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

• Published in: Physical chemistry chemical physics : PCCP Vol. 16; no. 33; pp. 17688 - 1774
• Main Authors: Arcis, H, Zimmerman, G. H, Tremaine, P. R
• Format: Journal Article
• Language: English
• Published: England 07.09.2014
• Subjects: Chlorides; Conductivity; Constants; Hydroxides; Ion pairs; Mathematical models; Resistivity; Strontium
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c4cp01703j

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) 0 2 and SrCl 0 2 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) 0 2 ] > K A2 [SrCl 0 2 ], 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 log 10   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) 0 2 are the first accurate values to be reported for hydroxide ion pairs with any divalent cation under these conditions. Stepwise ion pair formation constants for aqueous strontium chloride and strontium hydroxide have been determined from T = 295 K to T = 625 K at p = 20 MPa, using a high-precision flow AC conductivity instrument.