Phase Equilibria for the Aqueous Reciprocal Quaternary System Rb+

Jun 17, 2014 - The space phase diagram, the planar projection diagram, the water content diagram, and the diagrams of the physicochemical properties ...
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Phase Equilibria for the Aqueous Reciprocal Quaternary System Rb+, Mg2+//Cl−, Borate−H2O at 348 K Qinghong Yin,†,# Pengtao Mu,†,# Qi Tan,† Xudong Yu,† Zhongquan Li,‡ and Ying Zeng*,†,§ †

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, P. R. China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, P. R. China § Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu 610059, P. R. China ‡

ABSTRACT: Phase equilibria for the reciprocal quaternary system containing rubidium, magnesium, chloride, and borate in aqueous solution at 348 K was investigated by isothermal dissolution method. The compositions, densities, and refractive indices of the solution at equilibrium were measured experimentally. The space phase diagram, the planar projection diagram, the water content diagram, and the diagrams of the physicochemical properties (densities and refractive indices) vs composition were constructed using the measured data. Results indicate that the quaternary system is a complex type along with the double salt rubidium carnallite (RbCl·MgCl2·6H2O) formed at 348 K. The planar projection diagram consists of three invariant points, seven univariant curves and five crystallization zones corresponding to four single salts rubidium pentaborate tetrahydrate (RbB5O6(OH)4·2H2O), hungchaoite (MgB4O5(OH)4·7H2O), rubidium chloride(RbCl), bischofite(MgCl2·6H2O), and a double salt of rubidium carnallite (RbCl·MgCl2·6H2O). With a view to the crystallization zones, the crystallization zone of salt RbB5O6(OH)4·2H2O occupies the largest part, meaning rubidium borate can be more easily separated from solution than the other coexisting salts in this system at 348 K. The water content and physicochemical properties of the equilibrium solution corresponding the univariant curve EH3 change obviously with the increase of J(Mg2+), whereas the water content and physicochemical properties change only slightly on the other univariant curves. system are rarely reported. Feit9 studied the quaternary system K+, Rb+, Mg2+//Cl−−H2O at 293 K, 298 K and 373 K; Kalinkin10 investigated ternary system K+, Rb+//SO42−−H2O at 298 K; Song11 used the Pitzer equation to calculate the equilibrium solubility of ternary system Na+, Rb+//Cl−−H2O at 298 K. Some stable and metastable phase equilibria aimed at rubidium systems also have been studied by our research group, such as Li+, K+, Rb+//borate−H2O at 323 K;12 K+, Rb+, Mg2+//Cl−−H2O at 323 K;13 Li+, Rb+, Mg2+//Cl−−H2O at 323 K;14 Li+, K+, Rb+// Cl−− H2O at 298 K, 323 K and 348 K.15−17 The main compositions of the Pingluo underground brine can be simplified as the complex six-component system Li+, K+, Rb+, Mg2+//Cl−, borate−H2O.18 The quaternary system Rb+, Mg2+//Cl−, borate−H2O is one of the basic subsystems of the complex six-component system. The quaternary system Rb+, Mg2+//Cl−, borate−H2O consists of four ternary subsystems and the phase equilibria of the ternary systems have been studied in our previous work. Research results show that the ternary systems Rb+//Cl−, borate−H2O,19 Rb+, Mg2+//borate− H2O,20 and Mg2+//Cl−, borate−H2O are all of simple type, without double salt or solid solution found, whereas the ternary system Rb+, Mg2+//Cl−−H2O is of a complex type with the

