Frequency-dependent molar electrical conductivities for aqueous solutions of potassium borate, and sodium borate have been measured from ambient to near-critical temperatures and pressures to an accuracy of ±3 percent, using a unique high-precision flow-through AC conductance instrument. The concentration dependence of these conductivities was analyzed with the Turq-Blum-Bernard-Kunz ("TBBK") theoretical model to yield (i) limiting conductivities of the borate ion, λ(0)[B(OH)4(-)], and (ii) ion-pair formation constants, KA, for the species NaB(OH) and KB(OH) from T = 298 K to T = 623 K at a constant pressure p ∼ 20 MPa. The ion-pair formation constants for both borate salts were found to be consistent with previous literature studies at temperatures below 473 K. No significant difference in KA was observed between the species NaB(OH) and KB(OH). As temperature was increased from 473 up to 623 K, the degree of association increased significantly, and was found to be considerably higher than for any other 1-1 electrolyte previously studied. For instance, at 623 K, the association constant log KA[NaB(OH)] = 2.75 ± 0.21 was an order of magnitude higher than log KA[NaCl(0)] = 1.53 ± 0.03, and approximately equal to that of a 2 : 1 electrolyte, log KA[SrCF3SO3(+)] = 2.58 ± 0.06. Deviations in the limiting conductivities from Stokes Law show that the borate ion's unusual "structure making" effect, observed by other workers at sub-ambient conditions, persists up to temperatures above 500 K. The temperature dependence of the Walden product ratio is very different from that observed for other monovalent anions for which experimental data are available over this wide range of temperatures.

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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) 40 , 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 [LiOH 0 ], derived from critically evaluated literature data is also reported. These results provide selfconsistent thermodynamic constants suitable for modeling primary coolant chemistry for most current PWR reactor types.

Experimental values for the ionization constant of water, pKw,m, from T = 373 K to T = 674 K and from p = 5.75 MPa to p = 31.15 MPa, have been derived from direct measurements of the electrical conductivity of very pure water at the University of Guelph, the University of Delaware, and the Oak Ridge National Laboratory using high-precision high-temperature flow-through AC electrical conductance instruments based on the design by Wood and co-workers [J. Phys. Chem. 99, 11612 (1995)]. The results compare well with published high-temperature potentiometric and calorimetric studies up to 573 K and are consistent with the 1981 and 2006 IAPWS (International Association for the Properties of Water and Steam) pKw,m formulations to within better than 0.1 pK units up to 598 K and to better than 0.2 pK units at 623 K. Above 623 K, the 2006 and 1981 IAPWS formulations showed systematic deviations from the new results, which reached two and five orders of magnitude near the critical point, respectively. Based on these conductivity studies and critically evaluated literature data, revised parameters for the Marshall–Franck and Bandura–Lvov equations of state are reported, which reproduce the experimental data with standard uncertainties u(pK) = 0.018 and u(pK) = 0.016, respectively, over the experimental temperature range at water densities from 1.00 g cm−3 to 0.20 g cm−3, which corresponds to T = 373 K–674 K from psat to p = 31 MPa, and over the range T = 273 K–373 K at p = 100 kPa. These new experimental conductivity results are the most accurate values to be reported under near-critical conditions for densities between 0.50 g cm−3 and 0.20 g cm−3.

Frequency-dependent molar electrical conductivities for aqueous solutions of boric acid with small additions of sodium hydroxide or potassium hydroxide have been measured from 298 to 573 K at 12 MPa to an accuracy of ±3%, using a unique high-precision flow-through AC conductance instrument. The measurements were made to yield accurate polyborate equilibrium formation constants and limiting conductivities under the primary coolant conditions of pressurized water nuclear reactors (PWRs). Complementary measurements from T = 283 K to T = 313 K at p = 0.1 MPa were completed using a commercial conductivity probe to extend the data to lower temperatures. Up to 423 K, measurements were performed under conditions where the triborate ion B3O3(OH)4 – is the major anionic species in solution. At temperatures above 473 K, the conditions favored the formation of a different anionic species, which was assigned to be the diborate ion B2(OH)7 –. The analysis of the concentration dependence of the molar conductivity data using the TBBK equation yielded limiting conductivities, λ0, and formation constants, K b 31,m and K b 21,m , for triborate and diborate from 283 to 473 K, and from 473 to 573 K, respectively. These results are consistent with the only other experimental study that has reported equilibrium constants for aqueous polyborates above 373 K (Mesmer et al. Inorg. Chem. 1972, 11, 537). The limiting conductivities are the first to be reported in the literature.