Solubility of KH2PO4 in KCl, H3PO4, and Their Mixture SolutionsFang 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 KCland H3PO4 is basically required for designing and optimizing the preparationprocess of KH2PO4 from KCl and H3PO4 using solvent extraction. In thisstudy, the solubility of KH2PO4 in KCl, H3PO4, and their mixture solutions hasbeen measured respectively at temperatures of (288.2, 298.2 and 308.2) K. InKCl solutions, the solubility of KH2PO4 decreases as the concentration of KClincreases before KCl begins to precipitate. In H3PO4 solutions, the solubility ofKH2PO4 increases as the concentration of H3PO4 increases. In mixed solutionsof approximately equimolar KCl and H3PO4, the solubility of KH2PO4decreases a little first and then increases remarkably with the increase ofKCl (H3PO4) concentration. In the three systems studied, the solubility ofKH2PO4 always increases with increasing temperature.

■ INTRODUCTION

KH2PO4, as a kind of nutrient-rich phosphorus potassiumcompound as well as the starting material for the syntheses ofother potassium salts, penicillin, and sodium glutamate, iswidely used in the agricultural, chemical, pharmaceutical, andfood industries.1 Several production processes such asneutralization technology, direct chemical conversion method,crystallization method, ion exchange method, and theextraction technology can be applied to produce KH2PO4.

1,2

Among them extraction technology is considered to be highlypromising because of the use of inexpensive KCl instead ofKOH, low energy consumption, and high product purity.3−5,8 Asimple process of extraction technology is shown in Figure 1.The process involves two liquid phases. The aqueous phasecontains the solutes of KCl, H3PO4, and KH2PO4. The organicphase containing chemical extractant can extract HCl from theaqueous phase selectively. Solubility data of KH2PO4 in themixed solution of KCl and H3PO4 is basically required fordesigning 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. Thispaper provides the solubility of KH2PO4 in KCl, H3PO4, andtheir mixture solutions at (288.2, 298.2 and 308.2) K,respectively.

■ EXPERIMENTAL SECTION

Materials. Materials used in this study were KCl (AR grade,≥ 99.5% purity), KH2PO4 (AR grade, ≥ 99.5% purity), andH3PO4 (AR grade, mass fraction higher than 85%). KCl andKH2PO4 were purchased from Beijing Modern EasternFinechemical, and H3PO4 was purchased from BeijingChemical Works. Solutions were prepared using deionizedwater. All chemicals were used without further purification.

Equilibration. The samples for measuring the solubility ofKH2PO4 in KCl solutions were prepared as follows: (0 to 12) gof KCl was added to 30 g of water, and the mixture was stirreduntil a transparent solution was obtained; then 12 g of KH2PO4

(over the solubility absolutely) was added to the mixture. Thesamples 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 mixturewas stirred until a transparent solution was obtained; 18 g ofKH2PO4 (over the solubility absolutely) was then added to themixture. The samples for measuring the solubility of KH2PO4

in mixed KCl and H3PO4 solutions were prepared as follows: (0to 9) g of KCl and (0 to 14) g of 85% phosphoric acid wasadded to 30 g of water, and the mixture was stirred until atransparent solution was obtained; 15 g of KH2PO4 (over thesolubility absolutely) was then added to the mixture. Inaddition, the molar concentration of KCl and H3PO4 wasapproximately the same before KH2PO4 addition, according tothe stoichiometric ratio.All these samples were all kept in a shaking thermostat water

bath at 150 rev·min−1 during our experiments. The temperatureuncertainty was ± 0.1 K.To determine the equilibrium time, several samples of the

system KCl−H3PO4−KH2PO4−H2O at 298.2 K were analyzedfor Cl− and total PO4

3− after shaking for 90 h and 4 monthsseparately. The results shown in Table 1 indicate that thedifferences of concentrations of Cl− and total PO4

3− (m(Cl)and m(Pt)) are always quite small for the same sample. So theperiod of 90 h is set as to be sufficient to reach equilibrium inour experiments.

