Hydrofluoric and hydrochloric acid behaviour in concentrated saline solutions

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 + 1.81I^1/2/(1 + 3.73I^1/2) - 0.06I - 0.0136I² and log K* = 1.822 - 0.982I^1/2/(1 + 0.328I^1/2) + 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.