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

Abstract

Frequency-dependent electrical conductivities of solutions of aqueous strontium hydroxide and strontium chloride have been measured from T = 295 K to T = 625 K at p = 20 MPa, over a very wide range of ionic strength (3 × 10⁻⁵ to 0.2 mol kg⁻¹), using a high-precision flow AC conductivity instrument. Experimental values for the concentration-dependent equivalent conductivity, Λ, of the two electrolytes were fitted with the Turq–Blum–Bernard–Kunz (“TBBK”) ionic conductivity model, to determine ionic association constants, K_A,m. The TBBK fits yielded statistically significant formation constants for the species SrOH⁺ and SrCl⁺ at all temperatures, and for Sr(OH)⁰₂ and SrCl⁰₂ at temperatures above 446 K. The first and second stepwise association constants for the ion pairs followed the order K_A1(SrOH⁺) > K_A1(SrCl⁺) > K_A2[Sr(OH)⁰₂] > K_A2[SrCl⁰₂], consistent with long-range solvent polarization effects associated with the lower static dielectric constant and high compressibility of water at elevated temperatures. The stepwise association constants to form SrCl⁺ agree with previously reported values for CaCl⁺ to within the combined experimental error at high temperatures and, at temperatures below ∼375 K, the values of log₁₀ K_A1 for strontium are lower than those for calcium by up to ∼0.3–0.4 units. The association constants for the species SrOH⁺ and Sr(OH)⁰₂ are the first accurate values to be reported for hydroxide ion pairs with any divalent cation under these conditions.