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

Article

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

After 90 h shaking, the samples were kept in the stoppedthermostat water bath for about 24 h. And then, thesupernatant was sampled for analysis of ion concentrations;the residual solid was isolated by suction filtration, washed withethanol, 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 PO4

3− (m(Pt), mol·kg−1) were measured byion chromatography (type ICS-1100, Dionex). Samples wereanalyzed with dilution to obtain concentrations in thecalibration range. The calibration ranges are (2 to 8) ppm forK+, (6 to 24) ppm for Cl−, and (12 to 48) ppm for PO4

3−,respectively.For mixed solutions of KCl and KH2PO4, m(Cl) and m(Pt)

were measured and the concentrations of KCl and KH2PO4were calculated using eqs 1 and 2, respectively. For mixedsolutions of H3PO4 and KH2PO4, m(K) and m(Pt) weremeasured and the concentrations of KH2PO4 and H3PO4 werecalculated using eqs 3 and 4, respectively. For mixed solutionsof KCl, H3PO4, and KH2PO4, m(K), m(Cl), and m(Pt) weremeasured and the concentrations of KCl, KH2PO4 and H3PO4were 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 studymeans the mass of KH2PO4 dissolved per 100 g water. Therelative uncertainties of the concentration data in the two unitsare calculated as listed in the tables of this paper.

=m m(KCl) (Cl) (1)

=m m(KH PO ) (P )2 4 t (2)

=m m(KH PO ) (K)2 4 (3)

= −m m m(H PO ) (P ) (K)3 4 t (4)

= −m m m )(KH PO ) (K) (Cl2 4 (5)

= −

= + −

m m m

m m m

(H PO ) (P ) (KH PO )

(P ) (Cl) (K)3 4 t 2 4

t (6)

The above-mentioned data processing methods are establishedon the Law of Conservation of Matter, the assumption of totaldissociation of every salt in water, and the accordance of themeasurement of AAS and ICS. In Figure 2, with a selection of a

KCl solution, a KH2PO4 solution, and a mixed KCl andKH2PO4 solution, a comparison of K+ concentrations obtainedby two different methods is shown, where the abscissa and theordinate correspond to direct AAS measurements andcalculations by the conservation of matter using m(Cl) orm(Pt) measurements by ICS. All three points are located nearthe diagonal line, verifying the validity of our assumption andanalysis method.The residual solid from each sample was identified by

powder X-ray diffraction (type D8 Advance, Bruker). X-raypowder diffraction patterns were recorded on a diffractometer(using Cu Kα radiation) operating at 40 kV/40 mA. A scanning

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

Table 1. Temporal Study for Equilibrium Determination at298.2 K and Atmospheric Pressurea

after 90 h shaking after 4 month shaking

m(Cl) m(Pt) m(Cl) m(Pt)

sample mol·kg−1 mol·kg−1 mol·kg−1 mol·kg−1

1 0.106 1.523 0.106 1.5332 0.213 1.590 0.212 1.6033 0.314 1.683 0.318 1.6884 0.93 2.13 0.91 2.155 1.84 3.15 1.82 3.14

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

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

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

■ RESULTS AND DISCUSSION

Solubility of KH2PO4 in KCl Solutions. Solubility data ofKH2PO4 in KCl solutions at (288.2, 298.2, and 308.2) K ispresented in Table 2. We can see that for all the threetemperatures, KCl begins to precipitate together with KH2PO4

when KCl concentration increases to a limit. As KClconcentration is lower than the limit, the solubility ofKH2PO4 (the mass of KH2PO4 dissolved per 100 g water)always decreases with KCl concentration increasing, andincreases with increasing temperature.The results above can be explained by the common ion

effect. The dissolution equilibrium of KH2PO4 can be describedby

↔ ++ −KKH PO (s) (aq) H PO (aq)2 4 2 4 (7)

When solid KCl is added into saturated KH2PO4 solution anddissolved, the concentration of K+ increases, shifting theequilibrium above to the left. Hence, the solubility of KH2PO4

decreases. Therefore, KCl has an obviously inhibitive effect onthe dissolution of KH2PO4.Solubility of KH2PO4 in H3PO4 Solutions. The solubility

data of KH2PO4 in H3PO4 solutions at (288.2, 298.2, and308.2) K is presented in Table 3. The solubility of KH2PO4

(the mass of KH2PO4 dissolved per 100 g water) increases withthe increasing of H3PO4 concentration obviously.The following dissociation equilibria are frequently proposed

for H3PO4 solutions:6,710

↔ + =+ − KH PO H H PO , 0.00711 at 298.2 K3 4 2 4 1(8)

+ ↔ = −− − KH PO H PO H P O , 1.2 1.4 at 298.2 K3 4 2 4 5 2 8 2(9)

Assuming KH5P2O8 has a higher solubility than KH2PO4, eq 9could give an explanation of the promotional effect of H3PO4on the dissolution of KH2PO4. When phosphoric acid is addedinto saturated KH2PO4 solution, the equilibrium in eq 9 shiftsto the left. Thus, with the conversion of H2PO4- to H5P2O8-,the equilibrium in eq 7 shifts to the left and the solubility ofKH2PO4 is increased. Therefore, H3PO4 has an obviouslypromotional effect on the dissolution of KH2PO4.

