Phase Equilibrium in the Aqueous Ternary System NaH2PO4 + CO

Article pubs.acs.org/jced

Phase Equilibrium in the Aqueous Ternary System NaH2PO4 + CO(NH2)2 + H2O at 298.15 K Mei Liu,† Jian-hua Tang,* Chi Cui, Cheng Li, and Xi-zhou Chen Department of Chemical Engineering and Techonology, Sichuan University, Chengdu, Sichuan, 610065, People’s Republic of China ABSTRACT: In this study, the solubilities and densities of a ternary system (NaH2PO4 + CO(NH2)2 + H2O) at 298.15 K was investigated using isothermal solution saturation and moist residues. According to the experimental results, the phase diagram and the diagram of densities versus composition were plotted. It turned out that there were one cosaturated point (CO(NH2)2 + NaH2PO4·2H2O), two univariant curves, and two crystallization regions corresponding to NaH2PO4·2H2O and CO(NH2)2 in this ternary system. The system belonged to a simple eutectic type, in which no double salt was formed. This research filled the blank of phase diagram in this ternary system and demonstrated the absence of double salt in the system.

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INTRODUCTION

with a temperature range from 273.15 K to 373.15 K was used for phase equilibrium measurement. The temperature of this oscillator could be controlled within ± 0.3 K. A Philips X’Pert Pro MPD X-ray diffraction (XRD) analyzer was used for solidphase X-ray analysis. Experimental Methods. The solubility of this three-phase system was determined through the method of isothermal solution saturation.4−6 Under the temperature of 298.15 K (± 0.3 K), when pure NaH2PO4 was added into the saturated solution of pure CO(NH2)2, the solubility of CO(NH2)2 decreased because of the ion effect. With further addition of NaH2PO4, the CO(NH2)2 solute kept decreasing to a stop point. The solution is known as cosaturation solution, in which either NaH2PO4 or CO (NH2)2 can be dissolved anymore; thus the composition of this solution would not change under constant temperature and pressure. In phase diagrams, this point at which the solubility of solutes stays unchanged is called the invariant point. Before the solution reached the invariant point, since the method was adding pure NaH2PO4 into a saturated solution of pure urea, only NaH2PO4 was saturated in water, while the crystals remained unsaturated in the process. The method of testing the after part of this phase diagram was in reverse, which is putting pure urea into saturated solution of NaH2PO4. It was quite difficult to separate crystals from mother liquor completely. Therefore, the well-known Schreinemaker’s method came into being, which was also used in this experiment to help determining the composition of the solid phase indirectly.

Monosodium phosphate, which is also known as anhydrous monobasic sodium phosphate and sodium phosphate monobasic dehydrate, is a chemical compound of sodium with a phosphate counterion. Monosodium phosphate is an important chemical intermediate and is widely used in the fields of boiled water treatment, plating, and tanning. It is also used as a laxative and pH buffer1 when combined with other sodium phosphates. Urea, as an important nitrogen fertilizer, has many agrochemical properties, including decomposition, poly gather, and ammonia volatility. It could be applied to soil to close water and roots as much as possible to take care of young seedlings.2 It has been proven that the combination of phosphoric acid and urea could form a urea phosphate.3 However, no research on whether monosodium phosphate can react with urea to form a double salt has ever been done before, since the completely phase equilibrium data of NaH2PO4−CO (NH2)2−H2O system at 298.15 K have not been reported yet. The experiment to determine phase equilibrium data of the monosodium phosphate (NaH2PO4)−urea (CO(NH2)2)−water (H2O) system at 298.15 K could help fill in the blanks of data in this research aspect and, in the meantime, supply an accurate answer for whether urea phosphate exist in this system or not.

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METHODOLOGY Materials. Sodium phosphate monobasic dehydrate (NaH2PO4, 0.990 mass fraction) was obtained from Tianjin Bodi Chemical Holding Co. Ltd., China. Urea (CO (NH2)2, 0.990 mass fraction) was obtained from Chengdu Kelong Chemical Reagent Co. Ltd., China. Doubly deionized water (electrical conductivity ≤ 1·10−4 S·m−1) was employed in this experiment. Instruments. A constant temperature bath oscillator (SHZ88, Jintan Medical Instrument Corporation, Jiangsu, China) © 2012 American Chemical Society

Received: September 7, 2012 Accepted: November 30, 2012 Published: December 6, 2012 132

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Table 1. Mass Fraction Solubility of the Ternary NaH2PO4 + CO(NH2)2 + H2O System at Temperature T = 298.15 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 20