1. INTRODUCTION Brine, abundant with minerals as lithium, sodium, potassium, magnesium, borate and the rare alkali element rubidium, is an important liquid mineral resource and acts as an important raw material in inorganic industry. There are different kinds of brine resources distributed in China, such as salt lake brine, gas field brine, and underground brine. Especially, Pingluo underground brine, located in the Sichuan Basin, is famous for its huge reserves and excellent quality. The Pingluo underground brine is a promising deposit of brine, with 1.77 × 1011 m3 resource reserves, containing not only extraordinarily high potassium (up to 54.66 g·L−1), but also high boron and rubidium. The content of boron (up to 4.99 g·L−1) and rubidium (up to 37.50 mg·L−1) are 32.29 and 2.75 times that of the industrial grade for comprehensive utilization, respectively.1 It is well-known that the solubility data of inorganic salts and the phase diagram play an important role in exploiting salt-lake brine and underground brine resources. For example, Teeple2 measured the phase diagrams of the aqueous system Na+, K+//Cl−, CO32−, SO42−, borate−H2O at different temperature, and the relevant phase diagrams have been used to exploit the Searles Salt Lake. Jin3−5 studied the phase equilibria of the system Na+, K+, Mg2+//Cl−, SO42−−H2O at 288 K, 298 K and 308 K, and Song6−8 investigated the quinary system Li+, Mg2+//Cl−, SO42−, borate−H2O and its subsystems at 298 K to comprehensively utilize the resources in Chaidamu Saline Lake. Up to now, the researches on salt-water systems aimed at rubidium © 2014 American Chemical Society

Received: February 26, 2014 Accepted: June 4, 2014 Published: June 17, 2014 2235

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Table 1. Solid − Liquid Equilibrium and Physicochemical Properties for the Quaternary System Rb+, Mg2+//Cl−, Borate−H2O at 348 K and Pressure p = 0.1 MPaa Composition of equilibrated solution,

Jänecke index of dry salt

w(B) × 102

J(Rb22+) + J(Mg2+) = J(Cl22−) + J(B4O72−) =100

density no.

ρ/(g·cm−3)

refractive index

1,A 2 3 4 5 6 7,H1 8 9 10 11 12 13 14 15,H2 16,B 17 18,H1 19,C 20,H2 21 22 23,H3 24,D 25 26,H3 27,E 28 29 30 31 32 33 34 35 36 37 38,H3

1.7802 1.7743 1.7129 1.7011 1.6672 1.6665 1.6098 1.6019 1.5937 1.5503 1.5352 1.5144 1.5045 1.4951 1.5094 1.6559 1.6296 1.6098 1.4908 1.5094 1.5016 1.5091 1.5284 1.5373 1.5295 1.5284 1.1424 1.1904 1.1949 1.2126 1.2129 1.2411 1.2491 1.2661 1.3122 1.3458 1.4530 1.5284

1.3936 1.3965 1.4000 1.4050 1.4069 1.4083 1.4098 1.4099 1.4102 1.4085 1.4100 1.4180 1.4136 1.4260 1.4295 1.4110 1.4085 1.4098 1.4305 1.4295 1.4304 1.4287 1.4311 1.4390 1.4350 1.4311 1.3404 1.3455 1.3462 1.3470 1.3477 1.3558 1.3603 1.3710 1.3835 1.3935 1.4165 1.4311

w(Rb+) w(Mg2+) w(Cl−) w(B4O72−) w(H2O) J(Rb22+) 39.05 36.88 34.07 30.24 26.11 25.37 23.33 21.20 19.64 14.93 8.91 6.92 3.75 2.25 0.86 25.38 24.19 23.33 0.49 0.86 0.68 0.62 0.64 0.00 0.27 0.64 1.99 2.21 2.08 2.13 2.00 2.05 1.90 1.32 1.16 0.62 0.48 0.64

0.00 0.50 1.21 2.11 3.12 3.23 3.38 3.84 4.15 5.05 5.98 8.54 7.08 8.65 8.42 3.18 3.24 3.38 9.14 8.42 9.12 8.88 9.00 9.72 9.66 9.00 0.51 0.67 0.81 1.01 1.24 1.99 2.02 3.39 4.66 5.67 6.44 9.00

