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 θ_Cl,F(0.01), ψ_Na,Cl,F(0.002 33), and ψ_K,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 I^½/(1 + 3.73I^½)– 0.06I- 0.0136I² and log K*= 1.822 – 0.982I^½/(1 + 0.328I^½)+ 0.0713I+ 0.0169I². Pitzer model parameters β_MgF⁽⁰⁾(3.932), β_MgF⁽¹⁾(–25.17), and C^φ_MgF(–5.294) were also derived. A Henry's law expression for the reaction HX(g)→ H⁺(aq)+ X^–(aq)(K_H/mol² kg^–2 atm^–1) was applied to HCl (K_H= 2.04 × 10⁶) and HF (K_H= 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.