Solubility of KH2PO4 in KCl, H3PO4, and Their Mixture Solutions

Article pubs.acs.org/jced

Solubility of KH2PO4 in KCl, H3PO4, and Their Mixture Solutions Fang Zhao, Yangcheng Lu, Kai Wang, and Guangsheng Luo* State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R. China ABSTRACT: The solubility data of KH2PO4 in the mixed solution of KCl and H3PO4 is basically required for designing and optimizing the preparation process of KH2PO4 from KCl and H3PO4 using solvent extraction. In this study, the solubility of KH2PO4 in KCl, H3PO4, and their mixture solutions has been measured respectively at temperatures of (288.2, 298.2 and 308.2) K. In KCl solutions, the solubility of KH2PO4 decreases as the concentration of KCl increases before KCl begins to precipitate. In H3PO4 solutions, the solubility of KH2PO4 increases as the concentration of H3PO4 increases. In mixed solutions of approximately equimolar KCl and H3PO4, the solubility of KH2PO4 decreases a little first and then increases remarkably with the increase of KCl (H3PO4) concentration. In the three systems studied, the solubility of KH2PO4 always increases with increasing temperature.

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Equilibration. The samples for measuring the solubility of KH2PO4 in KCl solutions were prepared as follows: (0 to 12) g of KCl was added to 30 g of water, and the mixture was stirred until a transparent solution was obtained; then 12 g of KH2PO4 (over the solubility absolutely) was added to the mixture. The samples for measuring the solubility of KH2PO4 in H3PO4 solutions were prepared as follows: (0 to 11) g of 85% phosphoric acid was added to 30 g of water, and the mixture was stirred until a transparent solution was obtained; 18 g of KH2PO4 (over the solubility absolutely) was then added to the mixture. The samples for measuring the solubility of KH2PO4 in mixed KCl and H3PO4 solutions were prepared as follows: (0 to 9) g of KCl and (0 to 14) g of 85% phosphoric acid was added to 30 g of water, and the mixture was stirred until a transparent solution was obtained; 15 g of KH2PO4 (over the solubility absolutely) was then added to the mixture. In addition, the molar concentration of KCl and H3PO4 was approximately the same before KH2PO4 addition, according to the stoichiometric ratio. All these samples were all kept in a shaking thermostat water bath at 150 rev·min−1 during our experiments. The temperature uncertainty was ± 0.1 K. To determine the equilibrium time, several samples of the system KCl−H3PO4−KH2PO4−H2O at 298.2 K were analyzed for Cl− and total PO43− after shaking for 90 h and 4 months separately. The results shown in Table 1 indicate that the differences of concentrations of Cl− and total PO43− (m(Cl) and m(Pt)) are always quite small for the same sample. So the period of 90 h is set as to be sufficient to reach equilibrium in our experiments.

INTRODUCTION KH2PO4, as a kind of nutrient-rich phosphorus potassium compound as well as the starting material for the syntheses of other potassium salts, penicillin, and sodium glutamate, is widely used in the agricultural, chemical, pharmaceutical, and food industries.1 Several production processes such as neutralization technology, direct chemical conversion method, crystallization method, ion exchange method, and the extraction technology can be applied to produce KH2PO4.1,2 Among them extraction technology is considered to be highly promising because of the use of inexpensive KCl instead of KOH, low energy consumption, and high product purity.3−5,8 A simple process of extraction technology is shown in Figure 1. The process involves two liquid phases. The aqueous phase contains the solutes of KCl, H3PO4, and KH2PO4. The organic phase containing chemical extractant can extract HCl from the aqueous phase selectively. Solubility data of KH2PO4 in the mixed solution of KCl and H3PO4 is basically required for designing and optimizing the process. The solubility of KH2PO4 in various systems has been studied.9,11,12 However, the solubility of KH2PO4 in KCl, H3PO4, and their mixture solutions has not been reported. This paper provides the solubility of KH2PO4 in KCl, H3PO4, and their mixture solutions at (288.2, 298.2 and 308.2) K, respectively.

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EXPERIMENTAL SECTION

Materials. Materials used in this study were KCl (AR grade, ≥ 99.5% purity), KH2PO4 (AR grade, ≥ 99.5% purity), and H3PO4 (AR grade, mass fraction higher than 85%). KCl and KH2PO4 were purchased from Beijing Modern Eastern Finechemical, and H3PO4 was purchased from Beijing Chemical Works. Solutions were prepared using deionized water. All chemicals were used without further purification. © 2014 American Chemical Society

Received: October 12, 2013 Accepted: January 15, 2014 Published: January 29, 2014 439

dx.doi.org/10.1021/je400911m | J. Chem. Eng. Data 2014, 59, 439−443

Journal of Chemical & Engineering Data

Article

Figure 1. A schematic diagram of extraction technology for KH2PO4 production.