15.57 16.29 17.06 18.18 19.56 19.57 19.12 19.51 19.65 20.49 20.69 27.41 21.63 25.87 24.62 19.92 19.40 19.12 26.86 24.62 26.46 25.31 25.18 26.94 26.87 25.18 0.00 0.39 0.78 1.39 2.17 4.05 4.95 8.60 12.73 15.46 17.96 25.18

1.38 1.00 1.33 1.11 0.81 0.81 0.88 1.08 1.30 0.97 0.98 0.80 1.24 0.65 0.65 0.00 0.41 0.88 0.00 0.65 0.95 1.87 2.89 3.10 3.11 2.89 5.06 5.45 5.34 5.34 4.96 5.71 3.77 4.02 2.95 2.91 2.21 2.89

44.00 45.33 46.33 48.36 50.40 51.02 53.29 54.37 55.26 58.56 63.44 56.33 66.30 62.58 65.45 51.52 52.76 53.29 63.51 65.45 62.79 63.32 62.29 60.24 60.09 62.29 92.44 91.28 90.99 90.13 89.63 86.20 87.36 82.67 78.50 75.34 72.91 62.29

100.0 91.37 79.98 67.14 54.33 52.78 49.56 43.97 40.26 29.58 17.48 10.33 7.00 3.56 1.43 53.16 51.46 49.56 0.75 1.43 1.04 0.99 1.01 0.00 0.40 1.01 35.77 31.80 26.89 23.08 18.68 12.76 11.79 5.23 3.41 1.53 1.04 1.01

J(Mg2+)

J(Cl22−) J(B4O72−)

J(H2O)

equilibrated solid phase

0.00 8.63 20.02 32.86 45.67 47.22 50.44 56.03 59.74 70.42 82.52 89.67 93.00 96.44 98.57 46.84 48.54 50.44 99.25 98.57 98.96 99.01 98.99 100.0 99.60 98.99 64.23 68.20 73.11 76.92 81.32 87.24 88.21 94.77 96.59 98.47 98.96 98.99

96.11 97.27 96.55 97.29 98.15 98.15 97.94 97.53 97.08 97.89 97.88 98.68 97.45 98.86 98.81 100.0 99.05 97.94 100.0 98.81 98.39 96.74 95.01 95.01 94.98 95.01 0.00 13.69 24.12 36.25 48.95 60.82 74.20 82.40 90.42 92.09 94.67 95.01

1070 1067 1033 1020 996 1008 1075 1071 1076 1102 1182 799 1177 942 1035 1019 1061 1075 931 1035 920 953 926 837 837 926 15 753 12 473 11 150 9272 7955 5103 5158 3118 2197 1768 1514 926

RB + RI RB + RI RB + RI RB + RI RB + RI RB + RI RB + RI + Rb-Car RB + Rb-Car RB + Rb-Car RB + Rb-Car RB + Rb-Car RB + Rb-Car RB + Rb-Car RB + Rb-Car RB + Rb-Car + Bis RI + Rb-Car RI + Rb-Car RB + RI + Rb-Car Rb-Car + Bis RB + Rb-Car + Bis RB + Bis RB + Bis RB + Bis + MB Bis + MB Bis + MB RB + Bis + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + MB RB + Bis + MB

3.89 2.73 3.45 2.71 1.85 1.85 2.06 2.47 2.92 2.11 2.12 1.32 2.55 1.14 1.19 0.00 0.95 2.06 0.00 1.19 1.61 3.26 4.99 4.99 5.02 4.99 100.0 86.31 75.88 63.75 51.05 39.18 25.80 17.60 9.58 7.91 5.33 4.99

Note: Standard uncertainties u are u(T) = 0.50 K, ur(p) = 0.05, ur(ρ) = 2.0 × 10−4 g cm−3, ur(n) = 1.0 × 10−4; ur(Rb+) = 0.0050, ur(Mg2+) = 0.0050, ur(Cl−) = 0.0030, ur(B4O72−) = 0.0030. RB, RbB5O8·4H2O; RI, RbCl; Rb-Car, RbCl·MgCl2·6H2O; Bis, MgCl2·6H2O; MB, MgB4O7·9H2O.