Solubility of KH2PO4 in Mixed Solutions of KCl andH3PO4. Table 4 presents the solubility of KH2PO4 varying withthe concentration of KCl and H3PO4 at (288.2, 298.2, and308.2) K. As mentioned in the experimental section, the molarconcentration of KCl and H3PO4 was approximately the same(the concentration difference being verified as less than 0.2mol·kg−1).When the concentration of KCl (H3PO4) is low, the

solubility of KH2PO4 increases (the mass of KH2PO4 dissolvedper 100 g water) very slowly with the increase of theconcentration of KCl (H3PO4) at 288.2 K. By contrast, thesolubility of KH2PO4 is almost constant at 298.2 K and evendecreases a little at 308.2 K. However, when the concentrationof KCl (H3PO4) becomes higher, the solubility of KH2PO4always increases remarkably.These results indicate that the competition between the

promotional effect from H3PO4 addition and the inhibitiveeffect from KCl addition is highly dependent on theconcentration of KCl (H3PO4). At low concentrations, these

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 mol·kg−1 g·(100 g H2O)

−1 solid phase

288.2 K0.000 0.000 1.24 17.3 KH2PO4

0.333 2.9 0.97 15.6 KH2PO4

0.825 7.3 0.70 11.3 KH2PO4

1.65 15.1 0.46 7.7 KH2PO4

2.05 19.2 0.38 6.6 KH2PO4

2.31 22 0.33 5.6 KH2PO4

3.02 30 0.23 4.1 KCl+KH2PO4

298.2 K0.000 0.000 1.46 24 KH2PO4

0.450 4.2 1.18 20.0 KH2PO4

0.883 8.2 0.97 16.4 KH2PO4

1.73 16.4 0.63 10.8 KH2PO4

2.20 21 0.53 9.3 KH2PO4

2.48 25 0.47 8.4 KH2PO4

3.26 34 0.27 5.0 KCl+KH2PO4

308.2 K0.000 0.000 1.72 26 KH2PO4

0.325 3.1 1.42 25 KH2PO4

0.800 7.6 1.12 19 KH2PO4

1.59 15.3 0.77 13.4 KH2PO4

2.00 19.6 0.65 11.6 KH2PO4

2.26 22 0.57 10.2 KH2PO4

3.58 39 0.33 6.4 KCl+KH2PO4aThe 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.

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two effects are comparative; at high concentrations, thepromotional effect of H3PO4 is dramatically boosted anddominates the increasing of KH2PO4 solubility.The Identification of the Solid Phase. Figure 3a presents

a typical XRD pattern of the residual solid obtained in ourexperiments. We can see that all the main peaks were in goodagreement with the KH2PO4 standard data, from which we canconclude that the residual solid was high purity KH2PO4. Thiscould confirm the experiment as a determination of KH2PO4solubility. We also present a typical XRD pattern of the residualsolid containing KH2PO4 and KCl (Figure 3b).

■ CONCLUSIONSThe solubility of KH2PO4 was measured in KCl solutions (0mol·kg−1 to 3.6 mol·kg−1), H3PO4 solutions (0 mol·kg−1 to 1.7mol·kg−1) and mixed solutions of approximately equimolar KCland H3PO4 (0 mol·kg−1 to 1.9 mol·kg−1 for KCl) attemperatures of (288.2, 298.2 and 308.2) K. In KCl solution,the solubility of KH2PO4 decreases as the concentration of KClincreases before KCl begins to precipitate. In H3PO4 solution,the solubility of KH2PO4 increases as the concentration ofH3PO4 increases. In a mixed solution of approximatelyequimolar KCl and H3PO4, the variance trend of KH2PO4solubility with the KCl (H3PO4) concentration increasing iscomplex: it is small in the low KCl (H3PO4) concentration

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

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

H3PO4 KH2PO4

mol·kg−1 g·(100 g H2O)−1 mol·kg−1 g·(100 g H2O)

