Emfs for the cell without liquid junction: Pt, H~2 (101.325kPa) |Li~2B~4O~7(m~1), CaCl~2(m~2)|AgCl-Ag are reported at five temperatures from 278.15 to 318.15K. The standard association constant of ionic pair [CaB(OH)~4]^+, K~d is determined by fitting to the cell results. The dependence of pK~d on temperature T is expressed by an empirical equation: pK~d=0.6857-359.72/T-4.632×10^- ^3T. The other thermodynamic quantities of the association process are also calculated. The results show that the driving force for the ion association reaction is association entropy.

Key words: BORATES, CALCIUM COMPOUDS, ION PAIRS, ELECTROMOTIVE FORCES, ASSOCIATION CONSTANT, THERMODYNAMICS, SOLUTION THEORY