a

double salt RbCl·MgCl2·6H2O formed at 348 K. This work is a continuation of our previous research, the solubilities and the stable phase equilibrium of the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K were presented in details, including the physicochemical properties (densities and refractive indices) of this quaternary system.

tetrahydrate (RbB 5 O 6 (OH) 4 ·2H 2 O) 21 and hungchaoite (MgB4O5(OH)4·7H2O)22 were synthesized in our laboratory with purities higher than 99.00 % (w/w). The deionized water, with an electrical conductivity less than (1 × 10−4) S·m−1 and pH ≈ 6.60, was used in the experiments. The following instrumentations were used in this experiment: A THZ-82-type thermostatic water bath oscillator with the temperature range (RT ≈ 373 K) and temperature controlling precision ±0.5 K was used for the solubility experiments. The AL 104 type analytical balance of a resolution of 0.0002 g was applied to determine the weight of samples. The WAY type Abbe refractometer, which was conducted in a thermostat that electronically controlled the set temperature at (348 ± 0.5) K, with a precision of 0.0001 was used for the measurement of the refractive index of solution at equilibrium. The crystalline form of the solid

2. EXPERIMENTAL SECTION 2.1. Reagents and Apparatus. Magnesium chloride hexahydrate (MgCl2·6H2O; with initial purities higher than 98.00 % (w/w), final purities higher than 99.00 % (w/w) after recrystallized method) was supplied by Chengdu Kelong Chemical Reagent Plant, rubidium chloride (RbCl; with purities higher than 99.50 % (w/w)) was obtained from Jiangxi Dongpeng New Materials Co., Ltd. Rubidium pentaborate 2236

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Table 2. Composition of the Invariant Points for the Subsystems of the Quaternary System Rb+, Mg2+//Cl−, Borate−H2O at 348 K and Pressure p = 0.1 MPaa composition of equilibrated solution

Jänecke index of dry salt

w(B) × 102

J(Rb22+) + J(Mg2+)= J(Cl22−) + J(B4O72−) =100

no.

system

w(Rb+)

w(Mg2+)

w(Cl−)

w(B4O72−)

w(H2O)

J(Rb22+)

J(Mg2+)

J(Cl22−)

J(B4O72−)

J(H2O)

equilibrated solid phase

a b c d A B C D E H1 H2 H3

MgB4O7−H2O Rb2B4O7−H2O RbCl−H2O MgCl2−H2O RBI RMI RMI MBI RMB RMBI RMBI RMBI

0.00 3.53 38.42 0.00 39.05 25.38 0.49 0.00 1.99 23.33 0.86 0.64

0.04 0.00 0.00 10.06 0.00 3.18 9.14 9.72 0.51 3.38 8.42 9.00

0.00 0.00 15.94 29.32 15.57 19.92 26.86 26.94 0.00 19.12 24.62 25.18

0.25 3.21 0.00 0.00 1.38 0.00 0.00 3.10 5.06 0.88 0.65 2.89

99.71 93.26 45.64 60.62 44.00 51.52 63.51 60.24 92.44 53.29 65.45 62.29

0.00 100.0 100.0 0.00 100.0 53.16 0.75 0.00 35.77 49.56 1.43 1.01

100.0 0.00 0.00 100.0 0.00 46.84 99.25 100.0 64.23 50.44 98.57 98.99

0.00 0.00 100.0 100.0 96.11 100.0 100.0 95.01 0.00 97.94 98.81 95.01

100.0 100.0 0.00 0.00 3.89 0.00 0.00 4.99 100.0 2.06 1.19 4.99

342 949 25 057 1128 814 1070 1019 931 837 15 753 1075 1035 926

MB RB RI Bis RB + RI RI + Rb-Car Rb-Car + Bis Bis + MB MB + RB RB + RI + Rb-Car RB + Rb-Car + Bis RB + Bis + MB

