Journal of Chemistry
Volume 2018 (2018), Article ID 8749345, 5 pages
https://doi.org/10.1155/2018/8749345

Research Article

Solid-Liquid Phase Equilibria for the Ternary System KNO₃ + KH₂PO₄ + H₂O at 283.15, 298.15, and 313.15 K

Jun-feng Wan¹ and Fei Wang²

¹College of Oil and Gas Engineering, Shengli College China University of Petroleum, Dongying, Shandong 257061, China
²Shandong ShiYi Science and Technology Co., Ltd., Dongying, Shandong 257061, China

Correspondence should be addressed to Fei Wang; wangfeiboelg@163.com

Received 28 January 2018; Accepted 14 June 2018; Published 15 July 2018

Academic Editor: Elena Gomez

Copyright © 2018 Jun-feng Wan and Fei Wang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The solid-liquid phase equilibria of the ternary system KNO₃ + KH₂PO₄ + H₂O at the temperatures of 283.15, 298.15, and 313.15 K and the pressure of  0.1 MPa are researched by isothermal solution saturation. The equilibria solid phases are researched by Schreinemakers method (wet residues). The solubility data are determined. Based on these data, the phase diagrams and the crystalline areas are determined. The phase equilibria at different temperatures are compared and discussed. All results can provide basic data support for crystallization and further researches.

1. Introduction

Salt-water phase equilibria, an important predicting implement to apply for representing the thermodynamic behavior, play a very great guiding role for the relevant process [1, 2]. KNO₃ is a source of potassium and soluble nitrogen, both of which are vital plant nutrients, and this material is widely used in agricultural and industrial fields [3]. KH₂PO₄, as an important industrial material and compound fertilizer, is widely used in agricultural, chemical, pharmaceutical, and food industry [4, 5].

The solubility data of the ternary system KNO₃ + KH₂PO₄ + H₂O can be discovered in previous researches [6, 7] and were reported about 40–80 years ago. In their experiments, different experimental ways, different apparatuses, and different analytical ways were used to obtain data. However, the details of these experiments and no more recent data are not discovered for the ternary system. As we all know, experimental conditions and analytical ways have been gradually improved. Therefore, it is essential to supply more data. At the same time, the data presented are far from enough, so an extensive research at other temperatures needs to be done. The complete phase equilibria data of the KNO₃ + KH₂PO₄ + H₂O system at 283.15 K and 313.15 K have not been reported yet. The thesis could help fill in the blank of data in the research, and new experimental data are useful for scholars and engineers to cope with the problem of obtaining and storage of manures containing potassium nitrate and phosphates. Additionally, all results can provide basic data support for industry and further theoretical studies.

2. Methodology

2.1. Materials and Apparatus

Potassium dihydrogen phosphate (KH₂PO₄, ≥0.995 mass fraction) and potassium nitrate (KNO₃, ≥0.995 mass fraction) are from Tianjin Bodi Chemical Holding Co., Ltd., China. Doubly deionized water (electrical conductivity ≤1·10⁻⁴ S·m⁻¹) is employed in the thesis. The purities and sources of the chemicals are listed in Table 1.

Table 1: Purities and suppliers of chemicals.

Chemical
Mass fraction purity
CAS number
Source

KH₂PO₄
≥0.995
7778-77-0
Tianjin Bodi Chemical Holding Co., Ltd., China

KNO₃
≥0.995
7757-79-1
Tianjin Bodi Chemical Holding Co., Ltd., China

A HZS-HA thermostatic vibrator is used to measure phase equilibria and made in Donglian Electronic & Technology Development Co., Ltd., Beijing, China.

2.2. Experimental Method

The method, isothermal solution saturation [8–10], is employed for determining the solubility data. The famous Schreinemakers method (wet residues [11–13]) is applied to analyze the composition of the equilibria solid phase.

According to a fixed ratio and making sure that one of the components is in excess, the experimental components are added into a series of conical flasks (250 mL) gradually, and the sealed flask is placed into the thermostatic vibrator. The vibrator oscillates continuously at the specific temperatures: 283.15 K, 298.15 K, and 313.15 K. In a pre-experiment, the liquid phase of the samples is analyzed at every 2 h. It is shown that the phase equilibria are reached in 10 h. After equilibria, the oscillation is stopped and the system is allowed to stand for 3 h to make sure that all the suspended crystals settle. The liquid phase and wet residues are transferred to 250 mL volumetric flasks, respectively. Finally, the samples are quantitatively analyzed by chemical ways.

More details of the experimental method and the procedure of the preparation, collection, and transfer of samples are depicted in the previous theses [8, 10].

2.3. Analysis

The concentration of H₂PO₄⁻ is determined by the quinoline phosphomolybdate gravimetric way [14] with a relative standard uncertainty of 0.01. The potassium ion is analyzed by a gravimetric way with sodium tetraphenylboron [15–17], and the relative standard uncertainty is 0.01. The experimental results are achieved from the average value of three parallel measurements.

