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?

Abstract

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 flowthrough 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°(CF 3 SO 3 − ), and the strontium ion, λ°(Sr 2+ ). The TBBK fits to the concentration-dependent equivalent conductivity data for both NaCF 3 SO 3 and Sr(CF 3 SO 3 ) 2 required statistically significant ionic association constants for the species NaCF 3 SO 3 0 at temperatures T > 448 K, SrCF 3 SO 3 + at T > 448 K, and for Sr(CF 3 SO 3 ) 2 0 at T > 548 K. The stepwise association constants, K A , for the charged species SrCF 3 SO 3 + , were found to be greater or equal to than the ones for the neutral species Sr(CF 3 SO 3 ) 2 0 . The experimental value of K A for Sr(CF 3 SO 3 ) 2 0 was found to be similar to that for NaCF 3 SO 3 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.

Extrapolation Methods for AC Impedance Measurements Made with a Concentric Cylinder Cell on Solutions of High Ionic Strength

“…Additionally, because the measurements are acquired with a computer, the instrument permits averaging of many measurements, thereby increasing the accuracy [21], and in our experience, improving efficiency and reducing tedium. The instrument and cell have been described in detail previously [12,13], so only a short description will be given here.…”

“…Method D gives percent differences that are all below 1 %. For these reasons, method D was the preferred extrapolation used in our most recent studies [12,13].…”

“…In the course of measuring the impedance spectra of several solutes at high temperatures and pressures with the conductance instrument built at the University of Delaware in the lab of R. H. Wood, it was found that the extrapolation method was the largest contributor to the experimental uncertainty when measuring solutions with high concentrations [12,13]. This is especially important because high concentration measurements are required to accurately determine association constants involving the formation of triplet ions [14].…”

Ion-pair formation in aqueous strontium chloride and strontium hydroxide solutions under hydrothermal conditions by AC conductivity measurements

“…12,14,15 Measurements were carried out using a series of solutions at concentrations as low as 10 À5 mol L À1 , at temperatures from 295 K up to 625 K at a constant applied pressure of B20 MPa. The first part of this study 16 examined the properties of sodium and strontium triflate in the same series of experimental runs, to determine values for the limiting conductivity of the Sr 2+ ion required to interpret these results. The present paper reports the second and major part of this study, which yielded first and second stepwise association constants of strontium hydroxide and strontium chloride, K A,1 and K A,2 , under these very challenging and aggressive conditions.…”

Prediction of rate constants of important chemical reactions in water radiation chemistry in sub and supercritical water – non-equilibrium reactions

“…Many properties of water depend on temperature and density, and this can significantly change the rate constants of reactions. 10,11,13,15,[67][68][69][70][71][72][73][74][75][76][77][78][79][80] These include the efficiency factor (cage effect), 7 the diffusion rate of different species, and the dielectric constant that influences the reactions of charged species. In this work, we considered all these aspects.…”