6.49 3.84 4.47 4.51 4.11 3.94 3.96 4.05 4.00 4.07 4.01 3.96

1.1800 1.1936 1.2165 1.1969 1.2007 1.1477 1.1436 1.1398 1.1396 1.1395 1.1420 1.1531

Cl Cl Cl+Pd Cl+P P P P P P P P P

a

Standard uncertainties u are u (T) = 0.05 K, u(P) = 0.02 kPa, ur(w1) = 0.01, ur(w2) = 0.05, ur(nD) = 0.0002, ur(pH) = 0.02, ur(ρ) = 0.005. bw1, mass fraction of KH2PO4; w2, mass fraction of KCl. cND, not determined. dCl, KCl; P, KH2PO4.

Table 4. Solid−Liquid Equilibrium and Physico−Chemical Properties of the Ternary System KH2PO4 (1) + KCl (2) + H2O at T = 313.15 K and P = 88.35 kPaa composition of liquid phase no. 1,E2 2 3,C2 4,C2 5,C2 6 7 8 9 10 11 12 13 14 15 16,F2

100w1b 0.00 1.84 4.36 4.40 4.44 5.47 6.79 8.54 10.80 12.30 14.02 17.66 18.48 19.96 22.27 25.04

100w2

b

28.40 27.99 27.40 26.35 27.26 25.30 20.89 17.60 14.92 12.43 11.27 7.68 5.32 3.74 2.84 0.00

composition of wet residue 100w1 c

ND 0.75 45.75 58.49 50.19 78.92 79.90 79.23 79.69 81.18 77.77 82.07 76.46 76.51 86.56 ND

physico-chemical properties of liquid phase

100w2

nD

pH

ρ/(g·cm−3)

equibrium solid phase

ND 69.95 36.68 20.17 24.02 4.50 2.77 3.09 3.08 3.48 0.88 2.58 1.13 1.22 0.08 ND

1.3683 1.3695 1.3726 1.3708 1.3710 1.3714 1.3686 1.3655 1.3631 1.3623 1.3618 1.3610 1.3608 1.3605 1.3603 1.3612

6.49 3.75 4.23 4.23 4.32 3.93 3.91 3.81 3.76 3.79 3.84 3.83 3.85 3.87 3.86 3.93

1.1956 1.2001 1.2145 1.2106 1.2104 1.2070 1.1865 1.1797 1.1658 1.1653 1.1647 1.1622 1.1685 1.1730 1.1774 1.1839

Cl Cl Cl+Pd Cl+P Cl+P P P P P P P P P P P P

a

Standard uncertainties u are u (T) = 0.05 K, u(P) = 0.02 kPa, ur(w1) = 0.01, ur(w2) = 0.05, ur(nD) = 0.0002, ur(pH) = 0.02, ur(ρ) = 0.005. bw1, mass fraction of KH2PO4; w2, mass fraction of KCl. cND, not determined. dCl, KCl; P, KH2PO4.

apparatuses, different analytical methods. Table 5 shows the composition of eutonic point for the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K. The experimental method (isothermal method) employed is essentially the same to the one used at previous works.13 They all found there was one eutonic point for the ternary system at 298.15 K. In Table 5, the only data set found for comparison is the composition of eutonic point. The values in literature show a little inconsistency for the eutonic point at 298.15 K. Maybe it was caused by different apparatuses, different shaking time. But experimental data are somewhat similar from those obtained in literature. Particularly, the composition of invariant point is 100w(KCl) = 24.82, 100w(KH2PO4) = 3.57 in ref 13, and the composition of the corresponding equilibrated solution is 100w(KCl) = 24.37, 100w(KH2PO4) = 3.66 in this study. Therefore, we can see that experimental data agree well with literature data, which indicates that the experimental methods are reliable in this experiment. Comparison with the values

