Limiting Conductivities and Ion Association in Aqueous NaCF3SO3 and Sr(CF3SO3)2 from (298 to 623) K at 20 MPa. Is Triflate a Non-Complexing Anion in High-Temperature Water?

Frequency-dependent electrical conductivities of solutions of aqueous sodium trifluoromethanesulfonate (“triflate”) and strontium triflate have been measured from T = 298 K to T = 623 K at p = 20 MPa, over a very wide range of ionic strength [(2·10–5 to 0.35) mol·kg–1], using a unique high-precision flow-through AC electrical conductance instrument. Experimental values for the equivalent conductivity, Λ, of each electrolyte were used to calculate their equivalent conductivities at infinite dilution, Λ°, with the Turq–Blum–Bernard–Kunz (TBBK) ionic conductivity model. Values were derived for the limiting equivalent conductivity of the triflate ion, λ°(CF3SO3 –), and the strontium ion, λ°(Sr2+). The TBBK fits to the concentration-dependent equivalent conductivity data for both NaCF3SO3 and Sr­(CF3SO3)2 required statistically significant ionic association constants for the species NaCF3SO3 0 at temperatures T > 448 K, Sr­CF3SO3 + at T > 448 K, and for Sr­(CF3SO3)2 0 at T > 548 K. The stepwise association constants, K A, for the charged species Sr­CF3SO3 +, were found to be greater or equal to than the ones for the neutral species Sr­(CF3SO3)2 0. The experimental value of K A for Sr­(CF3SO3)2 0 was found to be similar to that for NaCF3SO3 0 at 548 K but increased more steeply with temperature. At temperatures above 548 K, association constants derived from the concentration-dependent equivalent conductivities were increasingly sensitive to the assumptions used to calculate solution densities. Procedures for minimizing these effects are reported. The temperature dependence of the experimental association constants and limiting equivalent conductivities from (298 to 623) K could be represented accurately as functions of solvent density and viscosity, respectively.