Reply to Dr. Stoesselfs comment on “Reaction paths and equilibrium end-points in solid-solution aqueous-solution systems”

In reply to the Critical Comment of R. K. Stoessell (this issue), limiting activity coefficients of bromide in halite (γ_NaBr) have been calculated by least-squares fitting of Simons et al.'s (1952) bromide distribution coefficient data for the Na(Cl,Br)-NaOH-H₂O system at 35°C. Regular and subregular solidsolution model fits give γ_NaBr = 7.4 and γ_NaBr = 8.8, respectively. The Br contents of halite at equilibrium with seawater at initial halite saturation, calculated from the regular and subregular fits, are 17 ppm and 14 ppm, respectively. A survey of literature data for trace bromide in halite shows a wide spread in distribution coefficients, with lower values (D_Br≈ 0.01) reported by Bloch and Schnerb (1953), Puchelt et al. (1972), and Lutz (1975), and higher values (D_Br− ≈ 0.03) reported by Braitsch and Herrmann (1963), Kühn (1968), Herrmann (1972), Herrmann (1980), Mccaffrey et al. (1987), valiashko et al. (1976), Valiashko and Lavrova (1976), and Fontes (pers. commun., 1990). The measurement of stoichiometric saturation states for halite (or sylvite) with trace bromide mole-fractions is not practical, given the insensitivity of the measured solubilities on the bromide mole-fractions. Distribution coefficient measurements, with proof of thermodynamic equilibrium, need to be obtained instead, to conclusively determine the thermodynamic-mixing properties of both Na(Cl,Br) and K(Cl,Br) solidsolution series at very low mole-fractions of bromide. The applicability of the stoichiometric saturation concept to the interpretation of precipitation processes is questionable, primarily because the concept requires solid-solutions to behave as one-component solids with fixed composition. Lippmann diagrams are useful in depicting stoichiometric saturation, endmember saturation, and thermodynamic equilibrium states in binary-solid-solution aqueous-solution systems. Lippmann diagrams can contribute a better understanding of these systems, regardless of the concentration of the endmember components.