Note: Standard uncertainties u are u(T) = 0.50 K, ur(p) = 0.05; ur(Rb+) = 0.0050, ur(Mg2+) = 0.0050, ur(Cl−) = 0.0030, ur(B4O72−) = 0.0030; RBI: Rb+//Cl−, borate−H2O. Notations: RMI, Rb+, Mg2+//Cl−−H2O; MBI, Mg2+//Cl−, borate−H2O; RMB, Rb+, Mg2+//borate−H2O; RMBI, Rb+, Mg2//Cl−; borate−H2O; RB, RbB5O8·4H2O; RI, RbCl, Rb-Car, RbCl·MgCl2·6H2O; Bis, MgCl2·6H2O; MB, MgB4O7·9H2O. a

phase was confirmed by the DX-2700 X-ray diffractometer with Cu Kα radiation, under the operation conditions 40 kV and 30 mA. 2.2. Experimental Method. The isothermal dissolution method was employed to investigate the phase equilibrium.12 The system points for the quaternary system were obtained by adding the third component gradually based on the ternary saturation points at 348 K. A series of artificial brine samples were put into the tightly sealed glass bottles and the bottles were placed in the THZ-82 type thermostatic water bath oscillator with the temperature (348 ± 0.5 K) and a constant oscillation frequency (120 rpm) to accelerate equilibration. The clarifying solutions were taken out periodically for chemical analysis; when the composition of the liquid sample remained constant, the equilibrium was reached. Experimental results show that the time to reach equilibria is more than 4 weeks with stirring. After equilibrium, the liquid and solid phases were separated by filtration at 348 K. The compositions of the liquid phase were measured by chemical analysis; the solid phases were dried at 348 K, and then determined by X-ray diffractometer. The physicochemical properties (densities and refractive indices) of the liquid phase were also determined. 2.3. Analytical Methods.23 Each sample was measured three times, and the average value of three measurements was considered as the final value of the analysis. The concentration of Rb+ was analyzed by sodium tetraphenylborate (STPB)− hexadecyl trimethylammonium bromide (CTAB) back-titration with a precision of ± 0.5 %; The concentration of Mg2+ was measured by ethylene diamine tetraacetic acid titration in the presence of mixed K−B(Acid chrome blue K − Naphthol green B) with a precision of ± 0.5 %; The composition of Cl− was determined by AgNO3 volumetric method with a precision of ± 0.3 %, and the composition of borate was determined by neutralization titration in the presence of mannitol with a precision of ± 0.3 %.

Figure 1. Stereo diagram of the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K.

corresponding solid phase of the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K are listed in Table 1. The component corresponding to the invariant points of the binary and ternary subsystems of this quaternary system were tabulated in Table 2. In Tables 1 and 2, the concentration of solution component was expressed in mass fraction w(B) (with w(Rb+) + w(Mg2+) + w(Cl−) + w(B4O72−) + w(H2O) = 1) and the Jänecke index was expressed by J(B) (with J(Rb22+) + J(Mg2+) = J(Cl22−) + J(B4O72−) =100). B can be Rb+, Mg2+, Cl−, B4O72− or H2O. The data of the Jänecke index ought to comply with the following correlations.

3. RESULTS AND DISCUSSION The compositions and physicochemical properties (densities and refractive indices) of the equilibrium solution and its 2237

