Abstract: The activity coefficients of aqueous LiCl in system LiCl-MgCl_2-H_2O were determined at 25 ℃ in the total ionic strength range from 0.05 to 6.0 mol·kg~(-1) by E.M.F. method using a lithium ion-selective electrode and Ag/AgCl electrode.
The electrode pairs used in present work were prepared in our laboratory and performance tests were carried out. linear near-Nernst response, good reproducibility, stability and selectivity were observed.
The Harned coefficients, Pitzer ternary interaction parameters and Scatchard parameters were evaluated from present measurement by using multiple regression. The whole range of the ionic strengths studied.
The experimental activity coefficients were compared with which calculated by Pitzer equation with known parameters from experimental osmotic coefficients.

Key words: Activity coefficients, Ternary system LiCl-MgCl₂-H₂O, Li sensitive electrode, E.M.F. method