Ion Hydration and Association in Aqueous Potassium Phosphate Solutions

Ionic hydration and ion association in aqueous solutions of KH₂PO₄, K₂HPO₄, and K₃PO₄ at 25 °C up to high concentrations have been investigated using dielectric relaxation spectroscopy (DRS). The three phosphate anions were found to be extensively hydrated, with total hydration numbers at infinite dilution of ∼11 (for H₂PO₄^–), ∼20 (HPO₄^2–), and ∼39 (PO₄^3–). These values are indicative of the existence of a second hydration shell around HPO₄^2– and especially PO₄^3–. Two types of hydrating water molecules could be quantified: irrotationally bound (ib, H₂O molecules essentially “frozen” on the DRS time scale) and “slow” (loosely bound water molecules with identifiably slower dynamics than bulk water). For H₂PO₄^– over the entire concentration range and for HPO₄^2– and PO₄^3– at concentrations <i>c</i> ≲ 1 mol L^–1, only “slow” H₂O was detected; however, at higher concentrations of the latter two anions, an increasing fraction of ib water appears, making up ∼50% of the total hydration number close to the saturation limit of K₂HPO₄. Contrary to common belief, all three salts showed significant ion pair formation, with standard association constants of the 1:1 species increasing in the order: KH₂PO₄⁰(aq) < KHPO₄^–(aq) < KPO₄^2–(aq). The main type of ion pair in solution shifted from solvent-shared ion pairs (SIPs) to double-solvent-separated ion pairs (2SIPs) in the same sequence