MSP 49.08 46.48 43.17 41.5 39.31 38.18 35.42 34.55 32.54 31.4 30.87 29.18 25.85 22.22 17.81 13.51 8.01 6.17 1.5 0

U 0 4.2 7.7 12.95 17.14 20.96 27.16 31.11 33.8 34.83 35.54 37.8 37.79 41.82 46.55 48.1 49.35 50.82 53.23 53.87

composition of wet residue, 100wb MSP d

ND 67.72 65.95 66.11 66.01 66.78 63.06 59.78 64.36 57.14 27.95 24.01 20.26 16.32 10.58 8.4 4.23 3.23 0.81 ND

densities ρ/(g·cm−3)

U

equibrium solid phase

ND 1.24 2.87 3.76 4.97 5.24 8.75 12.26 9.54 15.03 48.44 48.87 51.35 57.15 68.65 67.63 73.21 74.12 74.77 ND

MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d MSP.d+U U U U U U U U U U

c

exp. value

calcd value

relative errore

1.3807 1.3602 1.3507 1.3541 1.3447 1.337 1.3361 1.3335 1.319 1.3154 1.3256 1.3044 1.2794 1.2484 1.2257 1.1964 1.1641 1.1471 1.122 1.1128

1.3807 1.3688 1.3486 1.3481 1.3401 1.3405 1.3329 1.3360 1.3255 1.3183 1.3156 1.3069 1.2783 1.2582 1.2338 1.2029 1.1628 1.1521 1.1225 1.1128

0 0.0063 0.0015 0.0044 0.0034 −0.0026 0.0024 −0.0019 −0.0049 0.0022 0.0075 0.0019 0.0009 −0.0079 −0.0066 −0.0054 0.0011 −0.0044 −0.0005 0

Standard uncertainties u are u(T) = 0.3 K, u(p) = 0.05 MPa, u(MSP.d) = 0.01 (mass fraction), u(U) = 0.01 (mass fraction), u(ρ) = 0.01 g·mL−1. w, mass fraction. cMSP, NaH2PO4; MSP.d, NaH2PO4·2H2O; U, CO(NH2)2. dND, not determined. eRelative error = (Calcd. value − Exp. value)/ Calcd. value.

a b

rinsed 5 to 7 times to ensure that all wet residues were transferred into flask. The wet residues that have not been weighed were treated using drying oven under constant pressure and temperaturel thus dry samples were obtained for X-ray diffraction (XRD). Analysis. The P2O5 concentration was measured through the quinoline phosphomolybdate gravimetric method,7 and the average deviation of the determination was less than 0.01. The urea concentration8 was determined by means of the distillation process, and the mean deviation of the determination was less than 0.01. The density was caculated by weighing the 2 mL saturated solution, and the absolute uncertainties in the density measurements were estimated to be within 0.01 g·mL−1. The average value of three measurements was considered as the final value of each analysis.

Since the point which reflected the composition of wet residues should be on the tie line joining the composition of the pure solid and the saturated liquid in the equilibrium, the pure solid composition could be found by drawing the tie line of saturated liquid and the wet residues, and the node of this tie line and zero water line should be the pure solid point. In this way, many straight lines were drawn, and some of them had a common intersection point which represented the same composition of the pure solid phase. Experimental Procedures. A known mass of urea (CO(NH2)2), sodium dihydrogen phosphate (NaH2PO4), and doubly deionized water were loaded into a conical flask (250 mL). The flask was put into the constant temperature water bath oscillator. The oscillator vibrated continuously with temperature controlled at around 298.15 K (uncertainty, ± 0.3 K), and the temperature of the complex was monitored using a mercury thermometer. In this experiments, after 8 h of vibration under constant temperature (298.15 K), when the concentrations of both P2O5 and urea in the solution both remained unchanged for another 2 h, the oscillator was stopped, and the temperature was kept at around 298.15 K for 1.5 h to allow the system to reach equilibrium. After the system reached equilibrium, the saturated solution was removed into a 100 mL volumetric flask using a 2 mL pipet at 298.15 K. The volumetric flask was weighed before and after the solution was added. The solution was diluted with deionized water in flask immediately, and the diluted solution was weighed. The wet residues were removed into a small beaker using a scoop. Similarly, the weight of beaker was measured and recorded before and after the residues were added to determine the mass of wet residues. The residues were dissolved in deionized water afterward, and the solution was transferred to a 100 mL volumetric flask. The small beaker was

