Density, pH, and Boron Species in the Ternary System NaBO2–Na2SO4–H2O at 298.15K and 323.15K

/e densities and pH values in the system NaBO2–Na2SO4–H2O at 298.15K and 323.15K were investigated. Combining the equilibrium constants for different boron species, the distributions of six boron species in the mixed solution were calculated with total boron concentration and pH values. /e molar fractions of the six boron species are mainly affected by the total boron concentration and temperature, but rarely affected by the concentration of SO4. /e dominant boron species in the mixed solution at the two temperatures is B(OH)4. /e mole fraction of B(OH)3, B5O6(OH)4, and B3O3(OH)4 can be neglected. /e polyborate ions are easier to form as the temperature increases./e results of distribution for boron species in this study and those with the Pitzer model can both be used to describe the distribution of boron species in the mixed solution.


Introduction
e Qaidam Basin in Qinghai Province is rich in boron resources. Boron is usually distributed in liquid brine boron deposits and solid borate deposits [1]. With the continuous development of social progress and high-tech, borates are widely used in glass production, agricultural and sideline products, and pharmaceuticals, and the application of boron-containing compounds is in aerospace and defense construction [2]. Borates are gaining more and more attention, and global demand for boron will continue to grow over time [3]. In chemical engineering, the formation conditions and transformation rules of borate and hydrated borate crystals were expounded, and borate minerals were comprehensively developed and utilized to select the optimal extraction process [4]. In the last century, high-quality solid boron deposits were almost depleted. erefore, it is urgent to solve the problem of developing liquid boron resources in western China as soon as possible. Aqueous boron species can be found in natural waters, including seawater, salt lakes, oilfield brine, and hydrothermal fluids [5]. e behavior of borate solution extremely complicated, and it can exist in several different species in aqueous solution, such as metaborate B(OH) 4 − , B 2 O(OH) 6 2− , B 3 O 3 (OH) 4 − , B 4  . e distributions of boron species in the system NaCl−NaSO 4 −NaBO 2 −H 2 O were also calculated in our previous work [9]. e preponderant boron species is B(OH) 4 − , whose mole fraction is more than 0.95. e calculated results for distribution of boron species in the system KCl−K 2 SO 4 −K 2 B 4 O 7 −H 2 O at 298.15 K show that the mole fraction of the boron species is mainly affected by the concentration of boron but no other anions in the solution [10].
Boron form depends on boron concentration, pH, temperature, and ionic strength [11]. With the pH values in the solution and equilibrium constant between different boron species, the distribution of boron species was calculated [12][13][14][15]. e calculated results in these literatures [13,14] are in agreement with those with the Pitzer model in the systems NaBO 2 −H 2 O and K 2 B 4 O 7 −H 2 O [9,10]. e concentration of boron species was calculated in references [12][13][14][15], and it was considered that the activity coefficients of all boron ions were 1.0. Although the calculation may not be correct, the calculated results can also describe the distribution of boron species in the solution.
e total boron concentration, pH value, metal cations, and temperature in the medium all affect the existence form and equilibrium relationship of borate anions, among which the total boron concentration and pH value are particularly important. At present, there are relatively few studies on the distribution of chemical species in the solution of multiple systems. NaBO 2 is an important boron compound, especially in the industrial production of NaBH 4 [16,17], and it is a promising hydrogen solid carrier due to its easy hydrolysis and adjustable hydrogen release [18]. e physicochemical properties and distribution in the system NaBO 2 −H 2 O were presented [13], and the distributions for boron species in the mixed system Na 2 SO 4 -NaBO 2 -H 2 O at 298.15 K were also calculated with the Pitzer model [9]. However, the physicochemical properties including pH and density in the system were not reported. e distribution balance of boron species in the solution changes with temperature. In this work, the pH values and density values of the system Na 2 SO 4 -NaBO 2 -H 2 O at 298.15 K and 323.15 K were presented, and the distribution of boron species was also calculated.

Materials and Apparatus.
e chemical reagents used in this experiment are given in Table 1. Experimental water was double deionized water (DDW) with the conductivity less than 1.2·10 −4 S·m −1 and pH � 6.60 at 298.15 K. e values of pH were determined by an imported pH meter (Orion 310P-01A from US, the accuracy of ±0.001 for pH determination).

