Studies On The Thermodynamics Of The Systems Containing Boron And Lithium Li₂B₄O₇-H₂O, Li₂B₄O₇-MgCl₂-H₂O By Isopiestic Method And E.M.F. And The Representat

There are many salt lakes on the QinghaiXizang Plateau. They are renowned for containing high concentrations of boron and lithium. These studies of the thermodynamics of the species in aqueous solutions containing boron and Lithium are of interest in predicting the thermodynamic behaviour of the high ionic strength brine of these salt lakes. In this article, thermodynamic properties on Li₂B₄O₇-H₂O and Li₂B₄O₇-MgCl₂-H₂O have been studied by isopiestic method and e.m.f.. Ion-interaction models based on Pitzer's model have been reported to describe the systems containing boron and lithium.1) At present commercial Li₂B₄O₇ on sale is A.R. grade. The purity is not satisfactory for the demand of thermodynamic studies, and should be purified further however it is difficult to be purified by normal re-crystallization. In this article, a purification method by an improved re-crystallization for Li₂B₄O₇ is reported. A set of equipment is designed and set up for the purification of Li₂B₄O₇. The optimum conditions for purifying Li₂B₄O₇ have been obtained. X-ray of the purified product shows that the product is hydrate lithium tetraborate. The TG and DTG curves of crystallization product show that the mole ratio of Li₂B₄O₇: H₂O is 1: 2.89. The purity of the purified product has been improved 0.5-1% higher than Li₂B₄O₇.of A.R. grade. ICP analysis of the purified product shows that the contents of Fe, Ca have come down by one-tenth.2) Isopiestic molalities, vapor pressures and osmotic coefficients of aqueous Li₂B₄O₇ solutions from 0.0275mol · kg^-1 to 2.9397mol · kg^-1 have been determined at 298.15K using an improved apparatus. The measurements have extended into the diluted and the supersaturated molality region. In general opinion, the molality of 0.1 mol · kg^-1 should be the lower limit for isopiestic measurement under normal circumstances for its complex design chamber even though there are successful investigations below the molalites. However, 6 sets of data for the Li₂B₄O₇(aq) below 0.1mol · kg^-1 have been obtained with satisfactory precision using the improvedapparatus. The rules of the changes in vapor pressure over aqueous Li2B4C>7 solutions were revealed, which indicated that the bond of borate anions with water are better than that of NaCl. The different formation reaction equilibria for the speciescontaining boron of boric acid (H3BO3) and borate anions B(OH) ^, B3O3(OH) and8405(011)4" in Li2B4O7(aq) solutions in different concentration ranges have beentaken into account respectively. The stoichiometric equilibrium constants for the formation reactions of the boron species and the ion-interaction parameters among ion lithium and various borate anions have been estimated simultaneously from the experimental data according the Pitzer equations of osmotic coefficient with minor modifications by iteration method and a least squares analysis to minimize the differences between the experimental values of osmotic coefficients and the calculated. 16 parameters were obtained with the standard deviation 0.0055 for the ion-interaction model. For applying conveniently the ion interaction model simplified from model II was proposed by combining the same kind of parameter terms with standard deviation 0.0150. The studies indicate that the parameterized ion-interaction model combining with chemical equilibria may well represent the thermodynamic properties of osmotic coefficients for the complex system containing polymeric boron anions.3) The saturated solution molalities of the Li2B4O7-MgCl2-H2O system at different mole fraction of MgCfe have not been reported because of its instabilities and uncertainties of its crystal. In this article, a new method studying the saturated solution molalities of the Li2B4O7-MgCl2-H2O system has been reported. The precision of the method is not lower than that of normal solubility equilibrium method.4) The isopiestic molalities and the water activities have been measured by isopiestic method and the water vapor pressures and the osmotic coefficients have been calculated from the measurements in the ion-strength range from 0.0581 mol?kg"1 to 0.6320mol*kg'1 at T=298.15K for the system Li2B4O7-MgCl2-H2O. Allthe 50 sets of data are at different MgCl2 mole fraction of 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.95, and distributes uniformly under the saturated molality lines.5) The AgCl/Ag electrode belongs to thermal electrode type were prepared according to the directions. The capabilities of the electrode have been tested by the cell: Li ISE | LiCl(aq) | AgCl/Ag from I =0.05 to 3 mol-kg'1 of LiCl and its performance showed near-Nerstian behaviour.6) The isopiestic measurement for the Li2B4O7-MgCl2-H2O system can supply information of the behaviours of the solvent H2O, for understanding the system, the actions of the salts of MgCU and Li2B4O? are important too. The e.m.f. method has been performed to study the system and 41 sets of data have been derived. The mean activity coefficients of LiCl for the system have been investigated at constant ionic strengths of 0.05 mol 4cg"\ 0.1 mol 4eg"\ 0.15 mol -kg"1, 0.2 mol -kg'1, 0.25 mol -kg"1, 0.3 mol -kg"1,0.35 mol ?kg"\ 0.4 mol "kg"1,0.45 mol -kg"1,1 mol 'kg'1,2 mol 'kg'1, and 3mol -kg"1 of the several MgCl2 ionic strength fraction (0.05, 0.1,0.2, 0.4, 0.6,0.8, 0.9, 0.95, 0.99) by the cell Li ISE | Li2B4O7(mA)(aq), MgCl2(mB)(aq) | AgCVAg at temperature of 298.15K. The relations between the mean activity coefficients of LiCl and the ionic strengths of Mg2+ and B4O72" have been reported.7) The studies on ion-interaction model based on Pitzer's of the system. For enlarging the using range of model, the model of system are derived from 50 sets of data obtained by isopiestic method, 41 sets of data from e.m.f. and combined with 22 sets of data from the isopiestic measurements of Li2B4C?7 -H2O system, 6 sets data from the isopiestic measurements of Li2B4O7-LiCl-H2O system. The formation reaction equilibria ofboron species of boric acid (H3BO3) and borate anions (B(OH) 4, B3C>3(OH) 4 and8405(011)4") have been taken into account for the system in different total boronconcentration ranges. The model basing on Pitzer's osmotic coefficient equation and activity coefficient equation of Li+ and Cl" with minor modification have been proposed with the ion-interaction parameters related to the ion interactions among theions Li+, Mg2+, Cl\ B(OH);, B3O3(OH);, and B4O5(OH)^. The values of the meanactivity coefficient of LiCl determined experimentally are in reasonable agreement with these calculated by using the model are in reasonable agreement. Pitzer's ion-interaction model has been extended to represent the thermodynamic properties of the complex system containing various borate anions. These studies are useful to improve the thermodynamic model for salt lake brine systems containing lithium and borates.g) The isopiestic molalities, water activities, and osmotic coefficients weredetermined at 298.15K for the synthetic brine systems of Yi Li-Ping, Dong Tai, and Dong Tai with Ca. The thermodynamic properties were compared between the three brines, and a cause of formation of the differences has been indicated physically and chemically. The results reveal that the water activities of the three brines tend to equilibrium.