The electromotive force measurements have been made with the cells Pt, H2│B(OH)3(m1), LiB(OH)4(m)2, LiCl(m3), MgCl2(m4)│AgCl, Ag and Pt, H2│B(OH)3(m1), LiB(OH)4(m2), MgCl2(m4)│AgCl, Agover the temperature range 278.15～318.15K. The results have been used to determined the standard association constants of ionic pair [MgB(OH)4^+], Kt. An empirical equation, pKt=A1+A2/T+A3T, was obtained, the other thermodynamic quantities of ionic association process were calculated and discussed. An important conclusion was drawed, that is, the association enthopy is the driving force for the ion association reaction.

Key words: SOLUTION, BORIC ACID, MAGNESIUM CHLORIDE, LITHIUM CHLORIDE, LITHIUM BORATE, ASSOCIATION CONSTANT