Table 1. Temporal Study for Equilibrium Determination at 298.2 K and Atmospheric Pressurea after 90 h shaking m(Cl) sample 1 2 3 4 5

mol·kg

−1

0.106 0.213 0.314 0.93 1.84

after 4 month shaking

m(Pt) mol·kg

−1

1.523 1.590 1.683 2.13 3.15

m(Cl) mol·kg

−1

0.106 0.212 0.318 0.91 1.82

(4)

m(KH 2PO4 ) = m(K) − m(Cl)

(5)

m(H3PO4 ) = m(Pt ) − m(KH 2PO4 )

m(Pt) mol·kg

m(H3PO4 ) = m(Pt ) − m(K)

−1

= m(Pt ) + m(Cl) − m(K)

1.533 1.603 1.688 2.15 3.14

(6)

The above-mentioned data processing methods are established on the Law of Conservation of Matter, the assumption of total dissociation of every salt in water, and the accordance of the measurement of AAS and ICS. In Figure 2, with a selection of a

a The relative standard uncertainties ur are ur[m(Cl)] = 0.01, ur[m(Pt)] = 0.005.

After 90 h shaking, the samples were kept in the stopped thermostat water bath for about 24 h. And then, the supernatant was sampled for analysis of ion concentrations; the residual solid was isolated by suction filtration, washed with ethanol, and air-dried for XRD characterizations. Analytics. The concentration of K+ (m(K), mol·kg−1) was measured by atomic absorption spectrometry (type Z-5000, Hitachi High-Technologies). The concentration of Cl− (m(Cl), mol·kg−1) and total PO43− (m(Pt), mol·kg−1) were measured by ion chromatography (type ICS-1100, Dionex). Samples were analyzed with dilution to obtain concentrations in the calibration range. The calibration ranges are (2 to 8) ppm for K+, (6 to 24) ppm for Cl−, and (12 to 48) ppm for PO43−, respectively. For mixed solutions of KCl and KH2PO4, m(Cl) and m(Pt) were measured and the concentrations of KCl and KH2PO4 were calculated using eqs 1 and 2, respectively. For mixed solutions of H3PO4 and KH2PO4, m(K) and m(Pt) were measured and the concentrations of KH2PO4 and H3PO4 were calculated using eqs 3 and 4, respectively. For mixed solutions of KCl, H3PO4, and KH2PO4, m(K), m(Cl), and m(Pt) were measured and the concentrations of KCl, KH2PO4 and H3PO4 were calculated using eqs 1, 5, and 6. The concentration data in this study is presented in the units of both mol·kg−1 and g solute·(100 g H2O)−1. The latter one is used because KH2PO4 solubility as we discuss in this study means the mass of KH2PO4 dissolved per 100 g water. The relative uncertainties of the concentration data in the two units are calculated as listed in the tables of this paper. m(KCl) = m(Cl)

(1)

m(KH 2PO4 ) = m(Pt )

(2)

m(KH 2PO4 ) = m(K)

(3)

Figure 2. Verification of validity of our assumption and analysis method. ●, KCl solution; ▲, KH2PO4 solution; ◆, mixed solution of KCl and KH2PO4. The straight line is y = x.

KCl solution, a KH2PO4 solution, and a mixed KCl and KH2PO4 solution, a comparison of K+ concentrations obtained by two different methods is shown, where the abscissa and the ordinate correspond to direct AAS measurements and calculations by the conservation of matter using m(Cl) or m(Pt) measurements by ICS. All three points are located near the diagonal line, verifying the validity of our assumption and analysis method. The residual solid from each sample was identified by powder X-ray diffraction (type D8 Advance, Bruker). X-ray powder diffraction patterns were recorded on a diffractometer (using Cu Kα radiation) operating at 40 kV/40 mA. A scanning 440



Article

Table 2. Solubility of KH2PO4 in KCl Solutions at (288.2, 298.2, and 308.2) K and Atmospheric Pressurea KCl

KH2PO4

mol·kg−1

g·(100 g H2O)−1

0.000 0.333 0.825 1.65 2.05 2.31 3.02

0.000 2.9 7.3 15.1 19.2 22 30

0.000 0.450 0.883 1.73 2.20 2.48 3.26

0.000 4.2 8.2 16.4 21 25 34

0.000 0.325 0.800 1.59 2.00 2.26 3.58

0.000 3.1 7.6 15.3 19.6 22 39

mol·kg−1 288.2 K 1.24 0.97 0.70 0.46 0.38 0.33 0.23 298.2 K 1.46 1.18 0.97 0.63 0.53 0.47 0.27 308.2 K 1.72 1.42 1.12 0.77 0.65 0.57 0.33

g·(100 g H2O)−1

solid phase

17.3 15.6 11.3 7.7 6.6 5.6 4.1

KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KCl+KH2PO4

24 20.0 16.4 10.8 9.3 8.4 5.0


26 25 19 13.4 11.6 10.2 6.4


a The relative standard uncertainties ur are ur = 0.01 for concentration of KCl and ur = 0.02 for concentration of KH2PO4 in units of mol·kg−1, ur = 0.03 for concentration of KCl and ur = 0.04 for concentration of KH2PO4 in units of g·(100 g H2O)−1.