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RESULTS AND DISCUSSION

The phase equilibrium experimental data of the ternary NaH2PO4−CO (NH2)2−H2O system at 298.15 K are shown in Table 1, including the density for this system. The ion concentration values in this equilibrium system were measured in mass fraction. According to the data listed in Table 1, the ternary system phase diagram is given in Figure 1. As shown in Figure 1, W, A, and B represent H2O, pure solid of NaH2PO4, and pure solid of CO(NH2)2, respectively. Points R and S are solubility of different single salts. R represents the solubility of CO(NH2)2, which is 52.67 in mass fraction (100w). S represents the single salt mass fraction (100w) of NaH2PO4 that dissolved in pure water, which is 47.28. T is an invariant point at 298.15 K, which could reflect the cosaturated solution of NaH2PO4 and CO(NH2)2. The curve between points R and T indicate that NaH2PO4 has been saturated in the water, while CO(NH2)2 has been precipited. Additionally, 133

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Figure 2. Density vs composition.

Figure 1. Phase diagram for the ternary CO(NH2)2 + NaH2PO4 + H2O system at 298.15 K. ■, wet residue phase composition at 298.15 K; A, pure solid of NaH2PO4; B, pure solid of CO(NH2)2; W, water; R, solubility of CO(NH2)2 in water; S, solubility of NaH2PO4 in water; T, cosaturated point of CO(NH2)2 + NaH2PO4.

system at 298.15 K. wi is the salt of i in the solution in mass fraction. Constants Ai of CO(NH2)2 and NaH2PO4 for calculation of solution density are 0.002039 and 0.006633, respectively.

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the curve between points T and S presents the saturation process of CO(NH2)2 as well as the precipitation of NaH2PO4. As shown in Figure 1, the solubility curve RT links the component points of the liquid phase and wet residue phase and then extends to intersect with X and Y axes, and the intersection is approximately the solid-phase component for the CO(NH2)2. The same method was used to determine the equibrium solid-phase component of curve TS, and the result turned out to be NaH2PO4·2H2O. With the help of XRD test, the solid phases of A and B were proven to be CO(NH2)2 and NaH2PO4·2H2O, respectively. During the XRD test, every wet residue sample was dried under 298.15 K and used for XRD test to determine the existence of the double salt. It turned out that this system was a simple eutectic type. It can be seen from Figure 1 that the area of WRS represents unsaturated region at 298.15 K and RBT represents crystalline region of CO(NH2)2, while ATS stands for crystallization region of NaH2PO4·2H2O. Area ATB is the mixed crystallization region of CO(NH2)2 + NaH2PO4·2H2O. It seems that sizes of pure CO(NH2)2 and pure NaH2PO4·2H2O were equal to each other, but the pure CO(NH2)2 region is a little bit larger than pure NaH2PO4·2H2O with precise enumeration. Figure 2 shows the relationship between the density and the mass concentration of NaH2PO4 in the solution. Along with the percentage of NaH2PO4 increasing in the solution, the density tends to increase. But at the cosaturated point, which is represented by T, the curve decreases slightly afterward and then increases again. Based on the following empirical equations of density in electrolyte solutions developed in the previous study,9 the density of the solution was calculated. The experimental data were compared with the calculated data to determine the relative error. All of the data mentioned above are listed in Table 1. ln

ds0 = d0

CONCLUSIONS The equilibrium of the ternary system NaH2PO4 + CO(NH2)2 + H2O at 298.15 K was studied. The data of solubility and density of this system were obtained in this process. Based on the data achieved, the phase diagram was constructed, the solidphase which is balanced with saturated solution was detected, and crystalline regions of both solid-phases were determined. There was only one invariant point in this phase diagram, and there was no double salt formed in this ternary system. All results obtained in this experiment can provide fundamental data support in this ternary system, since no data record has not yet been found. These fundamental data could be used in future studies of separation and crystallization, and so forth.

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AUTHOR INFORMATION

Corresponding Author

*E-mail (J.T.): [email protected]. Notes

The authors declare no competing financial interest. † E-mail (M.L.): [email protected].

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REFERENCES

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∑ Ai ·wi

where d0 = 0.997044 g·cm−3, which is the density of water at 298.15 K; Ai is the constant of each possible component i in the 134

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(7) ESO/TC 47.ISO 3706-1976. Phosphoric Acid for Industrial Use (Including Foodstuffs)-Determination of Total Phosphorus (V) Oxide Content-Quinoline Phosphomolybdate Gravimetric Method; ISO Information Handing Services: Switzerland, 1976. (8) GB/T 2441.1-2001. Determination of Urea-Determination of total nitrogen content; ISO Information: China, 2001. (9) Lin, L. J.; Fang, C. H.; Fang, Y.; Qin, X. F. A new model for prediction density of electrolyte solutions. J. Salt Lake Res. 2006, 14 (2), 56−61.

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