Hydrofluoric and hydrochloric acid behaviour in concentrated saline solutions

Abstract

Ion-selective electrode measurements of mean NaF and Cl/F activities in concentrated NaCl media at 25 °C have been used to estimate Pitzer activity coefficient model parameters θCl,F(0.01), ψNa,Cl,F(0.002 33), and ψK,Cl,F(–0.003 3). Dissociation constants (K1) of HF and stability constants (K*) of MgF+ were determined in NaCl media at 25 °C up to I= 5.0 mol kg–1 and are described by: log K1=–3.165 + 1.81 I½/(1 + 3.73I½)– 0.06I- 0.0136I2 and log K*= 1.822 – 0.982I½/(1 + 0.328I½)+ 0.0713I+ 0.0169I2. Pitzer model parameters βMgF(0)(3.932), βMgF(1)(–25.17), and CφMgF(–5.294) were also derived. A Henry's law expression for the reaction HX(g)→ H+(aq)+ X–(aq)(KH/mol2 kg–2 atm–1) was applied to HCl (KH= 2.04 × 106) and HF (KH= 9.61). Partial pressures and speciation of the acids in acidified NaCl–MgCl2 media were then calculated over a range of concentrations. The results suggest that fluoride (as HF) is expelled preferentially to chloride from naturally acidified marine aerosols.