Ion Hydration and Association in Aqueous Potassium Phosphate Solutions

Ionic hydration and ion association in aqueous solutions of KH2PO4, K2HPO4, and K3PO4 at 25 degrees 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 similar to 11 (for H2PO4-, similar to 20 (HPO42-), and similar to 39 (PO43-). These values are indicative of the existence of a second hydration shell around HPO42- and especially PO43-. Two types of hydrating water molecules could be quantified: irrotationally bound (ib, H2O molecule essentially "frozen" on the DRS time Scale) and "slow" (loosely bound water molecules With identifiably slower dynamics than bulk water). For H2O4- over the entire concentration range and for HPO42- and PO43- at concentrations c less than or similar to 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, similar to 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) < KPO42-(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.