region of KCl + KH2PO4 (II), and unsaturated regions (IV). The crystal region of KH2PO4 is larger, which indicates that KH2PO4 is of low solubility. These results show that KH2PO4 is easy to saturate and crystallize from solution. Zhao3 studied the ternary system KCl + KH2PO4 + H2O at (288.2, 298.2, 308.2) K. Thus, a comparison between the four temperatures was drawn in Figure 5a and Figure 5b. There is significant consistency between our solubility data at (298.15, 313.15) K and the data from ref 3 at (288.2, 298.2, 308.2) K for the ternary system KCl + KH2PO4 + H2O. As we can see in Figure 5a, the solubility of KCl and KH2PO4 gradually increase with temperature increasing and the solubility of KCl decreased in saturated solution with the addition of pure KH2PO4. In Figure 5b, the literature and experimental data at 298.15 K was compared. According to Figure 5b, our data show values a little higher than the literature data at 298.15 K. After analyzing the original literature, we know different methods were used to obtain solubility data of the ternary system between this work and the literature.3 Maybe the deviation was caused by different D



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Figure 3. Phase diagram for the ternary KCl + KH2PO4 + H2O system at 298.15 K and P = 88.35 kPa: □, moist solid phase, 100w1, KH2PO4 wt %; 100w2, KCl wt %; W, H2O; A, pure solid of KCl; U, pure solid of KH2PO4; C1, eutonic point of KH2PO4 and KCl; E1, solubility of KCl in water at 298.15 K; F1, solubility of KH2PO4 in water at 298.15 K.

Figure 4. Phase diagram for the ternary KCl+ KH2PO4 + H2O system at 313.15 K and P = 88.35 kPa: □, moist solid phase; 100w1, KH2PO4 wt %; 100w2, KCl wt %; W, H2O; A, pure solid of KCl; U, pure solid of KH2PO4; C2, eutonic point of KH2PO4 and KCl; E2, solubility of KCl in water at 313.15 K; F2, solubility of KH2PO4 in water at 313.15 K. Figure 5. (a) SLE data for the ternary system KCl+ KH2PO4 + H2O system: 100w1, KH2PO4 wt %; 100w2, KCl wt %; black ■, 288.2 K, ref 3; red ●, 308.2 K, ref 3; green ▲, 298.15 K, exp. value; blue ▼, 313.15 K, exp. value. (b) SLE solubility for the ternary KCl + KH2PO4 + H2O system at 298.15 K: 100w1, KH2PO4 wt %; 100w2, KCl wt %; black □, ref 3; red ○, exp. value. (c) SLE solubility for the ternary KCl+ KH2PO4 + H2O system at 313.15 K: 100w1, KH2PO4 wt %; 100w2, KCl wt %; red ○, ref 13; black □, exp. value.

given by literature is a good support for the quality of the data measured in this work. Except for these literature data, the solubility for the ternary system KCl + KH2PO4 + H2O at 313.15 K were also investigated in ref 13. A comparison between the literature and experimental data at 313.15 K is shown in Figure 5c. As we can see from Figure 5c, a good resemblance is obtained between the literature data and experimental data over the saturated solution for the ternary system at 313.15 K. But the values in literature show a little inconsistency compared with our data. In Vol. 31 of the IUPAC Solubility Data Series, the ternary system KCl + KH2PO4 + H2O at 313.15 K had been studied by the method of isothermal method. In Table 5, the only literature

data of eutonic point at 313.15 K was set found for comparison. The composition of the corresponding eutonic point is 100w(KCl) = 25.97, 100w (KH2PO4) = 4.21 in ref 13. And the composition of the corresponding eutonic point at 313.15 K is 100w(KCl) = 26.35, 100w(KH2PO4) = 4.40 in this study. E



