Values of electromotive force of the cell without liquid junction Li-ISE│Li₂B₄O₇(m_A)(aq.), MgCl₂(m_B)(aq.)│AgCl/Ag were determinaed at 298.15 K. The mean activity coefficients of LiCl in the system with ionic strengths from 0.05 to 3 mol•kg^－1 at different ionstrength fraction Y_B of MgCl₂ have been calculated, together with the tendencies of these values along with the ion strengths and the concentrations of Mg^2＋ and B₄O₇^2－2B₄O₇-MgCl₂-H₂O system, but also from the isopiestic studies for systems Li₂B₄O₇-MgCl₂-H₂O, Li₂B₄O₇-LiCl-H₂O and Li₂B₄O₇-H₂O. The species of boric acid H₃BO₃ and borate anions B(OH)₄^－ B₃O₃(OH)₄^－ and B₄O₅(OH)₄^2－ have been taken into account in different total boron concentrations. To consider the stoichiometric molality equilibrium constants for the formation of the aqueous species containing boron, Pitzer’s osmotic equation and activity equation have been modified slightly to represent the system, and the ion interaction parameters have been obtained using multiple regression analysis. The standard deviation of the calculated and experimentally determined activity coefficients of LiCl in Li₂B₄O₇-MgCl₂-H₂O system was 0.0167, which reflects that the set up model for the system with boron was adaptable to this four ion system.

Key words: Li₂B₄O₇-MgCl₂-H₂O system, electromotive force, isopiestic, mean activity coefficients, ion interaction model