The effect of high pressure on the ion pair equilibrium constant of alkali metal fluorides: A spectrophotometric study

Abstract

Bromophenol blue indicator was used in UV-visible spectrophotometric measurements to study ion association constants of alkali metal fluorides. The equilibrium constants for the ion pair formation of the alkali metal fluorides were determined as a function of ionic strength at one atmosphere pressure and 25°C. The effect of pressure on these association constants was measured at a constant total ionic strength of 1.0 mol-kg⁻¹ over a pressure range of 1 to 2000 atmospheres at 25°C. The pressure dependences of the stoichiometric association constants of the alkali metal fluorides are given by: lnK ^* _LiF =0.77–2.47×10⁻⁴P−2.12×10⁻⁸P²; lnK ^* _NaF =0.53–1.08×10⁻⁴P−1.66×10⁻⁸P²; lnK ^* _KF =0.24–4.41×10⁻⁵P−7.15×10⁻⁸P²; lnK ^* _RbF =−0.17–8.65×10⁻⁵P−4.51×10⁻⁸P²; and lnK ^* _CsF = −0.37–1.14×10⁻⁴P−6.82×10⁻⁸P², where P is the pressure in atmospheres. The stoichiometric molar volume and compressibility changes for ion pair formation of the alkali metal fluorides were evaluated from the pressure dependence of K ^* _MF data. The thermodynamic association constants were also calculated making use of activity coefficient data from the Pitzer equations. The partial molal volume and compressibility changes for ion pair formation of each alkali metal fluoride are reported.