Structure of the solubility diagram in the Na2SO4-Na2CO3-NaHCO3-H2O system at 0, 25 and 50 ?? 104 L. Soliev, M. T. Jumaev, R. O. Turaev, H. R. Makhmadov, B. B. Dzabborov Tajik State Pedagogical University named after S. Ayni 121 Prospect Rudaki, Dushanbe, 734003, Tajikistan E-mail: Soliev.Lutfullo@yandex.com; jumaev_m@bk.ru Structure of the solubility diagram in the Na 2 SO 4 ‑Na 2 CO 3 ‑NaHCO 3 ‑H 2 O system AT 0, 25 and 50 °С The solubilities in invariant points of Na 2 SO 4 -Na 2 CO 3 -NaHCO 3 -H 2 O system were investigated at 0, 25 and 50 °С. The phase equilibria in the said system were discussed, and phase diagrams at given temperatures were constructed. Keywords: solubility, phase equilibria, liquid phase, chemical analysis, crystallo-optical analysis, phase diagram Received: 07.07.2018. Accepted: 23.07.2018. Published: 30.07.2018. © Soliev L., Jumaev M. T., Turaev R. O., Makhmadov H. R., Dzabborov B. B., 2018 Introduction Four-component system Na2SO4- Na2CO3-NaHCO3-H2O is a part of more complex six-component system Na, Са// SO4, CO3, HCO3, F-H2O. Equilibria in the latter determine the conditions of  alu- minium production liquid waste disposal. The waste water of cryolite recycling plants contains fluorides, carbonates, bicarbonates and sulphates of sodium and calcium [1, 2]. Crystallization and dissolution processes in such waste water are governed by the phase equilibria both in  six-component system Na, Са//SO4, CO3, HCO3, F-H2O and in its constituents, five- and four-com- ponent systems. In our earlier studies [3, 4] the phase diagrams in similar systems were construc- ted. This study presents the results of inves- tigation of Na2SO4-Na2CO3-NaHCO3-H2O system at 0, 25 and 50 °C using solubility method. The main goal of this work was to establish the concentration parameters of geometrical images and separation of the crystallization fields of individual equilib- rium solids in the phase diagrams. Results and discussion The system investigated contains the fol- lowing equilibrium solid phases: Nk – nah- colite NaHCO3 (0, 25, 50 °C); Mb – mirabilite Na2SO4·10H2O, С·10 – Na2CO3·10H2O (0, 25 °C); Tr – throne NaHCO3·Na2CO3·2H2O (25, 50  °C); Th  – thenardite Na2SO4, Bur  – burkeite 2Na2SO4·Na2CO3, С·1  – Na2CO3·H2O (50 °C) [5, 6]. The following reagents were used in  xperiments: Na2SO4·10H2O (“chemi- cally pure” grade), Na2СО3 (“pure” grade), NaHСО3 (“pure” grade). The xperiments were carried out according to “saturation method” described in detail elsewhere [8]. Based on the data available [5, 6], we prepared the mixtures of precipitates with saturated solutions according to the invari- D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 2. 02 Soliev L., Jumaev M. T., Turaev R. O., Makhmadov H. R., Dzabborov B. B. Chimica Techno Acta. 2018. Vol. 5, No. 2. P. 104–108. ISSN 2409–5613 105 Fig. 1. Micrographs of equilibrium solid phases, corresponding to the invariant points of system Na2SO4-Na2CO3-NaHCO3-H2O at 0 (1, 2, 3, 8), 25 (1, 2, 3, 4, 9, 10) and 50 (2, 4, 5, 6, 7, 11, 12, 13) °C 106 ant points in the three-component systems Na2SO4-Na2CO3-H2O, Na2SO4-NaHСО3- H2O and Na2CO3-NaНСО3-Н2О at 0, 25 and 50 °C. Then, transferring the non-var- iant points from the three-component sec- tion to the four-component section [3, 4], the saturated solutions prepared had been kept in a thermostat at a given temperature until the equilibrium was reached. Thermostating was carried out in U-8 ultra-thermostat. Stirring was performed using a  PD-09 magnetic stirrer for  50– 120 h. Temperature was maintained with 0.1 °C accuracy using a contact thermo- meter. Crystallization of solid phases was observed with a  POLAM-R 311 micro- scope. After the equilibrium in  a  given system was achieved, the solid phases were photographed with a Sony-DSC-S500 digital camera. Equilibrium was assumed to be attained when the phase composition of the precipitates was constant. A Buchner funnel with an ash-free filter paper (Blue Band) connected to  a  vacu- um pump has been used for  separation of  the liquid phase and solid phase. The precipitate after filtration was washed with 96 % ethanol and then dried at 120 °С. The Table 1 Solubility in central (invariant) points in system Na2SO4-Na2CO3-NaHCO3-H2O № of points Structure of a liquid phase, mas. % Phase composition of depositsNa2SO4 Na2СО3 NaНСО3 H2O 0 °C E1 3 2.73 – 5.58 91.69 Mb+Nk E2 3 – 5.6 4.6 89.8 Nk+ С·10 E3 3 2.8 6.0 – 91.2 С·10+Mb E1 4 2.12 5.13 4.37 88.38 Mb+Nk+С·10 25 °C E1 3 16.4 18.3 – 65.3 Mb+С·10 E2 3 20.68 – 4.16 75.16 Nk+Mb E3 3 – 17.62 4.62 77.76 Tr+Nk E4 3 – 22.46 2.84 74.7 С·10+Tr E1 4 21.2 20.07 5.51 50.22 Mb+Tr+С·10 E2 4 20.9 22.54 4.77 50.68 Nk+Tr+Mb 50 °C E1 3 29.65 – 4.05 66.30 Тh+Nk E2 3 22.47 10.52 – 67.61 Тh+Bur E3 3 5.87 28.52 – 65.61 Bur+С·1 E4 3 – 16.92 6.30 76.78 Nk+Tr E5 3 – 31.80 0.85 67.35 Tr+С·1 E1 4 12.64 21.31 2.51 54.76 Тh+Nk+Bur E2 4 4.30 24.36 0.64 58.03 Bur+Tr+С·1 E3 4 7.52 9.14 3.24 60.08 Tr+Nk+Bur 107 standard techniques used for the chemi- cal analysis of products are described else- where [8–10]. Results of  the crystallooptical analy- sis [11] of equilibrium solid phases (mi- crophoto) are presented in Fig. 1, and the results of the chemical analysis of the satu- rated solutions are given in Table 1. On the basis of  the data obtained, the diagrams of solubility in the Na2SO4- Na2CO3-NaHCO3-H2O system at 0, 25 and 50 °C were constructed. Salt parts of these diagrams are shown in  Fig.  2. The loca- tion of non-variant points on the diagrams were determined by  the center of  mass method [12]. Fig. 2. Solubility diagrams in Na2SO4-Na2CO3- NaHCO3-H2O system: а – at 0 °C; b – at 25 °C; c – at 50 °C a b c 108 References 1. Morozova VA, Rzhechitskii EP. [Solubility in the NaF – Na2SO4 – NaHCO3 – H2O system at 0 °C]. Zhurnal Prikladnoi Khimii [Journal of Applied Chemistry]. 1976;49(5):1152–4. Russian. 2. Morozova VA, Rzhechitskii EP. [Solubility in the systems NaF – NaHCO3 – H2O, NaF – Na2SO3 – H2O and NaF –Na2CO3 – H2O at 0 °C]. 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