Abstract

A conductimetric technique has been used to determine binary diffusion coefficients of aqueous hydrofluoric acid and aqueous potassium fluoride at 25°C. Measurements were made at concentrations from 0.001 to 0.1 mol dm −3. In dilute solutions ionization of HF molecules to F − and highly mobile H + (HF ⇌ F − + H +) leads to a rapid increase in the overall rate of diffusion of the HF component. Provided the association equilibrium HF + F − ⇌ HF − 2 is included in the analysis, the results are in good agreement with behaviour predicted by theory for diffusion of incompletely dissociated electrolytes. The limiting diffusion coefficient of the HF molecule, 1.68 (±0.02) × 10 −9 m 2 s −1, has been determined. Contrary to an earlier report, diffusion of dilute KF solutions is correctly described by Onsager-Fuoss theory.

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