−1 solid phase

288.2 K0.000 0.000 1.24 21 KH2PO4

0.093 1.12 1.32 22 KH2PO4

0.197 2.43 1.37 23 KH2PO4

0.49 6.4 1.47 27 KH2PO4

0.76 10.4 1.58 30 KH2PO4

1.03 15.0 1.67 34 KH2PO4

1.33 20.6 1.76 38 KH2PO4

1.72 29 1.86 44 KH2PO4

298.2 K0.000 0.000 1.46 25 KH2PO4

0.062 0.77 1.51 26 KH2PO4

0.148 1.86 1.54 27 KH2PO4

0.205 2.61 1.55 28 KH2PO4

0.46 6.2 1.65 31 KH2PO4

0.72 10.1 1.72 34 KH2PO4

1.02 15.3 1.79 37 KH2PO4

1.29 20.5 1.89 42 KH2PO4

1.68 29 1.98 48 KH2PO4

308.2 K0.000 0.000 1.72 31 KH2PO4

0.088 1.15 1.75 32 KH2PO4

0.253 3.4 1.79 33 KH2PO4

0.51 7.2 1.82 35 KH2PO4

0.73 10.8 1.92 39 KH2PO4

1.02 15.9 2.01 44 KH2PO4

1.32 22.1 2.08 48 KH2PO4

1.68 31 2.20 56 KH2PO4aThe relative standard uncertainties ur are ur = 0.02 for concentrationof 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 forconcentration of KH2PO4 in units of g·(100 g H2O)

−1.

Table 4. Solubility of KH2PO4 in Mixed Solutions of KCland H3PO4 at (288.2, 298.2, and 308.2) K and AtmosphericPressurea

KCl H3PO4 KH2PO4

mol·kg−1g·(100gH2O)

−1 mol·kg−1g·(100gH2O)

−1 mol·kg−1g·(100gH2O)

−1solidphase

288.2 K0.000 0.000 0.000 0.000 1.24 20.5 KH2PO4

0.110 1.00 0.084 1.00 1.23 20.6 KH2PO4

0.341 3.3 0.305 3.8 1.22 21.4 KH2PO4

0.676 6.9 0.66 8.9 1.17 21.9 KH2PO4

0.93 10.1 0.91 13.0 1.13 22.4 KH2PO4

1.30 15.4 1.28 19.9 1.09 23.6 KH2PO4

1.53 19.0 1.50 25 1.03 23.3 KH2PO4

1.77 24.9 2.00 37 1.05 27 KH2PO4

298.2 K0.000 0.000 0.000 0.000 1.46 25 KH2PO4

0.060 0.57 0.065 0.80 1.46 25 KH2PO4

0.106 1.01 0.087 1.08 1.45 25 KH2PO4

0.322 3.2 0.305 3.9 1.38 25 KH2PO4

0.680 7.1 0.63 8.7 1.29 25 KH2PO4

0.913 10.1 0.88 12.8 1.25 25 KH2PO4

1.26 15.2 1.25 19.7 1.22 27 KH2PO4

1.51 19.3 1.50 25 1.16 27 KH2PO4

1.82 26.4 2.01 38 1.13 31 KH2PO4

308.2 K0.000 0.000 0.000 0.000 1.72 31 KH2PO4

0.104 1.02 0.073 0.94 1.69 30 KH2PO4

0.208 2.10 0.195 2.6 1.65 30 KH2PO4

0.315 3.2 0.326 4.4 1.61 30 KH2PO4

0.652 7.2 0.65 9.3 1.54 31 KH2PO4

0.873 10.1 0.90 13.7 1.47 31 KH2PO4

1.23 15.4 1.26 20.8 1.39 32 KH2PO4

1.44 19.2 1.45 25 1.40 34 KH2PO4

1.90 29 1.82 36 1.39 39 KH2PO4aThe relative standard uncertainties ur are ur = 0.01 for concentrationof KCl, ur = 0.02 for concentration of H3PO4 and ur = 0.02 forconcentration of KH2PO4 in units of mol·kg−1, ur = 0.03 forconcentration 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.

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

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will be helpful for designing and optimizing KH2PO4production by the extraction process.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: gsluo@tsinghua.edu.cn. Tel.: +86-10-62783870.FundingWe gratefully acknowledge the support of the National NatureScience Foundation of China (21036002) for this work.NotesThe authors declare no competing financial interest.

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