Experimental Method.
To affirm the effect from Na 2 SO 4 to the boron species distribution in the system NaBO 2 -Na 2 SO 4 -H 2 O, the mix solution with different ratios of Na 2 SO 4 and NaB(OH) 4 (m(Na 2 SO 4 )/m(NaB(OH) 4 )) was prepared. First, the saturated solution in the systems NaBO 2 -H 2 O and Na 2 SO 4 -H 2 O was prepared, respectively. en, the mixed solution with different ratios of Na 2 SO 4 and NaB(OH) 4 was prepared by mixing the two saturated solutions and DDW according to the solubility data of the NaBO 2 -Na 2 SO 4 -H 2 O ternary system at 298.15 K and 323.15 K [19,20]. e concentration of NaBO 2 and Na 2 SO 4 was tabulated, as shown in Figure 1. e mixed solution was then used for physicochemical property measurement. e densities (ρ) were measured using a density bottle with an uncertainty less than ±0.002 g·cm −3 . e pH values were measured three times with the pH meter, and the uncertainty between the measurement results is 0.002. e concentration of SO 4 2-was determined by the barium sulfate gravimetric method, and the relative error is less than ±0.0005 [21]. e concentration of BO 2 was obtained by the modified mannitol gravimetric method [21,22] with the relative error less than 0.003.

Density.
e density values of the NaBO 2 -Na 2 SO 4 -H 2 O ternary system at 298.15 K and 323.15 K are given in Tables 2  and 3, respectively. e density diagrams with the concentration of boron (m(B)) as the abscissa were plotted, as shown in Figure 2. In the same ratio of Na 2 SO 4 and NaB(OH) 4 , the densities increase as m(B) increases, as shown in Figure 2. e densities also increase at the same concentration of one salt as the concentration of the other salt increases. However, the densities decrease at the same concentration with the increase of temperature. With the changing trends in the density diagrams, shown in Figure 2, the densities can be used to roughly estimate the concentrations of Na 2 SO 4 and NaB(OH) 4 in the ternary system.