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On the basis of the Jänecke index in Tables 1 and 2, the space phase diagram of the quaternary system at 348 K was constructed in Figure 1. Figure 2 is the planar projection diagram of Figure 1. There are four binary and four ternary subsystems of the quaternary system Rb+, Mg2+//Cl−, borate−H2O. In Figure 1, points a, b, c, and d are invariant points of the binary subsystems; Points A, B, C, D, and E are invariant points of the ternary subsystems; Points H1, H2, and H3 are invariant points of the quaternary system Rb+, Mg2+//Cl−, borate−H2O. It is well-known that borates often exist as all kinds of polyanions in solution, such as B(OH)4−, B4O5(OH)42−, B3O3(OH)4−, B5O6(OH)4− et al., and they can be crystallized in different solid forms.24 The crystallographic forms of the solid phases were determined by the X-ray diffraction method, which shows that the crystallographic forms of rubidium borate and magnesium borate of this quaternary system at 348 K are RbB5O6(OH)4·2H2O and MgB4O5(OH)4·7H2O, and their corresponding molecular formula can be abbreviated to RbB5O8·4H2O and MgB4O7·9H2O, respectively. As shown in Figures 1 and 2, this system is of a complex type with the double salt RbCl·MgCl2·6H2O formed at 348 K. The stable phase diagram consists of five crystallization zones, seven univariant curves, and three invariant points. The five crystallization zones correspond to salts RbB5O8·4H2O, MgB4O7·9H2O, RbCl, RbCl·MgCl2·6H2O, and MgCl2·6H2O. The crystallization zones of salt MgCl2·6H2O is the smallest, and the crystallization zone of salt RbB5O8·4H2O is the largest, which shows that MgCl2·6H2O has the largest solubility in water than the other salts, and RbB5O8·4H2O can be easier to separate from solution in this system at 348 K. The seven univariant curves, namely, curves AH1, BH1, H1H2, CH2, H2H3, DH3, and EH3, coexist with two salts and an equilibrated solution, respectively. The three invariant points H1, H2, and H3 are cosaturated with three salts and one equilibrated liquid, and they are all commensurate invariant points according to the phase rule. Figures 3 to 5 show the X-ray diffraction pattern of invariant points H1, H2, and H3, respectively. As shown in Figures 3 to 5,

Figure 2. Planar projection diagram of the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K. ●, experimental point; , univariant curve.

Letting w(Mg 2 +) w(Rb+) 1 × + 2 85.47 24.30 w(B4 O27 −) w(Cl‐) 1 = × + 2 35.45 155.24

[M] =

(1)

+

J(Rb22 +) =

w(Rb ) 1 × × 100 2 85.47[M]

(2)

J(Cl 22 −) =

w(Cl−) 1 × × 100 2 35.45[M]

(3)

J(H 2O) =

w(H 2O) × 100 18.02[M]

(4)

Figure 3. X-ray diffraction pattern of the invariant point H1 (RbCl·MgCl2·6H2O, RbCl, and RbB5O8·4H2O). 2238

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Figure 4. X-ray diffraction pattern of the invariant point H2 (MgCl2·6H2O, RbB5O8·4H2O, and RbCl·MgCl2·6H2O).

Figure 5. X-ray diffraction pattern of the invariant point H3 (MgCl2·6H2O, RbB5O8·4H2O, and MgB4O7·9H2O).

w(H2O) = 53.29 %. At the invariant point H2, the mass fraction composition of the equilibrated solution is w(Rb+) = 0.86 %, w(Mg2+) = 8.42 %, w(Cl−) = 24.62 %, w(B4O72−) = 0.65 %, w(H2O) = 65.45 %. At the invariant point H3, the mass fraction composition of the equilibrated solution is w(Rb+) = 0.64 %, w(Mg2+) = 9.00 %, w(Cl−) = 25.18 %, w(B4O72−) = 2.89 %, w(H2O) = 62.29 %. Figure 6 is the water content diagram of the system at 348 K, with the J(H2O) as ordinate, and the J(Mg2+) as abscissa. On the univariant curve EH3, the water content changes obviously with the increase of J(Mg2+) and reaches the minimum at the point H3, whereas the changes of water content become slight on the other univariant curves. Density and refractive index value are basic and important physicochemical property of electrolyte solutions and always change with the temperature and composition of solution.