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This further illustrates that temperature can influence the composition of the corresponding equilibrated solution for the ternary system KCl + KH2PO4 + H2O. The solubility of KCl in water increases a little, but the solubility of KH2PO4 changes greatly with increasing temperature, which is from 20.49 wt % to 25.40 wt %. As a result, the crystalline region of KH2PO4 becomes larger and the eutonic point moves upward from point C1 to C2 as the temperature increases from 298.15 K to 313.15 K. On the basis of Figure 3 and Figure 4, we found that the crystallization field of KH2PO4 is larger than that of KCl at the two different temperatures. Mainly because KH2PO4 has low solubility, which shows that it is easy to saturate and crystallize from solution. 3.2. Physical and Chemical Properties of the Solutions. Experimental results of physicochemical properties (pH, nD, ρ) for the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K were tabulated in Table 3 and Table 4. According to these data, relationships between physicochemical properties (pH, nD, ρ) and the mass concentration of KH2PO4 in the solution are shown in Figure 7, Figure 8, and Figure 9. From Figures 7 to 9, we can see that the physicochemical properties of the solid−liquid phase equilibrium solution vary regularly with the composition of KH2PO4 mass fraction.

Table 5. Composition of Eutonic Point for the Ternary System KH2PO4 (1) + KCl (2) + H2O at (298.15 and 313.15) K and P = 88.35 kPa T = 298.15 K

T = 313.15 K

100w(KCl)

100w(KH2PO4)

100w(KCl)

100w(KH2PO4)

this worka

23.49 24.37

3.60 3.66

27.40 26.35 27.26

4.36 4.40 4.44

ref 13

24.82 23.90

3.57 4.47

25.97

4.21

a

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.02 kPa, ur(KCl) = 0.05, ur(KH2PO4) = 0.01.

We suspect that it is possible for the previous work not to have achieved equilibrium or reached complete solid−liquid equilibrium in the ternary system at 313.15 K. It is necessary for the experiment to run for a long time for the ternary system to achieve solid−liquid equilibrium. Maybe the difference was caused by different shaking time or different analytical methods. The data in Table 3 and Table 4 were plotted in Figure 6 for comparison. In Figure 6, the solubility of KH2PO4 always

Figure 6. Phase diagram for the ternary KCl + KH2PO4 + H2O system at 298.15 K and 313.15 K and P = 88.35 kPa: □, 298.15 K; red ○, 313.15 K; C1, eutonic point of KH2PO4 and KCl at 298.15 K, E1, solubility of KCl in water at 298.15 K; F1, solubility of KH2PO4 in water at 298.15 K. C2, eutonic point of KH2PO4 and KCl at 313.15 K; E2, solubility of KCl in water at 313.15 K; F2, solubility of KH2PO4 in water at 313.15 K; 100w1, KH2PO4 wt %; 100w2, KCl wt %; , 298.15 K, ......., 313.15 K.

increases with KCl concentration decreasing, and increases with increasing temperature. When solid KH2PO4 was added into the saturated solution of KCl, the concentration of K+ increased. The dissolution equilibrium of KH2PO4 and KCl can be shown in eqs 1 and 2. The equilibrium in eq 1 shifts to the left. Thus, the solubility of KCl decreases with the further addition of KH2PO4 in the system. The results can be explained by the common ion effect at a certain temperature. In other words, KH2PO4 has an obviously inhibitive effect on the dissolution of KCl. KCl(s) ↔ K+(aq) + Cl−(aq)

(1)

KH 2PO4 (s) ↔ K+(aq) + H 2PO4 −(aq)

(2)

Figure 7. (a) Density vs composition at 298.15 K: black □, exp. value at 298.15 K; red ○, calcd. value at 298.15 K; (b) density vs composition at 313.15 K; black □, exp. value at 313.15 K; red ○, calcd. value at 313.15 K. F



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8, density and refractive index values of the solution reached the highest values at the eutonic point C1 (C2) with the increase of KH2PO4 concentration. The density of the eutonic points are 1.2165 g·cm−3 at 298.15 K and 1.2145 g·cm−3 at 313.15 K. Refractive index values of eutonic points are 1.3700 at 298.15 K and 1.3726 at 313.15 K. Figure 9 shows relationships between the pH values and KH2PO4 concentration in the solution at 298.15 K and 313.15 K. The pH values range from 6.49 to 3.96, which is mainly due to the hydrogen ion concentration increasing with the dissociation of KH2PO4 in the system, causing the system to become obviously acidic. Therefore, the pH values of this ternary system tend to decrease significantly with the increase of KH2PO4 concentration before reaching the invariant point C1 (C2), and then, it decreases slowly. 3.3. Theorized Calculation of Density and Refractive Index. On the basis of the experimental data, it can be found that density and refractive index of the ternary system vary regularly with KH2PO4 concentration, and it reaches a maximum value at the eutonic point at (298.15 and 313.15) K. According to the following empirical equations, the density values and refractive index of the solution were also calculated.28 ln(d /d0) =

