Ion hydration and association in aqueous potassium phosphate solutions

Abstract

Ionic hydration and ion association in aqueous solutions of KH 2 PO 4, K 2 HPO 4, and K 3 PO 4 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 H 2 PO 4 -), ∼20 (HPO 4 2-), and ∼39 (PO 4 3-). These values are indicative of the existence of a second hydration shell around HPO 4 2- and especially PO 4 3-. Two types of hydrating water molecules could be quantified: irrotationally bound (ib, H 2 O molecules essentially "frozen" on the DRS time scale) and "slow" (loosely bound water molecules with identifiably slower dynamics than bulk water). For H 2 PO 4 - over the entire concentration range and for HPO 4 2- and PO 4 3- at concentrations c ≲ 1 mol L -1, only "slow" H 2 O 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 K 2 HPO 4. 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: KH 2 PO 4 0 (aq) < KHPO 4 - (aq) < KPO 4 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.