the abscissa ordinate is the 2θ from 10° to 60°; the vertical ordinate is the intensity. The XRD pattern of the invariant H1 shown in Figure 3 was well matched to the standard diffraction pattern of RbB5O8·4H2O, RbCl, and RbCl·MgCl2·6H2O with powder diffraction file (pdf) number“43-0415″, “06-0829″, “801703″, respectively. It shows that salts RbB5O8·4H2O, RbCl, and RbCl·MgCl2·6H2O coexist at the invariant point H1. Similarly, as shown in Figures 4 and 5, salts RbB5O8·4H2O, RbCl·MgCl2·6H2O, and MgCl2·6H2O coexist at the invariant point H2 and salts RbB5O8·4H2O, MgCl2·6H2O, and MgB4O7· 9H2O coexist at the invariant point H3, respectively. The mass fraction composition of the equilibrated solution corresponding to invariant points H1, H2, and H3 are listed as follow. At the invariant point H1, the mass fraction composition of the equilibrated solution is w(Rb+) = 23.33 %, w(Mg2+) = 3.38 %, w(Cl−) = 19.12 %, w(B4O72−) = 0.88 %, 2239

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Figure 6. Water-content diagram of the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K. ●, experimental value; , experimental relationship curve.

Figure 8. Refractive indices vs composition diagram for the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K. ●, experimental value; , experimental relationship curve.

The densities and refractive indices of this quaternary at 348 K were determined, and the data are shown in Table 1. On the basis of the experimental data in Table 1, the relationship between physicochemical properties and the composition of magnesium ions in solution were constructed in Figures 7 and 8.

4. CONCLUSION The compositions, densities, and refractive indices of the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K were investigated by the isothermal dissolution method. The quaternary system is a complex type with the double salt RbCl·MgCl2·6H2O formed at 348 K. The phase diagram consists of five crystallization zones, seven univariant curves, and three invariant points. Three invariant points, cosaturated with three salts and one equilibrium solution, are all commensurate invariant points. The crystallographic forms of rubidium borate and magnesium borate in this quaternary system at 348 K are RbB5O6(OH)4·2H2O and MgB4O5(OH)4·7H2O, and their corresponding molecular formula can be abbreviated to RbB5O8·4H2O and MgB4O7·9H2O. Salt RbB5O8·4H2O has the largest crystallization zone and presents the smallest solubility among the coexisting salts, which shows that it can be more easily separated from solution than the other coexisting salts in this system at 348 K. The physicochemical properties of the equilibrium solution change regularly with the Jänecke index of magnesium ions.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-28-84078940. Fax: 86-28-84079074.

Figure 7. Densities vs composition diagram for the quaternary system Rb+, Mg2+//Cl−, borate−H2O at 348 K. ●, experimental value; , experimental relationship curve.

Funding

Research funding for this work from the National Natural Science Foundation (41173071) and Key Foundation (41030426) of China, National High Technology Research and Development Program of China (2012AA061704), China Geological Survey (12120113087700), the Research Fund for the Doctoral Program of Higher Education from the Ministry of Education of China (20115122110001), and the Sichuan youth science and technology innovation research team funding scheme (2013TD0005) is gratefully acknowledged.

Figure 7 is the density versus composition diagram of the equilibrated solution. The density decreases with the increase of J(Mg2+) on univariant curves except for curve EH3. On the univariant curve EH3, the density sharply increases with the increase of J(Mg2+), which is because of the higher solubility of MgCl2·6H2O than MgB4O7·9H2O at 348 K. Figure 8 is the refractive index versus composition of the solution at equilibrium. As the change of J(Mg2+), the refractive index of the solution regularly changes. On the univariant curve EH3, the refractive index increases obviously until it reaches the maximum at the invariant point H3.

Notes

The authors declare no competing financial interest. # Q.Y. and P.M. are co-first authors. 2240

dx.doi.org/10.1021/je5001928 | J. Chem. Eng. Data 2014, 59, 2235−2241

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dx.doi.org/10.1021/je5001928 | J. Chem. Eng. Data 2014, 59, 2235−2241