∑ (Ai ·Wi )

(3)

ln(n/n0) =

∑ (Bi ·Wi )

(4)

where d and d0 refer to the density of the solution or the pure water at 298.15 K and 313.15 K; n and n0 refer to the refractive index of the solution or the pure water at 298.15 K and 313.15 K. The d0 and n0 values of the pure water at 298.15 K are 0.99707 g·cm−3 and 1.33250, respectively. The d0 and n0 values of the pure water at 313.15 K are 0.99224 g·cm−3 and 1.33061, respectively. Ai and Bi represent the constants of each possible component i in the system, and they can be obtained from the saturated solubility of the binary system at 298.15 K and 313.15 K29,30. Ai and Bi of KCl and KH2PO4 for calculation of density and refractive index of solution are shown in Table 6. Wi is the

Figure 8. (a) Refractive index vs composition at 298.15 K: black □, exp. value at 298.15 K; red ○, calcd. value at 298.15 K; (b) refractive index vs composition at 313.15 K: black □, exp. value at 313.15 K; red ○, calcd. value at 313.15 K

Table 6. Constants for Calculation of Density and Refractive Index of Equilibrium Solutions in the Ternary System at 298.15 K and 313.15 K constant

KCl

KH2PO4

Ai Bi Ai Bi

0.0063758 0.00098966 0.006565 0.0009835

0.0074084 0.0008568 0.0070529 0.0009077

T = 298.15 K T = 313.15 K

salt of i in the solution in mass fraction.Where xe and xc separately represent experimental value and calculated value, N is the number of experiment points. The RD is the relative deviation between the calculated and the experimental values, which is defined as follows: Figure 9. pH vs composition: black □, at 298.15 K; red ○, at 313.15 K.

RD =

xe − xc xe

(5)

The relative average deviation (RAD) is defined as follows:

Similar trends are shown in Figure 7 and Figure 8. In general, the density and refractive index values increase with increasing KH2PO4 concentration and then produce a decreasing trend beyond the eutonic point. As it is shown in Figure 7 and Figure

N

RAD = G

∑i = 1 |RD| N

(6) DOI: 10.1021/acs.jced.5b00113 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Table 7 and Table 8, respectively. As for the calculated results, the maximum RD is 0.0270. According to eq 6 and eq 7, the RAD and rmsd of the density and refractive index were calculated. The maximum rmsd is 0.0148. It indicates that calculated values of density and refractive index compare quite well with the other experimental values. Calculated results and experimental values of density and refractive index were plotted in Figure 7 and Figure 8, respectively. As shown in Figure 7 and Figure 8, calculated results agree with experimental values with very little differences. Thus, the empirical equations are applicable to the density and refractive index values of the equilibrium solutions in the KCl−KH2PO4−H2O system at 298.15 K and 313.15 K.

The quality of the calculated value to the experimental data was judged via the root-mean-square deviation (rmsd) value which was calculated as follows: ⎡ 1 rmsd = ⎢ ⎢⎣ N − 2

⎤1/2

N

∑ (x

e

c 2⎥

−x)

i=1

⎥⎦

(7)

A comparison of the experimental and calculated values of density and refractive index in the ternary system at (298.15 and 313.15) K were shown in Table 7 and Table 8. According Table 7. Comparison of the Experimental and Calculated Values of Density and Refractive Index in the Ternary System at 298.15 Ka density/(g·cm−3)