3. Results and Discussion

In Figure 1, the experimental data are compared with the literature [10, 18] and it is discovered that the experimental values are in good agreement with the literature, which illustrates that experimental ways and devices are feasible in this thesis.

Figure 1: Solubility for KH₂PO₄ or KNO₃ in pure water at 283.15, 298.15, and 313.15 K. literary solubility of KH₂PO₄ in water [18]; experimental solubility of KH₂PO₄ in water; literary solubility of KNO₃ in water [10]; experimental solubility of KNO₃ in water.

The phase equilibria data are listed in Table 2. Based on these data, the ternary phase diagram is illustrated in Figure 2 and the phase diagrams at other temperatures are illustrated in Figure 3 and are similar to that in Figure 2.

Table 2: Mass fraction solubility of the ternary KNO₃ (1) + KH₂PO₄ (2) + H₂O system at the temperatures of  283.15, 298.15, and 313.15 K and the pressure  of 0.1 MPaa.

Number
Composition of liquid phase, 100 b
Composition of wet residue phase, 100
Equilibrium solid phase

100
100
100
100

283.15 K

1
17.29
0.00
NDc
ND
KNO₃

2
16.43
1.94
47.40
1.35
KNO₃

3
15.46
3.95
48.59
2.53
KNO₃

4
14.73
5.81
49.91
3.54
KNO₃

5
13.92
7.92
48.15
4.89
KNO₃

6
13.34
9.33
49.28
16.68
KH₂PO₄ + KNO₃

7
13.34
9.33
14.92
44.48
KH₂PO₄ + KNO₃

8
11.85
9.94
7.27
44.99
KH₂PO₄

9
9.72
10.87
5.83
46.93
KH₂PO₄

10
7.00
11.98
4.45
44.73
KH₂PO₄

11
4.70
12.89
3.13
42.97
KH₂PO₄

12
2.51
13.90
1.63
45.32
KH₂PO₄

13
0.00
15.35
ND
ND
KH₂PO₄

298.15 K

1
0.00
20.05
ND
ND
KH₂PO₄

2
3.64
17.29
1.94
59.39
KH₂PO₄

3
7.52
15.19
3.76
58.47
KH₂PO₄

4
10.59
13.63
4.95
60.07
KH₂PO₄

5
14.60
11.91
6.83
58.97
KH₂PO₄

6
17.61
10.68
8.40
57.71
KH₂PO₄

7
20.41
9.68
10.09
55.69
KH₂PO₄

8
22.16
9.07
18.06
56.70
KH₂PO₄ + KNO₃

9
22.16
9.07
53.23
16.34
KH₂PO₄ + KNO₃

10
23.33
7.00
53.67
4.30
KNO₃

11
24.33
4.97
55.24
3.03
KNO₃

12
25.96
2.11
56.11
1.35
KNO₃

13
27.25
0.00
ND
ND
KNO₃

313.15 K

1
39.30
0.00
ND
ND
KNO₃

2
37.37
2.53
60.69
1.68
KNO₃

3
34.86
5.46
59.12
3.54
KNO₃

4
33.22
7.50
55.24
18.96
KH₂PO₄ + KNO₃

5
33.22
7.50
29.03
43.56
KH₂PO₄ + KNO₃

6
31.14
8.42
17.18
49.62
KH₂PO₄

7
28.09
9.60
15.30
50.88
KH₂PO₄

8
25.40
10.56
14.61
48.53
KH₂PO₄

9
22.83
11.60
12.48
51.73
KH₂PO₄

10
19.95
12.85
11.54
49.62
KH₂PO₄

11
17.38
14.23
9.72
52.23
KH₂PO₄

12
14.55
15.75
8.09
53.33
KH₂PO₄

13
10.90
17.99
6.02
54.93
KH₂PO₄

14
7.31
20.09
3.95
56.95
KH₂PO₄

15
3.29
22.70
1.94
54.51
KH₂PO₄

16
0.00
25.32
ND
ND
KH₂PO₄

aStandard uncertainties u(T) = 0.3 K, ur(p) = 0.05, ur() = 0.01, and ur() = 0.01. b, mass fraction of KNO₃; , mass fraction of KH₂PO₄; cND, not determined.

Figure 2: Equilibrium phase diagram of the ternary system KH₂PO₄ + KNO₃ + H₂O at 313.15 K. equilibrium liquid phase composition; moist solid phase composition; A, pure solid of KH₂PO₄; B, pure solid of KNO₃; W, water; M, solubility of KH₂PO₄ in water; N, solubility of KNO₃ in water; S, cosaturated point of KH₂PO₄ and KNO₃.

(a)

(b)

Figure 3: Equilibrium phase diagram of the ternary system KH₂PO₄ + KNO₃ + H₂O at (a) 283.15 and (b) 298.15 K. equilibrium liquid phase composition; moist solid phase composition; A, pure solid of KH₂PO₄; B, pure solid of KNO₃; W, water; E and G, solubility of KH₂PO₄ in water; F and K, solubility of KNO₃ in water; T and D, cosaturated point of KNO₃ and KH₂PO₄.