rate of 0.02°·s−1 was applied to record the patterns in the 2θ angle range from 10° to 90°

H3PO4 ↔ H+ + H 2PO−4 ,

(8)

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H3PO4 +

RESULTS AND DISCUSSION Solubility of KH2PO4 in KCl Solutions. Solubility data of KH2PO4 in KCl solutions at (288.2, 298.2, and 308.2) K is presented in Table 2. We can see that for all the three temperatures, KCl begins to precipitate together with KH2PO4 when KCl concentration increases to a limit. As KCl concentration is lower than the limit, the solubility of KH2PO4 (the mass of KH2PO4 dissolved per 100 g water) always decreases with KCl concentration increasing, and increases with increasing temperature. The results above can be explained by the common ion effect. The dissolution equilibrium of KH2PO4 can be described by KH 2PO4 (s) ↔ K +(aq) + H 2PO−4 (aq)

K1 = 0.00711 at 298.2 K

H 2PO−4

↔

H5P2O−8 ,

K 2 = 1.2−1.4 at 298.2 K (9)

Assuming KH5P2O8 has a higher solubility than KH2PO4, eq 9 could give an explanation of the promotional effect of H3PO4 on the dissolution of KH2PO4. When phosphoric acid is added into saturated KH2PO4 solution, the equilibrium in eq 9 shifts to the left. Thus, with the conversion of H2PO4- to H5P2O8-, the equilibrium in eq 7 shifts to the left and the solubility of KH2PO4 is increased. Therefore, H3PO4 has an obviously promotional effect on the dissolution of KH2PO4. Solubility of KH2PO4 in Mixed Solutions of KCl and H3PO4. Table 4 presents the solubility of KH2PO4 varying with the concentration of KCl and H3PO4 at (288.2, 298.2, and 308.2) K. As mentioned in the experimental section, the molar concentration of KCl and H3PO4 was approximately the same (the concentration difference being verified as less than 0.2 mol·kg−1). When the concentration of KCl (H3PO4) is low, the solubility of KH2PO4 increases (the mass of KH2PO4 dissolved per 100 g water) very slowly with the increase of the concentration of KCl (H3PO4) at 288.2 K. By contrast, the solubility of KH2PO4 is almost constant at 298.2 K and even decreases a little at 308.2 K. However, when the concentration of KCl (H3PO4) becomes higher, the solubility of KH2PO4 always increases remarkably. These results indicate that the competition between the promotional effect from H3PO4 addition and the inhibitive effect from KCl addition is highly dependent on the concentration of KCl (H3PO4). At low concentrations, these

(7)

When solid KCl is added into saturated KH2PO4 solution and dissolved, the concentration of K+ increases, shifting the equilibrium above to the left. Hence, the solubility of KH2PO4 decreases. Therefore, KCl has an obviously inhibitive effect on the dissolution of KH2PO4. Solubility of KH2PO4 in H3PO4 Solutions. The solubility data of KH2PO4 in H3PO4 solutions at (288.2, 298.2, and 308.2) K is presented in Table 3. The solubility of KH2PO4 (the mass of KH2PO4 dissolved per 100 g water) increases with the increasing of H3PO4 concentration obviously. The following dissociation equilibria are frequently proposed for H3PO4 solutions:6,710 441



Article

Table 3. Solubility of KH2PO4 in H3PO4 Solutions at (288.2, 298.2, and 308.2) K and Atmospheric Pressurea H3PO4 mol·kg−1

g·(100 g H2O)−1

0.000 0.093 0.197 0.49 0.76 1.03 1.33 1.72

0.000 1.12 2.43 6.4 10.4 15.0 20.6 29

0.000 0.062 0.148 0.205 0.46 0.72 1.02 1.29 1.68

0.000 0.77 1.86 2.61 6.2 10.1 15.3 20.5 29

0.000 0.088 0.253 0.51 0.73 1.02 1.32 1.68

0.000 1.15 3.4 7.2 10.8 15.9 22.1 31

Table 4. Solubility of KH2PO4 in Mixed Solutions of KCl and H3PO4 at (288.2, 298.2, and 308.2) K and Atmospheric Pressurea