4. CONCLUSIONS The solid−liquid equilibria for the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K were studied by the isothermal solution saturation method and Schreinemaker’s wet residue. The ternary equilibrium data and physicochemical properties (pH, nD, ρ) of the saturation solution were obtained experimentally. On the basis of the experimental data, the phase diagrams and the diagrams of physicochemical properties vs composition in the system were plotted. The results show that the phase diagrams consist of one eutonic point, two univariant curves, and two crystallization regions corresponding to KCl and KH2PO4. According to the comparison experimental and literature data, the experimental methods are reliable in this experiment. At the same time, a large number of new experimental solubility values have been measured for the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K. The physicochemical properties of the solid−liquid phase equilibrium solution vary regularly with the concentration of KH2PO4. Solid−liquid equilibrium data are indispensable for the production of KH2PO4 by the method of extraction with industrial KCl and industrial H3PO4 aqueous mixtures.

refractive index

no.b

exp value

calcd. valuec

RD

exp value

calcd. valuec

RD

1 2 3 4 5 6 7 8 9 10 11 12

1.1800 1.1936 1.2165 1.1969 1.2007 1.1477 1.1436 1.1398 1.1396 1.1395 1.142 1.1531

1.1800 1.1901 1.2129 1.1934 1.1972 1.1443 1.1402 1.1365 1.1363 1.1362 1.1387 1.1531

0.0000 0.0109 0.0223 0.0002 0.0270 −0.0074 0.0049 0.0041 0.0090 −0.0046 0.0013 0.0000

1.3678 1.3681 1.3696 1.3700 1.3639 1.3601 1.3594 1.3578 1.356 1.3553 1.3557 1.3561

1.3678 1.3673 1.3680 1.3694 1.3636 1.3609 1.3568 1.3558 1.3537 1.3553 1.3541 1.3561

0.0000 0.0006 0.0012 0.0004 0.0002 −0.0006 0.0019 0.0015 0.0017 0.0000 0.0012 0.0000

a

Relative standard uncertainties ur are ur(nD) = 0.0002, ur(ρ) = 0.005. No. column corresponding to the no. in Table 3. cCalculated (calcd) value.

b

Table 8. Comparison of the Experimental and Calculated Values of Density and Refractive Index in the Ternary System at 313.15 Ka density/(g·cm−3) no.b

exp value

calcd valuec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1.1956 1.2001 1.2145 1.2106 1.2104 1.2070 1.1865 1.1797 1.1658 1.1653 1.1647 1.1622 1.1685 1.1730 1.1774 1.1839

1.1956 1.1908 1.2051 1.2012 1.2010 1.1976 1.1773 1.1705 1.1568 1.1563 1.1557 1.1532 1.1594 1.1639 1.1683 1.1839

■

refractive index

AUTHOR INFORMATION

Corresponding Author

RD

exp value

calcd valuec

RD

0.0000 −0.0066 −0.0085 −0.0051 −0.0116 −0.0088 −0.0063 −0.0027 −0.0130 −0.0076 −0.0127 −0.0170 −0.0019 0.0020 −0.0046 0.0000

1.3683 1.3695 1.3726 1.3708 1.371 1.3714 1.3686 1.3655 1.3631 1.3623 1.3618 1.361 1.3608 1.3605 1.3603 1.3612

1.3683 1.3700 1.3724 1.3710 1.3722 1.3709 1.3666 1.3644 1.3636 1.3621 1.3627 1.3624 1.3602 1.3599 1.3616 1.3612

0.0000 −0.0004 0.0002 −0.0001 −0.0009 0.0004 0.0014 0.0008 −0.0004 0.0001 −0.0006 −0.0010 0.0004 0.0004 −0.0009 0.0000

*E-mail: [email protected]. Tel.: (0086 0951) 3951036. Fax: (0086 0951) 3951036. Funding

This research was financially supported by the “Western Light” talent cultivation program of Chinese Academy of Sciences (CAS), 2013. Notes

The authors declare no competing financial interest.

■

REFERENCES

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a

Relative standard uncertainties ur are ur(nD) = 0.0002, ur(ρ) =0.005. No. column corresponding to the no. in Table 3. cCalculated (calcd) value.

b

to eq 5, the density relative deviation and refractive index relative deviation at (298.15 and 313.15) K were also given in H



Article

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