In Figure 2, A, B, and W represent solid KH₂PO₄, solid KNO₃, and H₂O, respectively. Point S, an invariant point at 313.15 K, reflects the cosaturated solution of KNO₃ and KH₂PO₄. M represents the solubility of KH₂PO₄ in water at 313.15 K. N represents the solubility of KNO₃ in water at 313.15 K. The saturated liquid line MSN consists of two branches. MS corresponds to the saturated KH₂PO₄ solution and visualizes changes of KH₂PO₄ concentration with the KNO₃ concentration rising in the equilibria solution. SN corresponds to the saturated KNO₃ solution and indicates changes of KNO₃ concentration with the KH₂PO₄ concentration rising.

As indicated in Figures 2 and 3, along the curve MS, the composition points of wet residue phase and liquid phase are connected and then extended, and the intersection of the lines is the solid phase component for KH₂PO₄. The same is used for determining the equilibria solid phase component of SN, that is, KNO₃. Similarly, the equilibria solid phases of A and B at 298.15 K are KH₂PO₄ and KNO₃, respectively; the equilibria solid phases of A and B at 283.15 K are same. Consequently, the ternary system is a simple eutectic type, and neither double salt nor solid solution is formed at the investigated temperature range. In Figure 2, WMSN denotes the unsaturated region at 313.15 K. AMS represents the crystalline region of KH₂PO₄, while NSB denotes the crystalline region of KNO₃. Zone ASB denotes the mixed crystalline region of KNO₃ + KH₂PO₄. The crystalline region of KH₂PO₄ is larger than the crystalline region of KNO₃ at each researched temperature.

In Figure 4, the literature [7] and experiment data at 298.15 K are compared. A good resemblance is obtained between the literature and experimental data. But the values in the literature show a little inconsistency when compared with our data. After the original literature, maybe the difference is cause by different shaking time and analytical methods. A comparison among the phase equilibria of KNO₃ + KH₂PO₄ + H₂O at 283.15, 298.15, and 313.15 K are shown in Figure 5, which further elucidates that temperatures can affect the equilibria. Raising from 283.15 K to 313.15 K, the unsaturated area becomes larger apparently, and the crystalline region of KH₂PO₄ becomes larger, while the crystalline region of KNO₃ becomes smaller, which illustrates that it is feasible to purify the mixed solution by changing crystalline temperature. The invariant point moves right from point T, to D, to S, which elucidates that the salting-out effect of KH₂PO₄ on KNO₃ does not change significantly. The equilibria solid phase remains the same, and only anhydrous potassium nitrate and potassium dihydrogen phosphate exist as solid phases.

Figure 4: Solubility for the ternary system KH₂PO₄ + KNO₃ + H₂O at 298.15 K.

Figure 5: Solubility for the ternary system KH₂PO₄ + KNO₃ + H₂O at 283.15, 298.15, and 313.15 K. 283.15 K; 298.15 K; 313.15 K; W, T, D, and S have the same meaning as described in Figures 2 and 3.

The differences in KNO₃ and KH₂PO₄ solubility between the aqueous solutions and invariant points are presented in Figure 6. The differences in KNO₃ solubility between the aqueous solutions and invariant points are 3.95%, 5.09%, and 6.08% at 283.15, 298.15, and 313.15 K, respectively, which elucidates KH₂PO₄ has a salting-out effect on KNO₃. The differences in KH₂PO₄ solubility between the aqueous solutions and invariant points are 6.02%, 10.98%, and 17.82% at 283.15, 298.15, and 313.15 K, respectively, which elucidates that KNO₃ has a salting-out effect on KH₂PO₄ and the effect is stronger at higher temperatures. The salting-out effect of KNO₃ on KH₂PO₄ is stronger than that of KH2PO4 on KNO₃.

Figure 6: Comparison of the KH₂PO₄ and KNO₃ solubility in aqueous solutions and at cosaturated points T, D, and S.

4. Conclusions

The solid -liquid phase equilibria of KNO₃ + KH₂PO₄ + H₂O at 283.15, 298.15, and 313.15 K are researched. The solubility data are obtained. Based on these data, the phase diagrams and the crystalline areas of both solid phases are determined. There are two crystalline areas, one invariant point and two univariant curves. The ternary system is a simple eutectic type, and the crystalline area of KH₂PO₄ is larger than the crystalline area of KNO₃ at each researched temperature. Raising from 283.15 K to 313.15 K, the equilibria solid phase remains unchanged, and the crystalline area of KH₂PO₄ expands, while the crystalline area of KNO₃ decreases. KNO₃ has a salting-out effect on KH₂PO₄, and the effect is stronger at higher temperatures. The salting-out effect of KNO₃ on KH₂PO₄ is stronger than that of KH₂PO₄ on KNO₃. All results can provide basic data support for separation and further researches.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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