KH2PO4 mol·kg−1 288.2 K 1.24 1.32 1.37 1.47 1.58 1.67 1.76 1.86 298.2 K 1.46 1.51 1.54 1.55 1.65 1.72 1.79 1.89 1.98 308.2 K 1.72 1.75 1.79 1.82 1.92 2.01 2.08 2.20

g·(100 g H2O)−1

solid phase

21 22 23 27 30 34 38 44

KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4

25 26 27 28 31 34 37 42 48

KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4 KH2PO4

31 32 33 35 39 44 48 56


KCl mol·kg−1

g·(100g H2O)−1

mol·kg−1

0.000 0.110 0.341 0.676 0.93 1.30 1.53 1.77

0.000 1.00 3.3 6.9 10.1 15.4 19.0 24.9

0.000 0.084 0.305 0.66 0.91 1.28 1.50 2.00

0.000 0.060 0.106 0.322 0.680 0.913 1.26 1.51 1.82

0.000 0.57 1.01 3.2 7.1 10.1 15.2 19.3 26.4

0.000 0.065 0.087 0.305 0.63 0.88 1.25 1.50 2.01

0.000 0.104 0.208 0.315 0.652 0.873 1.23 1.44 1.90

0.000 1.02 2.10 3.2 7.2 10.1 15.4 19.2 29

0.000 0.073 0.195 0.326 0.65 0.90 1.26 1.45 1.82

a

The relative standard uncertainties ur are ur = 0.02 for concentration of H3PO4 and ur = 0.02 for concentration of KH2PO4 in units of mol· kg−1, ur = 0.04 for concentration of H3PO4 and ur = 0.04 for concentration of KH2PO4 in units of g·(100 g H2O)−1.

H3PO4 g·(100g H2O)−1 288.2 K 0.000 1.00 3.8 8.9 13.0 19.9 25 37 298.2 K 0.000 0.80 1.08 3.9 8.7 12.8 19.7 25 38 308.2 K 0.000 0.94 2.6 4.4 9.3 13.7 20.8 25 36

KH2PO4 mol·kg−1

g·(100g H2O)−1

solid phase

1.24 1.23 1.22 1.17 1.13 1.09 1.03 1.05

20.5 20.6 21.4 21.9 22.4 23.6 23.3 27


1.46 1.46 1.45 1.38 1.29 1.25 1.22 1.16 1.13

25 25 25 25 25 25 27 27 31


1.72 1.69 1.65 1.61 1.54 1.47 1.39 1.40 1.39

31 30 30 30 31 31 32 34 39


a

The relative standard uncertainties ur are ur = 0.01 for concentration of KCl, ur = 0.02 for concentration of H3PO4 and ur = 0.02 for concentration of KH2PO4 in units of mol·kg−1, ur = 0.03 for concentration of KCl, ur = 0.04 for concentration of H3PO4, and ur = 0.04 for concentration of KH2PO4 in units of g·(100 g H2O)−1.

two effects are comparative; at high concentrations, the promotional effect of H3PO4 is dramatically boosted and dominates the increasing of KH2PO4 solubility. The Identification of the Solid Phase. Figure 3a presents a typical XRD pattern of the residual solid obtained in our experiments. We can see that all the main peaks were in good agreement with the KH2PO4 standard data, from which we can conclude that the residual solid was high purity KH2PO4. This could confirm the experiment as a determination of KH2PO4 solubility. We also present a typical XRD pattern of the residual solid containing KH2PO4 and KCl (Figure 3b).

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CONCLUSIONS The solubility of KH2PO4 was measured in KCl solutions (0 mol·kg−1 to 3.6 mol·kg−1), H3PO4 solutions (0 mol·kg−1 to 1.7 mol·kg−1) and mixed solutions of approximately equimolar KCl and H3PO4 (0 mol·kg−1 to 1.9 mol·kg−1 for KCl) at temperatures of (288.2, 298.2 and 308.2) K. In KCl solution, the solubility of KH2PO4 decreases as the concentration of KCl increases before KCl begins to precipitate. In H3PO4 solution, the solubility of KH2PO4 increases as the concentration of H3PO4 increases. In a mixed solution of approximately equimolar KCl and H3PO4, the variance trend of KH2PO4 solubility with the KCl (H3PO4) concentration increasing is complex: it is small in the low KCl (H3PO4) concentration

Figure 3. XRD patterns. (a) residual KH2PO4; (b) residual solid containing KH2PO4 and KCl: ◆, KH2PO4 standard data; +, KCl standard data.

region, but becomes increases remarkably in the high KCl (H3PO4) concentration region. These observations and data 442



Article

will be helpful for designing and optimizing KH 2 PO 4 production by the extraction process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-62783870. Funding

We gratefully acknowledge the support of the National Nature Science Foundation of China (21036002) for this work. Notes

The authors declare no competing financial interest.

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REFERENCES

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Solubility of KH2PO4 in KCl, H3PO4, and Their Mixture Solutions

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