Ion-Pair Formation Constants of Lithium Borate and Lithium Hydroxide under Pressurized Water Nuclear Reactor Coolant Conditions

A custom-made high-precision flow AC conductance instrument was used to measure the frequency-dependent molar electrical conductivities of aqueous solutions of lithium borate from T = 298 K to T = 548 K at a constant pressure, p = 20 MPa. Ion-pair formation constants of lithium borate, K A,m [LiB­(OH)4 0], were derived from these measurements using the Turq–Blum–Bernard–Kunz (TBBK) conductivity model. Our results are consistent with previous low-temperature studies and are the first to be reported at temperatures above 318 K. Under ambient conditions, the degree of association of LiB­(OH)4 0 is half an order of magnitude higher than that of NaB­(OH)4 0 and KB­(OH)4 0, but at T ≥ 448 K, the association constants of all three ion-pairs are equal within the combined experimental uncertainties. The results have been used to derive new equations to represent the temperature dependence of the limiting conductivity of lithium, and the ion-pair formation constant of lithium borate under PWR operating conditions. A new model for the ion-pair formation constant of lithium hydroxide, K A,m [LiOH0], derived from critically evaluated literature data is also reported. These results provide self-consistent thermodynamic constants suitable for modeling primary coolant chemistry for most current PWR reactor types.