Diffusion in dilute aqueous solutions of phosphoric acid. Verification of the limiting law for diffusion of weak electrolytes

Abstract

The diffusion coefficient of aqueous phosphoric acid has been measured by a conductimetric technique at 25 °C over the concentration range from 0.0012 to 0.0826 mol dm^–3. Below 0.01 mol dm^–3 dissociation of neutral H₃PO₄ molecules to H⁺ and H₂PO^–₄ produces a sharp increase in the diffusion coefficient of the phosphoric acid component. When a small correction is included to allow for the association reaction H₃PO₄+ H₂PO^–₄⇌H₅P₂O^–₈, the results are in good agreement with predictions based on the limiting law for diffusion of a weak electrolyte. Analysis of the data gives (0.87 ± 0.01)× 10^–9 m² s^–1 for the limiting diffusion coefficient of the aqueous phosphoric acid molecule. Within experimental error this value is identical to the limiting diffusion coefficient of H₂PO^–₄.