pH Data and Boron Species Distribution.
e experimental pH of the NaBO 2 -Na 2 SO 4 -H 2 O ternary system with different ratios is given in Tables 2 and 3. From the pH data, the pH diagrams are shown in Figure 3 to show the relationship between m(B) and pH at 298.15 K and 323.15 K. On the same scale, the pH value increases with the increase of the total boron concentration. But more than 1.2 mol·kg −1 ·H 2 O growth rate showed a downward trend. On the same total boron concentration line, the pH value decreases with the increase of Na 2 SO 4 mole fraction. e data show that the pH of the solution in the system NaBO 2 -Na 2 SO 4 -H 2 O is mainly affected by m(B). e mole fraction of Na 2 SO 4 for hydrolysis is lesser than that of NaBO 2 .
From the literature [12][13][14][15], B(OH) 3 was assumed as the reactants B(OH) 4 4 2-, and B 5 O 6 (OH) 4 were formed with B(OH) 3 and H 2 O. e reaction equation between boron species can be represented as e ion equilibrium constants among different boron species were reported under different ionic strengths in different media by potentiometric titration of hydrogen electrode. According to the calculation method in the literature [12][13][14][15], the total boron concentration, the measured pH value, and the equilibrium constant are mentioned in the literature [23,24], as shown in equation (2). e mole fractions of different boron species were calculated using equation (3).
In equation (2), K 11 , K 31 , K 32 , K 32 , and K 51 represent the equilibrium constant equations for B(OH) 4 -, 4 2-, and B 5 O 6 (OH) 4 -. e relationship between molar fraction of different boron species and total boron concentration is shown in Figure 4. e mole fraction (x) of B(OH) 4 in the mixed solution at 298.15 K is no less than 0.98 in the concentration range (0.1349∼3.3448 mol·kg −1 ). e x(B(OH) 3 ) can reach about 0.01 when m(B) is less than 0.14 mol·kg −1 , but decreases to a very small value with the increase of m(B). x(B(OH) 3 ) will be no more than 0.001 if the m(B) is more than 1.3 mol·kg −1 . e x for B 3 O 3 (OH) 5 2-, which was not considered in the calculation with Pitzer model [9], cannot be neglected in this calculation. x(B 3 O 3 (OH) 5 2-) increases as m(B) increases and reach about 0.11 when m(B) is about 4.0 mol·kg −1 . e mole fraction for B 3 O 3 (OH) 4 is always below 0.0005 and can be neglected. e x(B 5 O 6 (OH) 4 -) is about 1.0 × 10 −9 , which can be considered that B 5 O 6 (OH) 4 does not exist in the mixed solution at 298.15 K. In Figure 3 4 in the solution can also be considered to be nonexistent at 323.15 K.
From the changing trend for the six boron species in the mixed solution in the system NaBO 2 -Na 2 SO 4 -H 2 O at 298.15 K and 323.15 K, the boron species may react with the equations (4)- (7). e reaction is shown in Figure 5.
In the diluted solution, B(OH) 3 can be formed by hydrolysis of B(OH) 4 -. As the concentration of B(OH) 4 increases, B 3 O 3 (OH) 4 forms with B(OH) 4 and B(OH) 3 with equation (5). e solution is alkaline because of hydrolysis of B(OH) 4 -, so B 3 O 3 (OH) 4 will combine OHto form B 3 O 3 (OH) 5 2-. As the increase of concentration of B 3 O 3 (OH) 5 2-, B 4 O 5 (OH) 4 2forms with B 3 O 3 (OH) 5 2and B(OH) 4 -. Because of the OHformation and consumption, the concentration of OHfirst increases and then decreases as m(B) increases.
e dominant boron species in the system NaBO 2 -Na 2 SO 4 -H 2 O at the two temperatures is B(OH) 4 with the mole fraction no less than 0.90. From the results of solubility calculation [9,20], it can be considered that boron species mainly exists in the form of B(OH) 4 in the mixed solution. e mole fractions for the six boron species in the mixed solution with different ratios show a slight difference at two temperatures. e results show that the mole fractions of the six boron species in the system NaBO 2 -Na 2 SO 4 -H 2 O are mainly affected by m(B) in the solution, but rarely effected by m(SO 4 2-). From 298.15 K to 323.15 K, the distribution of boron species in the mixed solution changed a lot. As temperature increases, the distribution of boron species becomes complicated. e polyborate ions are easily formed as the temperature increases in the NaBO 2 -Na 2 SO 4 -H 2 O system. e results show that the mole fraction of the six boron species in the system NaBO 2 -Na 2 SO 4 -H 2 O is mainly affected by the temperature.     e distribution of boron species in the system NaBO 2 -Na 2 SO 4 -H 2 O at 298.15 K was also calculated with the Pitzer model in our previous work [9]. In the calculation [9], x(B(OH) 4 − ) is more than 0.96 in the system NaBO 2 -Na 2 SO 4 -H 2 O, which shows that the boron species in the solution in the system can be considered as a single   with the Pitzer model [9], as shown in Figure 6, which shows that the two methods are reliable for the distribution of boron species calculation. However, there are still some differences for the calculation with the two methods. Only four boron species B(OH) 3 , B(OH) 4 -, B 3 O 3 (OH) 4 -, and B 4 O 5 (OH) 4 2were considered in the Pitzer model because of not enough parameters in the literature [9], but six boron species exist in this work. From the calculated results in this work, B 3 O 3 (OH) 5 2cannot be neglected in the mixed solution. e activity coefficients for boron species were not considered in this work, but the activity coefficients were calculated with the Pitzer model. e pH values can be calculated with the Pitzer model. However, the calculation cannot be obtained in this work if the pH values were not known. e results from the two methods show a slight difference, but the two methods can both be used to describe the distribution of boron species in the mixed solution. e mole fractions of boron species are mainly affected by the concentration of total boron and temperature, but rarely affected by the concentration of SO 4 2-. e polyborate ions are easier to form as the temperature increases. e dominant boron species in the system NaBO 2 -Na 2 SO 4 -H 2 O at the two temperatures is B(OH) 4 -. e mole fraction of B(OH) 3   Ref. [9] is work Figure 6: Comparison of the mole fraction of B(OH) 4 − in the system NaBO 2 -Na 2 SO 4 -H 2 O at 298.15 K calculated in this work and with the Pitzer model [9]. , this work; , ref. [9]. solution in the ternary system can be considered to be nonexistent. In the solubility calculation, it can be considered that there is only one boron species B(OH) 4 in the system. e calculation of distribution for boron species in this study and those with the Pitzer model can both be used to describe the distribution of boron species in the mixed solution.

Conclusions
e results on physicochemical properties and boron species distribution calculation in the system NaBO 2 -Na 2 SO 4 -H 2 O can supply theoretical reference for separating sodium metaborate salts from brine and development of universal thermodynamic models of brine systems with various boron species.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.